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									     NASA Architecture for Solar System Time Synchronization and
                            Dissemination: Concept of Operations




                              Larry Felton, Lee Pitts, Frank VanLandingham

                                 Computer Sciences Corporation (CSC)
                              Huntsville Operations Support Center (HOSC)
                                  NASA Marshall Space Flight Center




                                                     November 2007




Dwarf planet Pluto and satellites. Credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the HST Pluto Companion Search Team



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                                                  ABSTRACT
NASA requires a standardized approach and infrastructure for disseminating time information and synchronizing
time between NASA‘s assets distributed throughout the Solar System. These are essential to meet NASA's scientific
and exploration requirements, increase interoperability between disparate mission assets, and ensure efficient use of
resources. To achieve this objective, a NASA Time Architecture and corresponding Time Concept of Operations
(ConOps) must be developed. This paper describes requirements for a conceptual NASA Time Architecture and
associated operations concepts based on user needs and requirements abstracted from a survey of current and future
missions. The requirements and concepts presented in this paper can serve as the basis for user services in
developing an individual mission operations plan, as well as inform and guide the development of standard time
synchronization and distribution services provided by NASA via the Space Communications and Navigation
infrastructure. The concept of Mission Domains is introduced to differentiate regions of the Solar System within
which NASA has or will have sufficient assets for a "local" time infrastructure. Operational concepts are presented
to illustrate and discuss methods of time transfer within and between mission domains. The paper concludes with
recommendations for improving and gaining a broader understanding of the Time Architecture Requirements and
the associated ConOps. This work was supported under NASA Marshall Space Flight Center Contract
NNM04AA07C.




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                                                               TABLE OF CONTENTS

1.      Introduction .........................................................................................................................................................4
2.      Approach and Objective .......................................................................................................................................5
3.      Survey of Current and Future Missions ...............................................................................................................5
4.      Requirements .......................................................................................................................................................7
5.     Architecture..........................................................................................................................................................9
   5.1.     Accuracy / Precision ................................................................................................................................. 10
   5.2.     Timescale .................................................................................................................................................. 12
   5.3.     Transmit/Receive Stations ........................................................................................................................ 13
   5.4.     Relays ....................................................................................................................................................... 13
   5.5.     Radiometric .............................................................................................................................................. 14
   5.6.     User Platforms .......................................................................................................................................... 15
   5.7.     Correlation and Transfer Methods ............................................................................................................ 15
   5.8.     User Accommodations ............................................................................................................................. 17
6.     Operations Concept Development ..................................................................................................................... 17
   6.1.     Mission Domains ...................................................................................................................................... 18
   6.2.     Basic Tenets.............................................................................................................................................. 20
   6.3.     Intra- and Inter-Domain Relationships ..................................................................................................... 24
7.     Summary ............................................................................................................................................................ 25
8.      Recommendations .............................................................................................................................................. 25
9.      Glossary ............................................................................................................................................................. 27
10.     Abbreviations and Acronyms ............................................................................................................................. 28
11.     References .......................................................................................................................................................... 31
12.     Acknowledgements ............................................................................................................................................ 33
Appendix – Mission Timing Survey Detailed Data ..................................................................................................... 36
  A-1. Time Precision ................................................................................................................................................. 39
  A-2. Navigation Precision ....................................................................................................................................... 41
  A-3. Time Requirements ......................................................................................................................................... 44
  A-4. Communication Services ................................................................................................................................. 46
  A-5. Factors Considered in Spacecraft Clock Correlation Determination ............................................................... 49
  A-6. Factors That Cause Problems in Spacecraft Clock Correlation Determination ............................................... 53

                                                                    LIST OF FIGURES
Figure 1. Mission Timing Survey Respondents and Mission Web Sites........................................................................7
Figure 2. Time Architecture Functional Requirements ................................................................................................7
Figure 3. Time Architecture Derived Requirements .....................................................................................................9
Figure 4. Time Architecture Components................................................................................................................... 10
Figure 5. Mission Time Precision Classification ....................................................................................................... 11
Figure 6. Relays in a Challenged Network ................................................................................................................. 14
Figure 7. Time Transfer Method Characteristics ...................................................................................................... 16
Figure 8. Time Transfer Delays ................................................................................................................................. 16
Figure 9. Mission Domain Definitions ....................................................................................................................... 18
Figure 10. Time Architecture Mission Domains......................................................................................................... 19
Figure 11. Time and Infrastructure Relationships Within and Between Domains ...................................................... 25
Figure 12. Mission Timing Survey Contributors ........................................................................................................ 34




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1. Introduction
NASA Headquarters established the Space Communications Architecture Working Group (SCAWG) to help define
an agency-wide space communications and navigation architecture. For NASA to adopt and implement this
infrastructure, a de facto time dissemination architecture is required. Thus, the SCAWG established a Time Team to
identify the time architecture requirements. An initial study was performed and the results were published in the
Time Team's Interim Report [1], which states:

    NASA’s users are a heterogeneous mix with regard to needs for and uses of time. … the time architecture
    is not merely a consequence of the communications and navigation design, but rather is an integrated
    architectural aspect responsive to a set of requirements arising from the needs of its users and implemented
    in a manner both cost-effective and efficient in providing desired services..

Because of the close linkage to the users, the Team recommended that a mission survey be undertaken to understand
the broad nature and scope of user requirements, and that a Time Architecture Concept of Operations (ConOps) be
developed. This document presents the ConOps, and reflects the survey results to date. Note that since the
publication of the interim report, the SCAWG has been reorganized under NASA Headquarters' Space Planning
Working Group, which now charters this work as part of the Position, Navigation and Time (PNT) Study Group's
responsibilities.

On June 27, 2007, the PNT Study Group sponsored the first NASA Forum on Time Synchronization and
Dissemination for the Solar System Technical Information Meeting (TIM), which was held at NASA Headquarters
in Washington, D.C. The goals were to assemble interested participants to discuss issues and concepts; provide an
opportunity to exchange ideas; identify potential issues to be resolved in formal requirements and systems planning
forums; and to seek community consensus on concepts and the way forward. Topics discussed ranged from
fundamental requirements and time transfer methods to operational issues and considerations. A preliminary version
of the ConOps was presented at the TIM [2], and valuable feedback was supplied by the participants.

It is anticipated that the time architecture requirements and the discussion of architecture concepts will inform and
guide the development of standard time synchronization and distribution services provided by NASA via the Space
Communications and Navigation (SCaN) infrastructure. The PNT Study Group is responsible for developing the
architectural requirements for time synchronization and distribution. The SCaN Networking Architecture and
Standards Program is responsible for translating these requirements into a set of standard time services that will be
provided by the SCaN infrastructure and for developing the internationally agreed upon standard protocols that
SCaN will provide its user community. This ConOps serves as a bridge between the time architectural requirements
and the standard time services. It is a basis for translating the requirements of the Time Architecture into operational
services that will be provided to current and future users.

A beneficial side-effect of compiling, reviewing and discussing the timing requirements for currently operating and
future missions is the resulting spectrum of information (Section 3, Appendix): timing accuracies and precisions
required and achieved; methods of timekeeping and tracking; time dissemination, distribution, and correlation issues
and corresponding solutions; and suggestions for future timing systems to facilitate timing or to improve accuracy.
This information is necessary to specify a Time Architecture that will successfully support future science missions
and the Vision for Space Exploration (VSE), which says in part:

        Develop the innovative technologies, knowledge, and infrastructures both to explore and to support
         decisions about the destinations for human exploration; and

        Promote international and commercial participation in exploration to further U.S. scientific, security, and
         economic interests.

This effort should also be viewed in the context of the U.S. Space-Based Positioning, Navigation, and Timing (PNT)
Policy, signed on December 8, 2004, which includes the goals:


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       Ensure U.S. PNT services exceed, or are at least equivalent to, those of foreign space-based PNT services

       Continue to improve the performance of space-based PNT.

The following sections state the ConOps objective; summarize the results of the mission timing survey; list the
functional and derived requirements for a NASA Solar System-based time architecture; and describe the
characteristics of a notional time architecture‘s components and present corresponding operations concepts. The
paper concludes with a summary and recommendations for further refining the Time Architecture requirements and
the ConOps. Ancillary information is also provided, including a glossary of terms, a compilation of acronyms and
abbreviations used in this paper, and a list of references.

2. Approach and Objective
The goal of this ConOps is to identify and define the operational concepts to which the time architecture must apply.
The NASA Time Architecture and standard time services must satisfy the operational requirements of future
missions. Therefore, to identify the operational requirements, a survey was conducted of current and planned
missions (Section 3). From the acquired information, requirements (Section 4) and notional time architecture
components (Section 5) were developed. Based on these requirements and components, the Operational Concepts
were outlined as a function of Mission Domain (Section 6).

This approach for the development of the Time Synchronization and Dissemination ConOps provides information
and operations environments that are ―essential to identify the most important factors or dimensions that bound the
analysis space―[3] and support the development of the Time Architecture. Broader success will be achieved by
having a Time Architecture and standard time services for which the requirements and design defined are
synchronized with the operational needs of all planned and future missions as defined in the Time ConOps.

In summary, the objective is to produce an operations concept that:

       Supports the generation of the Time Architecture and results in architectural requirements and operations
        concepts that are in synchronization.

       Is adaptable to missions with more stringent requirements.

       Provides for interoperability between diverse missions.

       Promotes uniformity in function.

       Reduces cost by focusing on needed requirements and attempting to efficiently accommodate with standard
        time services those requirements that are common across missions.

3. Survey of Current and Future Missions
An objective of this effort was to gather operational information about the timing requirements of currently
operating and planned missions. This information can be used to determine time architecture requirements based
upon current mission experience and upon future mission requirements. Such a database thus serves as an important
guide for constructing an appropriate time architecture. The Mission Timing Survey Detailed Data tables in the
Appendix summarize the data thoughtfully provided by the respective mission contacts. The limitations in the
current database reflect the time and resources available for this study. Useful follow-up would entail obtaining
information from more missions, talking to the responding missions to further understand their mission specifics,
and working on data consistency across missions. That said, however, there is sufficient data from a reasonable
mission spectrum to support this study effort.

Mission selection comprised two phases – mission identification and contact identification. The NASA
Headquarters Science Mission Directorate website provided an initial list of missions. This was supplemented with
missions supported and tracked by NASA Goddard Space Flight Center‘s (GSFC‘s) Near Earth Networks Services.

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These were the primary sources for mission identification. Missions were added to the list from those suggested by
colleagues familiar with the task.

Two principal sources were used to identify the appropriate contact – contacts known to the team members and
those cited in the respective mission web site. The use of known contacts proved more reliable than those from the
mission web sites. The identification of mission contacts was a major factor in culling the number of missions
selected to participate in the survey.

Forty-three missions were contacted and responses were received from 32. Figure 1 lists the missions that responded
to the survey and provided the data referenced in this report. Section 12 acknowledges those who supported and
responded to this survey—their knowledge and time made it possible.

   Mission                                    Web Site                                 Launch           Domain

ACE                http://www.srl.caltech.edu/ACE/ASC/related_sites.html          1997 Aug.                D3
Aquarius           http://aquarius.nasa.gov/                                      2009                     D1
Cassini            http://saturn.jpl.nasa.gov/                                    1997 Oct.                D3
Chandra            http://chandra.harvard.edu                                     1999 Jul.                D2
CloudSat           http://cloudsat.atmos.colostate.edu/                           2006 Apr.                D1
Con. X             http://constellation.gsfc.nasa.gov/                            2015 After               D3
Dawn               http://www.dawn.jpl.nasa.gov                                   2007 Sep.                D3
Gaia               http://gaia.esa.int/                                           2011 Dec                 D3
GFO                http://gfo.bmpcoe.org/gfo                                      1998 Feb.                D1
GLAST              http://glast.gsfc.nasa.gov/                                    2007 Oct.                D1
                                                                                  2010 Dec. (Core)
GPM                http://gpm.gsfc.nasa.gov                                       2014                     D1
                                                                                  (Constellation)
Herschel
Space              http://www.esa.int/science/herschel                            2008 Jul.                D3
Observatory
IMAGE              http://image.gsfc.nasa.gov/                                    2000 Mar                 D2
INTEGRAL           http://integral.esac.esa.int                                   2002 Oct.                D2
Juno               http://juno.wisc.edu/                                          2010                     D3
                                                                                  No Earlier Than
LISA               http://lisa.nasa.gov                                                                    D3
                                                                                  2015
LRO                http://lunar.gsfc.nasa.gov/                                    2008 Oct.                D2
Mars Phoenix       http://phoenix.lpl.arizona.edu/                                2007 Aug.                D4
Mars Rovers        http://marsrovers.jpl.nasa.gov                                 2003 Jun. / Jul.         D4
MESSENGER          http://messenger.jhuapl.edu/                                   2004 Aug.                D3
MMS                http://stp.gsfc.nasa.gov/missions/mms/mms.htm                  2013 Jul.                D2
Planck             http://www.esa.int/science/planck                              2008 Jul.                D3
Pluto New
                   http://pluto.jhuapl.edu/                                       2006 Jan.                D3
Horizons
Rosetta            http://www.esa.int/SPECIALS/Rosetta/index.html                 2004 Feb.                D3
RXTE               http://xte.gsfc.nasa.gov/docs/xte/xte_1st.html                 1995 Dec.                D1
STEREO             http://stereo.jhuapl.edu/                                      2006 Oct.                D3
Swift              http://swift.gsfc.nasa.gov                                     2004 Nov.                D1
Terra              http://terra.nasa.gov/                                         1999 Dec.                D1
THEMIS             http://themis.ssl.berkeley.edu/index.shtml                     2007 Feb                 D2
                   http://ulysses-ops.jpl.esa.int/
Ulysses                                                                           1990 Oct.                D3
                   http://ulysses.jpl.nasa.gov/




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      Mission                                Web Site                                  Launch           Domain

                                                                                  1977 Aug.
                                                                                  Voyager 2
Voyager -1, -2      http://voyager.jpl.nasa.gov                                                            D3
                                                                                  1977 Sep.
                                                                                  Voyager 1
WISE                http://wise.ssl.berkeley.edu/                                 2009 Nov.                D1
                     Figure 1. Mission Timing Survey Respondents and Mission Web Sites

The ―Domain‖ column in Figure 1 identifies the specific mission‘s spatial domain for the purposes of this ConOps.
These designations are defined and explained in Section 6.2.

4. Requirements
Time architecture functional requirements were abstracted from [1] and are enumerated in Figure 2. The third
column of the table cross-references the functional requirements to the derived requirements, which are provided in
Figure 3.

                                                                                                    Derived
 ID                        Time Architecture Functional Requirement
                                                                                                  Requirement

FR1      The future time and frequency architecture shall be an integral part of the space        All
         communication and navigation infrastructure, retaining the possibility of
         standalone dissemination systems.
FR2      The architecture shall accommodate user requirements, comprising Coarse (1               DR3, DR4,
         second to 1 millisecond), Fine (1 millisecond to 1 microsecond), and Precision           DR5
         (1 microsecond to 1 nanosecond) resolutions.
FR3      The architecture shall be scalable and accommodate on-demand high precision              DR3, DR4,
         user time requirements.                                                                  DR5, DR7
FR4      Terrestrial time scales at nanosecond-level accuracies shall be available to             DR3, DR4,
         users.                                                                                   DR5
FR5      The principles of general relativity to include time dilation, gravitational redshift,   DR1
         and Sagnac effect shall be applied for time synchronization and dissemination
         among timing systems on spacecraft and solar system platforms as required.

                             Figure 2. Time Architecture Functional Requirements

The time architecture functional requirements have been expanded and augmented as reflected in the derived
requirements detailed in Figure 3 [2]. These requirements are based on feedback from the Mission Timing Survey
(Section 5, Appendix), the TIM attendees, review comments, research based on the review of mission, network and
ground system reports and papers, and discussions with colleagues in the aerospace industry. The derived
requirements are also compatible with Exploration Requirement Ex-0047: The Space Communications Architecture
(SCA) shall provide timing and synchronization services [4]. The third column in Figure 3 refers to relevant
architectural components for which operations concepts are subsequently described in Section 5.




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                                                                                         Architecture
ID                   Time Architecture Derived Requirement
                                                                                         Component
DR1   A standard time scale shall be defined and traceable to an                       Timescale
      internationally recognized scale of atomic time. Standard conversion
      algorithms shall be provided to manage conversions between the                   User Platforms
      standard time scale and common time representations. Parameters and              User
      constants required by the algorithms shall be obtainable through                  Accommodations
      standard services.
DR2   Monotonic time shall be used for machine-machine interfaces.                     Timescale
         International System ―seconds clock‖ without leap seconds
         Current clock implementations range from 172800 seconds (two-
                   32
          day) to 2 -1 seconds (~ 136.099 years)
DR3   A non-limited time duration shall be defined to minimize discontinuities.        Timescale
         Representation should be virtually open-ended for advancing time
         An example of a time duration would be:
                   32
              o   2 -1 seconds (CUC 4-byte TAI format) has an epoch
                  duration of ~136 years.
DR4   Time services shall be accessible and usable in the constrained space            Transmit/Receive
      environment.                                                                      Stations
      Considerations include:                                                          Relays
         One-way links                                                                Correlation and
                                                                                        Transfer Methods
         Low bandwidth links
                                                                                       User Platforms
         Variable Latency
                                                                                       User
         Hop-by-hop transfers
                                                                                        Accommodations
         Periodic discontinuities
         Management of communication link loading
DR5   Application of time to mission systems shall be on a user-selected basis         Accuracy/ Precision
      and shall minimize impact or interference caused by discontinuities
      based in drift, clock rate differences, or time since last synchronization.      User Platforms

         User can obtain feedback and apply time based on best knowledge              User
                                                                                        Accommodations
         Provide for detection and mitigation of ―soft‖ hardware failures
DR6   All systems shall be capable of being calibrated and verified.                   Accuracy/ Precision
DR7   International compatibility shall be implemented and maintained to               Timescale
      maximize opportunities for interaction.
                                                                                       Correlation and
         Constellation has developed the need to exchange time between                 Transfer Methods
          spacecraft
                                                                                       Transmit/Receive
         Must incorporate a well-defined mechanism for defining and                    Stations
          maintaining a time standard within a domain
                                                                                       User
                                                                                        Accommodations


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                                                                                               Architecture
 ID                    Time Architecture Derived Requirement
                                                                                               Component
DR8      The accuracy/precision of the Delay Tolerant Network (DTN) shall be                Accuracy/ Precision
         approximately one [TBD] second.
                                                                                            Transmit/Receive
            The precise number depends on the adaptive algorithms used and                  Stations
             the recovery time.
                                                                                            Relays
            The Constellation Program has a ―TBR‖ DTN requirement. Some
             forward-looking standards should be accepted early even if they are            User Platforms
             not mature.                                                                    Correlation and
                                                                                             Transfer Methods
                                                                                            User
                                                                                             Accommodations

                              Figure 3. Time Architecture Derived Requirements

5. Architecture
The following sections discuss the notional time architecture components, as summarized in Figure 4.

  Architecture Component                                      Description                               Section

                                   
                                                                              -3
Accuracy / Precision                   Coarse: Millisecond (msec) – 10 sec                              5.1
                                   
                                                                         -6
                                       Fine: Microsecond (μsec) – 10 sec
                                   
                                                                         -9
                                       High: Nanosecond (ηsec) – 10 sec
                                   
                                                                        -12
                                       Ultra: Picosecond (psec) – 10          sec => (+20 years)
Timescale                             Continuous                                                       5.2
                                      No Leap Second
Transmit/Receive Stations             ―Well distributed‖ (Minimize no-contact intervals)               5.3
                                      Time architecture encompasses the Solar System
                                      Time services
                                      Broad range of bandwidths
Relays                                Intermediate transfer points                                     5.4
                                      Supports store and forward traffic of a DTN
                                      May be located in space, or at or on a celestial body
Radiometric                           One-Way and Two-Way Ranging for Navigation                       5.5
                                      One-Way Navigation and Timing Service
User Platforms                        Spacecraft – Bus                                                 5.6
                                      Non-terrestrial Ground Networks / Rovers
                                      Payloads
                                      Human support devices (laptops, consoles, etc)




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    Architecture Component                                      Description                                  Section
Correlation     and     Transfer        Network Time Protocol (NTP) service [5], [6]                        5.7
Methods
                                        Institute of Electrical and Electronics Engineers (IEEE)-
                                         1588 service [7]
                                        GPS [8]
User Accommodations                     Applying time updates on a non-interference basis                   5.8
                                        Backup and contingency support
                                        Maintaining mission operations when time updates are
                                         unavailable
                                        Ground Services for Processing Spacecraft Time

                                    Figure 4. Time Architecture Components

     5.1. Accuracy / Precision
The terms ―accuracy‖ and ―precision‖ figure prominently in the discussion of the Time Architecture requirements
and components. Accuracy refers to how closely a measurement, calculation or test result agrees with the accepted
reference value. Precision refers to the degree to which further measurements agree with each other. It is possible to
be accurate, but not precise; precise, but not accurate; both; or neither. The term bias is often used to refer to the
difference between the mean of a set of measurements and the reference value (accuracy); precision is often
characterized in terms of the measurement process‘ standard error.

The survey polled the missions as to the precision of the operational determination of time, the precision of the orbit
/ trajectory determination, and the mission timing requirements.

    For time precision, the survey requested operational information about the early mission operations and
     initialization, experiments, attitude determination and sensor calibration (AD&SC) and attitude control, and
     commanding.

    For navigation precision, the survey requested operational information about early operations, real-time,
     definitive determination, predictive, and orbital events.

    For time requirements, the survey requested information about a mission‘s command system, instruments, and
     frequency standards.

The tables in Sections A-1, A-2, and A-3 present the detailed information.

Functional Requirement FR2 (Section 4) states that the time architecture shall accommodate user requirements from
Coarse (1 second to 1 millisecond), to Fine (1 millisecond to 1 microsecond), to Precision (1 microsecond to 1
nanosecond) [1]. These categories are a mechanism to classify current and future missions. A fourth category, Ultra
(1 nanosecond to 1 picosecond), is added to accommodate missions planned prior to 2030 that may have more
precise time requirements. The classification of a mission to a particular category depends upon its timing
characteristics. Figure 5, Mission Time Precision Classification, illustrates this classification.

Eight missions stated the precision with which ―time‖ was to be correlated with UTC. Using this measurement of the
time accuracy, 2 of the missions are classified as Coarse; 6 are classified as Fine; and 1 is classified as Fine if the
correlation of Spacecraft Time to UTC is used or High if the post measurement correlation to UTC is used.




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       Mission                        UTC                    Category                Most Stringent Time                       Category
ACE                                                                         100 msec                                            Coarse
Aquarius                                                                    1 sec                                               Coarse
                          -    R/T – Timing (3):           Fine / High
                               30E-6 sec (30 µsec)
                               wrt UTC
Cassini                   -    Post Measurement –                           60E-9 sec = 60 sec wrt UTC                          High
                               Timing (3): 60E-9
                               sec (60 sec) wrt
                               UTC
Chandra                                                                     10 µsec                                              Fine
CloudSat                                                                    Coarse                                              Coarse
Con. X                   UTC ±100 sec                          Fine        wrt UTC ±100 sec                                    Fine
Dawn                                                                        time reconstruction – 60 msec                       Coarse
                         Time stamp to UTC (end-                Fine
Gaia                                                                        <60 sec                                             High
                         to-end) < 2 μsec
                         UTC: ± 20 sec                         Fine        Residual radial orbit error < 1.8 cm
GFO                                                                                                                              Ultra
                                                                            (6E-11 sec = 60 psec) 1
                         <10 μsec wrt UTC, 1                   Fine
GLAST                                                                       <10 μsec wrt UTC, 1 RMS                             Fine
                         RMS
GPM                                                                         +/- 1 km (3.3E-6 sec = 3.3 sec)       1
                                                                                                                                 Fine
Herschel Space
                                                                            0.1 msec = 100 sec                                  Fine
Observatory
IMAGE                                                                       1.0 sec                                             Coarse
INTEGRAL                                                                    < 32 μsec                                            Fine
                         Correlate to UTC < 100               Coarse        2-3 m for range data (6.7E-9 sec =
Juno                                                                                                                             High
                         msec                                               6.7 sec – 1E-8 sec = 10 sec) 1
LISA                                                                        < 90 km in position (300 sec) 1                     Fine
                         100 msec of UTC                      Coarse        Initial definitive: < 500 meters (1.7E-
LRO                                                                                                                              Fine
                                                                            6 sec = 1.7 sec) 1
Mars Phoenix                                                                100 sec                                             Fine
Mars Rovers                                                                 sub-secs                                            Coarse
MESSENGER                                                                   +/-1 msec post-processed                            Coarse
MMS                                                                         Fine (1 msec to 1 sec) 1                           Coarse
Planck                                                                      0.1 msec = 100 sec                                  Fine
Pluto New
                                                                            +/-10 msec post-processed                           Coarse
Horizons
Rosetta                                                                     3 msec                                              Coarse
RXTE                                                                        +/- 450 m (1.5E-6 sec = 1.5 sec) 1                  Fine
STEREO                                                                      +/-0.1 sec = 100 msec                               Coarse
Swift                                                                       1 msec                                              Coarse
                         ±100 sec wrt UTC                      Fine        ± 150 m, per axis, 3
Terra                                                                                                                            High
                                                                            secsec 1
THEMIS                                                                      +/-10 km (3.3E-5 sec = 33 sec) 1                    Fine
Ulysses                                                                     100 msec standard                                   Coarse
Voyager -1, -2                                                              48 sec                                              Coarse
WISE                                                                        +/- 30 km (9.9E-5 sec = 99 sec) 1                   Fine
1
    To have a common unit of comparison, navigation errors given in linear measure have been converted to light-travel time.

                                       Figure 5. Mission Time Precision Classification


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There are several alternative ways of classifying a mission in the various categories. Suppose the goal is to
determine the operational concepts and the time architecture requirements that are needed to support the ―most
stringent‖ time requirements. Then a classification scheme such as the following may be considered.
For each mission, determine:
    1. TP, the most stringent timing precision based on or expected during operation for early mission operations
         and initialization, mission experiment and data collection, AD&SC and attitude control, and commanding.
    2. TN, the most stringent timing precision in support of navigation based on actual or expected estimates
         during early mission operations, real-time operation, predictive estimation, definitive determination, and
         predictions and reconstruction of orbital events.
    3. TR, the most stringent timing requirement for the commanding and on-onboard instrument operation.
    4. TS, the most stringent of the three values T P, TN, and TR; i.e., TS = Minimum (TP, TN, TR).
Classify the mission according to the value of TS. Using this method, 14 of the surveyed missions are classified as
Coarse, 13 as Fine, 4 as High, and 1 as Ultra.
This also provides insight into the various mission accuracy requirements depending on the function.
     For 11 missions the most stringent timing precision equals the most stringent value (T S = TP).
     For 12 other missions the most stringent value equals the most stringent navigation precision (T S = TN).
     For 3 missions the most stringent value equals the most stringent timing requirement (T S = TR).
     For 3 missions the most stringent value equals the most stringent time precision and most stringent
         navigation precision (TS = TN = TP).
     For 2 missions the most stringent value equals the most stringent navigation precision and the most
         stringent timing requirement (TS = TR = TN).
     For 1 mission, all values are equal (T S = TN = TR = TP).

    5.2. Timescale
For mission operations, the most highly desirable timescale is continuous, has a well-defined epoch and has a
constant, stable unit of measurement. These characteristics are also consistent with the Time Architecture Functional
Requirements (Section 4). The timescales from the mission survey have these characteristics.

    1.   Global Positioning System (GPS) – For three missions, time is coordinated using GPS receivers.

    2.   International Atomic Time (TAI) – The onboard clock is an atomic clock. One mission plans to maintain
         TAI on-board. Another mission uses the GPS atomic standard.

    3.   Onboard Time – This is the timescale established by the onboard clock. It corresponds to the use of
         Mission Elapsed Time (MET). Three missions refer to onboard time. Two missions reference MET.

    4.   Terrestrial Time (TT) – One mission uses TT onboard.

    5.   Universal Time Coordinated (UTC) – The international standard timescale to which 8 missions correlate
         the onboard spacecraft time.

Each of the aforementioned characteristics can be influenced by various factors. Depending upon the spacecraft
design, the timescale with the factors that produce the most desirable characteristics will be selected for use.
The survey provides information about the factors used in the spacecraft clock correlation determination and those
factors that may cause a problem in this determination. This information is detailed in Sections A-5 and A-6.
        Epoch is well defined. For Spacecraft Time, it is typically the time of launch.

        Continuity can be influenced by on-board clock ―rollover‖ and leap seconds.

             o    For on-board clock roll over, 1 mission accounted for it; 3 missions appropriately sized the
                  number of bytes for the time packet so that rollover would not be a problem for the expected life


                                                  12 of 57
                  of the mission; and 1 mission modified the ground software to adjust the time when rollover
                  occurred.

             o    For leap seconds, 6 current missions and 1 past mission stated that leap seconds were a problem
                  that was encountered. Two future missions stated that leap seconds were being considered in
                  mission planning. For one mission the ―use of TAI eliminates jumps due to leap seconds‖. One
                  mission is considering the use of GPS receivers; however, it expresses a concern that ―there have
                  been instances of missions using GPS technologies where leap seconds have been improperly
                  handled…. The proper handling of this case will be considered when selecting GPS receivers‖.

        The unit of measurement for spacecraft time is typically the unit of the spacecraft clock counter. The
         constancy of this unit is influenced by temperature, radiation, oscillator voltage, loading and aging. By
         measuring quantities such as the ratio of the change in frequency, the stability of the oscillator—and
         therefore the counter—can be monitored. Eight missions considered this factor in spacecraft clock
         correlation determination.

It is important to note fourteen of the thirty-one missions reported no problems with time correlation. Five of these
missions are currently operational. Two are planned for launch in Fall, 2007. Seven are scheduled for launch in 2008
or later. Both operational and future missions indicate how the system will minimize timing errors. One example, for
a mission with multiple spacecraft, is to exchange time between the spacecraft via the inter-satellite link to minimize
clock errors between the spacecraft.

    5.3. Transmit/Receive Stations
Transmit/Receive stations are the intermediate elements between mission operations and user platforms. They may
be collocated with Mission Operations but are generally remote. They usually go by the nomenclature ―Ground
Station (G/S)‖. To provide adequate coverage of space borne systems, it is important that ground stations be
geographically diverse in location. From a capabilities standpoint, a time architecture supporting G/S must be able to
provide Internet Protocol (IP) uplink and a high precision time source for scoring data.

As shown in Table A-4, the most common ground stations in the survey are those associated with the NASA Deep
Space Network (DSN) (13 missions use DSN for communication, 1 uses it for backup, 1 will use elements of it as
part of the ground network) and the NASA Tracking and Data Relay Satellite System (TDRSS) (7 missions use
TDRSS for communication). Nine missions use other ground stations that are not part of the TDRSS or DSN
network. Six of these 9 missions use these stations for communications. Two use them either as supplement to or as
a contingency backup for TDRSS. One mission will use the ground station as an element in its ground network.

In time correlation determination, key factors contributed by the ground station are the ground receipt time, the
antenna geographic position used in the determination of the light travel time, and the calibrated parameters
associated with the ground station system. With respect to the ground receipt time, 5 survey missions that are
currently operating responded that errors in the ground receipt time had been a problem. One survey mission that is
yet to be launched responded that error in the ground receipt time is a factor to be considered. With respect to the
antenna position and calibrated parameters, 3 survey missions that are currently operating responded that errors in
antenna and signal delays had been a problem. 2 survey missions that are yet to be launched responded that errors in
the ground infrastructure are factors to be considered

    5.4. Relays
Within the NASA communication architecture there will exist a number of systems necessary to interconnect end-
points. These relays will be a combination of legacy relays such as TDRS and a new type of relay, which will act
much like routers in a modern network. However these new types of DTN supporting relays must have enhanced
capabilities to support the challenged network of interplanetary space. Figure 6 illustrates the relationship of relays
in the space communication architecture [9].




                                                  13 of 57
                                    Figure 6. Relays in a Challenged Network

The dotted lines represent logical connections and the solid lines represent physical connections. The relays connect
the ground stations to other relays and User platforms. As defined in Section 5.6, the User platforms can be any of a
number of devices. As each terrestrial element must be aware of current time, so must the DTN relays be cognizant.
This is required to support security, DTN protocols, and time distribution to user platforms. Non-DTN relays will
act as repeaters; the DTN relays will provide store and forward capability for network traffic.

All relays must support functions enabling the passing of data below the network layer. In the simplest mode a DTN
relay acts as a non-DTN relay allowing the use of radiometric techniques such as Pseudo Random Noise (PRN)
ranging. However to support a time architecture that has intermittent connections to Terrestrial end users and user
platforms, the DTN relays must host a precision time source that is manageable from the control centers to ensure
that time deviations are small throughout the communication systems. These deviations are probably well within the
Coarse accuracy range since they represent timeout values for the various protocols.

    5.5. Radiometric
There are various types of continuous wave radiometric ranging to include harmonic (fixed and swept tone) and
non-harmonic (PRN and BINary Optimum Range-BINOR) ranging. In addition, there are pulse type ranging
schemes for Radio Frequency (R/F) and laser tracking. This discussion is focused on PRN ranging.
For this discussion, one-way radiometric ranging is defined as the process of sending a PRN signal to a spacecraft
and recording the event on the spacecraft for subsequent downlink in telemetry. The subsequent arrival time of the
telemetry is noted on arrival at the G/S as a Ground Receipt Time (GRT). GRT and time of transmission of the PRN
are forwarded to the Mission Control Center (MCC). A corollary capability is for a spacecraft to broadcast a PRN
signal to the ground and measure the GRT directly before forwarding the measurement to an MCC.

These methods of ranging have several attractive features. First in its simplest form, little cognizance by onboard
systems is required. Second, it is simple to operate and definitive in solution. Finally it can be extended to two-way
ranging through the use of a turnaround ranging system in the space borne communications hardware and return the
PRN signal to a G/S. The use of these techniques can provide timing and ranging benefits from Low Earth Orbit
(LEO) to deep space.

There are several liabilities as well. For long roundtrip times as required for deep space vehicles, it may be necessary
to have more than one G/S support an activity. In addition, measurements embedded in telemetry may be obscured
and delayed by encryption on the downlink, if required. This is further complicated since the ground architecture
may not have decryption capability widely dispersed beyond the MCC. Finally, uplink of a PRN signal reduces the
total power available for the carrier-tracking loop, which may only be important for locked loop operations, notably
near Earth.

A number of products or measurements can be produced based from these and similar methods:
   Time of Arrival (TOA)
   Time Difference of Arrival (TDOA)
   PRN correlation


                                                   14 of 57
   Doppler shift, and Accumulated Delta Range (ADR)
Through the use of these products, approximate position can be determined and a subsequent clock correlation
made, assuming the requisite algorithms are utilized.

    5.6. User Platforms
Spacecraft evolution has rapidly advanced over the last 50 years from limited applicability and throwaway designs
to highly modular systems based on open standards and COTS components. As a result, even the internet and
associated protocols are going into interplanetary space on manned and unmanned platforms. Today‘s and
tomorrow‗s platforms will be a host of computers for many systems and payloads.

To coordinate operations and provide security, many of these systems will rely to a high degree on accurate and
precise timing for security, logging, and basic operations. This is primarily because they are computer based and not
merely electronic components whose outputs are correlated. Even on single unmanned spacecraft, multiple
processing systems are used for command and control, sensors, and navigation. As indicated in Figure 5, correlation
to UTC requirements never exceed midrange Fine and only one requires High post processing.

For manned spacecraft timing, tolerances are generally lower. Currently the International Space Station (ISS) [10] is
based off of GPS time and is uncorrected for leap-seconds. The Command and Control Multiplexor/DeMultiplexor
(C&C MDM) is the user of GPS time and is the source for command systems. An offset of 100 microseconds is
allowed for normal operations. The C&C MDM has a command check window of one (1) minute for time
authentication check. Time tagged commands are schedulable on the second. Crew laptops are found onboard the
ISS [11]. These laptops are synchronized to an onboard file server through a time utility. The file server is set
manually to ―GMT‖.

In future space missions, manned missions will continue to have multiple onboard systems to include C&C,
fileservers, and personal computing devices. Due to the critical nature of mission operations and the limited
manpower resources, a distributed time synchronization is required here as well.

On the ground side, Mars and Luna will have extensive wired and wireless networks. These will consist of manned
and unmanned missions. Manned missions will include a range of fixed and mobile devices, including those found
on surface-access craft such as landers and rovers, as well as robotic assistants and personal computing devices such
as laptops. Unmanned missions will consist of fixed stations (e.g., meteorological, communications relay, science
instruments, and sensor nets) and mobile robotic devices. Analogous C&C and time synchronization and
dissemination requirements will exist in these venues.

    5.7. Correlation and Transfer Methods
As indicated in survey results, the primary methods of determining and tracking time are radiometric and
correlative, respectively. This is particularly true above LEO. However, studies over the last ten years have indicated
alternatives for those spacecraft equipped to support synchronization. Both two-way and one-way synchronization
techniques have been demonstrated. For spacecraft, most have been in the Coarse accuracy range.

For two-way synchronization, a highly noticeable example is the Operating Missions as Nodes on the Internet
(OMNI) [12]. This program utilized a version of NTP ported from BSD 4.4 on the UoSAT-12. Tests demonstrated
that clocks can be handily synchronized in LEO orbit using NTP. Further simulations by the same laboratory
showed that above LEO in translunar space, NTP is also well behaved [13]. Protocols such as NTP do not merely
transfer time but also compensate for transfer delay as well as clock instability.

Another synchronization standard is IEEE-1588. 1588 has many of the same characteristics as NTP3 or NTP4, as
seen in Figure 7. However, both NTP and IEEE-1588 are more ―chatty‖ than traditional correlative techniques since
they require a dialog. These protocols experience difficulty in interplanetary space where long baselines provide a
high degree of asymmetry on each transfer leg due to protocol assumptions.



                                                  15 of 57
   Characteristic                             NTP                           IEEE-1588                    GPS
Spatial Extent                    Wide Area Network                  A Few Subnets             Wide Area Network
Communication                     Wide Are Network / Internet        Local Area Network        Satellite
Target                            A few milliseconds                 Sub-microsecond           Sub-microsecond
Control Style                     Client-Server or Peer-Peer         Master-Slave              Client
Administration                    Configured                         Self-Organizing           None
Latency Correction                Yes                                Yes                       Yes
Security                          Yes                                No but discussed for V2   No
Hardware                          No                                 For highest accuracy      RF receiver and
                                                                                               processor
Update Interval                   Varies                             ~2 seconds                ~1 second
                                       Figure 7. Time Transfer Method Characteristics

A third technique, which has been used by the United States Naval Observatory (USNO) since the early 1960s, is
Two Way Satellite Time Transfer (TWSTT). TWSTT is currently used between USNO and the Alternate Master
Clock at Schriever Air Force Base among other uses [14]. According to the USNO, various incarnations of TWSTT
have been used since the early 1960s with the National Physical Laboratory of the United Kingdom and the Radio
Research Laboratory in Japan. Like NTP and 1588, it requires active participation from end nodes and reciprocal
actions. However, in the solar system, non-reciprocal time delays would be the norm. Contributions from general
relativity and atmospheric refraction will be applied differently because the paths will diverge across interplanetary
distances and frames of reference. Figure 8 describes the generic case of Two Way Time Transfer (TWTT). TD1
differs from TD2 because of the extremely long baseline in the D4 domain (a local time domain in interplanetary
space). For a Lunar site in the D2 domain, divergence between TD1 and TD2 is much less due to the lunar orbit low
eccentricity and short roundtrip light travel time. The technique has reciprocal time delays to Luna and high
accuracy can be achieved.

                        Clock 1                                                                              Meas1 = T2 - (T1+TD1)
                                                  TDX = One way propoagation delay
                                                  T1 = Time of clock1
                                                  T2 = Time of clock2
                                                  T2 – T1 = .5 (Meas2 - Meas1)
                                                  TD1 is not equal to TD2
                                                                                                   Clock 2


Meas2 = T1 - (T2+TD2)

                                               Figure 8. Time Transfer Delays

A fourth technique is the US GPS, which utilizes a one-way time transfer method for synchronization [15]. One-way
transfer is perhaps the simplest method, where clock 1 sends a time signal to object 2 through a medium at a
distance. For a transfer through a vacuum, the latency is approximately 3.336 microseconds per kilometer. Using
this method, GPS satellites broadcast timing signals on a phase modulated L-band carrier. The satellites also
broadcast a time code referenced to the satellite clock and information enabling the user to obtain an estimated GPS
system time that implies USNO‘s UTC time. The user's receiver compares the arrival time of the GPS signal to the
local clock. Using estimation techniques and an approximation of the sender and receiver location allows for a fast
timing solution. When the final solution is complete and, accounting for propagation delays relating to range,
ionospheric and tropospheric refraction and decrease in light speed (if applicable), and hardware delays, then time
can be ascertained with uncertainties measured in nanoseconds. In continuous operation and with multiple satellite
in-views, time solutions are stable and reliable without requiring an Ultra Stable Oscillator (USO) onboard the
receiving platform.

One drawback identified for one-way time transfer is that there is no feedback loop, such as is available with two-
way transfer systems. However, a system like GPS is an externally monitored and managed system. System
feedback is provided by two mechanisms. First, the GPS master continuously monitors all GPS satellites. In
addition, an external observer (USNO) monitors important indicators. For GPS the USNO monitors three satellite
timescale references: individual satellite time, GPS ensemble time, and UTC (GPS). If USNO or the GPS master

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identifies an impacting failure or trend, corrective actions are taken, to include removing the affected GPS satellite
from the operational constellation. Second, the user accommodations of Section 5.8 apply. The user systems must be
intelligent enough to detect anomalies in time sources and must apply reasonable error checking. At the system
level, these indications will limit the application of the specific time source. The system status will be telemetered to
the applicable MCC.

    5.8. User Accommodations
A notional time architecture must accommodate time correlation and time synchronization. Each methodology must
be cognizant of the operational environment of the user platform. However, correlative techniques are insensitive to
onboard activities because they do not perturb the clock operation. Correlation merely models the clock behavior
and events or activities are scheduled accordingly. With time synchronization, the user platform is an active
participant and applies alteration to the clock model parameters dynamically. It is true that the same environmental
and spatial components apply such as the thermal environment of components, relativistic effects, and divergent
path lengths providing various latencies. However, onboard the user platform any number of activities can be upset
by adjusting time during critical periods..
Onboard, three accommodations should be incorporated to support synchronization or correlation. These are:
applying time updates on a non-interference basis; providing backup and contingency operations support; and
implementing requirements for onboard equipment and procedures to maintain mission operations when time
updates are unavailable.

         1.   Applying time updates on a non-interference basis – The application of updates to onboard systems
              requires cognizance of state. Due to the opportunistic nature of the DTN, time synchronization
              activities will be on an ad hoc basis. Therefore, the application of updates to critical onboard processes
              and activities must insure that those requiring high precision be consistent with specifications. An
              example of this is applying a timing update during an attitude maneuver where the control law might
              experience instabilities if the duration varied. A second example is an instrument readout over a fixed
              period of time. Varied durations of time intervals would yield data inconsistencies.

         2.   Supporting Primary/Secondary/Contingency operations (Prime, Backup, and Safe mode-like, non-
              computer mode) – For onboard timekeeping, a primary system and backup system should be available.
              For example, time missions using GPS have two GPS systems. This redundancy can ensure mission
              operations continuity if one system malfunctions or fails. If both systems malfunction, a failsafe mode
              of operations is required. This would either be an alternate system to synchronize time or allow the
              MCC to correlate vehicle clock (oscillator) time to UTC. This may be as simple as a direct correlation
              (subject to environmental and spatial considerations), which incorporates clock characteristics. Even if
              the ability to synchronize the spacecraft clock is reestablished, it may be necessary to initialize a
              recovering system with correlation.

         3.   Implementing stability requirements when time updates are unavailable – Clock stability requirements
              for onboard systems should reflect both of two possible conditions:

                    a.   Long term stability requirements for contingency operations

                    b.   Required stability to meet the flight profile requirements assuming a loss of the ability to
                         synchronize time based on loss of communication assets, e.g., relay satellites or GPS
                         suboptimal configuration

6. Operations Concept Development
Using the survey, information was gathered from missions of varying characteristics. From these characteristics and
the functional and derived requirements, basic time architecture operations concepts tenets were developed.
Missions were categorized as to their mission domains, and domain characteristics as well as interactions within and
between domains have been detailed. These elements are addressed in the following sections.



                                                   17 of 57
    6.1. Mission Domains
The spatial domain in which a mission operates is a primary item that controls the definition of its ConOps. The time
architecture will utilize assets within each domain and not require extension. This requires a set of common services
that are ubiquitous within a domain. Figure 9 defines the mission domains.

      ID     Domain Name                                          Description
     D1     Near Earth               Launch head
                                           o   Ground systems provide time
                                           o   Isolated clocks for ascent
                                     Encompasses
                                           o   Ascent
                                           o   Low Earth Orbit
                                           o   Medium Earth Orbit
                                           o   Geostationary Earth Orbit
     D2     Translunar               Encompasses
                                           o   High Earth Orbit
                                           o   Translunar
                                           o   Lunar environs
                                           o   Lunar Descent
                                           o   Lunar bases and sorties
                                           o   Earth-Lunar Lagrange
     D3     Interplanetary        A single system or a group of systems treated as a single system
                                     Sun-Earth Lagrange
                                     Heliocentric
                                     Interplanetary (e.g., Martian transit)
     D4     Local Time               A group of systems large enough to support a time infrastructure
                                     Isolated from terrestrial systems by distance and relativity
                                     Coordinated Martian Time and other managed time domains

                                      Figure 9. Mission Domain Definitions

There are two basic types which support this ConOps: Local Time Domains where there are sufficient resources to
support an infrastructure (D4), and Interplanetary Domains where resources are insufficient for a local infrastructure
(D3). Figure 10 illustrates the relationship of the mission domains. The Near Earth Domain (D1) and Translunar
Domain (D2) are special cases of D4 because of the extensive infrastructure currently present or anticipated, and
thus the many options for timing that they provide or will provide.




                                                  18 of 57
                    Time Architecture Mission Domains

                                          L2
                                          +

                                         Moon
                    Domain 2

                    Domain 1
                                                 Geo
                                         Earth
                                                                        Mars
                                                                      Domain 4




                                                            Inter-Domain


                      Domain 3

                                          Domain 4
                                                                         Jupiter
                                                                         Domain 4




                                 Figure 10. Time Architecture Mission Domains

        6.1.1. Domain 1
Domain 1 is composed of the Earth and near Earth space. This includes space out to geosynchronous orbits. This is
an arbitrary distinction based on activity level or population of space borne assets. D1 is the domain with the most
assets and usable operations methods, including time. For international usage, these assets are truly expansive. They
reflect a broad range of formats and capabilities.
A primary D1 asset is the Global Navigation Satellite System (GNSS), of which the GPS constellation is the most
venerable and reliable. GPS can be used as a time source from low Earth orbit with high precision and accuracy
[16]. However as the altitude rises, inviews vanish. Because GPS signals at altitudes above the GPS constellation are
10 to 100 times weaker and less densely populated, GPS receivers have not been feasible for use above LEO until
recently.

However, there are at least two options to mitigate this problem. First, other assets of GNSS will be available. This
includes the Russian Global Orbiting Navigation Satellite System (GLONASS), and the European Space Agency‘s
Galileo. GLONASS operates at roughly the same orbital altitude as GPS (19100 km vice 20200 km, respectively).
Galileo operates at an orbital altitude of 23000 km. These systems provide for an increased number of opportunities.
A second option is a new generation of space-borne GPS receivers. Through the use of GPS side lobes, time
determination can be supported with a high degree of precision even in geosynchronous orbits [17].

However, GPS is not acceptable for systems where mass and power consumption are in extremely limited supply
such as SmallSATs or vehicles, which will be in Domain 1 for a brief time or during contingency operations where
power may be at a premium. A second need for an alternate timing method is in support of equipment failures that
have limited the ability to acquire GNSS satellites. Methods usable include radiometric ranging and subsequent
correlative techniques as well as NTP [13].


                                                 19 of 57
         6.1.2. Domain 2
Domain 2 ranges from Geosynchronous orbit to Lunar orbit. By definition it encompasses three zones; 1) CisLunar
space, 2) the Lunar environs, and 3) all other regions of the Lunar orbit to include the Earth-Luna Lagrange points.
Due to Exploration plans for a permanent presence on the Moon [15] an extensive infrastructure will exist in Zone 2.
For a CisLunar (Zone 1) transfer time of 96 hours (345600 secs) and an USO with a stability of 10 -13 at 10 secs, a
deviation of approximately 35 nanoseconds can be expected. This is well within the in-situ clock synchronization
requirement of 1 microsecond defined by the Lunar Architecture Team [18]. Note that Zone 3 time synchronization
will fall within Domain 3 methods.

However, in Zones 1 and 3, time can be maintained via two conventional and well understood methods: radiometric
and NTP. In our surveys, many users use radiometric methods to correct time to Fine. For Coarse precision, NTP
and IEEE 1588 are sufficient throughout all zones. NTP has the additional benefit of being a software
implementation that requires no additional power and, with the exception of a few specific implementations, no
additional hardware.

         6.1.3. Domain 3
Domain 3 represents the environment of any spacecraft where infrastructure is insufficient to support a collaborative
solution between multiple spacecraft. An example of this would be a spacecraft in an Earth to Mars Hohmann
transfer orbit or a spacecraft conducting a Jovian survey. For each domain, corrections for range, Doppler shift, and
relativistic effects are required on a platform basis. This does not necessarily include isolated clustered systems.
Such systems as LISA may well constitute their own D4 type domain.

         6.1.4. Domain 4
Domain 4 represents a remote time domain with resources adequate to provide a managed time standard. A
particular D4 would maintain a master-slave clock relationship with a terrestrial master clock provided by DSN. An
example where this might occur would be an Aerocentric time domain [19]. The Martian slave clock would act as
the master clock in the Aerocentric domain and would be selected based on USO stability. A secondary stable clock
would be designated as well. The clocks need not be space based but could exist in a Martian ground based facility
in a less hostile environment. The spacecraft clocks could be of a lower performance and therefore be less
expensive. The creation of a D4 type domain would allow for a limited implementation of required relativistic
correction [20] on a domain basis thereby eliminating the need to correct in each remote platform.

The interface/or access point during normal operations (and probably most contingency operations) will be through
relays imbedded in a DTN. Such an interface point should require the necessary infrastructure to support time
services, including access to the high precision Martian master clock(s) with Fine capability In addition the ability to
disseminate time is paramount. These timing services should equate to many of those provided in domain D1 to
include a one-way time transfer much like GPS, a network time protocol (NTP/IEEE 1588), and radiometric
methods supporting contingency operations.

    6.2. Basic Tenets
The ConOps basic tenets are:

    1.   The Time Architecture will support assets within each domain and not require extension. This requires a set
         of common services that are ubiquitous within a domain.

    2.   MOCCs/MCCs will not be required to have precision timing sources.

            Ground site uplinks are the sources of time. It is at these sites that all ground times will be scored and
             forwarded to the users and Mission Operations Control Centers (MOCCs).



                                                   20 of 57
            MOCCs/MCCs will be expected to accommodate timing requirements.

    3.   Time recovery will be supported, including:

            Basic clock correlation technologies (SN/DSN).

            Adaptive technologies to compensate for domain unique contributions, such as relativity, ephemeris,
             and environmental factors (e.g., NTP).

            Restart and synchronization.

    4.   Timing methods will be published.

            Equations and implementation model (precisions).

            Included as an appendix to the service agreement/user guides.

            Deviations in methods constitute a modification of service agreements.

            Well defined and publicized controlled mechanism.

    5.   The time architecture will be a component of a DTN.

In the Exploration program communication with spacecraft and experiments is to be conducted in a manner
consistent with the challenged network of a DTN. Common examples of DTN usage are email and cellular phone
networks. The current internet can be described or bounded by four assumptions [21]:

    1.   A near continuous bi-directional path is available end-to-end between source and destination that exists for
         the duration of a communication session.

    2.   Short and consistent times are required for round trips to send data and receive acknowledgments for
         reliable communication. Retransmission based on immediate feedback from data receivers is an effective
         means for repairing errors.

    3.   Relatively symmetric and consistent data rates in both directions are the norm.

    4.   Low loss or corruption of data.

Generally it can also be assumed that endpoint-based security mechanisms are sufficient for meeting most security
needs. However, these assumptions impose restrictions that must be relaxed for effective space communications. For
example, assumption 2 would prohibit most space communications because of the larger distances and longer
transmission times. A DTN architecture is designed to relax these assumptions. It is based on the following
modifications to the four basic assumptions:

    1.   Path is intermittent and defined by geometry and usage.

    2.   Long and variable round trip times are defined by the segment traversed. Times may increase or decrease
         on any segment.

    3.   Uplink and downlink rates are almost never symmetric between two entities. Exceptions are usually peer
         relationships (EVA-EVA). Physics is usually the culprit.

    4.   Loss and corruption of data has the potential for much higher incidence due to the harsh environment,
         service degradation, Signal-to-Noise Ratio and inaccuracies in pointing.


                                                  21 of 57
A fifth assumption may be required based on the need to provide Fine or Ultra precision.

    5.   The capability to implement timing services at a level lower than the network layer may be required.

The DTN does not merely repeat and forward or store data / traffic, but also must be contextually aware of the data.
For instance, certain commanding might be real-time in nature: wake-up calls, recovery of systems or perhaps even
time synchronization. A real-time interface may require concurrent end-to-end connectivity, minimal processing
with repeatable latencies and time ordering. However, the end-nodes may not be up or active simultaneously during
signal traversal.

A store and forward (s/f) mode would be for non-real-time data such as a computer data load or voice mail (the
applications are endless). Non-qualifiable latencies in the s/f model would make forwarding time correlated
functions impossible without precise event markers. Therefore, it appears that time dissemination and/or correlation
must be in real-time.

All missions will originate from D1. However, all missions may not operate in D1. For instance, a lunar rover will
be launched from Earth but will never use time determining resources in D1 or Cislunar, but only in the D2
environment. Therefore services will be provided for trans-domain, single domain, and multiple domain platforms
[22].

All types of missions will be supported and will require a diverse set of timing methods. These mission types are not
precluded by mass or domain. Each mission must support precision timing solutions that are sufficiently stable to
support their own mission requirements.

As examples, consider four possible scenarios for Exploration. These scenarios are formulated based on the requisite
precision and the domain within which platforms can operate or traverse.

Scenario 1 – Pad launch of a booster vehicle with spacecraft from Kennedy Space Center (KSC) to LEO orbit.

             Time delivered to spacecraft via umbilical. Protocol IRIG/NTP/IEEE-1588

             Launch event

                   i. Use a stable on-board oscillator for timing events

                  ii. No timing system updates until injection or separation

             Payload separation

             Booster tracked, re-entry at time TBD

             Payload

                   i. Acquires GPS sources

                  ii. Acquired by Mission Systems (MS)

                 iii. Updates position and time

Scenario 2 – Launch of a booster vehicle with multiple SmallSats from KSC to LEO orbit.

         a.   Time delivered to spacecraft via umbilical. Protocol IRIG/NTP/IEEE-1588

         b.   Launch event



                                                  22 of 57
                  i. Use a stable on-board oscillator for timing events in each SmallSat

                 ii. No timing system updates until injection or separation

        c.   Payload separation

                  i. Deploy SmallSats

        d.   Booster tracked, re-entry at time TBD

        e.   Each SmallSat

                  i. Acquired by MS

                 ii. Acquires position via PRN ranging and the User Spacecraft Clock Calibration System

                 iii. Correlates time to 100 msec

                 iv. Initializes onboard time with NTP

Scenario 3 – Pad launch of a booster vehicle with spacecraft from KSC to LEO orbit to Lunar orbit.

            Time delivered to spacecraft via umbilical. Protocol IRIG/NTP/IEEE-1588

            Launch event

                  i. Use a stable on-board oscillator for timing events

                 ii. No timing system updates until injection or separation

            Payload separation

            Booster tracked, re-entry at time TBD

            Payload

                  i. Acquires GPS sources

                 ii. Acquired by MS

                 iii. Updates position and time

            Executes a Hohman transfer orbit

                  i. Receives updates from GPS/Radiometric

                 ii. Receives updates from Lunar relay and beacons

            Execute Lunar orbit insertion burn

                  i. Enters Lunar orbit

                 ii. Receives updates from Lunar relay and beacons




                                                  23 of 57
Scenario 4 – Deployment of Spacecraft in Mars orbit - This scenario assumes that the spacecraft has transited
Domains 1 and 2, and is transiting Domain 3.
              Time correlated to spacecraft via DSN prior to arriving at near Mars space
              Trajectory correction prior to acquiring timing assets in D4 domain
              Synchronize clock to D4 timing standard
              Execute Mars orbit insertion burn
                      i. Enters Mars orbit
                   ii. Synchronize clock to D4 timing standard

    6.3. Intra- and Inter-Domain Relationships
Figure 11 summarizes aspects of relationships within a domain (intra-domain) and between domains (inter-domain).
These relationships are with respect to time coordination, transfer, application, and distribution; and unique aspects
of domain infrastructure (or its lack), plus respective service levels and accuracies. Some of these have been
addressed in the preceding subsections of this Section.

As the domain concept evolved, there developed a clear need to address these relationships. Firstly, some missions
will transition from one domain to another during their flight profiles, such as in the Scenario 3 translation to lunar
orbit. Secondly, there are unique aspects to further scrutinize, such as user discretion in applying time updates
(Section 5.8). For example, a time update can be applied to a laptop whenever the crew has a spare moment;
however, prudence suggests that a time update to a spacecraft‘s Reaction Control System and Avionics would be
deferred during an orbit adjustment burn. Another example is based on the lack of domain-specific infrastructure in
D3; any relativistic or other corrections are therefore the responsibility of the individual mission platforms.

                  Intra-Domain                                           Inter-Domain

TIME

Coordination      Human and machine interfaces                           Machine interfaces

Transfer          Single method preferred                                Required rate must support most
                                                                         rigorous user

Application       User discretion – systems must be protected            Stratum Clock Model
                  from disruptions caused by arbitrary time
                  application-induced discontinuities

Distribution           Part of infrastructure                           Infrastructure to infrastructure via
                                                                         established protocol
                       Requires components/platforms to support
                        technology

INFRASTRUCTURE

Unique            Each D3 domain may be unique but design                Must support transitions, e.g:
                  reuse will reduce costs
                                                                         D1 launch  D2  D3  D4 (Scenario
                                                                         5)

Service
                  D1, D2, D4:                                               D1 – Well Defined
Level


                                                   24 of 57
                Intra-Domain                                           Inter-Domain

                    Ubiquitous Protocol                                  D2 – Being Defined [18]

                    Standard Services                                    D3 – Mission platform(s), no domain
                                                                           infrastructure; must apply relativistic
                                                                           and other corrections

                D3 – Unique                                               D4 – Remote Domain with at least
                                                                           minimal infrastructure

Accuracy        Individual Platforms may have high relative               Required Rate is Absolute Rate
                rates (local)                                              (System Timing)

                                                                          Does not support the higher
                                                                           precision timing rates that
                                                                           measurements may require—that
                                                                           burden is on the mission

               Figure 11. Time and Infrastructure Relationships Within and Between Domains


7. Summary
The ConOps presented herein is based on the time architecture functional requirements and considerations
developed by the SCAWG and augmented by the PNT Study Group.

The ConOps Team has:

        Initiated a survey of current and planned missions to assess their requirements as they pertain to a NASA
         Time Architecture, potential standard time services, and associated operational concepts.

        Investigated and expanded the requirements.

        Developed a concept for applying time architecture concepts to Solar System domains.

        Highlighted the operation within and between these domains.

8. Recommendations
The ConOps Team recommends follow-up investigations into the technologies required for a NASA Solar System-
wide Time Architecture:
    1.   Merge PNT Study Group architecture efforts with SCaN service definition activity – the PNT Study Group
         (or its successor) should continue to develop and refine the architectural requirements for time
         synchronization and distribution while the SCaN Networking Architecture and Standards program
              o translates these requirements into a set of standard time services to be provided by the SCaN
                   infrastructure, and
              o supports development of internationally agreed upon standard protocols.
    2.   Discuss timing concepts with the international community – given the VSE and U.S. Space-Based PNT
         Policy goals and objectives, it is essential to be aware of what others are doing, e.g., ESA, JAXA, and
         CONAE (Argentina), and to consult on the development of standard protocols. One particular forum to



                                                25 of 57
     address is the Consultative Committee for Space Data Systems (CCSDS) to ensure interoperability between user
     platforms and relays in the appropriate time formats.

3.   Prototype the time architecture – Set up a timing Simulation Integration Laboratory to support the
     transition from the theoretical architecture to the candidate designs and implementation approaches. Include
     any implementations in future architecture demonstrations.

4.   Complete the investigation of time transfer technologies detailed in the SCAWG Time Team Interim
     Report [1].

5.   Expand the mission timing survey database to ensure a responsive and comprehensive time architecture
     requirements and SCaN time services.




                                             26 of 57
9. Glossary
                          Conformity to fact. The closeness of agreement between a test result and the accepted
Accuracy
                          reference value.
                          The difference between the mean of the measurements and the reference value.
Bias
                          Establishing and correcting for bias is necessary for calibration.
                          Network characterized by challenges such as intermittent connectivity, network
Challenged Network
                          heterogeneity, and large delays.
Data Scoring              Marking good data with the Ground Receipt Time prior to forwarding it to the users.

Delay Tolerant Network    A specific implementation to address Challenged Network issues.

Gravitational Time Delay See Shapiro Effect.

                          The spatial domain in which a mission operates. In this context, the domain defines the
Mission Domain            available time distribution and dissemination infrastructure. Missions may transit
                          multiple domains during launch, orbital transfer, orbital insertion, and landing.

                          The ability to determine current and desired position—relative or absolute—and apply
Navigation
                          corrections to course, orientation, and speed to attain a desired position [23].

                          The ability to accurately and precisely determine one‘s location and orientation
Positioning
                          referenced to a coordinate system [23].

                          The ability of a measurement to be consistently reproduced. The number of significant
Precision                 digits to which a value has been reliably measured. The closeness of agreement
                          between independent test results obtained under stipulated conditions.

                          The act or process of separating something into its constituent parts. The fineness of
Resolution
                          detail that can be distinguished.
                          Gravitational time delay that increases transit time and imparts a Doppler Shift to a
                          signal passing near a massive object; e.g., when radar and radio beams pass closer to
Shapiro Effect
                          the Sun. Activities such as cross-solar system time synchronizations or distance
                          determinations using ranging data must correct for this effect.
                          Association of the Time, T1, from System One with the Time, T2, from System Two.
Time Correlation
                          The difference between the Times, T1 and T2, is not constrained but is determined.

Time Dissemination        The dispersal of Time throughout a region.

                          Simultaneity of the Time, T 1, from System One with the Time, T 2, from System Two.
Time Synchronization
                          The difference between the Times, T 1 and T2, is to be minimized.

Time Transfer             Transmission of Time from System One to System Two.

                          The ability to acquire and maintain accurate and precise time from a standard and
Timing
                          within user-defined timeliness parameters. Timing includes time transfer [17].

User Spacecraft Clock     A method designed for calibrating the spacecraft clock with UTC to microsecond
Calibration System        accuracy by using TDRSS PRN epochs.




                                               27 of 57
10. Abbreviations and Acronyms                           ESA        European Space Agency

                                                         ESAC       European Space Astronomy Centre

ACE        Advanced Composition Explorer                 ESTEC      European Space and Technology Centre

AD&SC      Attitude Determination        &   Sensor      EVA        Extra Vehicular Activity
           Calibration
                                                         FRn        Nth Functional Requirement
AU         Astronomical Unit
                                                         G&C        Guidance & Control
BINOR      BINary Optimum Range
                                                         G/S        Ground Station
bps        bits per second
                                                         GFO        GeoSat Follow-On
BSD        Berkeley Software Distribution
                                                         GHz        Gigahertz
C&C        Command and Control
                                                         GLAST      Gamma-ray Large Area Space Telescope
CAPS       Cassini Plasma Spectrometer
                                                         GLONASS    [Russian] Global Orbiting Navigation
                                                                    Satellite System
CCSDS      Consultative Committee for Space Data
           Systems
                                                         GNC        Guidance Navigation Control
Chandra    Chandra X-Ray Observatory
                                                         GNSS       Global Navigation Satellite System
cm         centimeter                                    GPM        Global Precipitation Measurement
Con. X     Constellation-X Observatory                   GPS        Global Positioning System

CONAE      Comisión Nacional de Actividades              GRT        Ground Receipt Time
           Espaciales [Space Agency of Argentina]
                                                         GSFC       Goddard Space Flight Center
ConOps     Concept of Operations
                                                         HEO        Highly Elliptical Orbit
CPU        Central Processing Unit
                                                         hr         hour
CSC        Computer Sciences Corporation
                                                         HST        Hubble Space Telescope
CUC        CCSDS Unsegmented Code
                                                         ID         Identification
Dn         Nth Domain
                                                         IEEE       Institute of Electrical and Electronics
DRn        Nth Derived Requirement                                  Engineers, Inc.

DSN        Deep Space Network                            IMAGE      Imager for Magnetopause-to-Aurora
                                                                    Global Exploration
DTN        Delay Tolerant Network
                                                         INTEGRAL   INTErnational Gamma-Ray Astrophysics
EDL        entry, descent and landing                               Laboratory

EOS        Earth Observing System                        IP         Internet Protocol

ERT        Earth-received time                           IRIG       Inter-Range Instrumentation Group




                                              28 of 57
ISS         International Space Station                     psec     picosecond

JAXA        Japan Aerospace Exploration Agency              R/F      Radio Frequency

km          kilometer                                       R/T      Real-Time

KSC         Kennedy Space Center                            RMS      Root Mean Square

L2          Sun-Earth Lagrange point                        RSS      Root Sum Squared

LENA        Low Energy Neutral Atom                         RTLT     Round-trip Light Time

LEO         Low Earth Orbit                                 RXTE     Rossi X-ray Timing Explorer
LISA        Laser Interferometer Space Antenna
                                                            S/C      Spacecraft
LRO         Lunar Reconnaissance Orbiter
                                                            s/f      store and forward
LVLH        Local Vertical Local Horizontal
                                                            SCA      Space Communications Architecture
m           meter
                                                            SCaN     Space Communications and Navigation
MCC         Mission Control Center
                                                            SCAWG    Space Communications          Architecture
MDM         Multiplexor/DeMultiplexor                                Working Group

            MErcury Surface, Space ENvironment,
MESSENGER                                                   sec      second
            GEochemistry, and Ranging

MET         Mission Elapsed Time                            SLE      Space Link Extension

mm          millimeter                                      SMA      S-band Multiple Access

MMS         Magnetospheric MultiScale Mission               SN       Space Network

MOCC        Mission Operations Control Center               SSA      S-band Single Access

MRO         Mars Reconnaissance Orbiter                     STEREO   Solar TErrestrial RElations Observatory

MS          Mission Systems
                                                            Swift    Swift Gamma-Ray Burst Mission

msec        millisecond
                                                            TAI      International Atomic Time
MSFC        Marshall Space Flight Center
                                                            TBD      To Be Determined
NASA        National Aeronautics and Space
                                                            TBR      To Be Resolved
            Administration
                                                            TC       Telecommand
NTP         Network Time Protocol
                                                            TDRSS    Tracking and Data Relay Satellite System
OMNI        Operating Missions as Nodes on the
            Internet
                                                            Terra    Flagship EOS Mission
PNT         Position, Navigation and Time [Study                     Time History of Events and Macroscale
            Group] / Positioning, Navigation, and           THEMIS
                                                                     Interactions during Substorms
            Timing [Policy]
                                                            TIM      Technical Information Meeting
PRN         Pseudo Random Noise


                                                 29 of 57
TM      Telemetry

TONS    TDRSS Onboard Navigation System

        Transportable Payload Operations
TPOCC
        Control Center

TT      Terrestrial Time

TWSTT   Two Way Satellite Time Transfer

UHF     Ultra High Frequency

US      United States

USN     Universal Space Network

USNO    United States Naval Observatory

USO     Ultra-Stable Oscillator

UTC     Universal Time Coordinated

VCDU    Virtual Channel Data Unit

VSE     Vision for Space Exploration

WISE    Wide-field Infrared Survey Explorer

wrt     with respect to

ηsec    nanosecond

μsec    microsecond

      sigma




                                              30 of 57
11. References
   [1]    B. L. Brodsky, A. Gifford, R. A. Nelson, A. J. Oria, L. Pitts, R. S. Orr, J. Prestage, J. Schier, F.
          VanLandingham, ―Issues in Defining a NASA Time Architecture‖, Interim Time Team Report submitted
          to the Space Communications Architecture Working Group, October, 2006.

   [2]    L. Pitts, L. Felton, F. Vanlandingham, ―NASA Architecture for Solar System Time Synchronization and
          Dissemination: Concept of Operations (ConOps)‖ presented at the NASA Forum on Time Synchronization
          and Dissemination for the Solar System Technical Information Meeting at NASA Headquarters,
          Washington D.C., June 27, 2007.

   [3]    Space Communication Architecture Working Group (SCAWG), NASA Space Communication and
          Navigation Architecture Recommendations for 2005-2030, Final Report, 15 May 2006.

   [4]    Exploration Architecture Requirements Document (EARD), ESMD-EARD-0001, Draft Revision H
          02.02.2007.

   [5]    RFC 1305 - Network Time Protocol (Version 3) Specification, Implementation and Analysis

   [6]    RFC 2030 - Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI

   [7]    Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, IEEE-1588,
          first edition, September 2004

   [8]    Global Positioning System - Standard Position System Signal Specification; 2nd Edition; June 2, 1995.

   [9]    D. J. Israel, A. J. Hook, K. Freeman, J. J. Rush, The NASA Space Communications Data Networking
         Architecture, NAS Technical Report NAS-06-014, August 2006.

   [10] SSP-41154, Software Interface Control Document Part 1 United States On-Orbit Segment to United States
       Ground Segment Command and Telemetry, Revision M, September 01, 2004

   [11] JSC 36381, Operations Local Area Network (OPS LAN) Interface Control Document, International Space
       Station Program, February 2000

   [12] J. Rash, R. Parise, K. Hogie, E. Criscuolo, J. Langston, C. Jackson, and H. Price, ―Internet Access to
        Spacecraft‖, 14th Annual American Institute of Aeronautics and Astronautics (AIAA)/Utah State
        University (USU) Conference on Small Satellites, August 2001.

   [13] K. Hogie and E. Criscuolo, ―Time Transfer Over Space Links,‖ presentation to the Time Team, December
        2006.

   [14] P. Koppang and P. Wheeler, ―Working Applications of TWSTT for High Precision Remote
        Synchronization‖, IEEE International Frequency Control Symposium, May 1998.

   [15] A. Gifford, S. Pace, J. McNeff, ―One-Way GPS Time Transfer 2000‖, 32nd Annual Precise Time and Time
        Interval (PTTI) Meeting, November 2000.

   [16] T. J. Grenchik and B. T. Fang, ―Time Determination for Spacecraft Users of the NAVSTAR Global
        Positioning System (GPS).‖

   [17] "Navigator GPS Receiver for Fast Acquisition and Weak Signal Tracking Space Applications" by L.
        Winternitz, M. Moreau, G. Boegner, and S. Sirotzky, in Proceedings of ION GNSS 2004, the 17th



                                                31 of 57
     International Technical Meeting of the Satellite Division of The Institute of Navigation, Long Beach,
     California, September 21–24, 2004, pp. 1013-1026

[18] Jim Schier, ―Time in the Lunar Architecture‖ presented at the NASA Forum on Time Synchronization and
     Dissemination for the Solar System Technical Information Meeting at NASA Headquarters, Washington
     D.C., June 27, 2007.
[19] R. J. Cesarone, S. N. Rosell, S. A. Townes, D. J. Bell, D. S. Abraham, and G. J. Kazz, Design
     Considerations for the Mars Network Operations Concept, Jet Propulsion Laboratory, California Institute
     of Technology, 2004.
[20] R. A. Nelson and T. A. Ely, ―Relativistic Transformations for Time Synchronization and Dissemination in
     the Solar System,‖ Proceedings of the 38th Annual Precise Time and Time Interval (PTTI) Systems and
     Applications Meeting (U. S. Naval Observatory, Washington, DC, 2006).

[21] Forrest Warthman, ―Delay Tolerant Networks (DTNs), A Tutorial‖, Warthman Associates, March 2003.

[22] A. Gifford, R. A. Nelson, R. S. Orr, A. J. Oria, B. L. Brodsky, J. J. Miller, and B. Adde, ―Time
     Dissemination Alternatives for Future NASA Applications,‖ Proceedings of the 38th Annual Precise Time
     and Time Interval (PTTI) Systems and Applications Meeting (U. S. Naval Observatory, Washington, DC,
     2006.

[23] National Space-Based Positioning, Navigation, and Timing (PNT) Coordination Office (www.pnt.gov).




                                            32 of 57
12. Acknowledgements
The authors would like to acknowledge Mike Kearney‘s support throughout this project. We began our participation
in the PNT Study Group at his request and with his support. When the ConOps idea was put forward, he became a
leading advocate. He is the US Secretariat for CCSDS and Technical Assistant in the Marshall Space Flight Center
(MSFC) Mission Operations Laboratory Office of the Manager (EO-01). This report was generated under the
auspices of the PNT Study Group, and we would like to acknowledge their feedback and ongoing interest in the
project. After starting the survey, we found that identifying an appropriate mission contact was a challenging task.
Dr. Martha Chu, Chief Engineer of GSFC‘s Information Systems Division, enthusiastically embraced the task and
recommended contacts for missions. An individual, when identified, was contacted via e-mail. The message
recipient responded to the Mission Survey or requested that someone else gather the information and respond. In all
cases, we would like to acknowledge the interest and effort made by these contributors. Figure 12 lists those who, to
our knowledge, collected the timing information and responded to the request and/or who forwarded the request to
the appropriate person for response. We gratefully acknowledge their contributions (and sincerely regret any
omissions).

          Mission                        Initial Contact                                   Respondent
                          Martha Chu
ACE                                                                       Jacqueline Maldonado Snell
                          Robert Sodano
                          Amit Sen                                        David M. Durham
Aquarius                  Project Manager                                 Aquarius Project System Engineer
                          Jet Propulsion Laboratory                       Jet Propulsion Laboratory
                                                                          Robert T Mitchell
Cassini
                                                                          Project Manager
Chandra                                                                   Bill Davis
                          Deborah Vane                                    Mark J. Rokey
CloudSat
                          Project Manager                                 Mission Manager
                          Jean Grady
Con. X                    Project Manager                                 Gabriel (Gabe) Karpati
                          GSFC
Dawn                                                                      Keyur Patel
                                                                          Project Manager
                          Rudolf ("Rudi") Schmidt
Gaia                                                                      Robert Furnell
                          ESA Project Manager
GFO                                                                       Morton D. Rau
GLAST                                                                     Norman Rioux
GPM                       Art Azarbarzin                                  Candace Carlisle
                          Project Manager                                 Deputy Project Manager
                                                                          Stefan Thürey
Herschel Space            Thomas Passvogel                                Planck S/C Systems Engineer
Observatory               Project Manager                                 Herschel / Planck Project, European
                                                                          Space Agency
                          Dr. Martha I. Chu
                          Chief Engineer
                          Information Systems Division, Code 580
                                                                          David G. Simpson, Ph.D.
                          Goddard Space Flight Center
IMAGE                                                                     IMAGE/LENA, Cassini/CAPS
                                                                          Goddard Space Flight Center
                          Richard.J.Burley
                          HST Deputy Operations Manager Goddard
                          Space Flight Center
                                                                          Peter Kretschmar
                          Arvind Parmar
INTEGRAL                                                                  Science Operations Manager
                          Mission Manager
                                                                          ESAC
                                                                          Rick Grammier
Juno                                                                      Project Manager
                                                                          Jet Propulsion Laboratory



                                                    33 of 57
         Mission                   Initial Contact                            Respondent
                                                               Nicholas M Jedrich
LISA
                                                               Goddard Space Flight Center
                                                               Ralph Casasanta
LRO                                                            CSC - GSFC LRO Ground System
                                                               Engineering
                                                               Barry Goldstein
Mars Phoenix                                                   Project Manager
                                                               Jet Propulsion Laboratory
Mars Rovers                                                    John Callas
                                                               Project Manager for both rovers
                                                               Jet Propulsion Laboratory
                                                               Stanley B. Cooper
MESSENGER                                                      The Johns Hopkins University Applied
                                                               Physics Laboratory
MMS                                                            Karen Halterman
                                                               Project Manager
                                                               Goddard Space Flight Center
                                                               Thomas Passvogel
                                                               Project Manager
Planck                                                         European Space & Technology Centre
                                                               (ESTEC)
                                                               The Netherlands
                                                               Stanley B. Cooper
Pluto New Horizons                                             The Johns Hopkins University Applied
                                                               Physics Laboratory
                     John Ellwood,
                                                               Andrea Accomazzo
Rosetta              Project manager: ESA/ESTEC (PK)
                                                               Spacecraft Operations Manager
                     The Netherlands
RXTE                                                           Jean Swank
                                                               Project Scientist,
                                                               GSFC
                                                               Stanley B. Cooper
STEREO                                                         The Johns Hopkins University Applied
                                                               Physics Laboratory
Swift                Joe Dezio
                     Project Manager:                          Neil Gehrels
                     Goddard Space Flight Center
                     Martha Chu

Terra                William. J. Guit                          Eric Moyer
                     EOS Aqua/Aura Mission Director
                     Goddard Space Flight Center
                                                               Peter Harvey
THEMIS                                                         Project Manager
                                                               University of California, Berkeley
                                                               Ed Massey
                                                               Voyager Project Manager
Ulysses                                                        Jet Propulsion Laboratory

                                                               Bruce Brymer
                                                               Ed Massey
                                                               Voyager Project Manager
Voyager -1, -2                                                 Jet Propulsion Laboratory

                                                               Jefferson.C.Hall-Jr
                                                               William Irace
WISE                                                           Project Manager
                                                               Jet Propulsion Laboratory

                           Figure 12. Mission Timing Survey Contributors

                                           34 of 57
The authors would also like to thank:
     CSC senior staff members Keith Hogie and Ed Criscuolo of the OMNI Project, who consulted on NTP and
        provided helpful comments on our ConOps presentation to the NASA Forum on Time Synchronization and
        Dissemination for the Solar System TIM, which served as an early draft of this report.
     Dr. William S. Davis, CSC/Chandra X-Ray Observatory mission support, for his review and helpful
        comments on the Mission Timing Survey and this study.
     Johns Hopkins University/Applied Physics Laboratory‘s Stanley B. Cooper, who in discussions at and after
        the TIM, provided comments and encouraged us to increase the emphasis on Domains D3 and D4.




                                              35 of 57
Appendix – Mission Timing Survey Detailed Data
As described in Section 3, the mission information in this paper is taken from the responses to the Mission Timing
Survey, shown below as it was distributed. This includes the websites listed in Figure 1, the precision information
presented in Figure 5, and the information detailed in Sections A-1 through A-6.

Some question responses were descriptive in nature, and some of these required editing for a more succinct
presentation of the information. Edits focused primarily on using acronyms, deleting unnecessary words, rephrasing,
and other techniques. In each case, every effort was made to retain the original meaning and intent. It was therefore
sometimes necessary to retain the original text despite its length. All changes are the responsibility of the authors.
Numerical values and the associated units were not changed.
                              Mission Timing Survey Topics and Questions

1.0     Mission Name
        If there is a web site, what is the URL?
2.0     Mission Objectives
        Briefly, what are the primary mission objectives?
3.0     Mission Timeline and Duration
        3.1 What are the primary mission phases and their durations?
        3.2 What are the critical events?
4.0     Mission Environment
        What is the mission environment?
             Low Earth Orbit (LEO) – up to 1,000 miles above Earth‘s surface; typically satellites used in
                 telecommunications and Earth sensing
             Medium (or Middle) Earth Orbit (MEO) – between 1,000 miles and 22,300 miles above Earth‘s
                 surface; typically geographical positioning systems not stationary in relation to the rotation of the
                 Earth
             Highly Elliptical Orbit (HEO) – typically a satellite system used in telecommunications for its high
                 dwell time over an area not on the equator
             Far Earth Orbit – a geocentric but not geosynchronous orbit; includes Lagrange points and Lunar
                 orbits
             Interplanetary – orbit which is non-geocentric but within the solar system
             Extrasolar – region of space outside the solar system
5.0     Mission Configuration
        Further characterize the mission in these areas:
        5.1 Number of spacecraft [single | multiple satellites and interrelationships]
        5.2 Bus configuration
        5.3 Required Standards [e.g., AOS | HDLC | IP | Key Management | Spacewire | CFDP | NTP | IEEE 1588 |
             AMS | etc.]
        5.4 Security model [e.g., CCSDS | IPsec | SCPS-SP | End-to-end | etc.]
6.0     Communication Modes
        Which of the following communication modes must be supported?
                 Normal Operations
                 Science Delivery
                 Contingency Operations
7.0     Commanding Modes




                                                   36 of 57
       Which of the following commanding modes must be supported?
            Stored program commands
            Real-time commands
            Autonomous commands / Directive initiation
8.0    Time Precision
       What time precision is required for the following?
       8.1 Early operations / initialization
       8.2 Experiment / measurement
9.0    Navigation Precision
       What navigation precision is required for the following?
       9.1 Early operations
       9.2 Real-time
       9.3 Post measurement
10.0   Time Correlation Computation Factors
       10.1 What factors are considered in the spacecraft clock correlation determination?
       10.2 What events, if any, have caused or do you anticipate causing a problem in this determination? [e.g.,
       onboard clock rollover | end of year | leap year/second]
11.0   Time Requirements
       11.1 Characterize the time available for fulfilling the mission objectives versus the time required to perform
             ―housekeeping‖ activities.
       11.2 Characterize the communications response time.
       11.3 Identify any specific time requirements in the following areas:
            Command Systems
            Instruments
            Frequency Standards
            Other
.




                                                 37 of 57
The following sections summarize detailed information from the Mission Timing Survey.

A-1.    Time Precision
A-2.    Navigation Precision
A-3.    Time Requirements
A-4.    Communication Services
A-5.    Factors Considered in Spacecraft Clock Correlation Determination
A-6.    Factors That Cause Problems in Spacecraft Clock Correlation Determination




                                               38 of 57
A-1. Time Precision


                                                                                 Time Precision
       Mission
                            Experiment /                          AD&SC /
                                                                                                               Commanding
                           Measurement                        Attitude Control
ACE                   100 msec
Aquarius              N/A                             1 sec                            N/A
Cassini
                                                                                       - Stored executed within a few secs with resolution of
                      100 µsec                                                           0.25625 sec
Chandra                                               0.25 sec
                      goal: 10 µsec                                                    - Real-time executed to a precision of few secs and correct
                                                                                         order.
CloudSat              Coarse                          Coarse                           Coarse
                      Photon arrival tagged wrt
Con. X                                                Minutes                          Minutes
                      UTC ±100 sec
Dawn                                                                                   - During early operations and cruise to the asteroids, will
                                                                                         keep absolute uncertainty in command execution time < 1
                  - onboard time - 2 sec                                                 min
                  - time reconstruction - 60          onboard time - 2 sec             - Larger errors, anticipate:
                    msec.                                                                o None for S/C
                                                                                         o Difficult for ground-based (real-time) telemetry observers.
                                                                                       - Relative command timing error <= 100 msec
Gaia               <60 sec                           <100 msec                        <1 sec
                  - Satellite Clock ±10 sec /
                     12 hrs
GFO
                  - Correlate Altimeter data
                     UTC: ± 20 sec
                                                      Star tracker processing
GLAST                 <10 μsec wrt UTC, 1 RMS                                         1 sec
                                                      relative time ~1 msec
GPM               - Core - 1 msec.
                  - Constellation - TBD, likely
                     similar to Core.
Herschel Space     Time reconstruction: 5
                                                      Time reconstruction: 5 msec      2 sec
Observatory        msec
IMAGE              1 sec



                                           39 of 57
                                                                                Time Precision
        Mission
                             Experiment /                        AD&SC /
                                                                                                            Commanding
                            Measurement                      Attitude Control
                     Total: ~100 μsec:
                     - 50 μsec for on-board
INTEGRAL             processing                        N/A                            ~ 1 sec
                     - 50 μsec for on-ground
                     processing
Juno                                                                              msec level
LISA
                     S/C time within 100ms of                                         Ground commands to adjust time to keep time difference
LRO
                     UTC                                                              <100ms
Mars Phoenix         1 msec                            100 sec                       1 msec
Mars Rovers          mins or secs and
                                                       N/A for surface phase          secs
                     sometimes sub-secs
                                                                                      - +/-1 sec to +/-10 sec
MESSENGER            +/-1 msec post-processed          +/-0.1 sec to +/-100 sec
                                                                                      - G&C commanding can be tighter.
MMS                  Fine (1 msec to 1 sec)           Fine                           Coarse
                     Time reconstruction: 5            Time reconstruction: 5
Planck                                                                                2 sec
                     msec                              msec.
                                                                                      - +/-1 sec to +/-10 sec
Pluto New Horizons   +/-10 msec post-processed         +/-0.1 sec to +/-100 sec
                                                                                      - G&C commanding can be tighter.
Rosetta              3 msec                            3 msec                         1 sec
RXTE                                                   Attitude on 1/4 sec
                     1 sec                                                           All commanding is on 1 sec boundaries.
                                                       boundary
                                                                                      - +/-1 sec to +/-10 sec
STEREO               +/-0.5 sec real-time              +/-0.1 sec to +/-100 sec
                                                                                      - G&C commanding can be tighter.
Swift                - 1 msec onboard
                     - 0.2 msec on ground
Terra                ±100 sec wrt UTC
                     <10 sec absolute for
THEMIS                                                 8 msec                         1 sec/24 hrs
                     substorm timing
Ulysses              100 msec                          DSN standard                    DSN standard
                                                                                       - Stored: commands: 48 sec
Voyager -1, -2
                                                                                       - Real-time commands: immediate or hours clock
WISE                                                                               0.6 sec



                                            40 of 57
A-2. Navigation Precision

                                                                   Navigation Precision
  Mission                                                                    Post
              Early operations                 Real-time                                         Predictive               Orbit Events
                                                                          measurement
ACE
Aquarius                                                          Responsibility of CONAE
                                                                                                                  Prediction: Titan & targeted
                                                                                                                  icy satellites flybys - 3 sec
                                                                                                                  (1)
                                     - Timing (3): 30E-6 sec wrt UTC - Timing (3): 60E-
                                                                                                                  Saturn closest approach (for
                                                                          9 sec wrt UTC
                                                                                                                  Titan inbound orbits) - 15
Cassini                              - Frequency - f(3): (f/f): 9E-13 - Frequency - f
                                                                                                                  sec (1)
                                       wrt TAI                            (3): (f/f): 3E-13                     Saturn closest approach (all
                                                                          wrt TAI
                                                                                                                  other orbits) - 2 sec (1)
                                                                                                                  Reconstruction: - 300 msec
                                                                                                                  (1)
                                                                        < 3 km (light travel  < 30 km after 7   - Most within a few secs
Chandra
                                                                        time - 10 µs)         days              - Radiation events > 1 hr
CloudSat      Coarse                 Coarse                             Coarse                Coarse              Coarse
Con. X                            Commensurate with photon arrival tagged wrt UTC ±100 sec                        +/- 10 km or more
Dawn                             Because can’t use GPS or any other Earth-based, near-real-time navigation network, this is N/A.
                                     <50m, assumes ranging and
Gaia
                                     Doppler tracking
                                     (Doppler Orbit / Operational
                                     Orbit) After tilt and bias removal Radial component
                                     along a 3,000 km arc (i.e., filter of orbit error < 10
GFO                                  length) of altimeter height data,  cm RMS on
                                     the residual radial orbit error <  wavelengths less
                                     1.8 cm RMS on horizontal spatial than 40,000 km
                                     scales of 1,000 km or less.
              120 km navigation 12 km navigation accuracy for                                                     120 km navigation accuracy
              accuracy for star      realtime Guidance Navigation                                                 for GNC to indicate science
GLAST         tracker                Control (GNC) control, LVLH        < 3.3 km                                  instruments passage in and
              initialization and     determination, Ku antenna                                                    out of South Atlantic
              Thruster checkout pointing, Solar array pointing                                                    Anomaly




                                          41 of 57
                                                                     Navigation Precision
  Mission                                                                      Post
               Early operations                  Real-time                                          Predictive              Orbit Events
                                                                            measurement
GPM            Core:                                                                            Accurate enough
               - ~2.5 km to geolocate instrument measurements to                                to schedule
                 within ½ pixel (achieved with GPS)                                             ground station
               - +/- 1 km for altitude (controlled FOT)                                         resources if
               Constellation - TBD                                                              necessary.
Herschel
                                   - Early operations - secs                                    5 msec / 24 hrs,
Space                                                                    5 msec                                      ~ 10 sec
                                   - At L2, about 10 sec (round-trip)                           max.
Observatory
IMAGE                                          +/- 50 km for all
                                                                   Orbit propagation
INTEGRAL                                                           delay prediction: <
                                                                   32 μsec
               Standard DSN performance - 2-way X-band Doppler & Range data with ~0.1 mm/sec noise (with a 60 sec count time) on the
Juno
               Doppler & 2-3 m for range data
                                                                                        Expect normal precision < 90 km in position and ~
LISA                                                                                    2 cm/sec in velocity. Precision in radial direction
                                                                                        most important
                                                                                                              Predictive support Laser
                                                                                        - < 800 m along
                                                                                                              Ranging activities to produce
                                                                                           track / 84 hr
LRO                                                                                                           specific acquisition data < 4
                                                                                        - Initial definitive:
                                                                                                              km for any portions of 10 day
                                                                                           < 500 meters.
                                                                                                              data product
Mars Phoenix                           1 sec                                                        0.1 sec
Mars Rovers                                                    N/A for surface mission.
MESSENGER
MMS                                             Fine precision is required for navigation in all the above cases
                                   - Early operations - secs                                      5 msec / 24 hrs,
Planck                                                                   5 msec                                      ~ 10 sec
                                   - At L2, about 10 sec (round-trip)                             max.
Pluto New
Horizons
Rosetta
RXTE                                                                     +/- 450 m              1 min boundary
STEREO
Swift                              3 arc-min                             3 arc sec



                                          42 of 57
                                                                       Navigation Precision
  Mission                                                                        Post
                 Early operations                  Real-time                                       Predictive          Orbit Events
                                                                              measurement
                                       TONS Terra:
                 Orbital Elements      Position: ± 150 m, per axis, 3
                 - Terra Position: ±   Crosstrack Velocity: ± 0.16
                   50 km, RSS, 3      m/sec, 3
                                                                            Same precision as
Terra              after 2 days        TONS TDRS:
                                                                            real-time
                 - TDRS Position:      Position: ± 75 m, RSS, 3 after 1
                   ± 120 km, RSS,      day propagated onboard
                   3  after 2 days    Velocity: ± 0.011 m/sec, RSS, 3
                                       after 1 day propagated onboard
                 +/- 0.4 deg (Set by
                                       +/- 0.4 deg (Set by 11-meter dish
THEMIS           11-meter dish                                              +/-10 km            +/-10 km        +/-10 km
                                       acquisition)
                 acquisition)
Ulysses                                                                    All DSN standard
Voyager -1, -2
WISE                                                                          – +/- 30km




                                              43 of 57
A-3. Time Requirements

                                                                        Time Requirements
    Mission          Command
                                                            Instruments                       Frequency Standards                 Other
                      Systems
ACE
Aquarius          ~1 sec Typically                         ~1 sec Typically                           TBD
                                                                                            Onboard clock stability - 15
Cassini
                                                                                                  msec / 8 hrs
                                                                                            Onboard oscillator stability
Chandra                                         ACIS 20 msec, HRC 16 µsec
                                                                                                – 10E-9 / 24 hrs.
CloudSat                                                                      None
Con. X                                              Mins                                                N/A
Dawn                     No
                 on-board time tag    - Instruments time stamp <50ns / 6hr spin period
Gaia
                 <1 sec               - Time stamp to UTC (end-to-end) <2μsec
GFO                                                                      ± 10 sec
GLAST                                           < 10 msec wrt UTC, 1 RMS                     Timing relative to UTC
GPM
                                                                                                                             Accuracy of time-
                                                                                             Stability, drift, ageing of     reference-marker
Herschel Space
                       1 sec               Onboard between instruments: 1 msec                onboard local clocks:        from master-clock to
Observatory
                                                                                                      <10E-6                 downlink-data: 0.1
                                                                                                                                   msec
IMAGE                                           1.0 sec
INTEGRAL                                                                                                                   Real time only
                                      Distribute to instruments
                                      - S/C clock time at 0.5 Hz;
                                      - S/C time-at-tone message; received 0.20 and
                                        [1.99] secs before corresponding S/C time tone.     - Telecommunications - X-
                 Correlate to UTC <
                                      - Acquire coherent Doppler and Ranging                  band (7.2 GHz)
Juno             100 msec.
                                        measurements at Earth distance < 6.46 AU, with      - Radio Science - X/Ka (32
                                        Sun-Earth-Probe angle > 3°, with end-to-end           GHz)
                                        accuracy: Two-way X-band Doppler: 0.1
                                        mm/sec 1, 60 sec integration time Range: 5 m
                                        1
LISA


                                         44 of 57
                                                             Time Requirements
    Mission         Command
                                                  Instruments                    Frequency Standards                  Other
                     Systems
LRO
Mars Phoenix
Mars Rovers
MESSENGER
MMS
                                                                                                                 Accuracy of time-
                                                                                 Stability, drift, ageing of     reference-marker
Planck                 1 sec           Onboard between instruments: 1 msec        onboard local clocks:        from master-clock to
                                                                                          <10E-6                 downlink-data: 0.1
                                                                                                                       msec
Pluto New
Horizons
Rosetta
RXTE
STEREO
Swift
                  stored command
                   execution: S/C
Terra
                   clock within ±8
                    msec of UTC
                 Stored commands:                                                S/C and instrument: 8
THEMIS
                   1-sec timebase                                                     Megahertz
Ulysses            DSN standard                 100 msec standard                   DSN standard
Voyager -1, -2     16 bits per sec
WISE                                                    0.6 sec




                                     45 of 57
A-4. Communication Services

       Mission                                                  Communication Service(s)

ACE              DSN
                 CONAE Ground Station
Aquarius         NASA Ground Network in support of launch; critical mission events, i.e., deployments, initial turn-ons, monthly orbit
                 maintenance maneuvers, etc.; back-up for flight or ground anomalies to safe the flight system.
                 DSN
Cassini           Uplink communication rates between 7.8125 bps and 500 bps (7.8125, 15.625, 31.25, 62.5, 125.0, 250.0, and 500.0)
                  During nominal operations, configure uplink for 500.0 bps and reserve 7.8125 bps for safing communications
                 Shared DSN services primarily using 34 meter antennas using beam wave guide DSN subnets and 26 meter subnets as
Chandra
                 needed all in S-band
CloudSat         Air Force Control Satellite Network
Con. X           DSN
Dawn
Gaia             X-Band Uplink/Downlink
GFO
                 TDRSS S-band Single Access (SSA), S-band Multiple Access (SMA), Ku
GLAST
                 Universal Space Network (USN) Ground network Hawaii and Dongarra Australia.
GPM              Core spacecraft:
                  During normal operations, downlink radiometer data nearly continuously using TDRSS SMA service (230 kbps)
                  Once per orbit, TDRSS SSA service scheduled to downlink remainder of mission science data (2.3 Mbps)
                  Shorter duration contingency operations low-rate engineering data downlinked through TDRSS system – most likely using
                   SSA service.
                  Mission contingencies, which involve loss of high-rate science communications via TDRSS, would necessitate use of
                   commercial or NASA-owned ground stations for science data recovery. Data will be downlinked via S-band to these ground
                   stations at ~ 4.6 Mbps. Contacts would be conducted approximately once per orbit, but would be subject to visibility
                   constraints as well as station support schedule constraints.
                 Constellation spacecraft:
                  Data rates are TBD – but basic communications architecture (TDRSS SMA and SSA services, with commercial ground
                   networks used as a backup) will be employed.




                                        46 of 57
       Mission                                                      Communication Service(s)
                  All uplink- and downlink data are packetized according to the ESA/ European adaptations of:
                   o TC: CCSDS-201.0, -202.0, -203.0, (ESA-PSS-04-107)
                   o TM: CCSDS-101.0, -102.0 (ESA-PSS-04-103, -106)
                   o Time: CCSDS-301.0 (ESA-PSS-04-202)
                   o Data exchange: MIL Std. 1553 B
                  Max. TM-/TC-packet length: 1 Kbytes.
Herschel Space
                  At communication-level, besides command distribution and (periodic (housekeeping)) data acquisition from all onboard
Observatory
                   units, supported main services are:
                   o Parameter monitoring with autonomous action-triggering
                   o Mission timeline with stored command-sequences
                   o Configuration into standby-mode in case of anomalies
                   o Downlinking of science-data (packets) from mass-memory.
                  No file-transfer, or other complex/flexible/demand-driven (onboard) data exchanges or services
IMAGE            DSN
                  Real-time ground control
                  Data Provision from station to control centre:
INTEGRAL           o HK data: online timely
                   o Science data: online complete
                  European Ground Stations
Juno             DSN
LISA             Telemetry, periodic data downloads
LRO              White Sands, USN
                  Direct communication via X-Band with DSN through cruise to Mars.
Mars Phoenix
                  UHF relay through MRO and Odyssey on EDL and surface operations.
Mars Rovers       X-band direct-to-earth is stand two-way DSN service.
                  UHF relay is two-way with additional communication latency.
MESSENGER        DSN
MMS              Elements of the Space Network (TDRSS) and Ground Network (DSN and USN) will be required. All communications use S-
                 band




                                         47 of 57
        Mission                                                  Communication Service(s)
                   All uplink- and downlink data are packetized according to the ESA/ European adaptations of:
                    o TC: CCSDS-201.0, -202.0, -203.0, (ESA-PSS-04-107)
                    o TM: CCSDS-101.0, -102.0 (ESA-PSS-04-103, -106)
                    o Time: CCSDS-301.0 (ESA-PSS-04-202)
                    o Data exchange: MIL Std. 1553 B
                   Max. TM-/TC-packet length: 1 Kbytes.
Planck             At communication-level, besides command distribution and (periodic (housekeeping)) data acquisition from all onboard
                    units, supported main services are:
                    o Parameter monitoring with autonomous action-triggering
                    o Mission timeline with stored command-sequences
                    o Configuration into standby-mode in case of anomalies
                    o Downlinking of science-data (packets) from mass-memory.
                   No file-transfer, or other complex/flexible/demand-driven (onboard) data exchanges or services
Pluto New
                  DSN
Horizons
Rosetta
RXTE              Communications through TDRSS, Domesat.
                  DSN for emergency. No emergency has occurred.
STEREO            DSN
Swift             S-band
                  TDRSS + Italian Space Agency Ground Station in Malindi, Kenya
                  Contingency - USN
Terra             TDRSS
THEMIS            S-band Up/Down using Berkeley Ground Station, Wallops and Hartebeestok mostly.
Ulysses           Standard DSN and Space Link Extension (SLE)
Voyager -1, -2    DSN
WISE              TDRSS




                                         48 of 57
A-5. Factors Considered in Spacecraft Clock Correlation Determination


   Mission                        Spacecraft                                Downlink                     Ground                   Correlation
                                                                     Not correctly accounted
                                                                                                 AGRT = DSN's GRT -          AGRT = DSN's GRT -
                                                                     scientists are aware that
ACE                                                                                              Ground Delay - Rate         Ground Delay - Rate
                                                                     range delay must be
                                                                                                 Delay                       Delay
                                                                     considered
Aquarius
                                                                                                 Tracking station location
Cassini                                                              One-way light time
                                                                                                 (dynamic)
                                                                     Signal-travel time from      DSN receipt time
Chandra       Signal delay time on-board spacecraft                  spacecraft antenna to        DSN time delayssync
                                                                     DSN ground antenna            pulse delay
CloudSat
Con. X
Dawn           Largest uncertainty: short-term and/or long-
                term instability of onboard oscillator - shifts in
                                                                                                 Uncertainties in earth
                oscillator frequency due to voltage variations,
                                                                                                 receipt time of time
                temperature variations, radiation, and aging
                                                                                                 correlation packets
               Uncertainties in onboard creation of time
                correlation packets
                                                                      One way ranging
                                                                       errors,
                                                                                                 Ground Station Time
               On-board clock stability                              Propagation
                                                                                                 Stamping Errors
Gaia           Signal delay/jitters                                   delay/error (Position
                                                                                                 (delays, jitters, clock
               Time Code quantization in telemetry                    uncertainty,
                                                                                                 performance
                                                                       Troposphere,
                                                                       Ionosphere, etc),
GFO




                                            49 of 57
  Mission                       Spacecraft                              Downlink                 Ground                    Correlation
              Use GPS receiver to determine frequency of on
              board oscillator & keep one second count
              delimiters in step with GPS atomic standard.
              Relativistic correction made in GPS clocks will
              automatically cause GLAST clock to reduce
GLAST
              ticks per second by one tick every several hours
              so GLAST one second interval will appear to
              agree with UTC one second interval. This will
              allow simple calculation to convert the GLAST
              MET to UTC.
GPM           Core and Constellation, spacecraft clock
              correlation/calibration functions will be                                                              Ground operations
              performed by on-board GPS receivers. Based                                                             would only perform this
              on experience with other missions flying GPS                                                           activity routinely only if
              technologies, failures are essentially non-                                                            on-board GPS
              credible given the presence of redundant                                                               capability is lost.
              antennas/receivers, as is planned for GPM.
                                                                                          Errors in ground-
               Ageing
Herschel                                                         Errors in downlinking     infrastructure
               Onboard master-clock drifts
Space                                                            time information to      Errors in decentralized
               onboard time-distribution delays
Observatory                                                      ground                    users
               local onboard time delays
                                                                                          Time delays
                                                                                                                     Clock-Corr maintained
                                                                                                                     by syncing onboard
                                                                                                                     MET with Ground
                                                                                                                     Receipt time applied to
       1
IMAGE                                                                                                                VCDU’s by DSN, and
                                                                                                                     adding R/F transit time
                                                                                                                     between IMAGE located
                                                                                                                     (using predicted orbit)
                                                                                                                     and DSN antenna.
                                                                  Ground – S/C delay
INTEGRAL      On-board delay
                                                                  Antenna delay
Juno
                                                                                                                     Routine time-of-day
LISA
                                                                                                                     synchronization



                                         50 of 57
  Mission                         Spacecraft                                 Downlink                 Ground              Correlation
                                                                                              Stations required to
                                                                                              meet 1 msec
LRO
                                                                                              accuracy/precision for
                                                                                              command time stamp
Mars Phoenix
Mars Rovers    clock drift
                Environmental effects (temperature, radiation,
                 relativity, oscillator voltage and loading) on the
                 oscillator and also oscillator aging                                         Accuracy of DSN
                                                                      One-way-light-time
MESSENGER       Instrument timing uncertainties                                              station telemetry frame
                                                                      delay
                Uncertainty in distribution of time to                                       time tags
                 instruments
                Delays through the spacecraft
MMS            Because all four spacecraft contain GPS
               receivers, TAI is maintained on-board all four
               spacecraft. Therefore, no ground correlation is
               required under normal conditions
                                                                                               Errors in ground-
                Ageing
                                                                      Errors in downlinking     infrastructure
                Onboard master-clock drifts
Planck                                                                time information to      Errors in decentralized
                Onboard time-distribution delays
                                                                      ground                    users
                Local onboard time delays
                                                                                               Time delays
                Environmental effects (temperature, radiation,
                 relativity, oscillator voltage and loading) on the
                 oscillator and also oscillator aging                                         Accuracy of DSN
Pluto New                                                             One-way-light-time
                Instrument timing uncertainties                                              station telemetry frame
Horizons                                                              delay
                Uncertainty in distribution of time to                                       time tags,
                 instruments
                Delays through the spacecraft
                                                                                              On-board delays (TM bit
               On-board delays (TM bit rate dependant) in                                     rate dependant) in
Rosetta
               generating time reference                                                      generating time
                                                                                              reference
RXTE           Use TT on board




                                            51 of 57
   Mission                          Spacecraft                                 Downlink                    Ground                 Correlation
                  Environmental effects (temperature, radiation,
                   relativity, oscillator voltage and loading) on the
                   oscillator and also oscillator aging                                            Accuracy of DSN
                                                                        One-way-light-time
STEREO            Instrument timing uncertainties                                                 station telemetry frame
                                                                        delay
                  Uncertainty in distribution of time to                                          time tags
                   instruments
                  Delays through the spacecraft
Swift
Terra
                                                                        Range is major factor in
THEMIS
                                                                        clock update process
                                                                        Factored with one way                                Compared to predicted
Ulysses          Spacecraft clock imbedded in engineering data
                                                                        light time                                           ERT
Voyager -1, -2
                                                                         RTLT
WISE             S/C delays
                                                                         TDRSS delays




                                              52 of 57
A-6. Factors That Cause Problems in Spacecraft Clock Correlation Determination


   Mission        Spacecraft             Ground                  Leap Second              Comment                   No Factor

                                                                                      Increased precision
                                                                                       in slope - fixed
                                                                                       2003
                                                                                      Increased precision
                                                                                       in offset - fixed
                                                                                       2006
                                                                                      Numerical error in
                                                                                       calculation - fixed
                                  DSN Ground Receipt
                                                           Leap secs not correctly     2006
ACE                               Time pausing for 1 sec
                                                           accounted - fixed 2002     Account for range
                                  fixed 2002
                                                                                       delay – fix if
                                                                                       requested
                                                                                      TPOCC file only
                                                                                       has entries for 5
                                                                                       years - fixed 1998
                                                                                      TPOCC file only
                                                                                       has entries for 10
                                                                                       years - fixed 2006
                                                                                                             None; responsibility of
Aquarius
                                                                                                             CONAE
                                                                                                             None; account for roll
Cassini                                                                                                      over, end of year, and
                                                                                                             leap second




                                     53 of 57
  Mission        Spacecraft                  Ground                   Leap Second               Comment                    No Factor

                                      DSN ground signal
                                       delays had to be
                                       "calibrated" or made
            Onboard clock signal
                                       consistent among all                                Post launch,
            delays re-evaluated
                                       antennas and at          Applied work-around to     evaluated absolute
            post launch resulting
                                       different telemetry      process data across leap   timing analysis of all
            in a change of
                                       rates post launch by     seconds. Ground clock      timing errors. The
            values. [This was
Chandra                                Chandra in-flight        correlation software is    overall results of the
            based on] "What
                                       observations             scheduled to be modified   analysis were later
            onboard delay times
                                      Modified original        to handle future leap      confirmed by X-ray
            should be used to
                                       ground clock             seconds.                   observations of
            time stamp the
                                       correlation software                                pulsars.
            science data?".
                                       post launch to
                                       account for onboard
                                       clock rollover
CloudSat                                                                                                            None
                                                                                                                    Have not assessed yet
Con. X                                                                                                              (project is in pre-phase A
                                                                                                                    formulation)
            Sized onboard time
            message so that it
Dawn        will not roll over
            during eight year
            mission
            On-board time
                                     CCSDS/SLE time
            precision and                                                                   Orbit determination
                                     stamp (for ground
            transfer to instrument                                                           / Ranging errors
                                     reception time) is too
            is critical, detector                                                            must be very low,
Gaia                                 restrictive (rounding to
            samples must be                                                                  to allow correlation
                                     1microsecond). Use
            stamped with                                                                     to UTC
                                     private field to extend
            precision < 50sec /                                                            < 2 μsec
                                     precision.
            6hr spin cycle.
GFO                                                             Leap Second
                                                                                                                    No anticipated problems
                                                                                                                    as long as one of two
GLAST
                                                                                                                    GPS receivers are
                                                                                                                    working


                                        54 of 57
   Mission        Spacecraft              Ground         Leap Second             Comment          No Factor

GPM                                                In past – there have been
                                                   instances of missions
                                                   using GPS technologies
                                                   where leap seconds have
                                                   been improperly handled,
                                                   causing large errors in
                                                   orbit determination. Proper
                                                   handling of this case will
                                                   be considered when
                                                   selecting GPS receivers.
                                                                                           None of these events:
                                                                                           time information is
                                                                                           handled in adequate
                                                                                           numerical format, free of
Herschel                                                                                   human "end-of-year‖.
Space                                                                                      Conversion into "human-
Observatory                                                                                suitable" time formats is
                                                                                           only done on ground,
                                                                                           when data are presented
                                                                                           to operators and
                                                                                           scientists.
              IMAGE clock is not
              very accurate. Its
              oscillation frequency                End-of-Year/Leap
IMAGE         cannot be controlled.                Seconds were never
              Time drift driven by                 problem
              temperature of CPU
              board.




                                      55 of 57
  Mission          Spacecraft               Ground                    Leap Second         Comment          No Factor

                                      Incorrect
                                       measurement of
                                       Antenna delay
                                      Sporadically incorrect
                                       set-up of ground
                                       station, leading to
INTEGRAL
                                       fixed offset in
                                       measurement for an
                                       entire pass.
                                      Update of Ground –
                                       S/C delay in mission
                                       control system
Juno                                                                                                None
LISA                                                                                                No anticipated issues
LRO                                                                                                 None
Mars Phoenix
Mars Rovers                                                                                         None
                                                                Leap seconds because
                                                                ground system uses leap
MESSENGER                                                       seconds in multiple
                                                                workstations and
                                                                processing systems.
MMS                                                                                                 By exchanging time
               Time code counters
                                                                                                    between the spacecraft
               are sized to avoid                               Use of TAI eliminates
                                                                                                    (via the inter-satellite
               clock rollover over                              jumps due to leap
                                                                                                    link), clock errors
               maximum expected                                 seconds.
                                                                                                    between spacecraft are
               mission life time
                                                                                                    minimized.




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   Mission            Spacecraft                 Ground                Leap Second          Comment          No Factor

                                                                                                      None of these events:
                                                                                                      time information is
                                                                                                      handled in adequate
                                                                                                      numerical format, free of
                                                                                                      human "end-of-year‖.
Planck                                                                                                Conversion into "human-
                                                                                                      suitable" time formats is
                                                                                                      only done on ground,
                                                                                                      when data are presented
                                                                                                      to operators and
                                                                                                      scientists.
                                                                  Leap seconds because
                                                                  ground system uses leap
Pluto New
                                                                  seconds in multiple
Horizons
                                                                  workstations and
                                                                  processing systems.
                 Uncertainty in correction factors for on-board
Rosetta
                 and ground delays
RXTE                                                                                                  No problem with
                                                                                                      spacecraft clock
                                                                                                      determination
                                                                  Leap seconds because
                                                                  ground system uses leap
STEREO                                                            seconds in multiple
                                                                  workstations and
                                                                  processing systems.
Swift
Terra                                                             Leap Second
                 Based upon clock
THEMIS           width, do not expect
                 clock rollover
Ulysses                                                                                               None
Voyager -1, -2
WISE                                                                                                  none




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