Technology of Precise Orbit Determination by dfgh4bnmu

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									                Technology of Precise Orbit Determination



                V Seiji Katagiri         V Yousuke Yamamoto
                                                                 (Manuscript received March 19, 2008)



                Since 1971, most domestic orbit determination systems have been developed by
                Fujitsu and its technology is highly evaluated by people involved in the field. In the
                mid-1990s, with the increased precision and sophistication of satellite Earth obser-
                vations there was a need for greater accuracy in determining orbits that could not be
                provided by conventional systems. Thus, the Japan Aerospace Exploration Agency
                (JAXA; formerly known as the National Space Development Agency of Japan) has
                planned the development of a Global and high accUracy Trajectory determina-
                tion System (GUTS), which uses GPS and satellite laser ranging (SLR) technology.
                Fujitsu developed the core technology for this and operated the system. Further-
                more, Fujitsu has been engaged in the operation of GUTS and is committed to main-
                taining and improving the precision of orbit determination technology through
                continuous improvement of this system.
                This paper introduces the overview of the GUTS, core technologies related to orbit
                determination developed by Fujitsu and the current situation with regards to derived
                precision of orbit determination. It also describes the overview of a precise posi-
                tioning experiment system based on our experience of using the core technology of
                GUTS.




1. Introduction                                                  the beginning of the 1990s, the position accuracy
     The satellites launched by domestic space                   requirements for satellites at an altitude lower
development agencies are being operated within                   than 1000 km had been in the order of several
the framework of the task known as “track-                       100 m to 1 km. The systems supplied by Fujitsu
ing and control”. Tracking and control means                     satisfied such requirements.
identifying the operational status of the onboard                     In the mid-1990s, a higher level of sophisti-
equipment in a satellite in orbit, appropriately                 cation and precision was required for the Earth
controlling the satellite and maintaining it in                  observation missions of satellites. The optical
the specified orbit. To achieve these things, it is              observation and remote sensing of the Earth’s
essential to precisely determine the satellite’s                 surface using multi-spectrum observation as well
location and establish a link between a terrestri-               as precise observation on a 24-hour basis using
al antenna and the satellite.                                    radio waves were implemented by the Advanced
     The orbit determination described in this                   Earth Observing Satellite-II (ADEOS-II) and
paper refers to the technology to precisely identi-              Advanced Land Observing Satellite (ALOS). The
fy the movement of a satellite (its position and                 position determination accuracy requirements for
velocity). Since 1971, Fujitsu has been engaged                  orbit determination systems was less than 10 m
in the field of orbit determination systems at the               for ADEOS-II and less than 1 m for ALOS, which
main domestic space development agencies. Until                  could not be achieved by conventional technolo-

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S. Katagiri et al.: Technology of Precise Orbit Determination




gies. To satisfy these requirements, the Japan                  range, and does not correspond to the actual
Aerospace Exploration Agency (JAXA; former-                     range. Since the GPS satellite’s position and the
ly known as the National Space Development                      clock offset are broadcast from GPS satellites,
Agency of Japan) has planned the development of                 four unknown variables (the three-dimensional
a Global and high accUracy Trajectory determina-                position of the vehicle and the clock offset) are
tion System (GUTS),1),2) which was a precise orbit              calculated in the positioning of the car naviga-
determination system using Global Positioning                   tion system, resulting in the need to receive
System (GPS) data whose benefits had been                       positioning signals from at least four satellites
demonstrated in the United States. Upon start-                  simultaneously.
ing the conceptual design, Fujitsu participated in                   If seen from the altitude of GPS satellites
discussions and accumulated precise orbit deter-                (20 000 km), even satellites that move in an orbit
mination technology through establishing GUTS.                  with an altitude lower than 1000 km such as
     The following sections of this paper outline               Earth observation satellites have a geometrical
the current status with regards to the GUTS                     condition not much different from the terrestri-
system, core technologies of GUTS and the                       al condition. Accordingly, the position and clock
derived precision of orbit determination, and give              offset can be calculated using the same principle
an overview of the ongoing project for a precise                as that mentioned above.
positioning experiment system.
                                                                2.2 Precise orbit determination
2. Overview of precise orbit                                         Because GUTS requires more precise
   determination technologies                                   position calculations than in the case of a car
      In this section, one of the most familiar                 navigation system, it receives radio waves of
examples of precise orbit determination technol-                two frequency bands that are sent from a GPS
ogy (a car navigation system) is introduced at                  satellite and, for each of them, the calculation is
first, followed by an explanation of a sophisticat-             carried out using more precise data called “carrier
ed version of such technology and the method of                 phase” in addition to the pseudo range. Further,
verifying the accuracy of a precise orbit.                      because the accuracy of the GPS satellite’s
                                                                position and the clock offset broadcasted from the
2.1 Positioning using GPS satellites                            GPS satellite is insufficient, the GPS satellite’s
     A car navigation system is the most familiar               position and the clock offset are calculated more
example of positioning that uses GPS satellites.                accurately by using observation data from ground
In a car navigation system, radio waves (the                    GPS stations in various areas of the world.
positioning signal) sent from GPS satellites are                     Besides observation data, an extremely
received by a GPS antenna in the vehicle and the                precise satellite dynamical model and observa-
vehicle’s position is calculated based on that data.            tion model are necessary. In the case of GUTS,
The observation data used for this calculation                  the dynamical model and observation model that
is called “pseudo range” and the calculation is                 comply with the latest version of the Technical
carried out by multiplying the difference between               Note at that time (IERS Conventions 1996)
the time of the satellite which is included in the              issued by International Earth Rotation and
GPS wave and the time of the GPS receiver in                    Reference Systems Service (IERS) were intro-
the vehicle by the speed of light. Because there                duced. Some of the values used for these models
is a difference between the GPS satellite’s clock               may change on a daily basis, and so they need to
and the GPS receiver’s clock in general, the range              be updated every day by acquiring the latest data
calculated in this process is called a “pseudo”                 from overseas servers.

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                                                        S. Katagiri et al.: Technology of Precise Orbit Determination




   Figure 1
   GUTS configuration.



2.3 Method for precision validation                   an onboard GPS receiver (user satellite), ground
      While precise determination of a satellite’s    GPS stations, server computers for orbit determi-
orbit is possible using GPS data, another means       nation and acquiring Internet data, SLR station,
of observation with the equivalent precision is       and various pieces of software for processing
necessary to verify it. Satellite Laser Ranging       and operating precise orbit determination. In
(SLR) is often used for this purpose. SLR sends       addition, because GUTS is planned to be config-
a laser of a short pulse width to a satellite and     ured as a highly automated system, there should
measures the time until the pulse returns to it       be features to plan, execute and monitor obser-
after being reflected by a reflector on the satel-    vations on an unmanned basis. The system
lite with a precision of less than ten picoseconds.   overview indicated in Figure 1 was designed
In the case of low orbital satellites such as Earth   based on these concepts. The range of functions
observation satellites, the measurement precision     covered by GUTS in this figure is the five subsys-
is less than 1 cm.                                    tems described below:
                                                      1) Central monitoring and operation planning
3. Global and high accUracy                                subsystem, which plans operation schedule
   Trajectory determination                                for orbit determination and SLR of GUTS
   System (GUTS)                                           and monitors status of each subsystem.
    As mentioned in the previous section, the         2) GPS observation subsystem, which is
elements necessary for GUTS are a satellite with           comprised of the JAXA GPS stations

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S. Katagiri et al.: Technology of Precise Orbit Determination




      in the Tsukuba Space Center (Japan),                      in 2000.
      Perth (Australia), Santiago (Chile) and                        The orbit determination subsystem had a
      Maspalomas (Canary Islands); and a GPS                    client-server configuration comprised of Windows
      station control subsystem for collecting data             PC clients, UNIX computation servers and a data
      from these stations.                                      accumulation server.
3)    SLR observation subsystem, which is                            Clients are equipped with a user interface
      comprised of a JAXA SLR station in                        application.
      Tanegashima (Japan) and an SLR station                         Precisely determining orbits requires large
      control subsystem for remote-controlling                  matrix processing so the computation server
      this station.                                             performs parallel processing with multiple CPUs.
4)    Orbit determination subsystem, which                           The file management function of the data
      precisely determines and evaluates a                      accumulation server has a portability focusing
      satellite’s orbit by using GPS and SLR data.              on the analysis application, allowing optional
5)    External interface subsystem, which distrib-              configuration by the analyzing party as required
      utes precise orbit information of satellites              without any administrator authority.
      for users besides receiving various data                       The external interface subsystem obtains
      necessary for orbit determination from the                various data necessary for orbit determination
      Internet and other systems in the Tsukuba                 from various servers on the Internet. A series of
      Space Center.                                             security countermeasures are being implemented
                                                                based on Fujitsu’s know-how, for example,
3.1 Acquisition of technologies at                              unnecessary ports are closed on the external
    conceptual design phase                                     interfaces.
      Fujitsu has been involved with GUTS since
the beginning of its conceptual design phase                    4. GUTS core technologies
in 1994. During the conceptual design phase,                         This section describes the GUTS core
Fujitsu accumulated precise orbit determination                 technologies that have been mastered by Fujitsu.
technologies for research of the basic technologies             There are six core technologies: preprocessing
mentioned in the previous sections through activ-               of observation data, nominal orbit determina-
ities such as conducting research based on the                  tion, orbit generation, theoretical calculation of
latest literature3) and visiting the Jet Propulsion             observed data, rejection of bad data using obser-
Laboratory (JPL) in the United States. Further,                 vation residuals, and batch sequential filter and
since 1996, Fujitsu has accumulated orbit deter-                smoother.
mination technologies using SLR by improving                    1) Preprocessing of observation data
the existing orbit determination systems (NOCS2)                     In the preprocessing process of observation
and conducting orbit determination experiments                  data, GPS data obtained from GPS receivers on
based on SLR data to verify various SLR-related                 ground GPS stations and user satellites, and
models.                                                         SLR data obtained from SLR stations are prepro-
                                                                cessed. The preprocessing algorithm shown in
3.2 Subsystems developed by Fujitsu                             Table 1 is used.
     Upon completing the conceptual design                      2) Nominal orbit determination
phase as mentioned in the previous paragraph,                        While the orbit estimation of the satellites
Fujitsu started development of the orbit deter-                 has a non-linear nature and is based on estimates
mination subsystem and the external interface                   of a large number of variables, linear process-
subsystem from 1997 and announced the results                   ing is conducted to reduce the computation time.

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                                                          S. Katagiri et al.: Technology of Precise Orbit Determination




Table 1                                                 models and observation residuals (difference
List of preprocessing algorithms.
                                                        between the observation data and the theoreti-
                                                        cal value) necessary for estimation is generated.
                                                        The models used for the calculation of theoretical
                                                        value are shown in Table 3.
                                                        5) Rejection of bad data using observation
                                                             residuals
                                                             Double difference value often used for differ-
                                                        ential GPSnote 1) is used to judge and reject bad
                                                        data from GPS data. Meanwhile, in the case of
                                                        SLR data, the time-series shape of observation
                                                        residuals is used. These algorithms are devel-
                                                        oped from analysis of the actual data obtained
                                                        during operation, and greatly contribute to
                                                        system automation.
                                                        6) Batch sequential filter and smoother
                                                             Parameters estimated in the precise orbit
                                                        determination consist of various coefficients
Because the difference from the initial value           listed in Table 2 and Table 3 in addition to the
is estimated in linear processing, only a poor          satellite’s position and velocity and clock offset.
convergency is obtained when the initial value          The number of parameters reaches approxi-
widely deviates from the true value, resulting in       mately 1000 in total. The estimation processing
even a wrong convergence point in some cases.           is basically an array calculation and to reduce
For this reason, the initial value should be as         the computation workload, technologies such as
close as possible to the true value. The nominal        batch sequential filters and smoother and pseudo
orbit determination is an algorithm to obtain the       epoch are used.
initial orbit of each satellite which is close to the
true value based on the GPS satellite’s position        5. Accuracy of precise orbit
broadcast from the GPS satellite and the naviga-           determination
tion data (satellite position) of the onboard GPS            Since the initial release of GUTS in October
receivers of satellites such as ALOS.                   2000, Fujitsu has been engaged in operations
3) Orbit generation                                     and experiments related to precise orbit deter-
      Precise computation of a satellite’s orbit        mination using this system. In the operation of
is carried out through numerical integration            precise orbit determination, stable operation of
by adding various dynamical models shown in             ALOS has been realized within the required level
Table 2. As mentioned before, the precise orbit         of accuracy and significant contributions have
generation for an initial orbit has significant         been made in image processing of Earth remote
influence on the accuracy of the satellite’s orbit      sensing data. Meanwhile, in the experiments
determination.
                                                        note 1)    A method where a GPS receiver is arranged
4) Theoretical calculation of observed data
                                                                   at a point whose position is precisely known
      Theoretical calculation of GPS observed                      and a location whose position you want to
data (pseudo range and carrier phase) and SLR                      measure, and the relative position of these
                                                                   two points is calculated precisely based on
observed data (laser beam round-trip time) is                      the difference of the data obtained from each
carried out by giving consideration to various                     of these receivers.

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S. Katagiri et al.: Technology of Precise Orbit Determination




Table 2                                                         Table 3
List of dynamical models.                                       List of observation models.




of precise orbit determination, technologies to
precisely determine the orbits of various satel-
lites have been mastered including those for
GPS satellites, ADEOS-II, SAC-C, ALOS and
GRACE-B. Further, Fujitsu has been engaged
in researching the trends of various algorithms
shown in Table 1 through Table 3, for which
international researchers have continuously
made efforts for improvement so that the latest                 published by the Analysis Center of International
trends can be reflected in the system to enhance                GNSSnote 2) Service (IGS) is maintained within
its accuracy.                                                   5 cm RMSnote 3) on a stable basis. IGS is an organi-
      The targeted accuracy of GPS satellites is                note 2)   Global Navigation Satellite System.
of a level where the deviation from the values                  note 3)   Root Mean Square.

406                                                                         FUJITSU Sci. Tech. J., Vol.44, No.4, (October 2008)
                                                               S. Katagiri et al.: Technology of Precise Orbit Determination




zation that publishes precise positions of GPS               last two elements, they need to be more sophis-
satellites. At the start of the GUTS experiments,            ticated to be used in the positioning system.
the deviation was 30 cm RMS. Thereafter, tuning              Namely, instead of working off-line, the orbit
of the estimation parameters and the addition of             and clock should be estimated on a real-time
new algorithms have made it possible to achieve              basis. Then, the determined orbit and time infor-
the targeted accuracy in near future.                        mation should be uploaded to the satellites and
      The accuracy achieved by low orbital satel-            modulated with the positioning signals so that
lites is approximately 40 cm RMS4) by ADEOS-II,              the information can be delivered to the position-
30 cm P-Pnote 4), 5) by ALOS, and 4 cm RMS6) by              ing users.
GRACE-B. Concerning ADEOS-II and ALOS,                            By using the technologies accumulated
which gave us the incentive to establish GUTS,               through GUTS, since 1999 Fujitsu has been
we achieved an excellent accuracy that signifi-              participating in the development of ETS-VIII and
cantly exceeded the required accuracy. While                 HAC experimental ground system (Figure 2),
correct modeling of the complicated satellite                which is an experimental satellite positioning
shape, mass change and attitude control opera-               system and announced the results in 2003. In
tion (yaw steering) were the challenges to be                addition, Fujitsu has just started developing the
addressed by ALOS, the above-mentioned accura-               positioning part in the quasi zenith positioning
cy was finally achieved by overcoming these                  experiment system, which is a practical satellite
challenges.                                                  positioning system.


6. Overview of precise                                       7. Conclusion
   positioning experiment                                          This paper introduced the precise orbit
   system                                                    determination technologies that Fujitsu has
     In the meeting of the Committee on Space                accumulated through its involvement in the
Development held in March 1997, a decision was               establishment, operation and experiments of
made to start domestic development of a new                  GUTS.
positioning technology equivalent to GPS using                     The responsibility of Fujitsu in the precise
positioning signals from satellites. The position-           orbit determination-related systems ranges from
ing technology using positioning signals from                the planning phase of system implementation
satellites approached by Japan has an objective to           to system operation. GUTS cannot be imple-
improve the available time for positioning signals           mented or operated without core technologies for
in Asia and Oceania by reinforcing and supple-               a flight dynamics system. Fujitsu has a great
menting the GPS positioning system. For this                 advantage because it acquired these technolo-
purpose, geostationary orbits and quasi zenith               gies ahead of its competitors. In this process,
orbits are suitable as satellite orbits. There are           Fujitsu has succeeded in gaining customers’ trust
three core technologies necessary for the satel-             through its commitment to finding solutions for
lite positioning system: onboard atomic clock,               various problems and challenges with customers.
time management technology of multi-satellites,              Particularly in the technologies for using GPS
and technology of precise orbit and time determi-            and for operating positioning systems, Fujitsu
nation. Although basic technologies have been                has maintained close tie-ups with the customers
established in the framework of GUTS about the               to address these challenges. Fujitsu is eager to
                                                             keep such close relationships with its customers
note 4)     Peak-to-Peak. If the distribution of errors
            indicates a normal distribution, it is equiva-   also in the ongoing development of a quasi zenith
            lent to six times the RMS.                       positioning system and flight dynamics system

FUJITSU Sci. Tech. J., Vol.44, No.4, (October 2008)                                                                     407
S. Katagiri et al.: Technology of Precise Orbit Determination




      Figure 2
      HAC experiment ground system.



                                                                     Communication Engineers. B, Vol.J84-B, No.12,
for space Very Long Baseline Interferometry                          pp.2187-2195 (2001).
(VLBI) satellites (ASTRO-G).                                    3)   S.C. Wu et al.: Automated precision orbit deter-
                                                                     mination for TOPEX/POSEIDON with GPS.
     To conclude this paper, we would like to                        AAS 93-576, pp.183-194 (1993).
express our deep appreciation to the people                     4)   S. Nakamura et al.: Precise Orbit Determination
                                                                     for ADEOS-II. 19th International Symposium on
in the Consolidated Space Tracking and Data                          Space Flight Dynamics (2006).
Acquisition Department in the Japan Aerospace                   5)   R. Nakamura et al.: PRECISE ORBIT
                                                                     DETERMINATION          FOR    ALOS.        20th
Exploration Agency for their great support and                       International Symposium on Space Flight
cooperation which helped us to master the precise                    Dynamics (2007).
                                                                6)   N. Kudo, S. Nakamura, S. Katagiri: Precise Orbit
orbit determination technologies described in this                   Determination for GRACE-B. 19th International
paper.                                                               Symposium on Space Flight Dynamics (2006).


References
1)     S. Katagiri, Y. Yamamoto, H. Ogasawara,
       H. Kitabatake et al.: Overview of the NASDA’s
       precise orbit determination software. Proc. of
       the Twenty-second International Symposium
       on Space Technology and Science, Vol.1,
       pp.1157-1162 (2000).
2)     S. Katagiri, Y. Yamamoto, H. Kitabatake et al.:
       Development of Precise Orbit Determination
       System using GPS Satellites. The transactions
       of the Institute of Electronics, Information and


408                                                                        FUJITSU Sci. Tech. J., Vol.44, No.4, (October 2008)
                                                                    S. Katagiri et al.: Technology of Precise Orbit Determination




                          Seiji Katagiri                                                  Yousuke Yamamoto
                          Fujitsu Ltd.                                                    Fujitsu Ltd.
                          Mr. Katagiri received the Dr. Eng. de-                          Mr. Yamamoto received the B.S. degree
                          gree in Electronic Engineering from the                         in Physical Geography from Hiroshima
                          University of Electro-Communications,                           University, Hiroshima, Japan in 1989.
                          Tokyo, Japan in 1996. He joined Fujitsu                         He joined Fujitsu Ltd., Tokyo, Japan in
                          Limited, Chiba, Japan in 1996 and has                           1989 and has been engaged in devel-
                          been engaged in development of pre-                             opment of satellite orbit determination
                          cise orbit determination and prediction                         systems.
                          systems using GPS and satellite laser
                          ranging (SLR) data. He is a member
of the Institute of Electronics, Information and Communication
Engineers (IEICE) of Japan and the Astronomical Society of
Japan.




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