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					THALES RESEARCH AND TECHNOLOGY (UK)         White Paper




                       Real-Time Precise Orbit & Clock Determination
                       High Precision, Real-Time GNSS Precise Point Positioning




 www.thalesgroup.com
                   White paper

                   Real-Time Precise Orbit & Clock Determination

                                                     Author               Lois Scott
                                                     Date                 October 2005
                                                     Reference            NAV051001
                                                     Issue                Issue 1



                   Abstract
                                           T ha les Re sea rc h & T ech no log y ( UK) Lt d h a ve de ve lo p ed a
                                           Re a l-T ime Or b it & C lock De ter minat ion (R T OD) s ys t e m to
                                           p ro vid e re a l- time pr e cis e o rb it t ra ject or ie s a nd c lock
                                           p ar a mete r s f or G PS ( an d ot he r GNSS) s a t e llites , th e
                                           p ro vis io n of w hich e nab les g lo b al dec ime t re- le ve l
                                           p os it ion in g. The acc ur ac y o f c lass ica l GN SS po in t
                                           p os it ion in g is limit ed as a res u lt of th e p re senc e of
                                           u n mo de lle d e rr or s an d b iase s . The Pr ec is e Po int Po sit ion ing
                                           ( PPP) a pp r oach ut ilis es d u a l f re que nc y ionospheric-free
                                           c o mb inat ions t o mit igate ion os phe r ic e rr o rs a nd p re c ise
                                           ( rat he r th a n b ro ad ca st) sat e llite ep he me r is a nd cloc k da ta t o
                                           mit ig ate o rb it a nd c loc k er ro rs . To a ch ie ve t he h ig he st
                                           p os s ib le p os it io n in g accu rac y, f u rth er co r rect io ns a r e
                                           c a lc ulate d a nd a pp lie d t o ac co u nt f or tr op osp he r ic d e la y,
                                           s ate llit e at tit ud e a nd sit e d isp la ce me nt e rr or s . Th is pape r
                                           p ro vid e s a n o ve r view an d pe rf o rma nc e e va lua tion of the
                                           RT OD Sys t e m.



                   Keywords
                                                     Precise Point Positioning; Orbit Determination; Real-Time; GNSS; GPS



                          Thales
                                 Thales designs, develops and implements advanced electronic solutions, which meet the
                                 communications, information and security needs of people in countries throughout the world. Its
                                 activities cover three major markets: Information Technology and Services, Aerospace and
                                 Defence. The Thales group has operations in more than 30 countries and totals 65,000
                                 employees world-wide.

                          Thales Research and Technology (UK) Limited
                                 Thales Research and Technology (UK) Limited (TRT (UK)) is the UK arm of the Thales
                                 corporate research centre, and its activities focus on providing solutions in homeland security,
                                 multi-national joint service operations, Galileo and virtual collaboration. These are based on the
                                 key technologies of IP Network management and security, Navigation, secure communications
                                 and Sensors & Signal Processing. TRT (UK) offers a wide range of consultancy and
                                 development services to European Government Agencies and to industry throughout the world.

                                                             Thales Research and Technology (UK) Limited
                                                                     Worton Drive, Worton Grange
                                                                   Reading, Berkshire, RG2 0SB, UK

                                                         e-mail: navigation.TRTUK@thalesgroup.com




©Thales Research and Technology (UK) Limited, 2005                            2 of 14
                   White paper

                   Real-Time Precise Orbit & Clock Determination



                   Introduction
                   The accuracy of classical GNSS point positioning is limited by the presence of unmodelled
                   errors and biases. The Precise Point Positioning (PPP) approach utilises dual frequency
                   ionospheric-free combinations to mitigate ionospheric errors and precise (rather than
                   broadcast) satellite ephemeris and clock data to mitigate orbit and clock errors. To achieve
                   the highest possible positioning accuracy, it is necessary to correctly account for numerous
                   effects and error sources. Examples of the effects that must be correctly modelled in the
                   system are the tropospheric delay, satellite attitude and Reference Station site displacement
                   errors.
                   Under partial funding from the British National Space Centre (BNSC), TRT (UK) has
                   developed the key components required to make real-time Precise Point Positioning possible.
                   This work was enhanced by the provision of a precise timescale (linked to UTC) by the
                   National Physics Laboratory (NPL) and Numerical Weather Prediction (NWP) data provided by
                   the UK Meteorological Office.

                   System Overview
                   The Real-Time Orbit Determination (RTOD) system comprises a suite of software to process
                   raw GNSS observations from a network of ground based reference stations and output precise
                   satellite orbit position and velocity together with satellite clock correction parameters. To
                   enable the RTOD system to run in real-time and to allow the performance to be evaluated,
                   TRT (UK) implemented the Thales UK-based GNSS Ground Segment (TUGGS). TUGGS
                   provides a wrapper around the RTOD system. It comprises a suite of software, which
                   provides raw GNSS observations to the RTOD system and uses the RTOD output data to
                   compute inputs required by existing positioning software.

                                                                                                                             TUGGS System
                     NPL Enhanced TimeScale Infrastrcuture




                                                                                   TUGGS Input PC




                                                                                                                                                                      TUGGS Output PC




                                                                                                                                              RTOD System
                                                                                                                                                                                                          Proprietary
                                                                                                         TUGGS PC1
                                                                                                                                                                                                            RTCM
                                                                        Precise                                                                                                                          orbit & clock
                                                                          time-
                                                                                                                        RTOD: Real-Time                                                                   correction
                                                                          scale
                                                                                                    GPS data              Orbit & clock                                                                    message
                                                                         linked                     (archived)           Determination
                                                                                                                                                                                             TUGGS PC2   (via socket)
                                                                        to UTC

                                                                                                                                                                                         RTCM
                                                             NPL ETSI                               TUGGS                                                                               Message
                                                                                                     Data                                                                               Generator
                                                                                                    Decoder
                                                                                                                        RTOD PC1   RTOD PC4    RTOD PC5
                                                                                                                                                                                        Navigation
                                                                                  Global dual-frequency
                                                                                                                                                                                         Message
                                                                                  GPS data (36 stations)                                                                                Generator

                                                                                                                                                                                                            RTOD
                                                                                                                                                                    SV clock parameters,
                                                                                                                                                                                                          navigation
                                                                                                                                                                        SV precise
                                                                                                                                                                                                          message
                                                                                                                        RTOD PC2   RTOD PC3    RTOD PC6              position & velocity
                                                                                                                                                                                                         (via socket)




                                                                                                                     TUGGS: Thales UK-based GNSS Ground Segment running in real-time
                                                         TUGGS Reference Station Network


                                                                                                                      Figure 1: TUGGS System
                   As shown in Figure 1, the TUGGS system provides a framework within which the RTOD
                   system operates. On the input side, raw dual-frequency GPS observations are received from
                   a global network of GPS reference stations and a precise timescale linked to UTC (the UK
                   Enhanced Timescale Infrastructure based at NPL). This is provided to the RTOD system by




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                   Real-Time Precise Orbit & Clock Determination


                   the TUGGS Data Decoder. Note that the facility is provided to allow the raw input data to be
                   archived and later replayed.
                   The outputs from TUGGS system are provided by the RTCM Message Generator and the
                   Navigation Message Generator.
                   The RTCM Message Generator uses the precise satellite orbit and clock parameters
                   estimated by the RTOD software, to estimate precise clock and orbit corrections relative to the
                   GPS broadcast message. These corrections are packaged into proprietary RTCM messages
                   suitable for dissemination to the real-time Precise Point Positioning user.
                   The Navigation Message Generator uses the RTOD precise satellite orbit and clock
                   parameters to estimate a "replacement" GPS navigation message.

                                                                                                         RTOD System

                                            RTOD: Real-Time Orbit & clock Determination




                                                                                                                                 RTOD PC5
                                         RTOD PC1




                     SV observations &
                                                                  UK Met.               RTOD PC4
                      GPS Broadcast                                                                                 IERS
                         navigation                             Office NWP                                      earth rotation
                         messages
                      (from TUGGS)
                                                                observations                                      bulletins

                                                     RTOD PC1                                      RTOD PC4                                  RTOD PC5

                                         +IERS                  RTOD Data                                         Clock                                         Clock         SV clock
                                                                                                                                                                             parameters
                                          bulletins,
                                                                  Server                pre-processed          Preprocessor      geometry-free, iono-free      Process
                                         +planetary                                     line-of-sight                             carrier-phase filtered,
                                          ephemeris                                     measurements,                               line-of-sight data
                                                                                        pre-processed                                                                        SV precise
                                                                                        station co-ordinates                                                                  position
                                                                                                                                                                             & velocity
                                                                                        RTOD PC3
                                          RTOD PC2




                                                                                                                                  RTOD PC6
                                                                    Orbit                                        SV precise                                           Html
                                                                                                                                                        RTOD System
                                                                Preprocessor                                     ephemeris                                            GUIs
                                                                                                                                                         Monitoring

                                                     RTOD PC2                                      RTOD PC3
                                                                                                                                             RTOD PC6
                                                                Orbit Double-                                    Batch Orbit
                                                                                                                                                              RTOD GUI
                                                                 differences    iono-free,                        Process
                                                 Orbit                          carrier-phase filtered,
                                               Ambiguity                        double-difference data

                                               estimates Orbit Double-                     +IERS
                                                                                                                     JPL
                                                                                            bulletins,                                                        Process &
                                                          difference                       +geo-                  planetary                                   Application
                                                                                            potential
                                                           histories                        series
                                                                                                                 ephemeris                                   configuration



                                                                           Figure 2: Thales RTOD System
                   The RTOD system (see Figure 2) is a suite of software capable of estimating high precision
                   GNSS orbit and clock corrections in real-time, hence enabling global positioning at the
                   decimetre-level. The orbit determination algorithms contained in the RTOD system
                   incorporate the very latest advances in geodesy and satellite orbitography. The RTOD system
                   determines satellite orbit corrections using an orbit integration process and an orbit adjustment
                   process supported by data pre-processing, planetary ephemerides manipulation and the
                   satellite force-modelling. The RTOD system determines satellite clock corrections using a
                   clock synchronisation process known as a “composite clock”, supported by data measurement
                   pre-processing.

                   RTOD System Operation
                   The RTOD System consists of five main processing components (RTOD Data Server, Orbit
                   Pre-processor, Batch Orbit Process, Clock Pre-processor and Clock Process) together with a
                   configuration and monitoring component (RTOD GUI).
                   The RTOD Data Server receives raw GNSS observations from a global network of reference
                   stations and performs the pre-processing and synchronisation functions required by all
                   subsequent components. In addition, the RTOD Data Server reads NWP files generated by
                   the UK Met Office and uses data contained in these files to provide enhanced tropospheric
                   modelling. The pre-processed data is then provided in real-time to both the Orbit Pre-



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                   processor and Clock Pre-processor, which perform specific pre-processing for the Batch Orbit
                   and Clock processes respectively.
                   Based upon the data made available by the RTOD Data Server, the Orbit Pre-processor
                   generates a set of Orbit Observation files, which contain linearly-independent ionospheric-free,
                   carrier-phase filtered double differences. A set of files containing the associated ambiguity
                   estimates and double difference histories are also created.
                   Periodically, a new orbit adjustment is performed by the Batch Orbit Process. The Batch Orbit
                   Process reads the Orbit Observation files generated by the Orbit Pre-processor and an
                   initialising Precise Ephemeris file (typically generated by the previous orbit adjustment) to
                   estimate the satellite orbit and attitude trajectories over the orbit adjustment period. The Batch
                   Orbit Process saves the latest satellite orbit determination data to a Precise Ephemeris file.
                   This file is used by a number of the downstream processes, including the Clock Process, the
                   Navigation Message Generator and the RTCM Message Generator, and, is used to initialise
                   the next orbit adjustment.
                   The Clock Pre-processor generates a set of geometry-free, ionospheric-free carrier-phase
                   filtered line-of-sight observations, based upon the data made available by the RTOD Data
                   Server. Unlike the Batch Orbit Process, the Clock Process operates in real-time, so the Clock
                   Pre-processor provides these observations in real-time, every second.
                   The Clock Process uses the observations from the Clock Pre-processor to synchronise the
                   reference stations for which data is available and to estimate various clock parameters. The
                   computed parameters include the offset and drift for each satellite with respect to RTOD
                   network time, the offset between RTOD network time and GPS system time and the offset
                   between RTOD network time and UTC. The estimated satellite clock parameters are provided
                   in real-time to the Navigation Message Generator and RTCM Message Generator for
                   generation of their respective output messages.
                   The operation of the combined RTOD and TUGGS systems is distributed over a small number
                   of dedicated PCs. Details of the RTOD and TUGGS system operation, specifically data
                   products, associated data intervals and data periods are provided below. It should be noted
                   however that the system is highly configurable: many of the intervals discussed below are
                   typical values, which may be modified by the system operator as required.


                   The Batch Orbit Process performs an orbit adjustment every hour (on the hour boundary).
                   Each orbit adjustment utilises 48 hours of the latest Orbit Observation data from the Orbit Pre-
                   processor. The Orbit Pre-processor stores Orbit Observation data to file at 10-minute
                   intervals, with a new file being created hourly. Each orbit adjustment produces satellite
                   precise trajectories spanning the 48-hour Orbit Observation data period, plus an 8-hour
                   prediction period. Hence, at the end of each adjustment, a new Precise Ephemeris (SP3) file
                   is created containing 56 hours of precise satellite orbit trajectories. In order to eliminate
                   interpolation errors, the RTOD system is configured to discard the first and last 2 hours of the
                   generated SP3 files.


                   The Clock Process estimates clock parameters every second. Each clock update uses
                   instantaneous geometry-free, ionospheric-free carrier-phase filtered line-of-sight data from the
                   Clock Pre-processor. The Clock Process Composite Clock is configured with 14 Ensemble
                   reference station clocks, with weighting dominated by the very stable NPL receiver clock (the
                   precise-timescale clock linked to UTC). For correct operation of the Clock Process, data from
                   at least two of the Ensemble stations is required every epoch. In addition, satellite clock




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                   Real-Time Precise Orbit & Clock Determination


                   parameters can only be generated if the satellite is in view from at least three reference
                   stations.


                   The RTCM Message Generator determines precise clock and orbit corrections every second.
                   Each clock and orbit correction update uses precise satellite orbit trajectories from the SP3
                   files created by the Batch Orbit Process and instantaneous satellite clock parameters from the
                   Clock Process. This data is used to estimate precise clock and orbit corrections relative to the
                   GPS broadcast message, which are then output in proprietary RTCM messages every
                   second.


                   The Navigation Message Generator calculates a new GPS Broadcast ephemeris message
                   every 2 hours (chosen to mimic the GPS navigation message). Each ephemeris calculation
                   uses precise satellite orbit trajectories from the SP3 files created by the Batch Orbit Process
                   and instantaneous satellite clock parameters from the Clock Process. Each ephemeris
                   calculation performs a Keplerian fit of the orbit data over 4 hours, including 2 hours of
                   prediction, and a least squares adjustment of the clock model data over the previous 24 hours.
                   The calculated ephemeris parameters are then output as a "replacement" GPS navigation
                   message.

                   Performance Evaluation
                   During the development of the RTOD and TUGGS system, 3 months of GPS measurement
                   data was archived from 36 reference stations, together with the GPS navigation messages
                   and precise orbit and clock corrections generated by the Jet Propulsion Laboratory (JPL). In
                   addition, the UK Met Office supplied global Numerical Weather Prediction data covering the
                   same period. The performance of the RTOD and TUGGS system was evaluated, by replaying
                   a subset of this data through the performance evaluation architecture (see Figure 3).

                                                                TUGGS System                                                                                                      Analysis System

                                                      TUGGS: Thales UK-based GNSS Ground Segment
                                                           running in replay (pseudo real-time)
                     TUGGS Input PC




                                                                                                                                                    ANALYSIS PC1




                                                                                                                                                                                            ANALYSIS PC2




                                                                RTOD System
                                                                                                 TUGGS Output PC




                                                                                                                                                                   ANALYSIS PC1                            ANALYSIS PC2
                                          TUGGS PC1
                                                             RTOD: Real-Time
                                                               Orbit & clock
                                                              Determination                                                                               Positioning                         Statistical
                                                                                                                                                          Application                          Analysis          Precise Point
                                 GPS &
                                                                                                                                       RTOD                                                                       Positioning
                               RTCM data                                                                                TUGGS PC2
                                                                                                                                      RTCM
                                                                                                                                                             RTOD                                    RTOD                 RTOD
                               (archived)                                                                                           orbit & clock
                                                                                                                    RTCM             correction              ‘PPP’                                   ‘PPP’                 ‘PPP’
                                                                                                                                     message
                                                                                                                   Message                                 Computation                              Collation             Reports
                                      TUGGS
                                                                                                                   Generator                                  JPL                                     JPL                   JPL
                                       Data                RTOD PC1   RTOD PC4   RTOD PC5

                                      Decoder                                                                                                                ‘PPP’                NMEA               ‘PPP’                 ‘PPP’
                                                                                                                   Navigation                              Computation            Strings
                                                                                                                                                                                                    Collation             Reports
                                                                                                                    Message
                                                                                                                   Generator           RTOD                  RTOD                                    RTOD                 RTOD
                                                                                                                                     navigation              ‘SAP’                                   ‘SAP’                 ‘SAP’
                                                                                                                                     message
                                                           RTOD PC2   RTOD PC3   RTOD PC6                                                                  Computation                              Collation             Reports
                                                                                                                                                            Broadcast                          Broadcast            Broadcast
                                                                                                                                                              ‘SAP’                              ‘SAP’                 ‘SAP’
                                                                                              SV observations,                                             Computation                          Collation            Reports
                                                                                  JPL RTCM orbit & clock correction message,                                                                                       Stand Alone
                                                                                      GPS Broadcast navigation message
                                                                                                                                                                                                                   Positioning


                                                                      Figure 3: Performance Evaluation Architecture
                   The performance evaluation architecture uses specially modified versions of operational
                   positioning software to perform Precise Point Positioning (PPP) and Stand Alone Positioning
                   (SAP). GPS Broadcast navigation messages are used in Stand Alone Position computations,



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                   Real-Time Precise Orbit & Clock Determination


                   referred to as Broadcast Stand Alone Positions. JPL precise orbit and clock corrections are
                   used in Precise Point Position computations, referred to as JPL Precise Point Positions.
                   Thales RTOD “GPS replacement” navigation messages are used in Stand Alone Position
                   computations, referred to as RTOD Stand Alone Positions. Thales RTOD precise orbit and
                   clock corrections are used in Precise Point Position computations, referred to as RTOD
                   Precise Point Positions.
                   Each of the four position solutions identified above are analysed to produce daily position
                   reports. Examining the resulting time series and statistics enables:
                      • Comparison of the Broadcast Stand Alone Position performance vs. RTOD Stand Alone
                         Position performance.
                      • Comparison of the JPL Precise Point Position performance vs. RTOD Precise Point
                         Position performance.

                   Precise Orbit Comparison Scenario
                   The Precise Orbit Comparison Scenario consisted of replaying a subset of the archived data
                   through the performance evaluation architecture and evaluating the resulting precise orbits.
                   The computed precise orbits determined by the Batch Orbit Process were compared with the
                   'truth'precise orbits available from the International GNSS Service (IGS). The IGS Final-Orbit
                   used as truth is considered to have typical standard errors of about 5cm.
                   The Precise Orbit Comparison Scenario results presented below were generated using 48
                   hours of Orbit Observation data commencing at 00:00 on 01/10/04. To prevent interpolation
                   errors in the presentation of the results, the first and last 2 hours of the resulting precise
                   trajectories are not included. Thus the plots show the orbit differences determined over 46
                   hours of the Orbit Observation data period (7200s - 172800s), plus a 6 hour prediction period
                   (172800s - 194400s).

                                                                                       Along Track Orbit Error

                                                      1


                                                   0.75


                                                    0.5


                                                   0.25
                                 Orbit Error (m)




                                                      0


                                                   -0.25


                                                    -0.5


                                                   -0.75


                                                      -1
                                                       7200   21600   36000   50400   64800   79200    93600 108000 122400 136800 151200 165600 180000 194400
                                                                                                      Epoch (seconds)




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                   Real-Time Precise Orbit & Clock Determination



                                                                                      Across Track Orbit Error

                                                      1


                                                   0.75


                                                    0.5


                                 Orbit Error (m)   0.25


                                                      0


                                                   -0.25


                                                    -0.5


                                                   -0.75


                                                      -1
                                                       7200   21600   36000   50400   64800    79200    93600 108000 122400 136800 151200 165600 180000 194400
                                                                                                       Epoch (seconds)




                                                                                              Radial Orbit Error

                                                      1


                                                   0.75


                                                     0.5


                                                   0.25
                                 Orbit Error (m)




                                                      0


                                                   -0.25


                                                    -0.5


                                                   -0.75


                                                      -1
                                                       7200   21600   36000   50400   64800    79200    93600 108000 122400 136800 151200 165600 180000 194400
                                                                                                       Epoch (seconds)



                                                      Figure 4: RTOD orbit differences with respect to IGS Final Orbit
                   Figure 4 shows the time evolution of the difference in the computed satellite co-ordinates (in
                   the flight frame) estimated by the Batch Orbit Process compared to those estimated using the
                   IGS Final Orbit SP3 files. These plots show that errors along all axes are sub-metre. The
                   largest differences are to be found in the along-track direction. It is hypothesised that the
                   slight '            of
                           fanning-out' along-track differences seen at the start and end of processing (i.e.
                   7200s - 36000s and 165600s - 194400s) are due to poorer estimation of sub-daily variations in
                   the earth rotation parameters at these times. The current Orbit Adjustment Process estimates
                   these variations as constant over the entire process run (i.e. 48 hours). It is further
                   hypothesised that estimating these variations as constant over shorter periods rather than the
                   entire process run, would result in more accurate estimates.




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                   Real-Time Precise Orbit & Clock Determination



                                                                                                 RMS Orbit Error




                                                   0.50
                                                   0.45
                                                   0.40
                                                   0.35
                                 Orbit Error (m)

                                                   0.30
                                                   0.25
                                                   0.20
                                                   0.15
                                                   0.10                                                                                                     3-D
                                                                                                                                                          ATK
                                                   0.05                                                                                                 XTK
                                                                                                                                                      RAD
                                                   0.00
                                                          15 17 1   3   4   5   6   7   8   9 10 24 25 26 27 30 31 11 13 14 16 18 19 20 21 22 23 28
                                                                                                      SV PRN




                                          Figure 5: RMS of RTOD orbit differences compared to IGS Final Orbit
                   As is customary in the orbit determination community, we have used RMS values to provide a
                   measure of the quality of the computed satellite orbit. Figure 5 provides a summary of the
                   RMS radial (RAD), across-track (XTK), along-track (ATK) and absolute (3-D) difference for
                   each satellite, ordered by satellite type, i.e. Block II, Block IIA, Block IIR. It is clear that the
                   RTOD orbit determination process provides an orbit better than 30 cm 3-D (RMS) compared to
                   the IGS Final Orbit. Furthermore, no difference is seen across different satellite types. The
                   major component of the difference lies in the along-track and across-track directions that
                   couple weakly into GNSS range measurement errors.

                                                                    Axis                                   Average RMS Orbit Error
                                                                    Absolute (3-D)                                  18.9 cm
                                                                    Along Track                                     15.6 cm
                                                                    Across Track                                    9.3 cm
                                                                    Radial                                          3.6 cm
                            Table 1: Average RMS of RTOD orbit differences compared to IGS Final Orbit
                   Table 1 shows the average RMS orbit difference computed for the RTOD orbit determination
                   process compared to the IGS Final Orbit. The average 3-D orbit difference is less than 20 cm
                   RMS. The average error in the radial direction, which dominates the GNSS range
                   measurement errors, is 3.6 cm RMS. This gives a User Equivalent Range Error (UERE) of
                   less than 10 cm RMS.

                   Precise Point Position Comparison Scenario
                   The Precise Point Position Comparison Scenario consisted of twice replaying a subset of
                   archived data through the performance evaluation architecture. The first run was configured
                   to use the RTOD orbit and clock corrections and the second run was configured to use the
                   IGS Final Orbit together with the RTOD clock corrections. The resulting RTOD Precise Point
                   Position reports were also compared with the JPL Precise Point Position reports to determine
                   how much of the resulting position error is due to the RTOD clock and orbit determination
                   processes.
                   The Precise Point Position Comparison Scenario results presented below were obtained after
                   allowing the RTOD and TUGGS System to initialise for 48 hours, starting at 00:00 on 1st
                   October 2004.




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                   Figure 6 to Figure 8 show the Precise Point Position Comparison position error vs. time
                   reports, where the errors represent the difference between the computed Precise Point
                   Position and a static surveyed point. In all the plots, the x-axis represents the time in hours
                   since 00:00 on 3rd October 2004 and the y-axis represents the position error in metres.




                                                     Figure 6: RTOD 'PPP' (IGS orbit and RTOD clock)




                                                       Figure 7: RTOD 'PPP' (RTOD orbit and clock)




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                   Real-Time Precise Orbit & Clock Determination




                                                     Figure 8: JPL 'PPP' (JPL orbit and clock)
                   Comparison of Figure 6 and Figure 7 shows that the RTOD Precise Point Position profiles
                   generated with either the IGS or the RTOD orbit are very similar and highly correlated. It is
                   therefore concluded that the dominant positioning error source is not residual orbit error.
                   Comparison of Figure 6 to Figure 8 shows that the Precise Point Position profiles generated
                   using RTOD clock corrections exhibit a relatively high degree of short-term variation compared
                   to the Precise Point Position profiles generated using JPL clock corrections. It is believed that
                   these variations occur because of frequent changes in the number of observations available to
                   the satellite clock determination process, together with regular data dropouts. During these
                   data dropouts, data is either only available from a substantially reduced number of reference
                   stations, or for no reference stations whatsoever, for periods of several seconds. The effect of
                   data dropouts is further compounded by the TUGGS network being much less dense than that
                   available to JPL, which leads to instabilities in the RTOD satellite clock corrections and
                   consequently to a less stable position estimate.

                                                                        Position Error
                                                       RTOD 'PPP'        RTOD 'PPP'         JPL 'PPP'
                                             Axis      (IGS orbit &    (RTOD orbit &        (JPL orbit
                                                      RTOD clock)           clock)           & clock)
                                                      68%      95%      68%       95%     68%      95%
                                            2D       31 cm 42 cm      34 cm     52 cm    14 cm    27 cm
                                           Height    22 cm 50 cm      23 cm     52 cm    21 cm    39 cm
                                            3D       39 cm 58 cm      46 cm     65 cm    30 cm    44 cm
                                                     Table 2: Precise Point Position Statistics
                   Table 2 shows the RTOD and JPL Precise Point Position error statistics. The RTOD Precise
                   Point Position errors generated with either the IGS or the RTOD orbit are highly correlated.
                   However, the position generated with the IGS orbit provides a 10% lower 3-D position error
                   (95%) than that generated with the RTOD orbit. Interestingly, this is predominantly in the
                   horizontal components.
                   Table 2 also shows that the Precise Point Position performance using RTOD clock corrections
                   is worse than the Precise Point Position performance using JPL clock corrections, particularly
                   in the horizontal components. This, and the high correlation in the RTOD Precise Point
                   Position errors generated with either IGS or RTOD orbit, demonstrates that the dominant


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                   Real-Time Precise Orbit & Clock Determination


                   component of the position error arises from errors in the satellite clock determination process,
                   not the orbit determination process. The performance of the RTOD clock determination
                   process suffers due to the relatively poor observability of satellite clocks possible within the
                   current TUGGS reference station network, compared to that possible within the much denser
                   station network available to JPL. The performance was further compromised by regular
                   dropouts in the data received for the available reference stations. It is anticipated that
                   continuous data from a network of, say, twice the number of reference stations would produce
                   results much closer to those obtained using JPL corrections.

                   Stand Alone Position Comparison Scenario
                   The Stand Alone Position Comparison Scenario consisted of replaying a subset of archived
                   data through the performance evaluation architecture. The RTOD Stand Alone Position
                   reports generated using the RTOD navigation messages were compared to the Broadcast
                   Stand Alone Position reports generated using the GPS Broadcast navigation messages.
                   The Stand Alone Position Comparison Scenario results presented below were obtained after
                   allowing the RTOD and TUGGS System to initialise for 48 hours, starting at 00:00 on 1st
                   October 2004.
                   Figure 9 and Figure 10 show the Stand Alone Position Comparison position error vs. time
                   reports, where the errors represent the difference between the computed Stand Alone Position
                   and a static surveyed point. In all the plots, the x-axis represents the time in hours since 00:00
                   on 3rd October 2004 and the y-axis represents the position error in metres.




                                                     Figure 9: RTOD 'SAP' (RTOD navigation message)




©Thales Research and Technology (UK) Limited, 2005                      12 of 14
                   White paper

                   Real-Time Precise Orbit & Clock Determination




                                      Figure 10: Broadcast 'SAP' (GPS Broadcast navigation message)
                   Comparison of Figure 9 and Figure 10 shows that the RTOD Stand Alone Position profiles and
                   the Broadcast Stand Alone Position profiles are of comparable magnitude.



                                                                               Position Error
                                                                   RTOD 'SAP'             Broadcast 'SAP'
                                                     Axis       (RTOD navigation           (GPS Broadcast
                                                                     message)            navigation message)
                                                                 68%         95%          68%          95%
                                                 2D           2.02 m      3.16 m       1.79 m       3.18 m
                                                 Height       1.69 m      3.21 m       1.99 m       3.52 m
                                                 3D           2.76 m      4.06 m       2.59 m       4.58 m
                                                            Table 3: Stand Alone Position Statistics
                   Table 3 shows the RTOD and Broadcast Stand Alone Position error statistics. Stand Alone
                   Positions using the RTOD navigation message are more accurate (95%) than those using the
                   GPS Broadcast navigation message. The 3-D position generated using the RTOD navigation
                   message is 52cm more accurate (95%) than that generated using the GPS Broadcast
                   navigation message: approximately 12.5% better.




©Thales Research and Technology (UK) Limited, 2005                           13 of 14
                    White paper

                    Real-Time Precise Orbit & Clock Determination



                    Conclusions
                    TRT (UK) have developed a robust system that provides the key GNSS Ground Segment
                    functionality in real-time, offering precise clock and orbit corrections. The evaluation has only
                    been carried out to date with GPS satellite data. However, the RTOD system has been
                    designed to be fully extensible for inclusion of other satellite types.
                    Performance evaluation with real data has demonstrated 3-D orbit accuracies commensurate
                    with that required for a decimetre level positioning system. Average orbit error in the radial
                    direction, which dominates the GNSS range measurement errors, is shown to be available in
                    real-time at a level consistent with that quoted for the IGS final-orbit.
                    Performance evaluation with real data has demonstrated 3-D Precise Point Positioning
                    accuracies at the decimetre level achievable in real-time using the RTOD and TUGGS
                    systems. The dominant remaining contributor to the error within the RTOD and TUGGS
                    System is the clock determination process. The performance of the clock process is limited by
                    relatively poor observability of satellite clocks due to the limited size of the station network
                    currently available to TRT (UK), and dropouts in the measurement data received from the
                    reference stations.
                    Further to the generation of the precise orbit and clock corrections, TRT (UK) has
                    demonstrated the computation of the ephemeris and clock components of the GPS Broadcast
                    navigation message. Similar ‘stand-alone’ positioning performance was obtained using the
                    RTOD navigation message in place of the Broadcast GPS navigation message.




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©Thales Research and Technology (UK) Limited, 2005                                           14 of 14

				
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