1 GeoForschungsZentrum Potsdam (GFZ), Division 1, Telegrafenberg A6, 14473 Potsdam Contact: Reigber@gfz-potsdam.de, Tel. +49 (0) 331 2881100, Fax +49 (0) 331 2881111

2 Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Deutsches Fernerkundungsdatenzentrum (DFD) Kalkhorstweg 53, 17235 Neustrelitz Contact: Norbert.Jakowski@dlr.de, Tel. +49 (0) 3981 480151, Fax +49 (0) 3981 480123

GFZ and DLR Contribution to a GPS Ground Network to Support the CHAMP Mission

C. REIGBER1, R. GALAS1, R. KÖNIG1, N. JAKOWSKI2, J. WICKERT2, A. WEHRENPFENNIG2

Abstract The paper discusses various scientific/technical issues as network distribution, requirements for autonomous and permanently operating GPS stations, near real time data transfer and operational requirements and data flow for a GPS ground station network being currently installed in preparation of the CHAMP mission.

1. Introduction CHAMP is a geoscientific mission for the determination of gravity field, magnetic field and for GPS-based atmospheric sounding (Reigber et al., 1996). The satellite is scheduled for launch for end of 1999. Both the gravity field experiment as well as the GPS based radio occultation measurements require high precision orbit estimation. The highest precision of orbit parameters can be achieved by dynamic orbit determination procedures supported by a fiducial network of GPS ground stations. Additionally, the so obtained GPS observations shall enable double differencing to compensate satellite clock errors in radio occultation data analysis. GPS ground data for the GPS occultation experiment of the CHAMP mission will be delivered by a dedicated global network. The network stations will be established by NASA/JPL and GFZ Potsdam. GFZ jointly with its partner DLR/DFD Neustrelitz suggests a sub-network, composed of 8 stations. This ground tracking network shall support also other forthcoming low Earth orbiting missions, e.g. GRACE (Tapley and Reigber, 1998).

2. GPS Ground Network Requirements Besides the precise orbit determination the ground station network serves primarily to compensate clock errors of the TRSR receiver onboard CHAMP for a precise analysis of radio occultation measurements, the network has to be high operational and shall ensure global coverage of radio occultation measurements. To achieve an even data coverage of radio occultation measurements, some geometrical relationships between CHAMP satellite, ground stations, occulting and

reference GPS satellites have to be taken into account. Furthermore, the infrastructure of selected sites has to fulfil some criteria such as secure housing, continuous power supply, stable communication links via internet. So one has to find a compromise to ensure a satisfying data coverage on one hand, and to guarantee on the other hand a reliable operation of the GPS receivers and a fast data transfer via internet to the GFZ Potsdam. Taking into account these various aspects, GFZ Potsdam plans to realize a network topology illustrated in Fig. 1. To meet operational requirements with a data latency of less than 3 hours, the ground station data should be available at the processing data centers in GFZ and DLR within 1 hour after data taking. The only way to meet this requirement is the construction of an autonomous tracking station described in the following section. Because the radio occultation measurements need a rather high sample rate of 50 Hz, the sample rate of GPS phase measurements at ground stations is required to be at least ≥ 1 Hz, i.e. the ground network has to be equipped with an appropriate GPS receiver. Further requirements for a ground tracking station are: quick set-up, self control, high reliability, easy operating and maintenance. Moreover a remote control by an operator located in a network operation center must be possible.

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Fig. 1: Planned GFZ High Rate GPS Tracking Network.

Station distribution according to demand for global coverage of the occultation events

3. Ground Tracking Station To fulfil the above requirements a CHAMP ground

tracking station (Galas and Burghardt, 1999) is composed of: GPS BenchMark receiver from AOA, collecting

data with 1 second sampling rate. meteorological station of type TM200, industry standard personal computer running

under Linux operating system, power controller with a relais and watch dog, a battery with re-charger.

The monitoring computer has a true Internet address and can be accessed any time using a dedicated line.

On-Site Data PoolGPS Raw Data, Meteorological

Data, Log- & Status-Files

Internet access(dedicated line)with ftp and telnet services for remotecontrol and maintenance required

trMonitor( )GPS-receiver control

and offloading program.(Opt.: Data logger)

MetLogger( )Data logger for

the Tm200 weather station

send2noc( )Automatic transferprocedures to the

Network Operating Center

Fig. 2: Data Flow at an Autonomous Tracking Station

The flow diagram in Fig. 2 shows the relationships between the software modules running on a field computer. Each of the software modules is implemented as a separate program and works pretty much independently of the other ones. The main module tasks are: Monitoring the GPS receiver, trouble shooting,

GPS data downloading Data logging from a meteorological station GPS data compression Automatic data delivery to a network operating

center (including all log-files) This software architecture allows development, tests, upgrade or adding of new modules in a very comfortable way and guarantees high system reliability. The GPS data are downloaded onto PC hard disk every 15 minutes, then compressed by a very efficient procedure (Köhler, 1999) and sent via Internet to GFZ.

4. Network Data Flow and Processing A number of programs, taking care for the data handling, will continuously run at the CHAMP Network Operating Center. The data handling software (see Fig.3) is now implemented and under test. All GPS- and log-files are handled. The GPS files are converted to a receiver independent binary format and merged into hourly data files.

Dedicated Line Connections to the Internet

GFZ Data PoolIn raw format

Converted filesto an exchange format

Data ManagerDecoding, converting, QC. etc

Data Distribution

Field Station nField Station i

Field Station 1

Fig. 3: Data Flow in the GFZ Tracking Network

Next the pre-processing and quality control procedures are applied. As a result for each data file a log-file, containing information about cycle slips, bad data etc., is created. Additionally, the hourly GPS files are sampled at 30 second rate and merged into daily files. All received log-files from the tracking stations are automatically analyzed and some network status reports are created.

TM

CDDIS, EDC

SLR data

External Data

A/I Products

DLR Neustrelitz

IGS NetworkAnalysis Center GFZ

H/R F/D Fiducial NetGFZ & JPL

1Hz GPS data30s GPS data 0.1Hz GPS SST data Pole, UT1, flux, ...

Various SourcesSLR NetworkRaw Data Center

DLR Neustrelitz

GFZ PotsdamOrbit & Gravity Field

Rapid Science Orbit

Data & Processing Center

Data & Processing Center

Atmosph & Iono Profiling

Fig. 4: Orbit Processing Scheme Sampled high-rate fiducial network data provide the fast and reliably available data base for precise orbit determination, complemented by the IGS 30 s GPS data, on-board SST POD data, and precision SLR range data. The orbit processing chain (see Fig. 4) comprises acquisition and preprocessing of all these observational and auxiliary data, determination of precise orbit products for the occultation processing chain, and generation of special orbit predictions for the SLR network and for mission planning. Before the launch of CHAMP, a validated and ready to operate S/W system will be available to generate a dynamic orbit solution. The dynamic solution evaluates the satellites equation of motion by accounting for gravitational and non-gravitational forces. The GFZ approach realized in the EPOS-OC S/W package is characterized by using undifferenced pseudo-range and carrier phase measurements from GPS ground station and on-board

receivers for the estimation of GPS orbits and clock parameters and the LEO ephemeries. GFZ experience in the SST technique is a result of gravity field modeling activities from TOPEX/POSEIDON and GPS-MET SST data (e.g. Kang et al., 1997). The benefits from synergies of GPS and SLR data have been established by Zhu et al., 1997. The dynamic solution allows to estimate physical parameters (e.g. gravity field parameters as one of the main science objectives of the CHAMP mission) in addition to the parameters primarily looked at in occultation processing, by adjusting the trajectory solution from physical and empirical force modeling to the tracking observations in the least squares sense. The dynamic solution also provides an easy means to predict the orbit by forward integration based on the parameters derived before in the adjustment step.

Network

ORBITS

Raw DataGPS

GFZ

Fiducial(ISDC)

(RDC)DLR

GFZ(ISDC)

Vertical Profiles(e.g. Temperature

atmospheric

Water Vapour)

Processing ofGPS Occultation Data

phase delay

DLR (AIP)

Prediction

Preprocessing

Profiling

Fig. 5: Processing of GPS Occultation Data Atmospheric and ionospheric data products will be generated by an automatically working processor system, which is under development and will be installed at GFZ Potsdam and DLR/DFD Neustrelitz. For retrieving accurate data products high precision orbits are needed.

5. Conclusions The coverage of occultation events was simulated by applying the Radio Occultation Simulation Tool (ROST) developed at DLR/DFD Neustrelitz (Jakowski et al., 1998). Using the station distribution as displayed in Fig.1, ROST computations provide globally distributed radio occultation events satisfying the mission requirements to a high degree (see Fig. 6). Further improvement of global coverage will be reached by combining the GFZ station configuration with the JPL network. As it has been shown, with a data latency less than 1 hour, the above described network fulfils all operational requirements. So it can be also used for future operational multi-satellite radio occultation missions without any significant modifications.

Fig. 6: Global distribution of occultation events (30

days)

References GALAS R, BURGHARDT W., (1999). Autonomous CHAMP

Ground Tracking Station: System Documentation and Description, GFZ Scientific Technical Report (in preparation), GFZ Potsdam

JAKOWSKI N., WICKERT J., HOCKE K., REIGBER CH., FÖRSTE CH., KÖNIG R. (1998). Atmosphere/ Ionosphere Sounding Onboard CHAMP, 1998 Western Pacific Geophysics Meeting, EOS Suppl., Transactions, AGU, 79, No. 24, W20

KANG Z., SCHWINTZER P., REIGBER R., ZHU S.Y., (EOS). Precise Orbit Determination of Low-Earth Satellites Using SST Data, Adv. Space Res., 19, 1667-1670, 1997

KÖHLER W., (1999). Compression algorithms for GPS binary format, GFZ Scientific Technical Report, (in preparation)

REIGBER CH, BOCK R., FÖRSTE CH., GRUNWALDT L., JAKOWSKI N., LÜHR H., SCHWINTZER P., TILGNER C. (1996). CHAMP Phase B, Executive summary, GFZ Scientific Technical Report STR96/13, GFZ Potsdam

TAPLEY B. D., REIGBER CH. (1998). GRACE: Satellite-to-satellite tracking geopotential mapping mission, 1998 AGU Fall Meeting, Suppl. to EOS, Transactions, AGU Vol. 79, No. 45, 1998.

ZHAO C. (1998). The Sensitivity of Atmospheric Temperature Retrieved from Radio Occultation Technique to Orbit Errors of GPS and LEO Satellites, GFZ Scientific Technical Report STR98/23, GFZ Potsdam.

ZHU S.Y., REIGBER Ch., KANG Z. (1997). Apropos Laser Tracking to GPS Satellites, J. Geodesy, 71, 423-431, 1997.