GPS radio occultation with CHAMP

Ch. Reigber, J. Wickert, G. Beyerle, R. Galas, L. Grunwaldt, R. König, C. Marquardt, T.Schmidt

• Introduction
• Planned Data Processing
• Acknowledgement
• References

FIGURES

• Figure 1
• Figure 2
• Figure 3
• Figure 4
• Figure 5
• Figure 6
• Figure 7

Introduction

The German Low Earth Orbiting satellite CHAMP (Challenging Minisatellite Payload for Geophysical Research and Application) was launched on July 15, 2000 into an almost circular, near polar (i = 87°) orbit with an initial altitude of 454 km. CHAMP is a geoscientific mission for the determination of the Earth's gravity and magnetic field and for GPS-based atmospheric sounding. The designed lifetime of the satellite system is 5 years (Reigber et al., 2000).

The sounding of the Earth's atmosphere will be performed by using the GPS radiooccultation technique and is foreseen to start at the end of this year. Advantages of this innovative remote sensing technique include its global coverage combined with high vertical resolution, all-weather capability, long-term stability and high accuracy. This makes CHAMP a promising tool for use not only in numerical weather prediction, but also within other fields of atmospheric research.

The satellite will daily provide up to 250 globally distributed vertical profiles of temperatures with a vertical resolution between about 0.5 km in the troposphere and roughly 1.5 km in the stratosphere. In the lower troposphere the ambiguity of low and wet contribution to the atmospheric refractivity is resolved by introducing information from meteorological analysis.

The CHAMP occultation experiment uses the next generation of GPS receivers, allowing occultation measurements in combination with an optimized antenna configuration. Consequently a better data quality of the occultation signals is expected in relation to previous experiments as GPS/MET. CHAMP will also provide valuable experience for the upcoming international multi-satellite projects using GPS techniques for atmospheric sounding from space, e.g. GRACE, SAC-C, METOP and NPOESS.

Together with various partners (AWI Potsdam, GKSS Geesthacht, DWD Offenbach, MPI for Meteorology Hamburg, JPL Pasadena, IRE Moscow), GFZ will work on the improvement of GPS retrieval algorithms, continuous validation of atmospheric products and advanced methods of data assimilation techniques for GPS-based radio occultation data. These scientific efforts are coordinated within the Helmholtz Association Strategy Fund project GASP (GPS Atmosphere Sounding Project) .

Fig. 1 gives an overview on the scenario of GPS radio limb sounding measurements onboard the CHAMP satellite and related infrastructure on the ground.

The satellite data, measured by the NASA/JPL Black Jack GPS receiver onboard the satellite, are received at the download station Neustrelitz (Germany) and then transferred to the processing unit at GFZ via FTP. Additional data for Atmospheric Profiling are the GPS data of a global distributed fiducial GPS network (High Rate and Low Latency Network) and the orbit data of GPS and CHAMP satellites. These data are provided by GFZ, which operates the fiducial network and provides the rapid orbits.

Having the complete set of input data, the automatic Atmosphere Processing System at GFZ will start the generation of the atmospheric data products. The products including metadata information will be transferred to the Information System and Data Center (ISDC), which is also located at GFZ Potsdam. Here the data will be archived and also provided for the user community via.

The GPS Receiver TRSR-2 onboard CHAMP. In combination with the STAR accelerometer it serves as the main tool for high-precision orbit determination of the CHAMP satellite. Additional features are implemented for atmospheric limb sounding and the experimental use of specular reflections of GPS signals from ocean surfaces for GPS-altimetry.

The zenith pointing POD antenna is mounted on a choke ring. The aft pannel, which is tilted by 20°towards nadir, carries the high-gain helix antenna for occultation measurements. On the bottomside of the satellite (not visible here) there is a high-gain nadir-viewing helix antenna to enable future GPS altimetry measurements.

Fig. 4 shows the network topology of the ÒHigh Rate and Low LatencyÓ GPS ground network. The network is operated by JPL Pasadena and GFZ Potsdam with cooperating partners. The network supports the Rapid Orbit Determination for the GPS and the CHAMP satellites. Furthermore the ground data will be used for applying double differencing technique for the derivation of the atmospheric excess path of the occultation link. Currently the data are provided in data intervalls of 15 min with an average latency of about 10 min (Galas et al., 2000).

Planned Data Processing

The current software system for the forthcoming of CHAMP GPS radio occultation data is subdivided in three main components. The first component decodes und extracts the occultation measurements of the onboard GPS receiver. In addition it selects and synchronizes the space measurements with the ground based data. It is directly linked to the double differencing module, which combines the precise orbit information of the GPS and CHAMP satellite with the GPS phase data to extract the atmospheric excess phase of the occultation link for each occultation event. The inversion module for the derivation of the atmospheric parameters completes the software system and generates the vertical atmospheric profiles. The latter uses the geometric optics approach and the Abel inversion technique (Kursinski et al., 1997, Melbourne et al. 1994). All components were tested by raw data of the GPS/MET experiment (Rocken et al. 1997). The software system for calculating the precise orbits of the GPS and the CHAMP satellite is a separate system and is using a dynamic method (Kang et al., 1997).

The main processing steps of the retrieval are:

• Calibration of the input data resulting inthe atmospheric excess path of the occultation link using a double differencing technique
• Derivation of the bending angle applying correction of the Ionosphere
• Abel inversion to generate vertical refractivity profile assuming spherical symmetry of refractivity distribution
• Derivation of temperature and water vapor profiles using the equation of state.

Global distribution of possible CHAMP occultations for September 8, 2000. For the first period of the CHAMP occultation experiment we expect about 250 occultations per day.

Comparison of vertical temperature profiles assuming dry air, derived from occultation data of the GPS/MET Experiment. The blue crosses indicate the retrieval result of UCAR (Boulder) and the red crosses show the result of the GFZ processing software. In addition to the occultation profiles, the corresponding vertical profile from NCEP reanalysis is shown.

Acknowledgement

We are grateful to the GPS/MET team at UCAR, Boulder, Colorado for makingGPS/MET data available to us for extensive testing of our algorithms and processing procedures.

Contact: jens.wickert@gfz-potsdam.de

This study is carried out under BMBF grants 01SF9922/2 and FKZ 50EP9587.

References

GALAS R, REIGBER CH., WICKERT J., H/R GPS Ground Tracking Network for CHAMP, Proceedings of IGS Network Workshop Oslo, 12-14/07/2000, submitted to Physics and Chemistry of the Earth, 2000.

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.

KURSINSKI E.R., HAJJ G.A., SCHOFIELD J.T., LINFIELD R.P., HARDY K. R., Observing Earth's atmosphere with radio occultation measurements using the Global Positioning System, Journal of Geophysical Research, 102, 23429-23465, 1997.

MELBOURNE W., DAVIS E., DUNCAN C., HAJJ G., HARDY K., KURSINSKI E., MEEHAN T., YOUNG L., The application of spaceborne GPS to atmospheric limb sounding and global change monitoring, Publication 94-18, Jet Propulsion Laboratory, Pasadena, California, 1994.

REIGBER CH. et al., CHAMP Project Site, Internet: http://op.gfz-potsdam.de/champ, cited August 2000.

ROCKEN C., ANTHES R., EXNER M., HUNT D., SOKOLOVSKIY S., WARE R., GORBUNOV M., SCHREINER W., FENG D., HERMANN B., KUO Y.-H., ZOU X., Analysis and validation of GPS/MET data in the neutral atmosphere,Journal of Geophysical Research, 102, 29849-29860, 1997.