Antenna phase center calibration for precise positioning of LEO satellites
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Phase center variations of the receiver and transmitter antenna constitute a remaining uncertainty in the high precision orbit determination (POD) of low Earth orbit (LEO) satellites using GPS measurements. Triggered by the adoption of absolute phase patterns in the IGS processing standards, a calibration of the Sensor Systems S67-1575-14 antenna with GFZ choke ring has been conducted that serves as POD antenna on various geodetic satellites such as CHAMP, GRACE and TerraSAR-X. Nominal phase patterns have been obtained with a robotic measurement system in a field campaign and the results were used to assess the impact of receiver antenna phase patterns on the achievable positioning accuracy. Along with this, phase center distortions in the actual spacecraft environment were characterized based on POD carrier phase residuals for the GRACE and TerraSAR-X missions. It is shown that the combined ground and in-flight calibration can improve the carrier phase modeling accuracy to a level of 4 mm which is close to the pure receiver noise. A 3.5 cm (3D rms) consistency of kinematic and reduced dynamic orbit determination solutions is achieved for TerraSAR-X, which presumably reflects the limitations of presently available GPS ephemeris products. The reduced dynamic solutions themselves match the observations of high grade satellite laser ranging stations to 1.5 cm but are potentially affected by cross-track biases at the cm-level. With respect to the GPS based relative navigation of TerraSAR-X/TanDEM-X formation, the in-flight calibration of the antenna phase patterns is considered essential for an accurate modeling of differential carrier phase measurements and a mm level baseline reconstruction.
KeywordsAntennas Phase center variation Orbit determination LEO satellites TerraSAR-X CHAMP GRACE
The IGOR receiver, choke rings and SLR reflector of TerraSAR-X have been contributed by the GeoForschungsZentrum (GFZ), Potsdam, to enable a high precision orbit determination of this spacecraft. Precise GPS ephemerides for use within this study have been obtained from the Center for Orbit Determination in Europe at the Astronomical Institute of the University of Bern (AIUB). Satellite laser ranging measurements of the GRACE and TerraSAR-X were provided by the International Laser Ranging Service (ILRS). The support of the above institutions is gratefully acknowledged.
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