Impact of Satellite Attitude on Altimetry Calibration with Microwave Transponders

Abstract

Satellite altimetry plays a key role in monitoring changes in sea level and climate change. The quality of satellite altimetry products is commonly ensured through dedicated calibration. One such calibration is with microwave transponders acting as ground reference point targets. It is common practice that satellite ranges between the transponder phase center and the satellite center of gravity (CoG) are compared against the true geometric ranges to determine bias. Transponder ranges are, however, realized by the two phase centers of the altimeter and the ground transponder. So, to make this comparison feasible, the space origin of the measured range is transferred from the altimeter phase center (APC) to the satellite CoG by applying a constant offset, usually referred to as “CoG correction”. Instead of a fixed “CoG correction”, this work introduces the actual vector between APC and CoG in space, by examining the satellite attitude. Thus, the observed and geometric distances to the transponder are both referred to the APC. The case of Jason-3 and Sentinel-6A Michael Freilich (Sentinel-6A MF) with two transponders on Crete (CDN1) and Gavdos (GVD1) islands is examined. At first, the attitude of Jason-3 is determined by its quaternions. Then, analysis reveals that the transponder bias is correlated with the Jason-3 satellite attitude. The revised calibration brings about bias changes which fluctuate from about −2 mm to 1 mm in range and from −110μs to +110 μs in datation for Jason-3. Spectral analysis on the bias differences between the revised and conventional transponder calibrations reveals constituents with periods of 117, 39 and 23 days. Finally, the revised methodology on crossover calibrations over the GVD1 transponder results in an improvement between the mean bias of the ascending and descending orbits by 12% for Jason-3 and by 14% (preliminary) for Sentinel-6A MF.

CryoSat Long-Term Ocean Data Analysis and Validation: Final Words on GOP Baseline-C

“…It is important that we assess the data's usefulness for the long-term monitoring of changes in sea level and ice volume and for the data to be considered an essential climate variable or ECV [13] and to meet the requirements as defined in ESA's sea level climate change initiative (CCI) [14][15][16]. Our validation efforts can subsequently be combined with the absolute calibration of satellite altimeters over permanent in situ calibration facilities, as in Gavdos and Crete [17], Svalbard [18], and Corsica [19], as well as with results from the CryoSat-2 platform roll calibration [20] and from comparison with ICESAT-2 satellite laser altimetry [21].…”