# 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 altitude. The revised calibration brings about bias changes which fluctuate from about \(-2\) mm to \(1\) mm in range and from \(-110\) us to \(+110\) us 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. satellite altimetry; calibration; satellite attitude; Jason-3; Sentinel-6A MF 2022146369 2022146369 2022146369 2022146369 2022146369 2022146369 20222146369 20222146369 20222146369 20222146369 202222146369 202222146369 202222146369 20222  
are routinely used in geophysical analysis and climate studies are the sea surface height, wave height, wind speed, ionospheric total electron content, sea and land ice coverage and polar region topography [10; 11; 12; 13; 14; 15]. These products are the starting point for scientific research, interpretation and important decision making that can largely impact society. Thus, systematically assessing the accuracy and validity of altimetric products should be treated as indispensable elements of all operational satellite altimetry missions. The altimetric range, which is used for the direct calculation of sea surface heights, is continuously monitored for its quality to identify potential instrumental errors, drifts, etc. Altimetric ranges of one mission are also assessed with respect to other missions for identifying relative effects. Continuously monitoring the accuracy, homogeneity, and reliability of altimetric ranges is performed by external and independent facilities dedicated to Calibration/Validation (Cal/Val). The Cal/Val techniques developed for this purpose are generally classified into two main categories: those which rely on reference infrastructures deployed at sea (e.g., buoys) or on coasts (e.g., tide gauges) and those that rely on point targets operating on land (e.g., active transponders and corner reflectors). The present analysis is focused on the latter category (i.e., calibration of altimetric ranges using a transponder) and utilizes measurements acquired by two Ku-band microwave transponders of the Permanent Facility for Altimeter Calibration (PFAC) of ESA, installed on Crete (CDN1 site) and Gavdos (GVD1 site) islands. Although altimeter calibration using a transponder has been proposed in the past [16], its practical implementation has not evolved accordingly to align with algorithmic and technological developments.