A New Method of Precise Jason-2 Altimeter Calibration Using a Microwave Transponder

This study presents a new method of precise altimeter absolute calibration using a dedicated microwave transponder, which acts as an altimeter signal repeater that can be deployed at any sub-track position both in coastal regions and inland. The Austrian Academy of Sciences operates an altimeter transponder at the Gavdos calibration/validation facility located beneath a Jason cross-over point. We discuss the capabilities and strengths of the transponder technique in general and in the particular case of a dedicated calibration campaign carried out in 2011. For the accomplishment of this campaign, including 26 ascending and descending Jason-2 passes, the onboard Poseidon-3 altimeter had to be switched to the DIODE/DEM mode for every overflight. Four different methods have been developed to analyze the transponder generated waveforms, provided by S-IGDR and S-GDR products, respectively. The resulting biases of the altimetric ranges have proven to be stable (3 mm rms) and agree to a large extent among the proposed algorithms. However, the absolute bias value of 25.8 +/- 0.3 cm derived from the transponder calibration technique shows a significant deviation of several centimeters compared with the ones resulting from conventional techniques. Possible causes of this behavior are under further investigation.