Differential Calibration of Satellite Altimeters Using Corner Reflectors
Precise assessment of satellite altimeter performance is essential to understand and monitor global sea level rise with confidence. This investigation introduces a new approach based on at least two co-located point targets called differential range calibration (DiRaC), which monitors the performance of satellite altimeters. By analyzing differential biases between point targets, DiRaC mitigates common errors (e.g., those related to atmospheric effects, geophysical corrections, and orbit inaccuracies) that affect absolute range calibration. The methodology is demonstrated using two rectangular corner reflectors (CRs) deployed at a dedicated calibration site (called ALX) in Crete, Greece, under the framework of European Space Agency’s (ESA) Permanent Facility for Altimetry Calibration (PFAC). Fully focused synthetic aperture radar (FF-SAR) processing is applied to isolate the radar returns of each reflector, enabling both absolute range bias estimation for Sentinel-6 Michael Freilich (Sentinel-6 MF) and the application of DiRaC. An initial assessment revealed a variability of 6 mm in differential range bias, which, given the current sample size, corresponds to an estimated standard error of 1 mm. This variability is attributed to the combined effect of the intrinsic altimeter precision and a processing-related correlated component. A quantitative assessment of the individual contribution of these two effects requires an accurate characterization of the processing-related correlated component. Hence, with an additional correlation analysis, the standard error of 1 mm can be more accurately attributed to the Sentinel-6 MF altimeter performance.