Absolute Calibration of Jason-1 and Envisat Altimeter Ku-Band Radar Cross Sections from Cross Comparison with TRMM Precipitation Radar Measurements

Abstract

One year of collocated, rain-free nadir Ku-band backscatter cross-section measurements from the Tropical Rainfall Mapping Mission (TRMM) precipitation radar (PR) and both Jason-1 and Envisat RA-2 altimeter measurements have been compiled to compare these three sources of Ku-band radar cross section. With the exception of a +1.46 dB relative offset between Jason-1 and PR measurements and a ?1.40 dB offset between Envisat and PR ones, all three Ku-band measurements compare very well in terms of dependencies upon model wind speed estimates and significant wave height measurements. The altimeter radars and the rain radar thus provide consistent measurements, and observed biases can be rationalized as differences in the radar calibration. The precipitation radar, which also covers off-nadir measurements, has been absolutely calibrated using an active radar calibrator. Consequently, the observed relative offsets can be used to indirectly calibrate both Jason-1 and Envisat altimeter Ku-band radar cross sections in an absolute sense.