RapidScat backscatter measurement validation

Scatterometers are microwave radars deployed to estimate global wind speeds and directions over the sea surface by measuring normalized radar cross-section (σo) of the sea surface. Measured σo is determined by the sea surface roughness that is modulated mostly by wind, thus establishing a relationship between σo and wind conditions. Wind vectors are retrieved by inverting an empirical geophysical model function that relates measured σo to a wind vector (speed and direction) and radar measurement configuration (polarization, incidence, and azimuth angle). Careful scatterometer calibration is required for accurate wind retrieval. NASA’s RapidScat scatterometer operated on board the International Space Station between September 2014 and August 2016. It was a twin instrument of the legacy QuikScat scatterometer. This paper proposes a method to cross-calibrate σo measurements between the RapidScat and QuikScat, taking advantage of their overlapping orbits. The σo biases between the two instruments have been calculated from observations collected during a 19-month period between January 2015 and August 2016. Deviations from the common reference model are computed for both datasets, as a function of wind speed, relative wind direction, and incidence angles averaged over 1000 pairs of collocated QuikScat/RapidScat revolutions.