Radiometric Calibration of RapidScat Using the GPM Microwave Imager

Flying in a non-Sun-synchronous orbit, RapidScat is the first scatterometer capable of measuring ocean vector winds over the full diurnal cycle, instead of observing a given location at a fixed time of day. The non-Sun-synchronous orbit also enables the overlap with other satellite instruments that have been flying in Sun-synchronous orbits. RapidScat covered the latitude range between ±51.6° and was operated on board the International Space Station between September 2014 and August 2016. This paper describes the process that combines RapidScat’s active and passive modes, simultaneously measuring both the radar surface backscatter (active mode) and the microwave emission determining the system noise temperature (passive mode). This work also presents the radiometric (passive mode) cross-calibration using the GPM (Global Precipitation Measurement) Microwave Imager (GMI) as a reference to eliminate the measurement biases of brightness temperature between a pair of radiometer channels that are operating at slightly different frequencies and incidence angles. Since the RapidScat operates at 13.4 GHz, and the closest GMI channel is 10.65 GHz, GMI brightness temperatures were normalized before the calibration. Normalization was based on the radiative transfer model (RTM) to yield an equivalent brightness temperature prior to the direct comparison with RapidScat. The seasonal and systematic biases were calculated for both polarizations as a function of geometry, atmospheric, and ocean brightness temperature models. The calculated biases may be used for measurement correction and for reprocessing of geophysical retrievals.