Radiometric Calibration of RapidScat using GPM Microwave Imager

Due to it’s Non-Sun-synchornous orbit, RapidScat is the first scatterometer capable of measuring ocean vector winds over the full diurnal cycle istead of observing given location at the fixed time of day. Non-Sun-synchronous orbit enables also overlap with other satellite instruments that have been flying in Sun-synchronous orbits. Rapidscat covers range between ± 51.6  latitudes and was operated onboard the International Space Station between September 2014 and August 2016. This paper describes process that combines RapidScat’s active/passive mode, simultaneously measuring both the radar surface backscatter (active mode) and microwave emission from the system noise temperature (passive mode). This work presents the radiometric (passive mode) cross-calibration using the GPM Microwave Imager (GMI), to eliminate brightness temperature measurement biases between a pair of radiometer channels operating at slightly different frequencies and incidence angles. The GPM Microwave Imager (GMI) on the GPM Core satellite flies in a low inclination orbit, with conical-scanning dual-polarized beams. Since the RapidScat operates at 13.4 GHz and the closest GMI channel is 10.65 GHz. GMI Tb’s were normalized required before the calibration. The GMI brightness temperatures was translated using the radiative transfer model (RTM) to yield an equivalent Tb prior to direct comparison with RapidScat. Seasonal and systematic biases between two radiometers have been calculated for both polarizations as a function of geometry, atmospheric and ocean brightness temperature models. Calculated biases may be used for measurement correction and reprocessing.