NASA RapidScat is the first satellite scatterometer that flew in non-Sun-synchronous orbit. It’s unique orbit enabled co-located measurements with multiple satellite remote-sensing instruments that mostly fly in Sun-synchronous orbits. RapidScat primary mission was the retrieval of global ocean wind vectors from normalized radar backscatter measurements. Instrument operated onboard the International Space Station between September 2014 and November 2016 covering a latitude range between ±51.6o. This paper describes process that combines RapidScat’s active/passive mode, simultaneously measuring both the radar surface backscatter and microwave emission from the system noise temperature. 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 is a non-sun-synchronous orbital, dual-polarization, conical-scanning, multi-channel (ranging from 10 to 183 GHz). Since the RapidScat operates at 13.4 GHz and the closest GMI channel is 10.65 GHz, GMI Tb’s were required to be normalized before the calibration. The GMI brightness temperatures were translated using the radiative transfer model (RTM) to yield an equivalent Tb prior to direct comparison with RapidScat. Seasonal biases between two radiometers have been calculated for both polarizations as a function of atmospheric and ocean brightness temperature models. Calculated biases may be used for measurement correction and reprocessing. Trends from observations during a 20-month period have been described, and indicate that RapidScat instrument in both active and passive modes can be used to connect the sun-synchronous sensors that observe the oceans at different local times to remove inter sensor biases. Both L2A and L2B RapidScat data sets were provided by the NASA Physical Oceanography Distributed Active Archive Center (PODAAC) at the Jet Propulsion Laboratory.
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