Calibration of the RapidScat Microwave Brightness Temperature
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NASA RapidScat is the first satellite scatterometer that flown in non-Sun-synchronous orbit. Its unique orbit enabled collocated measurements with multiple satellite remote-sensing instruments that usually fly in Sun-synchronous orbits. RapidScat’s primary mission was retrieval of global ocean wind vectors from normalized radar backscatter measurements. The instrument operated onboard the International Space Station between September 2014 and August 2016 covering latitude range between ±51.6ᵒ . To serve as a cross-calibration reference with other instruments, RapidScat must be internally calibrated. This dissertation presents the first radiometric calibration of RapidScat brightness temperature measurements and describes the 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 dissertation also presents the cross-calibration using the GPM 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 direct comparison with RapidScat. A comprehensive study, monthly, and seasonal measurement biases between two radiometers have been evaluated using the Radiative Transfer Model (RTM). Systematic brightness temperature biases for both ascending and descending portions have been calculated as a function of geometry (polarizations, latitude, and incidence angle), atmospheric model (water vapor and cloud liquid water), and ocean brightness temperature model (wind speed, wind direction, and sea surface temperature) under different environmental parameter ranges. These deviations were averaged over two years of RapidScat revolutions. Trends from observations during a 20-month period between January 2015 and August 2016 have been described. Results obtained indicate that the 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 biases. Calculated biases may be used for measurement correction and for reprocessing of geophysical retrievals. Both L2A and L2B RapidScat data sets were provided by the NASA Physical Oceanography Distributed Active Archive Center at the Jet Propulsion Laboratory.