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 mostly 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 sub-satellite latitude range between ±51.6°. To serve as a cross-calibration reference with other instruments, RapidScat must be carefully calibrated. This paper 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). The radiometric calibration of RapidScat that enables the surface brightness temperature measurement is presented. Seasonal measurement biases have been evaluated using the Radiative Transfer Model (RTM). Systematic brightness temperature biases for both polarizations have been calculated as a function of geometry, atmospheric model, and ocean brightness temperature models. These deviations were averaged over 1084 RapidScat revolutions. Trends from observations during a 20-month period between January 2015 and August 2016 have been described. Results obtained indicate that most of the measured data in 2015 show an overall average agreement.