NASA RapidScat is the first Ku-Band wind scatterometer that flown in non-Sun-synchronous orbit. RapidScat is capable of measuring ocean vector winds over the full diurnal cycle, instead of observing a given location at a fixed time of day. The RapidScat operated onboard the International Space Station between September 2014 and August 2016. This unique platform facilitated global contiguous geographical sampling between the ±56 latitude. Non-Sun-synchronous orbit also enables the overlap with other satellite instruments that have been flying in Sun-synchronous orbits. To serve as a cross-calibration reference with other instruments, RapidScat must be carefully calibrated. This paper presents the double-difference methodology to scatterometry. The methodology has been adopted for the cross-instrument calibration between RapidScat and QuikScat scatterometers simultaneously orbiting the Earth on- board two independent satellite platforms. The statistical analysis and measurements biases have been evaluated between the two scatterometers. Calculated biases may be used for measurement correction and reprocessing. This work summarizes the comparison measurements data in the period from January 2015 to August 2016. This research was performed under the grant from the NASA Headquarters and oth RapidScat and QuikScat datasets were provided by the NASA Physical Oceanography Distributed Active Archive Center (PODAAC) at the Jet Propulsion Laboratory.