A Method for Continues Calibration of a Rotating Antenna Scatterometer in Application to CFOSAT Measurements

Spaceborne radar scatterometers require precise calibration to provide accurate sea surface wind speed retrievals. The Scatterometer (SCAT) onboard the China-France Oceanography Satellite (CFOSAT) presents additional calibration complexities as a rotating antenna instrument making moderate incidence angle dual-polarization backscatter measurements. Here we propose a two-step methodology for recalibrating SCAT backscatter measurements σ
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. First, we match probability distribution functions of SCAT σ
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to a standard geophysical model function at common wind speeds (7-8 m/s) to determine calibration coefficients as a function of incidence angle, antenna azimuth, and polarization. We then derive an instrument-specific Geophysical Model Function from the recalibrated σ
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accounting for variations from the native antenna gain and noise characteristics.Validating SCAT wind retrievals from the recalibrated σ
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against those using traditional fixed calibration shows significant improvements in accuracy over time, with reduced cross-track errors and decreased uncertainties for most wind speeds. The proposed recalibration technique enables precise wind retrievals from SCAT well before comprehensive engineering calibrations and provides a practical solution for overcoming unforeseen calibration issues to achieve high-accuracy satellite wind speed retrievals. This empirical two-step approach may be extensible to other innovative spaceborne scatterometer systems where signal callibration technics are not fully established.