Calibration and Validation of NOAA Oceansat-3 Scatterometer Wind Data Products

The OceanSat-3 scatterometer (OSCAT-3), launched by the Indian Space Research Organization (ISRO) on November 26, 2022, is a Ku-band radar designed to measure sea surface wind vectors by inverting the normalized radar cross-section (sigma0) though an empirical model function. Since May 2023, NOAA has been processing near real-time OSCAT-3 Level 1 B data to produce wind vectors at 12.5 km grid spacing. Additionally, rain rate retrieval algorithm has been developed to identify and flag contaminated wind retrievals. The OSCAT-3 sigma0 and brightness temperature calibration drift of 0.2 dB and ∼ 30 K respectively over the period of one-year time has been noted, resulting in an underestimation of rain rates and wind speeds, particularly in high wind conditions. This prompted NOAA to implement new calibration offset in August 2024 of -0.22 dB for V-pol and -0.88 dB for H -pol measurements. Addressing these biases improved wind speed standard deviation on average by 0.3 m/s relative to Numerical Weather Prediction (NWP) model. Similarly, corrected brightness temperature values (32.8 K for V-pol and 30.7 K for H-pol) have improved rain rate retrievals, increasing rain rate detection accuracy. Assuming calibration stability, these updates ensure reliable wind and rain rate retrievals, with the recommended rain flag threshold updated to 1.5 mm/hr.