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Intersatellite radiometric calibration for a satellite radar scatterometer

After the launch of NASA's SeaWinds radar scatterometer on the QuikSCAT satellite in 1999, a radiometer function, known as the QuikSCAT Radiometer - QRad, was implemented in the Science Ground Data Processing Systems to allow the measurement of the earth's microwave brightness temperature (Tb) using the radar system noise temperature [1, 2]. This paper will describe an inter-satellite radiometric calibration technique to validate the QRad brightness temperature algorithm and the QuikSCAT L2A Tb product. This approach allows the inter-comparison of two satellite sensors (radiometers) that have significant differences in their designs. To assess the quality of the QRad instrument, we compare its Tb measurements with the near simultaneous and collocated ocean brightness temperature observations from WindSat on the Coriolis Satellite, which serves as the brightness temperature calibration standard. Since the QRad and WindSat instruments were of different designs, brightness temperature normalizations were made for WindSat before comparison to account for expected differences in Tb because of incidence angle and channel frequency differences. Brightness temperatures for nine months during 2005 and 2006 were spatially collocated for rainfree homogeneous ocean scenes (match-ups) within 1° latitude x longitude boxes and within a ± 60 minute window. To ensure high quality comparison, these collocations were quality controlled and edited to remove non-homogenous ocean scenes and/or transient environmental conditions, including rain contamination. WindSat and QRad Tb's were averaged within 1° boxes and were used for the radiometric inter-calibration analysis on a monthly basis. Results show that QRad radiometric calibration is stable in the mean over the yearly seasonal cycle.