Cross Calibration of the OceanSAT-2 Scatterometer With QuikSCAT Scatterometer Using Natural Terrestrial Targets

The accuracy of ocean surface wind vectors measured by satellite-borne scatterometers depends on measured backscattering coefficient (sigma degrees). Hence, an in-flight calibration of a satellite scatterometer is essential as this is not guaranteed by its prelaunch absolute calibration. The postlaunch calibration of sigma degrees is also required to monitor the time evolution of the accuracy of measured sigma degrees. This is performed using relative calibration over land targets with minor spatiotemporal variation of sigma degrees. A few such targets are the Amazon rainforest, Greenland, Antarctica, etc. In this paper, relative calibration of sigma degrees from the OceanSAT-2 Scatterometer (OSCAT) has been carried out by comparing it with a similar quantity fromthe Quick Scatterometer (QuikSCAT) for November 2009. The differences between the average sigma degrees of QuikSCAT and that of OSCAT have been calculated globally to check the overall consistency. Over the calibration sites, the differences are within +/- 0.25 dB. Histograms of differences in ascending/descending passes and fore/aft looks of OSCAT have also been analyzed over the calibration sites. These indicate that look bias in OSCAT degrees sigma is within the range of +/- 0.5 dB. It is also evident that pass biases, i.e., differences between ascending and descending passes, exist over the Amazon rainforest for both QuikSCAT and OSCAT. This diurnal variation in sigma degrees may go up to 1.25 dB in OSCAT. Further, computations of daily average and standard deviation over the calibration site show that mean OSCAT sigma degrees is consistent with mean QuikSCAT sigma degrees, whereas the standard deviation in OSCAT is marginally higher. Further, time-series analysis of OSCAT sigma degrees shows its temporal stability.