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 (σ^°). 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 σ^° is also required to monitor the time evolution of the accuracy of measured σ ^°. This is performed using relative calibration over land targets with minor spatiotemporal variation of σ^°. A few such targets are the Amazon rainforest, Greenland, Antarctica, etc. In this paper, relative calibration of σ^° from the OceanSAT-2 Scatterometer (OSCAT) has been carried out by comparing it with a similar quantity from the Quick Scatterometer (QuikSCAT) for November 2009. The differences between the average σ^° 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 σ^° 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 σ^° 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 σ^° is consistent with mean QuikSCAT σ^°, whereas the standard deviation in OSCAT is marginally higher. Further, time-series analysis of OSCAT σ^° shows its temporal stability.