Relative Calibration Using Natural Terrestrial Targets: A Preparation Towards Oceansat-2 Scatterometer

Scatterometer instruments transmit a series of microwave pulses and measure the returned echo to determine the normalized radar cross section (sigma(0)) over the target to derive the near-ocean-surface wind vector. Accuracy of the derived wind vector over the data sparse oceans therefore depends on the accuracy of sigma(0) measurement. For this purpose, accurate calibration of the scatterometer is required. As a preparation toward calibration of the Oceansat-2 mission, of the Indian Space Research Organisation, a relative calibration technique has been proposed in this study by selecting homogeneous areas over the globe with isotropic radar response and temporally stable signature of sigma(0). For this purpose, the daily averaged sigma(0) and Level-2A (L2A) sigma(0) measurements of the QuikSCAT scatterometer have been used. Analyzing the monthly mean and standard deviation in sigma(0) for the period of 2005-2006, several regions are chosen which have a quasi-isotropic radar response and minimal temporal variation in sigma(0). The analysis shows that the selected areas over Antarctica and Greenland with permanent ice covers have temporally stable signatures of sigma(0). The regions like the Amazon forests and parts of Australia also show high temporal stability of sigma(0) but greater standard deviation than the snow-covered areas. The QuikSCAT L2A data have also been used to study the day-night variation and azimuthal dependence of the sigma(0) over these targets. The present work demonstrated that quasi-uniform natural sites such as Sahara, Amazon forest, Kutch, Greenland region, and Antarctica region, covering wide dynamic range of sigma(0), can be used for the purpose of calibration.