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 (σ⁰) 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 σ⁰ 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 σ⁰. For this purpose, the daily averaged σ⁰ and Level-2A (L2A) σ⁰ measurements of the QuikSCAT scatterometer have been used. Analyzing the monthly mean and standard deviation in σ⁰ for the period of 2005-2006, several regions are chosen which have a quasi-isotropic radar response and minimal temporal variation in σ⁰. The analysis shows that the selected areas over Antarctica and Greenland with permanent ice covers have temporally stable signatures of σ⁰. The regions like the Amazon forests and parts of Australia also show high temporal stability of σ⁰ 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 σ⁰ 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 σ⁰, can be used for the purpose of calibration.