On-orbit calibration of the Seawinds scatterometer antennas

Measurements of ocean, sea-ice and terrain geophysical parameters can be remotely sensed using an active microwave sensor known as the microwave scatterometer. Satellite scatterometers are spaceborne radars that measure the absolute backscattered power from the Earth surface; and from this measurement, the normalized radar cross-section (/spl sigma//spl deg/) of that surface is determined. Two satellite scatterometers have been operated by NASA to provide global measurements of ocean surface wind vectors (wind speed and direction). To meet the required wind vector accuracy, separate antennas providing these multiple looks must be well calibrated to within few tenths of a dB. In the past, pre-launch scatterometer antenna gain calibrations have proven inadequate; and post-launch corrections have been applied. This paper proposes a simple calibration method applicable to the upcoming Seawinds scatterometer launch. The method, used to eliminate gain biases in the two antenna beams, is based on the results from the NASA scatterometer (NSCAT) beam balance. The NSCAT instrument is introduced, followed by a description of the NSCAT calibration method. The Seawinds geometry is described, and NSCAT-derived models, applicable to the Seawinds (/spl sigma//spl deg/) incidence angle response, are presented.