Scatterometer Beam Balancing Using Open-Ocean Backscatter Measurements

Abstract

Calculation of vector winds from spaceborne fan-beam scatterometers requires that backscatter measurements from different antennas be relatively calibrated to high accuracy. A method is developed to perform postlaunch antenna calibration using global mean ocean backscatter measurements in conjunction with estimates of the statistical distribution of near-surface wind velocity and the model function relating backscatter to winds. Substantial analytic simplifications result from assuming that the wind speed and azimuth distributions are separable and that the upwind?downwind asymmetry term in the model function is small. The analytic model allows quantitative examination of the sensitivity of the technique to errors in the approximate wind distributions and empirical model function. The approach and its assumptions are tested using 13 months of ERS-1 backscatter data, surface wind estimates from two operational weather analyses, and three empirical C-band model functions. It is shown that the ERS-1 antennas are relatively calibrated to within 0.2 dB, which is consistent with other published results obtained using ground receiving stations and Amazon forest data. The results are nearly insensitive to realistic errors in the estimated wind velocity distributions and model function. Analysis suggests that the ocean antenna calibration technique should be accurate to about 0.2 dB using as little as 3 weeks of scatterometer data.