Scatterometer beam balancing using open-ocean backscatter measurements

Calculation of vector winds from spaceborne fan-beam scatterometers require s that backscatter measurements from different antennas be relatively calib rated to high accuracy. A method is developed to perform postlaunch antenna calibration using global mean ocean backscatter measurements in conjunctio n with estimates of the statistical distribution of near-surface wind veloc ity and the model function relating backscatter to winds. Substantial analy tic simplifications result from assuming that the wind speed and azimuth di stributions are separable and that the upwind-downwind asymmetry term in th e model function is small. The analytic model allows quantitative examinati on of the sensitivity of the technique to errors in the approximate wind di stributions and empirical model function. The approach and its assumptions are tested using 13 months of ERS-1 backscatter data, surface wind estimate s from two operational weather analyses, and three empirical C-band model f unctions. It is shown that the ERS-1 antennas are relatively calibrated to within 0.2 dB, which is consistent with other published results obtained us ing ground receiving stations and Amazon forest data. The results are nearl y insensitive to realistic errors in the estimated wind velocity distributi ons and model function. Analysis suggests that the ocean antenna calibratio n technique should be accurate to about 0.2 dB using as little as 3 weeks o f scatterometer data.