Pointing Calibration for Spaceborne Doppler Scatterometers

Doppler scatterometers have demonstrated the ability to measure wide-swath ocean surface currents from airborne platforms. Since platform velocities for spaceborne platforms are almost two orders of magnitude larger, errors in the knowledge of the pointing of the radar antenna result in ocean current errors also two orders of magnitude larger, and this presents a major challenge to achieving useful measurements of ocean currents. Here, we present a new calibration method to estimate pointing biases that removes the dominant pointing errors and allowing the retrieval of global ocean currents with modest requirements on the system stability. The method uses the fact that pointing errors have a signature on velocity that depends on cross-track distance (or azimuth angle) alone, while ocean currents do not, if averaged sufficiently along-track. This lack of correlation between error and true currents allows the use of along-track averages of residual radial velocity, after possibly subtracting prior estimates of the currents, for the inversion of the slowly varying pointing errors. The calibration method can be implemented in ground processing and does not impact the processing of onboard data. We illustrate the performance of the calibration on the performance of the proposed NASA/CNES ODYSEA Doppler scatterometer and assess its ability to meet the mission science goals.