Use of the Earth's surface as a reference to correct airborne nadir-looking radar radial velocity measurements for platform motion

A technique for correcting radar radial velocity Vr in airborne, nadir-pointing radar data using the surface of Earth as a reference is proposed and tested. Operating airborne Doppler radars requires correcting the radial velocity for platform motion. This can be accomplished with accurate beam-pointing and platform motion measurements. However, there are often residual pointing errors due to drift in inertial navigation systems (INS) and/or errors in platform-relative pointing. The technique proposed here takes advantage of the fact that the surface is stationary and the mean of the measured Vr at the surface Vrsurfmeas should be 0 m s(-1). Therefore, if a good estimate of the mean Vrsurfmeas is made, it can be subtracted from the measured Vr to correct for errors due to residual pointing errors. The Vrsurfmeas data contain many independent deviations from 0 m s(-1) due to various causes, including measurement variance and large deviations due to surface features. These deviations must be filtered out of Vrsurfmeas before the surface reference can be applied to correct the Vr data. A two-step filtering process was developed and tested. The first step removes large deviations in Vrsurfmeas and the second step removes the measurement noise. The technique was examined using data from three field campaigns and was found to improve the quality of Vr in all cases. The Vr bias was removed and the variance was substantially reduced. The approach is generally applicable to nadir-pointing airborne radar data.