Use of the Earth’s Surface As a Reference to Correct Airborne Radar Radial Velocity Measurements for Platform Motion

A new technique to correct airborne Doppler radar measurements for aircraft motion using the measured radial velocity (Vr) of the ground or ocean has been developed and deployed. The measured Vr is a combination of the motion of the particles in the radar resolution volume and the motion of the radar platform. The impact of platform motion is typically corrected using accurate antenna pointing, aircraft attitude and aircraft motion information. These measurements have small errors that result in errors in the corrected Vr.

When the radar is pointed at nadir, the surface (land or water) has an intrinsic Vr of 0 m s^-1 providing a reference with which to correct any residual Vr error. At a given radial, the measured surface Vr can be subtracted from the measured Vr throughout the radial beam. However, the raw measured surface Vr should not be used because its measurement variance would cause large errors. Thus the measurement variance must be removed, along with any outliers, before the surface Vr can be used to correct the Vr field. The method proposed herein applies two specially designed finite impulse response (FIR) filters to the measured surface Vr, the first to remove outliers and the second to remove the measurement variance. The resulting smooth surface Vr is subtracted from the Vr field to produce the final corrected measurement.

The technique has been applied to the W-band HIAPER Cloud Radar (HCR) flown on the National Center for Atmospheric Research (NCAR) High-Performance Instrumented Airborne Platform for Environmental Research (HIAPER) aircraft. Results have been analyzed from numerous test flights and the Nor’easter and Cloud Systems Evolution in the Trades (CSET) field programs. This talk will describe the technique, its applications and results.