Use of the Earth’s Surface as a Reference to Correct Airborne Nadir-Looking Radar Radial Velocity Measurements for Platform Motion

Abstract

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 [Formula: see text] should be 0 m s−1. Therefore, if a good estimate of the mean [Formula: see text] is made, it can be subtracted from the measured Vr to correct for errors due to residual pointing errors. The [Formula: see text] 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 [Formula: see text] 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 [Formula: see text] 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.

Ice in Southern Ocean Clouds With Cloud Top Temperatures Exceeding −5°C

“…The HCR measured the equivalent radar reflectivity factor (Z e ) and radial velocity (V r ) below the aircraft. The algorithm presented by Ellis et al (2019) was used to correct slight errors in the beam pointing angle away from nadir for the HCR. The method assumes that the surface echo is stationary and any offset from 0 m s −1 can be added or subtracted throughout the entirety of the beam using a surface V r of 0 m s −1 as a reference for correction.…”

Cloud and Precipitation Particle Identification Using Cloud Radar and Lidar Measurements: Retrieval Technique and Validation

“…It has collected data in four major and one minor field campaigns, ranging in location from the Southern Ocean to the tropics (Albrecht et al., 2019; Fuchs‐Stone et al., 2020; McFarquhar et al., 2020; Rauber et al., 2017). Care has been taken to optimize radar calibration and data quality, and provide additional data products, such as reanalysis fields or melting layer altitude (Ellis et al., 2019; Romatschke, 2021; Romatschke & Dixon, 2022; Romatschke et al., 2021). HCR observations are computed with a resolution of ∼20 m in range and a temporal resolution of 10 Hz.…”

Structure of an Atmospheric River Over Australia and the Southern Ocean: II. Microphysical Evolution

“…The radar employed on the G‐V was the High‐Performance Instrumented Airborne Platform for Environmental Research (HIAPER; University Corporation for Atmospheric Research (UCAR)/NCAR, 2014) Cloud Radar (HCR), an airborne, W‐band, Doppler research radar that fits within an underwing pod (Ellis et al, 2017). The HCR transmits at 94.4 GHz (3 mm wavelength) and was positioned at nadir during all transects through the AR.…”

“…The reflectivity data should thus not be interpreted quantitatively, as the radar signal penetrated through supercooled liquid water (SLW) layers above and rain below the radar bright band across the AR. The V r was corrected for platform motion following Ellis et al (2017) and Rauber et al (2017). Radar gates were masked if the normalized coherent power was less than 0.16 to eliminate noise.…”