Impact of Crosstalk on Reflectivity and Doppler Measurements for the WIVERN Polarization Diversity Doppler Radar

The Wind Velocity Radar Nephoscope (WIVERN) mission, one of the four ESA Earth Explorer 11 candidate missions, aims at globally observing, for the first time, simultaneously vertical profiles of reflectivities and line-of-sight (LOS) winds in cloudy and precipitating regions. WIVERN adopts a dual-polarization Doppler radar to overcome the short decorrelation time between successive radar pulses transmitted from low Earth-orbiting satellites with finite beamwidth antennas. WIVERN transmits a single polarization state at a time (H or V), receives in both the polarization states, and uses the polarization diversity pulse pair (PDPP) technique to estimate the Doppler velocity. The weaker cross-polar signals can sometimes interfere with the copolar ones, causing ghost signals in the measurements that hinder the system’s overall performance. In addition, with the envisaged radar trigger mode, parameters such as linear depolarization ratio (LDR) and differential reflectivity ( $Z_{\text {DR}}$ ) cannot be directly measured because of the nearly simultaneous transmission of H and V pulses. To overcome these challenges, this article presents a novel technique based on the optimal estimation (OE) algorithm for retrieving LDR, $Z_{\text {DR}}$ , and copolar reflectivity for radars operated in the PDPP mode. The performance of the proposed method is evaluated using a realistic climatology of profiles simulated from CloudSat data. Results demonstrate that copolar reflectivity can be accurately retrieved in regions with a good signal-to-noise ratio (SNR) and in the absence of simultaneous crosstalk interference in both the channels (which occurs very rarely). The LDR retrieval, on the other hand, is typically driven by the a priori with a substantial impact of measurements only for the surface returns. The impact of crosstalk is also assessed on the reduction of precise Doppler measurements. Findings confirm that a selection of the separation between the two polarization diversity pulses ( $T_{\text {HV}}$ ) of $20 \mu \text{s}$ achieves a good balance between the large errors originated by the strong dependence on the Doppler phase noise at small $T_{\text {HV}}\text{s}$ and those caused by the drop in correlation and unambiguous Nyquist velocity at large $T_{\text {HV}}$ .