Counting marmots…
The WIVERN (Wind VElocity Radar Nephoscope) 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 winds in cloudy and precipitating regions. WIVERN adopts a dual-polarization Doppler radar in order 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 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 co-polar ones, causing ghost signals in the measurements that hinder the system’s overall performance. Additionally, with the envisaged radar trigger mode, parameters such as Linear Depolarization Ratio (LDR) and Differential Reflectivity (Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DR</sub> ) 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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DR</sub> , and co-polar reflectivity for radars operated in PDPP mode. The performance of the proposed method is evaluated using a realistic climatology of profiles simulated from CloudSat data. Results demonstrate that co-polar reflectivity can be accurately retrieved in regions with a good signal-to-noise ratio and in the absence of simultaneous cross-talk interference in both 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 cross-talk is also assessed on the reduction of precise Doppler measurements. Findings confirm that a selection of the separation between the two polarization diversity pulses ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T<sub>HV</sub></i> ) of 20 μs achieves a good balance between the large errors originated by the strong dependence on the Doppler phase noise at small <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T<sub>HV</sub></i> s and those caused by the drop in correlation and unambiguous Nyquist velocity at large <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T<sub>HV</sub></i> .
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