Abstract. The planned and potential introduction in global satellite observing systems of conically scanning Ka- and W-band atmospheric radars (e.g., the radars in the Tomorrow.IO constellation, https://www.tomorrow.io/space/, last access: 1 June 2022, and the Wivern (WInd VElocity Radar Nephoscope) radar, https://www.wivern.polito.it, last access: 1 July 2022) calls for the development of methodologies for calibrating and cross-calibrating these systems. Traditional calibration techniques pointing at the sea surface at about 11∘ incidence angle are in fact unfeasible for such fast rotating systems. This study proposes a cross-calibration method for conically scanning spaceborne radars based on ice cloud reflectivity probability distribution functions (PDFs) provided by reference radars like the Global Precipitation Measurement (GPM) Ka-band radar or the W-band radars planned for the ESA-JAXA EarthCARE or for the NASA Atmosphere Observing System missions. In order to establish the accuracy of the methodology, radar antenna boresight positions are propagated based on four configurations of expected satellite orbits so that the ground-track intersections can be calculated for different intersection criteria, defined by cross-over instrument footprints within a certain time and a given distance. The climatology of the calibrating clouds, derived from the W-band CloudSat and Ka-band GPM reflectivity records, can be used to compute the number and the spatial distribution of calibration points. Finally, the mean number of days required to achieve a given calibration accuracy is computed based on the number of calibration points needed to distinguish a biased reflectivity PDF from the sampling-induced noisiness of the reflectivity PDF itself. Findings demonstrate that it will be possible to cross-calibrate, within 1 dB, a Ka-band (W-band) conically scanning radar like that envisaged for the Tomorrow.io constellation (Wivern mission) every few days (a week). Such uncertainties are generally meeting the mission requirements and the standards currently achieved with absolute calibration accuracies.
Abstract. The planned and potential introduction in the global satellite observing systems of conically scanning Ka and W band atmospheric radars [e.g. the radars in the Tomorrow.IO constellation, https://www.tomorrow.io/space/, and the Wivern (WInd VElocity Radar Nephoscope) radar, www.wivern.polito.it] calls for the development of methodologies for calibrating and cross-calibrating these systems. Traditional calibration techniques pointing at the sea surface at about 12° incidence angle are in fact unfeasible for such fast rotating systems. This study proposes a cross-calibration method for conically scanning spaceborne radars based on ice cloud reflectivity probability distribution functions (PDF) provided by reference radars like the GPM Ka-band radar or the W-band radars planned for the ESA-JAXA EarthCARE or for the NASA Atmosphere Observing System missions. In order to establish the accuracy of the methodology, radar antenna boresight positions are propagated based on four configurations of expected satellite orbits so that the ground-track intersections can be calculated for different intersection criteria, defined by cross-over instrument footprints within a certain time and a given distance. The climatology of the calibrating clouds, derived from the W-band CloudSat and Ka-band GPM reflectivity records, can be used to compute the number and the spatial distribution of calibration points. Finally, the mean number of days required to achieve a given calibration accuracy is computed based on the number of calibration points needed to distinguish a biased reflectivity PDF from the sampling-induced noisiness of the reflectivity PDF itself. Findings demonstrate that it will be possible to cross-calibrate within 1 dB a Ka-band (W-band) conically scanning radar like that envisaged for the Tomorrow.io constellation (Wivern mission) every few days (a week). Such uncertainties are generally meeting the mission requirements and the standards currently achieved with absolute calibration accuracy.
<p>The WIVERN (WInd VElocity Radar Nephoscope) concept, now in Phase&#160;0 of the ESA Earth Explorer program, promises to complement Doppler wind lidar by globally observing, for the first time, the vertical profiles of winds in cloudy areas. The mission will also strengthen the cloud and precipitation observation capability of the Global Observing System by providing unprecedented revisit time of cloud and precipitation vertical profiles. The mission hinges upon a single instrument, i.e., a dual-polarization Doppler W-band scanning cloud radar with a 3&#8201;m circular aperture non-deployable main reflector. The WIVERN antenna conically scans a large swath (of about 800 km) around nadir at an off-nadir angle of about 38<sup>o</sup> at 12&#8201;rpm (revolutions per minute). This viewing geometry allows daily revisits poleward of 50&#176;, 50-km horizontal resolution, and approximately 1-km vertical resolution. A key element is the use of closely spaced pulse pairs one of which is H polarised the other V polarised, so that the target does not have time to reshuffle, and, providing there is no significant cross-talk between the two returns, the high velocities associated with wind storms can be retrieved.&#160;</p> <p>In this paper we will discuss the scientific objectives of the mission and will outline some of the technical challenges of the measuring technique. In particular we will discuss how to correct for wind biases introduced by the satellite motion and wind shear across the beam, how to account for cross-talk between the H and V returns due to depolarisation by meteorological targets, how to calibrate the instrument and how to identify mis-pointing of the antenna that could affect Doppler accuracy. We will also present examples of Level 1 products via an end to end simulations applied to high resolution cloud resolving models and expected performances of the instrument in terms of cloud/precipitation and wind coverage.</p>
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