the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Apr 2025
EarthCARE’s Cloud Profiling Radar Antenna Pointing Correction using Surface Doppler Measurements
Abstract. The Earth Cloud Aerosol and Radiation Explorer (EarthCARE) mission, a joint effort between the European Space Agency (ESA) and the Japan Aerospace Exploration Agency (JAXA), aims to advance our understanding of aerosols, clouds, precipitation, and radiation using a comprehensive active and passive sensors payload. A key component of the payload is the 94-GHz Cloud Profiling Radar (CPR), which provides the first-ever Doppler velocity measurements collected from space. Accurate knowledge of the CPR antenna pointing is essential for ensuring high quality CPR Doppler velocity measurements. This study focuses on the geolocation assessment and antenna mispointing corrections during EarthCARE's commissioning phase and beyond, using Earth’s surface Doppler velocity measurements collected over the first nine months of the mission. While the instrument footprint is proven to be properly geolocated within about 100 meters, surface Doppler velocity observations reveal mispointing trends influenced by solar illumination cycles and thermoelastic distortions on the antenna. Correcting these effects significantly reduces biases, ensuring better Doppler velocity measurements, essential for understanding cloud microphysics and dynamics. The results, validated through the analysis of Doppler velocities in ice clouds, underline the critical role of pointing corrections for the success of the EarthCARE mission.
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CC1: 'Comment on egusphere-2025-1680 - Commonalities to Doppler wind lidar on Aeolus', Oliver Reitebuch, 27 Apr 2025
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I consider this as a very important and innovative contribution to the emerging field of active remote sensing from space of atmospheric winds with Doppler radar and lidar. Actually, a similar behavior of the wind bias along the orbit and with seasonal changes was found with the first Doppler wind lidar in space on Aeolus launched in August 2018. Here the thermo-elastic deformations of the primary mirror itself caused some complex wind bias variations along the orbit for the horizontal line-of-sight (HLOS) winds of several m/s as illustrated in Rennie et al. (2021), e.g. Fig. 4 or Weiler et al. (2021), e.g. Fig. 5. Although the vertical wind bias for CPR and the HLOS wind bias of Aeolus are caused by the antenna resp. primary mirror, there are some significant differences. The bias of the CPR is caused according to the authors by thermo-elastic deformations of the CPR antenna resulting in a mis-pointing of the LOS direction. This is mainly driven by the direct illumination or shading of the CPR antenna along the orbit by the Sun, which shows a seasonal dependency. The situation for Aeolus is more complex: Here the wind-bias is caused by a thermo-elastic deformation of the primary mirror shape, resulting in a change in the illumination (angle of incidence and divergence) of the Fabry-Perot and Fizeau interferometers used for detecting Doppler shifts from Rayleigh and Mie scattering. This thermo-elastic deformation of the Aeolus primary mirror is caused by changes of the infrared albedo of the Earth (outgoing long-wave radiation) - and thus depending on the atmosphere properties and sun illumination of the atmosphere. Despite this complex influence, it was possible to correlate the Aeolus wind bias with the temperature sensors mounted on the backside of the primary mirror (Fig. 2 in Weiler et al. 2021), which was a major breakthrough for correcting the wind biases (Rennie et al. 2021).
I would propose to include this commonality (and possibly also differences) in the introduction and discussion of the paper with the corresponding references.
It would be also nice to show a correlation of this bias with temperature sensors on the satellite structure and antenna, which could reflect this behavior along the orbit. Potentially also a correction of the LOS mis-pointing with these temperature sensors could be performed in the future.
A correction using the ground-return velocity from Aeolus is also discussed in the paper by Weiler et al. (2021), e.g. Fig. 13. As high-quality ground-returns from Aeolus are mainly limited to the polar regions with high albedo from ice and snow in the ultraviolet spectral region, and sea-surface returns can not be used due to their low SNR and non-negligible movement, a correction purely based on ground-returns is challenging for Aeolus. But such a correction scheme is still under investigation for the operational follow-on mission Aeolus-2, where higher SNR and higher vertical resolution for ground-return sampling is expected.
References:
Rennie, M., L. Isaksen, F. Weiler, J. de Kloe, Th. Kanitz, O. Reitebuch (2021): The impact of Aeolus wind retrievals in ECMWF global weather forecasts. Q. J. Roy. Meteorol. Soc., Vol 147, Issue 740, 3555-3586, https://doi.org/10.1002/qj.4142
Weiler, F., M. Rennie, Th. Kanitz, L. Isaksen, E. Checa, J. de Kloe, Ngozi Okunde, and O. Reitebuch (2021): Correction of wind bias for the lidar on-board Aeolus using telescope temperatures. Atm. Meas. Tech., 14, 7167–7185, https://doi.org/10.5194/amt-14-7167-2021
Citation: https://doi.org/10.5194/egusphere-2025-1680-CC1
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