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Calibration of a 35-GHz airborne cloud radar: lessons learned and intercomparisons with 94-GHz cloud radars

Ewald, Florian and Groß, Silke Martha and Hagen, Martin and Hirsch, Lutz and Delanoë, Julien and Bauer-Pfundstein, Matthias (2019) Calibration of a 35-GHz airborne cloud radar: lessons learned and intercomparisons with 94-GHz cloud radars. Atmospheric Measurement Techniques (AMT), 12 (3), pp. 1815-1839. Copernicus Publications. DOI: 10.5194/amt-12-1815-2019 ISSN 1867-1381

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Official URL: http://dx.doi.org/10.5194/amt-12-1815-2019

Abstract

This study gives a summary of lessons learned during the absolute calibration of the airborne, high-power Ka-band cloud radar HAMP MIRA on board the German research aircraft HALO. The first part covers the internal calibration of the instrument where individual instrument components are characterized in the laboratory. In the second part, the internal calibration is validated with external reference sources like the ocean surface backscatter and different air- and spaceborne cloud radar instruments. A key component of this work was the characterization of the spectral response and the transfer function of the receiver. In a wide dynamic range of 70 dB, the receiver response turned out to be very linear (residual 0.05 dB). Using different attenuator settings, it covers a wide input range from -105 to -5 dBm. This characterization gave valuable new insights into the receiver sensitivity and additional attenuations which led to a major improvement of the absolute calibration. The comparison of the measured and the previously estimated total receiver noise power (-95.3 vs. -98.2 dBm) revealed an underestimation of 2.9 dB. This underestimation could be traced back to a larger receiver noise bandwidth of 7.5 MHz (instead of 5 MHz) and a slightly higher noise figure (1.1 dB). Measurements confirmed the previously assumed antenna gain (50.0 dBi) with no obvious asymmetries or increased side lobes. The calibration used for previous campaigns, however, did not account for a 1.5 dB two-way attenuation by additional waveguides in the airplane installation. Laboratory measurements also revealed a 2 dB higher two-way attenuation by the belly pod caused by small deviations during manufacturing. In total, effective reflectivities measured during previous campaigns had to be corrected by +7.6 dB. To validate this internal calibration, the well-defined ocean surface backscatter was used as a calibration reference. With the new absolute calibration, the ocean surface backscatter measured by HAMP MIRA agrees very well (<1 dB) with modeled values and values measured by the GPM satellite. As a further cross-check, flight experiments over Europe and the tropical North Atlantic were conducted. To that end, a joint flight of HALO and the French Falcon 20 aircraft, which was equipped with the RASTA cloud radar at 94 GHz and an underflight of the spaceborne CloudSat at 94 GHz were performed. The intercomparison revealed lower reflectivities (-1.4 dB) for RASTA but slightly higher reflectivities (+1.0 dB) for CloudSat. With effective reflectivities between RASTA and CloudSat and the good agreement with GPM, the accuracy of the absolute calibration is estimated to be around 1 dB.

Item URL in elib:https://elib.dlr.de/127140/
Document Type:Article
Title:Calibration of a 35-GHz airborne cloud radar: lessons learned and intercomparisons with 94-GHz cloud radars
Authors:
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Ewald, FlorianDLR, IPAhttps://orcid.org/0000-0002-5899-0890
Groß, Silke MarthaDLR, IPAhttps://orcid.org/0000-0002-7467-9269
Hagen, MartinDLR, IPAhttps://orcid.org/0000-0003-4714-0775
Hirsch, LutzMax Planck Institute for Meteorology, Hamburg, GermanyUNSPECIFIED
Delanoë, JulienLATMOS/UVSQ/IPSL/CNRS, Guyancourt, FranceUNSPECIFIED
Bauer-Pfundstein, MatthiasMetek GmbH, Elmshorn, GermanyUNSPECIFIED
Date:20 March 2019
Journal or Publication Title:Atmospheric Measurement Techniques (AMT)
Refereed publication:Yes
Open Access:Yes
Gold Open Access:Yes
In SCOPUS:Yes
In ISI Web of Science:Yes
Volume:12
DOI :10.5194/amt-12-1815-2019
Page Range:pp. 1815-1839
Editors:
EditorsEmail
Amiridis, VassilisUNSPECIFIED
Publisher:Copernicus Publications
ISSN:1867-1381
Status:Published
Keywords:Cloud radar calibration, Ka-Band, Intercomparison of 35-GHz and 94-GHz, Internal calibration, External validation
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:Earth Observation
DLR - Research area:Raumfahrt
DLR - Program:R EO - Erdbeobachtung
DLR - Research theme (Project):R - Vorhaben Atmosphären- und Klimaforschung
Location: Oberpfaffenhofen
Institutes and Institutions:Institute of Atmospheric Physics > Lidar
Deposited By: Ewald, Florian
Deposited On:10 Apr 2019 11:39
Last Modified:10 Apr 2019 11:39

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