| Paper: | FR-P2.3 |
| Session: | Instrument Calibration II |
| Time: | Friday, March 30, 16:20 - 16:40 |
| Presentation: |
Oral
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| Topic: |
Sensor calibration: |
| Title: |
Radiometric Characterization of a Water-Based Conical Blackbody Calibration Target for Millimetre-Wave Remote Sensing |
| Authors: |
Karl Jacob; University of Bern | | |
| | Arne Schröder; University of Bern | | |
| | Axel Murk; University of Bern | | |
| | Leandro von Werra; AME GmbH | | |
| | Florian Reinhard; AME GmbH | | |
| | Philippe Raisin; AME GmbH | | |
| Abstract: |
We present a novel concept of water-based blackbody calibration targets to be used for the calibration of microwave remote sensing instruments. In microwave radiometry the absolute radiometric calibration relies on accurate blackbody targets. Regarding the performance of these calibration targets, a high microwave emissivity is as important as a homogeneous and well defined physical temperature. In spectroscopic observations a small coherent backscattering of the calibration target is even more critical since it leads to standing waves between the target and the receiver. Commonly used microwave absorbing materials include low density foams or plastics impregnated with carbon black or other weakly conducting fillers. But due to the low thermal conductivity of these materials they are not well suited for calibration targets. In order to improve the thermal performance most of the current high-quality targets make use of a thin absorber layer based on magnetically loaded epoxy resin, which is applied on a metal backing. But even there it is not trivial to ensure low thermal gradients, in particular when a variable brightness temperature shall be obtained which is different from the environment. In order to circumvent the conflict between the microwave and the thermal performance of the target an aqueous blackbody calibration source has been developed. Compared to the commonly used microwave absorbers water has the advantages of a high electromagnetic absorption and, at the same time, a high temperature uniformity when circulated. In addition the water temperature can be easily stabilized or varied with a closed cycle thermal control system which makes the target robust to temperature fluctuations in its environment
We will present the design of an operational water-based calibration target prototype. The prototype has been manufactured using a commercial 3D printer and it comprises a shell with a conical profile made of a low loss plastic. An exponential profile of the shell is used to compensate the comparatively high refractive index of water and obtain a high emissivity of the target. First measurements using a ground based radiometer operating at 110 GHz demonstrate the outstanding thermal properties of the calibration target for varying water temperatures between 10°C and 60°C. We show that the differences between the physical temperature of the water, measured with platinum resistance thermometers, and the measured brightness temperature are significantly smaller than with alternative target designs. In addition the measured baseline ripples are small compared to established blackbody concepts. Active measurements of the coherent backscattering have been performed to confirm its excellent electromagnetic performance. The proposed calibration target is a highly accurate reference source to be used in laboratories. It exhibits an electromagnetic performance of conventionally used epoxy-based targets, but without suffering from thermal gradients which are degrading the radiometric performance.
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