CHALLENGES OF ON-GROUND CALIBRATION OF POLARIMETRIC MICROWAVE RADIOMETERS

A. Brönnimann¹, R. Albers¹, L. Ciorba¹, G. Wylde², A. Murk¹

¹University of Bern, Institute of Applied Physics, ²Thomas Keating Ltd

The calibration of spaceborne microwave radiometers is essential for ensuring accuracy in Earth remote sensing. These instruments generally include on-board calibration systems, which can either be integrated into the receiver or free space blackbody calibration targets that are inserted in front of the antenna. However, these on-board systems require prior calibration on ground before the satellite is launched. This is usually done using the two-point calibration method, where the on-ground calibration targets (OGCT) provide a hot and a cold load. Polarimetric radiometers, such as the Copernicus Imaging Microwave Radiometer (CIMR), require more extensive setups with linearly polarised loads. The size of the OGCT is mainly dependent on the frequency and beam properties of the feeds. For the on-ground calibration of polarimetric radiometers, the OGCT system has to provide additional input signals with different polarisation states. One example of such an instrument is the Copernicus Imaging Microwave Radiometer (CIMR). It has a total of 50 polarimetric receivers at 1.4, 6.9, 10.7, 18.7 and 36.5 GHz, which have to be calibrated to an accuracy between 0.5 and 0.8 K [1]. The lowest frequency, the size of the CIMR feed cluster, and the divergence of the beams requires a very large OGCT in order to maximise coupling into the target.

Thomas Keating Ltd and the Institute of Applied Physics at the University of Bern have developed a concept for a large aperture polarimetric OGCT which would be suitable for the on-ground calibration of CIMR or similar instruments. In this paper we will discuss the general challenges of polarimetric on-ground calibration targets in these frequency bands and our proposed concept for CIMR. We will also show RF simulations and measurements of key components such as the microwave absorber, window materials and the polariser grid.

1. Donlon, C. J., Mission Requirements Document for the Copernicus Imaging Microwave Radiometer (CIMR), version 5.0, 2023, https://esamultimedia.esa.int/docs/EarthObservation/CIMR-MRD-v5.0-20230211_(Issued).pdf