title: Optical Design and Calibration of the Arctic Weather Satellite
Creators: name: Albers, Roland
Publisher: name: Universität Bern
Year: 2025
Subjects: subject: 530 Physics
subject: 620 Engineering

description: This thesis covers the quasi-optical design and calibration targets of the microwave radiometer on the Arctic Weather Satellite (AWS), which was launched on the 16th of August 2024. AWS is a prototype mission for a constellation called EUMETSAT Polar System Sterna (EPS-Sterna) consisting of 6 AWS models in 3 orbital planes which will be used operationally for numerical weather prediction (NWP). The radiometer operates in the 54, 89, 183 and 325 GHz bands in a cross-track scanning configuration. While the lower three frequency bands are common to other operational radiometers currently flying, it is the first spaceborne instrument to provide sounding data at 325 GHz. Another novel feature of the instrument is the wedge shaped Onboard Calibration Target (OBCT) with a new epoxy based absorber mixture. It is also the first radiometer to use a feedcluster that is directly illuminating a cross-track scanning mirror, instead of the conventional approach of using a Quasi-Optical Network (QON) to co-align the antenna beams. This approach allowed for a much more compact design at a lower cost compared to other microwave sounders, suitable for hosting on a smaller dedicated satellite platform and therefore enabling production in larger quantities for a constellation. However, this approach also introduced scan dependent performance variations, beam pointing offsets and elliptical beam contours, which need to be considered for the instrument calibration and accurate geolocation. Chapter 1 contains a brief summary of relevant concepts enabling microwave radiometry and gives an overview of radiometer receivers. It is focussed on providing the background information for the rest of the thesis and can be skipped by readers familiar with the subject matter. Chapter 2 describes the AWS mission in greater detail and puts it in context of other contemporary missions and the ultimate goal of the EPS-Sterna constellation. It also states the thesis objectives and shows first light data of the instrument from orbit. A detailed overview of the quasi-optics is provided in Chapter 3. The quasi-optics were simulated using the TICRA Tools software and this chapter elaborates on the output from the simulations. Firstly, the optimisation efforts of each component using Physical Optics (PO) simulations is described. The effect of the instrument structure is then considered by means of Method of Moments (MoM) simulations. The chapter ends with a presentation of the AWS farfield performance. Chapter 4 covers the calibration of the AWS radiometer. It describes the OBCT as well as Onground Calibration Targets (OGCTs), which have been designed and manufactured at the Institute of Applied Physics (IAP). A discussion of their performance concerning return loss and thermal aspects is provided. A conclusion of the thesis and recommendations for further work concerning the AWS quasi-optics is provided in Chapter 5, followed by the three peer-reviewed publications accompanying this thesis in Chapter 6.