Spectropolarimetry for Satellite Identification

Abstract

The exponentially increasing number of Earth-orbiting satellites, as well as the pronounced importance for space military operations, impose new challenges on a functional Space Domain Awareness (SDA). A key factor for an effective SDA is the successful identification of an Earthorbiting satellite, or object in general. Current methods are usually limited to either time-resolved photometry, spectrometry, or polarimetry. In this thesis, we present the Spectropolarimeter for Satellite Identification (SSI), whose design combines these detection methods with the aim to provide a unique ‘fingerprint’ of the satellite, thereby expanding the range of satellite identification methods. The SSI combines a dual-channel spectrometer with off-the-shelf polarization optics. Using a combination of an achromatic quarter-wave retarder, a highly chromatic multiple order retarder, a polarizing beam splitter, and two linear polarizers, all operating in the visible spectrum, a sinusoidal modulation of the linear polarization information is imprinted onto the measured spectrum. The amplitude of the signal scales with the degree of linear polarization and the phase with the angle of linear polarization. Furthermore, by using a dual-channel setup, the intensity spectrum can be obtained for the full spectrograph resolution of 9 nm. We introduce and verify the retrieval algorithm to attain the linear polarization information with an approximate resolution of 135 nm due to the modulation approach. Differential transmission between both channels is corrected by a proposed iterative transmission correction which is verified to have an effect of ≤ 1% on the accuracy of the retrieved linear polarization information. This thesis concentrates on geostationary satellites as observation targets, making use of the stable observation geometry to simplify calibration and analysis. We reason that the phase angle has a strong influence on the retrieved spectropolarimetric data and received photon flux. A thorough photon budget estimation is conducted with the conclusion that we are able to observe faint geostationary satellites with an apparent visual magnitude of 10, especially if the observed phase angle is kept minimal. An analysis of the most probable errors is conducted. We identify the convolution of the instrument response function with the modulated signal and the induced polarization by the telescope and instrument as the most severe ones. Laboratory wavelength and polarimetric calibration are conducted. Using a double glass plate setup to induce low levels of degree of linear polarization we performed laboratory validation measurements. We determined the overall laboratory polarimetric accuracy to be ≤ 1.2%. Additionally, we are able to distinguish signals ≥ 1.3 %. Preliminary observations of three bright stars are conducted. We are able to retrieve instrumentinduced polarization between 1% and 2.5% which is in the expected range. We conclude that there has been a misalignment between the telescope and instrument which causes a large discrepancy between expected and received photons and propose future measures to resolve the issue. The proof of concept to use linear channelled spectropolarimetry for satellite identification with the SSI will open up the way for further technical refinement of the instrument, an extension to low Earth orbit satellites, and the possible addition of phase angle resolved spectropolarimetric measurements to further improve the concept of a satellite‘s spectropolarimetric ‘fingerprint’. Our work is part of a collaboration between Leiden observatory and Delft University of Technology commissioned by the Royal Netherlands Air Force. Access to the telescope and support with the alignment activities were kindly provided by TNO.