title: Characterization and calibration of the AtmoLITE satellite instrument for the INSPIRESat-4 mission
Creators: name: Gong, Qiucheng
Publisher: name: Bergische Universität Wuppertal
Year: 2022

description: Temperature measurements in the mesosphere and lower thermosphere (MLT) region play an important role in understanding atmospheric photochemical and dynamical processes. A miniaturized limb sounder aboard the atmospheric coupling and dynamics explorer INSPIRESat-4 is developed by our research team to enable temperature detection in the MLT region by observing the oxygen atmospheric band (O<sub>2</sub> A-band) day and night airglow emissions near 762 nm. Temperature is obtained from the rotational structure of the emission band, which follows a Boltzmann distribution. The intensity ratios of different emission lines at different temperatures are used to retrieve atmospheric temperature.<br> The International Satellite Program in Research and Education (INSPIRE) is a worldwide consortium of universities and research institutions formed to advance space science and engineering. INSPIRESat-4 is the fourth satellite in the INSPIRE series of satellites. Atmospheric limb interferometer for temperature exploration (AtmoLITE) is one of the payloads carried by the INSPIRESat-4 mission.<br> AtmoLITE uses a spatial heterodyne spectrometer (SHS) which is a monolithic small bandpass high-resolution interferometer. The characterization of this instrument is the main subject of this work, which can be divided into three phases: before the SHS glueing for the AtmoLITE instrument, during and after the SHS glueing for the AtmoLITE instrument, and after finalising the integration of the SHS system. At each phase, corresponding measurement strategies and alignment procedures are presented.<br> Before the SHS glueing for the AtmoLITE instrument, wavefront error measurements are performed on the different SHS front and camera optics samples for the AtmoLITE instrument's qualification model (QM) and flight model (FM). Quantitative analysis of the wavefront errors for the different optics samples is carried out to verify that the assembled optics samples meet the design requirements.<br> During and after the SHS glueing for the AtmoLITE instrument, a pinhole light source setup is built to adjust the last lens of the front optics to find the optimum position with the smallest image point. The alignment result of the front optics shows that the measured spot size of 10 pixels matches the simulated result of 8 pixels with the alignment uncertainty about 2 pixels, which indicates that the alignment procedures for the front optics are correct and appropriate. After moving the vacuum compensator of the front optics, the measured spot size is two to three times larger than the simulated spot size. This will reduce our spatial resolution by a factor of about two or three. Meanwhile, the variation of the spot size at different temperatures is investigated. The results show that the spot size imaged by the pinhole setup increases as the temperature decreases. This suggests that due to the use of aluminium as an optomechanical material, the effect of its thermal expansion on the optical performance of the instrument should be further investigated for further missions.<br> After finalising the integration of the SHS system, a series of interferogram measurements have been performed at different wavelengths and temperatures in the vacuum environment, and corresponding calibration procedures are discussed which mainly include the dark and bias current subtraction, spike correction, removal of the DC and low frequency components, Littrow wavelength calibration, phase distortion correction and amplitude variation estimation. The Littrow calibration results show that there is a linear trend of temperature dependence. The expected thermal drift value for the Littrow wavenumber should be less than 1 cm<sup>-1</sup> over the nominal operating temperature range, but the measured thermal drift of the Littrow wavenumber is up to 6 cm<sup>-1</sup>. This means that the actual Littrow wavelength in the in-orbit measurements will shift towards the shorter wavelengths, resulting in the spectral overlap. Therefore, the ghost line removal needs to be considered for the in-orbit measurements. The calibrated results show that the validity of the calibration method and the final amplitude calibration matrix is obtained, which shows that the temperature dependence of the amplitude variation is almost negligible.