Spectro-polarimetry of HAbitable Planet Earth on Chandrayaan-3: instrument characteristics, calibration, and onboard performance

Abstract

The orbiter of the Chandrayaan-3 mission of the Indian Space Research Organisation carries an experimental payload called Spectro-polarimetry of HAbitable Planet Earth (SHAPE). This payload makes disk-integrated observations of Earth as an exoplanet, from the Moon as well as from the high-altitude Earth orbit. The instrument consists of an acousto-optic-tunable-filter-based near-infrared spectro-polarimeter making the measurements in the two orthogonally polarized directions using a pair of indium–gallium–arsenide detectors. The laboratory characterization of the flight model of the instrument included a relative measurement of the response of the two channels of the instrument. The field measurements of the instrument revealed a non-uniformity in the field response. In light of non-identical response of the two channels and the non-uniformity of the field response, a theoretical model of such a polarimeter is developed to gain insights into the performance of a polarimeter. Analysis of lunar observations obtained within the SHAPE field of view indicates that the transmission ratio between the two polarized channels lies in the range 0.8 to 1.2. Such deviations from unity can introduce offsets in the measured polarization of up to ∼10%. The suitability of SHAPE for studying the band polarization, defined as the relative polarization within a spectral absorption band, is studied. Using the Moon observations and the theoretical instrumental model, the suitability of measuring the relative band polarization is demonstrated. A study of systematic biases in the band polarization demonstrates a maximum offset of less than <1% in the measured value of the band polarization. The initial results of Earth observations, spectra and flux measurements across phase angles, and the methodology for retrieving the band polarization from these observations are also discussed.