Seméli Papadogiannakis, Swedish Defence Research Agency; Torbjörn Sundberg, Swedish Defence Research Agency; Per Hägg, Swedish Defence Research Agency; Hanna Sundberg, Swedish Space Corporation
Keywords: Polarimetry, spectroscopy, characterisation, geostationary, reddening
Abstract:
As space fills with more and more objects, the characterisation and classification of objects becomes increasingly important. Characterization can for example include object identification, operational status, geometry, attitude, and rotation. There are multiple interested actors including military (e.g., threat assessment), political (e.g., treaty compliance) and research (e.g., material science). We present here methods and results for polarisation and spectroscopy characterization of space objects using the Nordic Optical Telescope (NOT).
The NOT is a 2.56 m telescope primarily used for astronomical research, located in the Roque de los Muchachos Observatory, in La Palma, Spain. In collaboration with the staff on site, we observed three geostationary satellites (Intelsat 904, Intelsat 37e and TDRS13) at different local times and phase angles, which we define as the angle between the observatory, the satellite and the Sun.
We performed polarimetry measurements in the V-band with the ALFOSC camera to study the differences in the polarization signature of the three satellites. We show that there is a strong dependency between the degree of polarisation and the phase angle as much as 25% and that this angle variation is characteristic for each satellite.
We have also obtained spectra for the two Intelsat satellites at different times and phase angles, in order to study if the exposure to the space environment has caused a reddening of the satellite spectrum, as conjectured in the literature. The satellites chosen have been built around the same bus, but with a difference in 15 years of space exposure. We used the ALFOSC grism #4 for these observations, which covers a wavelength range of 2300-9700 Å.
Unlike previously reported, we find that the position of the peak and the shape of the spectrum vary more with the time of the observation than between satellites, which suggest that a single spectrum may not be sufficient for characterisation of unknown satellites. We also report rapid spectroscopical changes, up to a 20% decrease in flux in the near-infrared I band in two minutes, under good weather conditions, consistent with previous findings.
The measurements were repeated on several occasions in order to ensure consistency, and reduced according to standard procedure in astronomy. The raw images were bias-subtracted and corrected for gain. For the polarimetry, we performed aperture photometry relative to the background on eight equally-spaced angles with a calcite birefringent filter. The calibration was verified with respect to unpolarised standard stars, and the spectra was compared with Sun analogues.
The observed difference in polarisation between the satellites suggests that polarization could be used as a cost-effective means for non-resolved object characterization. Since the polarisation directly depends on the surface of the satellite, the variation in polarization could also be used together with modelling to determine information about the configuration of the solar panels and antennas. Polarimetry requires relatively short exposure times, which makes them relatively cheap, accessible to low-end instrumentation, and viable for large-scale studies. This is especially important to reach a better understanding of object characterisation in the ever more crowded space environment, as large number of objects may need to be characterised.
Further work could expand the measurements to a larger number of satellites and include modelling of the object geometry to see how it is linked to the polarization. It is encouraging that the polarimetry signatures are very distinct. Although the satellites in the spectroscopy comparison have similarities, they are not identical, which would have been optimal for a proper study of the reddening effect. Additional confounding factors may also include cloud coverage and differences in the solar panel composition, which makes it difficult to draw firm conclusions for the origin of the spectral differences.
Date of Conference: September 27-20, 2022
Track: Non-Resolved Object Characterization