Imaging Polarimetry of Geostationary Satellite Express-AM5

Abstract

This paper reports on the optical imaging polarimetry of geostationary satellite Express-AM5. Precise optical polarimetry was carried out using the Nayuta 2-m telescope at Nishi-Harima Astronomical Observatory and a polarimeter. V -band images were continuously taken in 312 minutes. The degree of polarization decreased with time until the minimum phase angle was reached at midnight, and then, it increased until morning. This change in the degree of polarization is well explained by the reflection of sunlight from monocrystalline solar panels of the satellite and possibly from other satellite components such as the antenna dishes and/or bus.

Short timescale imaging polarimetry of geostationary satellite Thor-6: The nature of micro-glints

“…On top of this long-timescale trend only some low-amplitude wiggles may be present, limited to low polarimetric amplitudes ( 0.3%) and short timescales (tens of seconds at most) on top of the general trend. The slowly increasing polarisation and the slow evolution of the polarisation angle in the MOPTOP interval is reminiscent of the polarisation curve of the geostationary satellite Express-AM5, presented by Kosaka et al (2020) (their Figure 2), which had very similar P lin and slowly varying polarisation angle at the phase angle of the MOPTOP observations of Fig 4).…”

“…It is important to point out that the Express-AM5 satellite has a different platform from Thor-6, though shares many of the main features, e.g a box-like bus, large antennas and large extended wing-like solar panels. We consider it likely that the longer timescale trend in the MOPTOP polarisation data of Thor-6 is similarly caused by reflection of the solar panels, using the same arguments as made by Kosaka et al (2020).…”

“…For a satellite in low-Earth orbit we expect similar polarimetric behaviours (after correcting for orbit orientation differences: geostationary satellites are located in a fairly narrow equatorial belt, whereas low earth orbit satellites cover a wide range of inclinations), with the key difference that they traverse the range of solar phase angles over a much shorter timespan, compressing the relevant timescales, which makes obtaining diagnostic data more challenging. Kosaka et al (2020) observed geostationary satellite Express-AM5 using a much larger telescope, the 2m Nayuta telescope, and a polarimeter, for ∼ 5 hours at a cadence of 90 seconds, forming one of the highest quality and highest cadence polarimetric datasets of a geostationary satellite to date. In their data, they see a minimum in the optical polarisation (P lin ∼ 1%) around the time of the minimum phase angle, with rapidly increasing linear polarisation after the minimum phase angle (with values increasing up to P lin ∼ 14% in the phase angle interval covered by their observations).…”

“…In their data, they see a minimum in the optical polarisation (P lin ∼ 1%) around the time of the minimum phase angle, with rapidly increasing linear polarisation after the minimum phase angle (with values increasing up to P lin ∼ 14% in the phase angle interval covered by their observations). The measurements from Kosaka et al (2020) provide full Stokes Q, U, I (see Section 4) and are calibrated onto the absolute polarisation degree and angle values system (IAU, 1973, where polarisation angle towards North is 0 • and East is 90 • ). However, at 90 second cadence, rotation of the satellite with respect to the observer can be significant and cause artefacts in the data; observations at higher time resolution are needed to avoid these.…”

“…However, single wavelength, lower cadence polarimetry datasets are an important first step, to identify the main satellite components responsible for the observed polarisation (e.g. Speicher et al, 2015;Beamer et al, 2018;Kosaka et al, 2020), to provide an inventory of empirical polarisation behaviour for a variety of satellite platforms (e.g. Speicher et al, 2015) and to postulate a sensible range of priors for more quantitative fitting methods.…”