Jasper Hudson, Macquarie University; Sarah Caddy, University of Melbourne; Lee Robert Spitler, Macquarie University; Zach Gazak, SSC/SYD 85/BI A&AS; Anthony Belo, Optus Satellite; Ryan Swindle, USSF/SSC; Erik Small, ENS Consulting; Justin Fletcher, Space Systems Command A&AS
https://doi.org/10.64861/ZVLO1145
Keywords: Huntsman telescope, multi-spectral, imaging, GEO, spectral positive identification
Abstract:
Ground-based optical observations are a key source of space domain awareness data, but operational use commonly focuses on astrometry and single-band brightness. Multi-band measurements may provide additional information about object identity, configuration and surface contributions, but their interpretation is complicated by geometry, surface materials, solar-panel orientation, and wavelength-dependent reflectance. For geosynchronous satellites, where repeated observations can be obtained over changing solar phase angle, multi-band photometry may provide an additional signature for distinguishing objects and interpreting changes in apparent brightness._x000D_
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This work investigates whether repeated two-band observations of GEO satellites provide stable and distinguishable optical colour signatures. Using the Huntsman Telescope, we observe a selected sample of active GEO satellites in the Sloan g and r filters. The study addresses whether satellites exhibit repeated g-r behaviour, whether satellites from the same fleet or spacecraft family occupy similar colour ranges or trajectories, and whether within-night colour changes are consistent with changes in solar phase angle or the relative contribution of visible spacecraft surfaces. Rather than assuming two-band colour uniquely identifies materials or attitude, we test when g−r provides useful information beyond single-band brightness._x000D_
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Observations are processed using the SENPAI satellite-imaging pipeline; Candidate detections are associated with catalogued objects using Two-Line Element data, which are used to calculate solar phase angle, range and Sun-object-observer geometry. Calibrated Sloan g and r magnitudes are derived using field stars, and paired measurements are combined to produce g-r colour indices. Photometric reliability is assessed using catalog-star residuals, zero-point uncertainty, filter-pairing time offsets, and comparisons between rate-frame and sidereal-frame calibration. The observational analysis is supported by the lumos-sat optical brightness model. The model uses simplified spacecraft geometries and wavelength-dependent reflectance assumptions to test whether changes in solar panel orientation or visible surface contribution could produce colour variations comparable to those observed. These experiments provide a physical basis for interpreting trends while retaining the degeneracies between attitude and material properties._x000D_
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The analysis will provide an observational assessment of the usefulness and limitations of Sloan g-r photometry for GEO characterisation. We will quantify the repeatability of colour measurements for individual objects, the separability of selected satellites and fleets, and the minimum colour variation that can be distinguished from calibration and filter-pairing uncertainty. For SSA/SDA, this work provides a practical test of whether calibrated two-band photometry can contribute an additional signature for object discrimination and monitoring without requiring resolved imaging or full spectroscopy. The results will identify when colour differences are repeatable, when they are dominated by geometry, and what calibration quality is required before small colour changes can be treated as physically meaningful._x000D_
Date of Conference: September 15-18, 2026
Track: Satellite Characterization