Observations of the Spin-Period Variations of Inactive Box-Wing Geosynchronous Satellites

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Observations of the Spin-Period Variations of Inactive Box-Wing Geosynchronous Satellites

Earl, Michael A. / Wade, Gregg A.

Ground-based broadband photometric observations of four inactive geosynchronous satellites of “box-wing” design were frequently performed between March 2012 and December 2013 to commence a long-term study of their attitude dynamics. The brightness of the satellites was observed to vary in a periodic fashion, which was interpreted to mean that reflected sunlight was being modulated due to satellite spin. The average observed spin periods inferred from the light curves ranged from 158 to 1548 s. The variation of each satellite’s inferred spin period was observed to be small or negligible over hourly timescales but varied significantly (from 15 to 25% of the average) and smoothly (possibly cyclically) over monthly to yearly timescales. The characteristics of the observed spin-period variations, including the amplitudes, timescales, and shapes, differed greatly between satellites and suggest a relationship between the average observed spin period and the variation amplitude. The observed spin-period variations were interpreted as being due to one or more external disturbance torques acting on the spacecraft. The most significant torque was found to be solar radiation pressure acting on the large-area solar panels. The magnitude of this torque produced sufficient angular accelerations to explain the observed spin-period variations. A first-order phenomenological model is proposed to explain the high-level aspects of the observed phenomena.