Minimization of Solar Radiation Pressure Effects for Gravity-Gradient Stabilized Satellites

Abstract

The considerations necessary for minimization of solar radiation pressure effects for gravity-gradient stabilized vehicles are presented here. Owing to the rather weak restoring forces available for gravity-gradient stabilized vehicles, solar pressure torques represent a prime source of attitude errors unless steps are taken to minimize their effects. The solar torque minimization procedure generally consists of four distinct steps for a given vehicle configuration: (a) Derivation of the solar torque expressions for the characteristic vehicle configuration, including such effects as diffuse reflection, multiple reflections, and so on; (b) identification of the relative contribution of the solar torques on the various surfaces, and facilitation of solar torque minimization by balancing torque contributions of similar time variation and opposite sign against one another; (c) minimization of the torque about the vehicle axis with the weakest restoring torque (usually the local vertical) via optimization of reflectance characteristics and other physical parameters (using a steepest descent or similar approach); and (d) determination of the vehicle attitude response for the nominal configuration and reflectances, suggesting any configurational changes which might reduce peak attitude errors if necessary. The minimization procedure is performed in this paper using the NASA / Hughes Applications Technology Satellite (ATS) as a prime example of a gravity-gradient-stabilized satellite in an environment where solar pressure is the predominant external disturbance. The application of the solar balancing techniques to the ATS configuration resulted in peak yaw torques of less than 1 dyne-cm for the synchronous altitude satellite, and corresponding peak attitude errors of less than 1 deg in all axes due to solar pressure torques. Although the torque minimization procedures presented here are applicable in the general sense, the application of the techniques to a specific configuration requires derivation of the solar torque expressions for that particular configuration; therefore, the torque minimization example for the NASA/Hughes ATS vehicle can serve as a guide for other configuration applications.