Coupled Control of Reflectivity Modulated Solar Sail for GeoSail Formation Flying

The use of the electrochromic device has been proposed for the attitude control of spacecraft for decades because its optical characteristics can be altered by electrical excitation. The technology was recently employed and verified successfully on Interplanetary Kite Craft Accelerated by Radiation of the Sun, the world's first interplanetary sailcraft. In addition to its ability to control the attitude of a solar sail, the reflectivity control device can also be used directly for the orbit control of the sail. In this study, a new approach for joint active orbit and attitude control of a sail using the reflectivity modulation technology is proposed. The force and torque models of a reflectivity modulated spinning disk sail experienced by solar radiation pressure are discussed, and two reflectivity modulation modes are provided and compared in terms of control performance. One mode, which was used in the Interplanetary Kite Craft Accelerated by Radiation of the Sun mission, is investigated for the active control of solar sails in a GeoSail formation flying application. The coupled attitude and orbit dynamics of the sail in a GeoSail orbit is discussed, and the equation of the relative orbit and attitude motion is given. The control performances of the two modes for GeoSail formation are numerically demonstrated and compared using a typical controller.