Pyramid Coarse Sun Sensing for NASA SSTI "Clark" Safe-Hold Mode

This paper describes an innovative approach to solving the Sun-pointing problem using a pyramid configuration of Coarse Sun Sensor (CSS) cells. In safe-hold mode, the Small Spacecraft Technology Initiative (SSTI) Clark spacecraft must keep its solar arrays towards the Sun using only CSS cells to sense the Sun. Unfortunately, Earth albedo degrades the accuracy of the Sun vector from these cells. Two CSS configurations are considered. A traditional configuration has cells mounted flat on each spacecraft face, with their normals along the spacecraft body axes. An alternative configuration has cells on the face of a shielded pyramid as well as the flat-mounted cells. A simulation is used to evaluate the safe-hold pointing performance with these two configurations. The CSS models generate illumination values while the CSS algorithms use these values to build a sensed Sun vector. The sensor models approximate illumination from the Earth albedo as well as from the Sun. The flatmounted configuration is compared with the pyramid (plus flat-mounted) configuration of CSS cells. Although the flat-mounted configuration satisfies the Clark requirements, the pyramid configuration reduces Sun pointing safe-hold performance errors by 50%. Moreover, the pyramid CSS configuration is robust to errors from the widely varying Earth albedo.