Onboard Attitude Error Computation for Automatic Spin Axis Control of Bhaskara

The Indian experimental remote sensing satellite, Bhaskara, uses two horizon crossing sensors mounted at 45?? and 135?? with respect to spin axis for onboard roll error computation and automatic control of the spin axis orientation within 3?? of the orbit normal. A new method for roll error computation and control and digital hardware for the same are described. The system has flexibility to vary the threshold, to adapt to altitude variations and various mission requirements, and to work with a single horizon sensor. Flight performance of the system is satisfactory.