Calculation of the Spin-Axis Orientation of the Telstar Satellites from Optical Data

The orientation of a satellite can be calculated if the aspect of two or more known reference vectors can be measured with respect to a body-fixed coordinate frame. On the Telstar satellites one such line may be determined by solar aspect cells, but the other is determined by ground observation of flashes of sunlight reflected from three mirrors mounted on the spinning satellite. The simple geometric considerations are discussed, as are the optimum placement of the mirrors, the amount of data to be used, and an error analysis. Spin-axis orientation data are used as the basis for steering the spin axis by ground command of the current in a coil of wire around the equator of the satellite.