Minimum star tracker specifications required to achieve a given attitude accuracy

We describe the principles of the computation of minimum specifications of the optical system and array detector of the star attitude sensor to achieve the required attitude accuracy for the given exposure. The computations assume that the attitude error is influenced only by random noise of diverse origin, whereas all systematic errors have been eliminated. We report the specifications required for trackers meant for different purposes. We show that the errors of existing star trackers exceed the lowest achievable error by one order of magnitude because of unaccounted biases. We discuss the types of these biases and the methods that can be used to take them into account or eliminate them.