Optimisation of Star Tracker Orientation and Attitude Trajectories for Space-To-Ground Optical Communication on CubeSats

Optical communication systems promise a step-change in data return capability for CubeSats but introduce exceptionally demanding pointing requirements. In some cases, sub-milliradian level pointing of the satellite is required to successfully acquire a laser signal from the ground. This is further complicated by the relatively high angular velocities and angular accelerations of the satellite-to-ground target vector during a typical communication pass.
Typically, star trackers are used to achieve the required attitude knowledge for initial acquisition of the laser signal from the ground, or for re-acquisition if the link is lost, due to cloud cover for instance. Therefore, availability of the star tracker throughout the full communication pass is desirable. For a given communication pass, star tracker availability can be maximised in two ways: first by minimising the angular velocity of the tracker, and second by minimising stray light from both the sun and the earth entering the tracker’s field of view.
During a communication pass, the requirement to point the laser boresight at the ground station constrains the attitude of the satellite in two out of three degrees of freedom. The unconstrained third degree of freedom is a rotation around the laser terminal boresight. The focus of the present work is on how this additional rotation may be used to optimise star tracker performance during a communication pass. A method is presented to minimise the angular velocity while avoiding sun and earth exclusion zones of the sensor, thereby maximising attitude estimation accuracy under dynamic conditions. Finally, from a mission design perspective the method above may be applied over the course of a full mission to all predicted communication passes to find the optimal orientation of the star tracker on the satellite. Developed strategies are tested via numerical simulations in MATLAB/Simulink.