Control of a fast steering mirror for laser-based satellite communication

Abstract

MIT Lincoln Laboratory has been contracted by NASA to test and build a platform capable of sending and receiving laser communication signals in space from Mars. The two main components of the pointing system on the spacecraft include an inertial reference frame to provide coarse laser control and a Fast Steering Mirror to remove any spacecraft jitter from the optical path. The optical path in the satellite must have no more that 400 nanoradians RMS motion from 1 Hz to 1 kHz. This thesis focuses on the feedback control of this Fast Steering Mirror (FSM). The feedback on the FSM comes from two different set of sensors. On power up, the FSM's angular position is controlled with feedback from local position sensors (KAMAN eddy current sensors). Optical feedback is accomplished with a laser beam and quad cell optical sensor. The optical sensor has an extremely small range of operation, and the mirror must first be pointed onto the active area of the quad cell before the optical feedback can be activated. This thesis investigates the controller being used for this FSM, the feedback loops for the different sensors and the pointing algorithms used to switch between feedback sensors.

(cont.) The analog control system in use has a crossover frequency of approximately 1 kHz. MIT Lincoln Laboratory and NASA would like to use an FSM with a closed loop bandwidth as high as possible to lower noise restrictions on other parts of the spacecraft. This thesis investigates the FSM dynamics in detail and applies a different control system to push out the bandwidth as far as possible.