Protruding through the starboard wall of the Attitude Control Module are two star trackers, whose sole function is to provide the on-board computer with information pertaining to the spacecraft's orientation. Each star tracker is a fixed solid-state camera which looks at the sky and reports to the on-board computer any stars it has detected and tracked. The computer then compares the positions and magnitudes of the identified stars to a digital catalog of 327 stars, and from that information estimates TOPEX/Poseidon's orientation in space. If this estimation differs from the on-board computer's internal model, corrective commands are issued to the Attitude Control System.
