What is a Star Tracker?

Star trackers or star sensors are optical devices that are used by satellites as a navigational tool to determine their orientation/attitude to the stars. The positions of stars are measured using a camera, photocells, and other onboard processing systems which identify the images and process the measured position in the reference frame of the spacecraft. 

There are around 50-60 navigational stars that have been mapped by astronomers and are primarily used to determine the satellite’s position. Star trackers can scan the starscape to pick out stars and constellations and compare them with these pre-defined navigational stars. But for complex missions such as the Mars mission, the entire starfield database has to be referenced to determine the orientation of a spacecraft.

Star trackers are primarily used for the attitude and orbit control systems of spacecraft but they have temporary outages due to interference effects of light reflecting from the satellite’s surfaces or exhaust plumes during propulsion sequences or by strong light from sources such as the sun, or the sun’s reflection off parts of the spacecraft. Therefore to reduce these effects, star trackers work in tandem with gyroscopes, as the latter provides temporary, and attitude information during star tracker outages. Sometimes two or more star trackers are used for redundancy or to increase the accuracy so that a spacecraft will have more accurate knowledge of its position.