Artist's concept of sailors navigating by the North star.

Most spacecraft steer by the stars. Or, to make sure they are still on course and pointed in the right direction, they may periodically check the position of the stars. So, what is a star camera——often referred to as a star tracker——and how does it work? A star camera or star tracker is a "celestial reference" device that recognizes star patterns, such as constellations.

Star patterns, and even single stars, are very helpful for navigation. In ancient times, sailors navigated by the North Star——it was their reference point. By looking at it they could tell if they were on course. This is similar to checking the position of our sun, which rises in the east and sets in the west, and being able to tell in which direction we are heading. The sun gives us point of reference——a starting point that we refer to and compare our position to as we travel.

To recognize star patterns, Compass uses an active pixel sensor (APS) in a wide-field-of-view (WFOV) miniaturized star camera. APS is a new type of compact imaging device with an array of photosensors. APS uses a fraction of the power used by standard charged coupled devices, enabling a major reduction in the power, size, weight, and cost of imaging and spectroscopy instruments. This makes APS very attractive for use on small, low power spacecraft and instruments.

The Compass WFOV star camera will observe a wider slice of sky, taking pictures of the star patterns in its view. And like a star tracker, the star camera will locate the positions of stars and report them to the spacecraft. The captured images will then be compared to a celestial map that resides in the spacecraft computer memory.

The Compass star camera's sensor will track both bright and dim objects in its field of view. It will also prevent extremely bright objects, such as the moon or Earth, from spilling over into the pixels of captured objects.

Compass star camera sends data to the gyroscopes, which can hold stable for just a short time, every few seconds. This helps to keep the gyroscopes accurate. Together, the star camera and gyroscopes keep the spacecraft stable and oriented in the right direction in space.