PCAD: Pointing Control and Aspect Determination
Determines the Chandra aspect solution, i.e. the position pointed to by the Chandra X-ray mirror (HRMA) optical axis. The PCAD has a system of gyroscopes to monitor short-term changes, and a 4.25 inch diameter optical telescope with a CCD imager (for longer term corrections and tying to an astrometric reference frame).
See the Proposers' Observatory Guide (POG) for technical details.