The IPS was designed to provide all necessary target acquisition and pointing control for the Astro telescopes, supplemented by the HUT acquisition TV camera and the ASTROS Star Tracker (AST) mounted on UIT. Three fixed-head star trackers (FHST's) comprise the optical sensor package (OSP) of the IPS, one bore-sighted with the telescopes, and the other two skewed at 12 angles to either side. Gyros provide information on short-term motions and preserve knowledge of the pointing direction during slews and other intervals when the OSP is not tracking stars. Once the gyro drift rates and the tracker orientations and sensitivities are calibrated in orbit and the IPS is pointed at a star with known coordinates, it knows in principle where it is pointed at all times. Knowledge of the pointing direction can be updated at every target acquisition.
After OSP calibration (OSPCAL), the IPS pointing direction is initialized with a procedure called Identification Initial, or ``IDIN''. This requires the bore-sight tracker to be pointed at a bright, isolated star. The subsequent stellar acquisitions used for routine observations use a procedure called Identification Operational, or ``IDOP''. IDOP's use the brightness, coordinate location, and separation angle of pre-selected guide stars identified in two or more of the star trackers to determine the IPS pointing direction. After a successful IDOP, the IPS adjusts the pointing direction to match the desired coordinates and enters optical hold by tracking the identified guide stars. Successful IDOP's should place the desired target within several arc seconds of the center of the HUT field of view. In optical hold, the IPS jitter about the current pointing direction is typically less than 1 rms in radius.
Once the IPS has completed a successful IDOP, the PS or MS identifies the target to be observed and centers it in the HUT aperture, using either a bias command if HUT has successfully identified the target or the manual pointing controller if it has not. The IPS knowledge of the pointing direction can then updated, and the observation can proceed. Since the IPS is already in optical hold, it provides stable pointing control for the remainder of the observation.