An optical device used on a space shuttle to align the navigational inertia measurement unit (IMU) to within 1.4°, enabling the star tracker to align the IMU more correctly. COAS has two crossing lines like a ‘+’. The device is normally mounted in the aft flight station, and the shuttle is manoeuvred manually by a flight crew member until a selected star is in the centre of the crossing lines. Two known stars are sighted and the shuttle's computer software determines the coordinates that can realign the IMU to within the required 1.4°.

COAS is also used to visually track targets, for instance during close or rendezvous operations. During the shuttle's ascent and deorbit thrusting periods, the device is mounted at the commander's station to display the shuttle's proper attitude orientation (see attitude control).