Star tracker

(navigation) An automatic sextant which has the ability to sight on and continuously to track selected stars throughout the day and night, providing continuous heading and position data with no intervention on the part of the airman. Also known as star tracker.

A device that automatically measures the angular separation of stellar observations with respect to a reference platform. It is also referred to as an astrotracker.

By using the star tracker in conjunction with a precise time reference (chronometer) and a dead-reckoning device consisting of gyroscopes and accelerometers (inertial navigator), a digital computer can correct many of the inertial navigator errors so that precise, autonomous (free from any radio position aids), terrestrial navigation can be achieved. The major errors corrected by the star tracker are introduced by the inertial navigator's gyroscopes that result in attitude deviations. In this configuration, called a stellar inertial system, very high precision aircraft autonomous navigation is achieved. Since the availability of radio position aids poses no problem for commercial aircraft, stellar inertial navigation technology is applied only on military vehicles. These navigation devices are used when radio position aids, such as Loran and the Global Positioning System (GPS), may be unavailable. See also Chronometer; Electronic warfare; Inertial guidance system.

Star trackers are also used for both military and nonmilitary applications on space probes, space-based interceptors, and satellites. In these applications the precise attitude capabilities of these devices provide the fiducial precision reference for pointing of the vehicle and Earth or planet sensors. On space missions, star trackers are the only sensor presently available that can provide arc-second attitude accuracy. See also Space navigation and guidance.

A gimbaled stellar inertial system is mounted on the stable element of an inertial navigator. The tracking system measures the telescope azimuth rotation angle and elevation angle to the acquired star with respect to a stable inertial reference. The tracker is programmed to observe different, widely angular separated stars in order to achieve accurate operation. The deviation of the measured stellar observations from their ideal stellar positions is utilized to enhance the performance of the inertial navigator.

The preponderance of modern systems do not contain gimbals. These systems are completely strapped down (that is, with no moving parts) and observe stars at random as the stars come into the rigid, fixed star tracker's field of view. These systems, based on their knowledge of the telescope's location and direction, know which stars should be in the field of view and where their images are located on the star tracker's optical detector. In order to make a three-axis attitude correction, there should be a least two stellar observations ideally separated by 90°. Smaller angular separations lead to a dilution of attitude correction precision. The multiple measurements need not be performed simultaneously since the star tracker corrects the attitude of each axis based on the stellar observations available. Thus, these strapdown startracker systems consist of either multiple telescopes with moderate fields of view or a single telescope with a very wide field of view for proper operation. See also Celestial navigation.

The preponderance of star-tracker photodetectors used to measure the stellar irradiance are solid-state, silicon semiconductor photosensors. These devices have their peak responsivity in the near-infrared region (0.6–1.0 micrometer). Imaging focal-plane arrays have up to a million photosensors on a single semiconductor silicon chip arranged in a rectangular grid matrix. Each of the individual photosensors on the focal-plane array is called a pixel. The devices are similar to the photosensors found in home video-camera recorders. See also Charge-coupled devices.