Abstract The attitude control, pointing control, and navigation systems of future advanced spacecraft will be characterized by a high degree of autonomy, very high accuracy, efficient commandability, and fast fault recovery. These characteristics are incompatible with the constraints of conventional star trackers which mandate a-priori definition and careful preparation of each on-board attitude fix, and only work if attitude uncertainties are small. With the availability of a highly-efficient, non-iterative star pattern recognition algorithm, accurate CCD star trackers, fast microprocessors, and high density memory chips, it is now feasible to build an Autonomous Star Tracker (AST) capable of determining its attitude rapidly and reliably without having any a-priori attitude knowledge. The Lockheed Palo Alto Research Laboratory is developing an AST prototype that will use the true sky to demonstrate its capabilities. In addition to being capable of attitude acquisition, the AST is designed to also perform autonomous attitude updating and to provide attitude information continuously. The paper describes a number of functions that are enabled or enhanced by the AST, including: gyroless/cheap-gyro attitude control, attitude safing, fast fault recovery, autonomous acquisition of celestial targets by space-borne astronomy telescopes, autonomous optical navigation, precision pointing to terrestrial targets, and uncalibrated attitude acquisition. In addition, an overview of the star pattern recognition algorithm is provided, a thorough, computer simulation program using a 248, 516 star catalog is described, and simulation results are presented. These simulations show that an anti-blooming capable AST with an 11.3 degree FOV diameter, a 4148 guide star database, and a MC68030 class microprocessor perfonns an attitude acquisition in 0.6 seconds with a success rate of 99.25%. Raising this number to 100% should be achieved easily by selecting the guide stars more carefully.