Space Telescope, ST, carrying five astronomy instruments, is a long life spacecraft designed to achieve and maintain ultra fine stabilization jitter performance. The RMS jitter specification for pointing performance is 0.007 arc-sec, 1 sigma, during periods of scientific observation. Spacecraft attitude control it achieved by on-board computer processing of attitude and rate sensor data to generate reaction wheel torque commands. The Space Telescope Antenna Pointing System, APS, is used to position a High Gain Antenna, MGA, for relaying science and engineering data from the spacecraft to the ground station.