MSX ATTITUDE DETERMINATION AND CONTROL HARDWARE

Although APL has built over 50 satellites of various types since 1960, the Midcourse Space Experiment (MSX) satellite requires the most comp lex and demanding attitude control system of all. Precise pointing abi lity is combined with substantial agility. The system must be able to point in the direction of a test ballistic missile launch, follow the predicted missile trajectory (as the satellite proceeds in its own Kep lerian orbit), and then receive closed-loop pointing instructions from the MSX optical and radio frequency sensors to follow the target as i t proceeds on its ballistic trajectory. To meet this challenge, the MS X attitude control hardware is the most advanced in the industry. The reaction wheels are custom-designed to provide twice the torque capabi lity of the Hubble Space Telescope wheels. A unique computer system in tegrates the attitude sensor data, provides solutions to the control l aws with data from gyros and wheels, and directs torque commands to th e four wheels every 0.5 s. This powerful onboard computational capabil ity could not: have been achieved 10 years ago.