The design of the digital control system for the DMSP spacecraft

The Defense Meteorological Satellite Program (DMSP) Block 5D Attitude Control works in conjunction with an on-board general-purpose computer used primarily for precision payload pointing and the integration of ascent and on-orbit equipment. The computer is reprogrammable and has multiple level interrupts, hardware double-precision arithmetics providing the necessary word size, and CMOS circuitry supplying low power consumption. Attitude determination is provided to an accuracy of 0.01 deg 3 sigma/axis by star mapper updates of a strap-down, gas-bearing inertial-reference gyro set and with a catalog of stars 4.3 magnitude or greater. The gyros use a digitally encoded analog rebalance loop with a low-pass filter with a cutoff at the sampling frequency. After orbit position is calculated from the ephemeris to derive an error signal, attitude control software generates a torque command to reaction control wheels, which are unloaded by magnetic air core coils. Component reliability is provided by redundant components and data channels which are commandable from the computer.