GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

9.1.8 Forward Communication LinkThe forward unit communicates with the aft unit via a pair of unidirectional parallel data busses, one aft-toforwardand one forward-to-aft. Timing signals, including the 34.1 kHz bridge excitation signal is provide viathis link.The GSS aft control unit (ACU) houses the GSS flight processor card, associated support circuitry, and thepower supply for the GSS string. A block diagram of this unit is shown in Figure 9-5. This unit is mounted onthe aft truss of the spacecraft, and thus is subject to wider temperature swings, approximately 265K to 300K9.1.9 RAD6000 ProcessorEach GSS employs a dedicated Lockheed-Martin Federal Systems (now BAE) RAD6000 processor, running at16.368 MHz. On board is 1 MB of EEPROM and 3 MB of static RAM. The processor communicates with thespacecraft via a Mil-Std-1553 serial link and with the GSS electronics via a parallel port on the processor card.This processor hold the gyro suspension system software and telemetry processing needed to receive commandsand send measurements to the ground via the spacecraft processor (CCCA). The telemetry system continuouslyprovides data at 2 to 5 second intervals of gyro position, control effort, suspension voltages applied, system state,analog monitor points, box temperatures, and other engineering information. Also available are high-speedsnapshots consisting of 12 seconds of full rate suspension control loop measurements and commands.9.1.10 Mux and TimingFigure 9-5. Block Diagram of the Aft Suspension Unit (ACU)The aft multiplexer and timing (AMT) card collects analog telemetry monitors from the aft box (power supplyvoltages, temperatures, etc) and generates the suite of timing signals for the GSS string.Gravity Probe B — Post Flight Analysis • Final Report March 2007 261