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

3.2.2.2 Gyro Suspension System RedundancyAn independent Gyro Suspension System (GSS) computer controls each of the gyros. Each GSS computer isconnected to a single gyro, so they cannot be used interchangeably. Each of the GSS computers has two gyrosuspension modes—analog and digital—that provide a form of redundancy in the gyro rotor suspension. Theanalog suspension mode is used primarily as a backup or safe mode for suspending the gyros. The digitalsuspension mode is computer-controlled; it puts less torque on the gyros than analog mode and enables theirposition to be controlled with extremely high precision. A failsafe mechanism, called the arbiter, is hard-codedinto the GSS firmware (programming at the chip level) that automatically switches the suspension system fromdigital to analog mode under certain pre-set conditions.3.2.2.3 Computer RedundancyThe GSS computers comprise four of the eight computers on-board the spacecraft. In addition there are twoflight computers and two SQUID Readout (SRE) computers. The main flight computer and its twin backup arecalled the A-Side (main) and B-Side (backup) CCCA (Command & Control Computer Assembly). Only one ofthese computers is running at any given time. If the A-side computer fails certain safemode tests, the B-sidecomputer automatically takes over, as was the case in March 2005. However, when the backup computer failedthese safemode tests, it was rebooted. We can only switch back to the main computer through manualcommands.3.2.2.4 SQUID Readout RedundancyEach of the twin SRE computers can control all four SQUID readouts, showing the spin axis orientations of allfour gyros. Like the main computers, only one of the SRE computers is running at any given time. When themain CCCA computer automatically switched over to the B-side in March 2005, the SRE computer did notswitch with it. However, in order to synchronize the timing between the B-side CCCA computer and the A-sideSRE computer, we commanded the SRE computer to reboot. Also, to ensure that we remained on the A-sideSRE computer, we disabled the safemode response that automatically switches from the A-side SRE computer tothe B-side SRE computer.3.2.2.5 Telescope Readout RedundancyRedundancy is also built into the Telescope Readout Electronics (TRE). The science telescope has two sets ofdetectors, designated A-side (primary) and B-side (backup). These detector packages, which are about thediameter of a dime, are comprised of pairs of silicon photo diodes that basically count photons from each half ofthe telescope's split beam, in both the X-axis (side-to-side) direction and the Y-axis (top-to-bottom) direction.Beam splitters and mirrors are used to redundantly direct all of these split light beams to the B-side detectors aswell as the A-side detectors. For comparison sake, we collected the data from both the primary and backupdetectors during telemetry communications passes, although we only used the data from one set of detectors tocontrol the spacecraft's pointing direction through the ATC system.3.2.2.6 Attitude Control System RedundancyThe ATC uses the navigation control gyros (called “rate” gyros), star trackers, and magnetometers to control thespacecraft's attitude when the telescope detectors are not controlling the spacecraft's attitude (e.g., the portion ofeach orbit when the spacecraft moves behind the Earth, eclipsed from view of the guide star). The spacecraftcontains two pairs of rate gyros, two star trackers, again designated A-side (primary) and B-side (backup), andtwo navigational magnetometers. The two rate gyros in each pair work together, independently controlling theX-axis and Y-axis position of the spacecraft; the redundant Z-axis position control from one of the gyros is notGravity Probe B — Post Flight Analysis • Final Report March 2007 79