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

5.1 The Spacecraft’s TaleWe begin this chapter with a GP-B Mission News story entitled “The Spacecraft’s Tale,” that was featured in ourWeekly Status Update, posted on our GP-B Web site and sent out to our email subscriber list on December 10,2004, following an anomaly with the spacecraft’s GPS system that occurred over the South Atlantic Anomalyregion of the Earth.Last Saturday began as a rather “ho-hum” California winter’s day. Orbiting the Earth every 97.5 minutes, theGP-B spacecraft passed directly over California around 6:30AM PST, but the Sun was already up, and the skywas too bright to see the satellite. In the GP-B Mission Operations Center (MOC), a skeleton crew, consistingof the on-duty Mission and Flight directors and one or two resident engineers, monitored several telemetrypasses (communications sessions) during the morning hours. Most were 25-minute satellite passes, duringwhich the spacecraft relays status information to the MOC through the NASA TDRS (Tracking and Data RelaySatellite) communications satellite system. And, during a 12-minute ground pass at 1:15PM PST, thespacecraft’s solid-state recorder relayed relativity data to the GP-B science database through a high-speedtelemetry connection with the Svalbard ground tracking station in Spitsbergen, Norway. All in all, it was anormal Saturday, and the atmosphere in the MOC was quite relaxed.Following the successful ground pass with Svalbard, the spacecraft continued on its southward route. Ataround 1:30 PM PST, Pacific time, the spacecraft was flying over South America—heading towards the SouthPole—when it entered the South Atlantic Anomaly (SAA). This is a region above the Earth where the fluxes oftrapped protons and other particles, emitted by the Sun, are much greater than anywhere else on Earth, due tothe asymmetry of the Earth’s protective Van Allen Radiation Belts. Thus, spacecraft are more vulnerable tobeing struck by protons when flying through this region.At 1:48PM, Pacific Time, an odd event silently occurred on-board the spacecraft, triggering four safemodes(pre-programmed command sequences designed to automatically place the spacecraft, its gyros, telescope, andother systems and instruments, in a stable and safe configuration in response to anomalous or out-of-limitsfeedback from various on-board sensors).Back in the MOC, the next telemetry pass was not scheduled until 3:16 PM, so the operations staff wascompletely unaware of this change in the spacecraft’s condition—for the time being. At 3:15PM, the MOC staffsettled into their seats for the upcoming satellite status telemetry pass. As the spacecraft’s antenna locked intothe TDRSS satellite and began transmitting, one-by-one, status monitors around the MOC began turning red,signaling the spacecraft had triggered its safemodes. There is a problem on-board.During the next 20 minutes, phones rang, pagers beeped, and soon, the MOC was teaming with activity. Anassessment of the safemodes that were triggered indicated that an error—never seen before--had occurred in amodule of the Attitude and Translation Control (ATC) computer system. The spacecraft’s GPS had registeredan off-the-scale velocity spike, which if correct, indicated that, for one brief moment, the spacecraft hadtraveled faster than the speed of light—or to use Star Trek terminology, it had “warped into hyperspace.” Infact, the GPS system had reported a single data point with an erroneously high velocity, which when squared,caused a computer overflow. The ATC computer module took exception to this data overflow and triggered asafemode test, which in turn activated a chain reaction response sequence.The MOC staff immediately scheduled several extra satellite communication passes so they couldcommunicate with the spacecraft more frequently. Then, over the ensuing 24 hours, they methodically workedthrough a series of tests and command sequences to return the spacecraft to its normal science operationmode. We initially assumed that the GPS receiver had suffered a proton hit in the SAA region, but furtheranalysis suggests that this was not the case. Rather, this anomaly was apparently caused by one of the fouraccessible GPS satellites being in the wrong position for proper GPS triangulation. The ATC system usuallycatches situations of this kind and disallows the data; but, this one was out of range before the data reached thefilter.The spacecraft has returned to normal operations. This incident was not detrimental to the GP-B experimentaldata. And, once again, the fact that an anomalous event occurred while the spacecraft was flying through theSAA region appears to be a coincidence—or is it?Gravity Probe B — Post Flight Analysis • Final Report March 2007 125