Hi,
Here is the Gravity Probe B update for Dec 10, 2004.
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GRAVITY PROBE B MISSION UPDATE FOR 10 DECEMBER 2004
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Now, almost 34 weeks in orbit, the GP-B spacecraft is in fine health, with all subsystems continuing to perform well. The spacecraft, which is in the middle of a 6-week un-eclipsed (full sun) period, is flying drag-free around gyro #3, maintaining a constant roll rate of 0.7742 rpm (77.5 seconds per revolution.) The temperature inside the Dewar is holding steady at just under 1.82 kelvin. All four gyros are digitally suspended in science mode. We have been collecting relativity data for 15 weeks. The data collection process is continuing to proceed smoothly, and the quality of the data continues to be excellent.
For much of the month of December, our Mission Planning staff has scheduled minimal spacecraft activity, outside routine monitoring of spacecraft functionality and downloading of science data. However, lest we become complacent, Mother Nature seems to have a way of stirring up the pot-as was the case last Saturday afternoon.
The Spacecraft's Tale
Last Saturday began as a rather "ho-hum" California winter's day. Orbiting the Earth every 97.5 minutes, the GP-B spacecraft passed directly over California around 6:30AM PST, but the Sun was already up, and the sky was too bright to see the satellite. In the GP-B Mission Operations Center (MOC), a skeleton crew consisting of the on-duty Mission and Flight directors and one or two resident engineers monitored several telemetry passes (communications sessions) during the morning hours. Most were 25-minute satellite passes, during which the spacecraft relays status information to the MOC through the NASA TDRS (Telemetry Data Relay Satellite) communications satellite system. And, during a 12-minute ground pass at 1:15PM PST, the spacecraft's solid-state recorder relayed relativity data to the GP-B science database through a high-speed telemetry connection with the Svaalbard ground tracking station in Norway. All in all, it was a normal Saturday, and the atmosphere in the MOC was quite relaxed.
Following the successful ground pass with Svaalbard, the spacecraft continued on its southward route. At around 1:30 PM PST, Pacific time, the spacecraft was flying over South America-heading towards the South Pole-when it entered the South Atlantic Anomaly (SAA). This is a region above the Earth where the fluxes of trapped protons and other particles, emitted by the Sun, are much greater than anywhere else on Earth, due to the asymmetry of the Earth's protective Van Allen Radiation Belts. Thus, spacecraft are more vulnerable to being 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, and other systems and instruments, in a stable and safe configuration in response to anomalous or out-of-limits feedback from various on-board sensors).
Back in the MOC, the next telemetry pass was not scheduled until 3:16 PM, so the operations staff was completely unaware of this change in the spacecraft's condition-for the time being. At 3:15PM, the MOC staff settled into their seats for the upcoming satellite status telemetry pass. As the spacecraft's antenna locked into the 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. An assessment of the safemodes that were triggered indicated that an error-never seen before--had occurred in a module of the Attitude and Translation Control (ATC) computer system. The spacecraft's GPS had registered an off-the-scale velocity spike, which if correct, indicated that, for one brief moment, the spacecraft had traveled faster than the speed of light-or to use Star Trek terminology, it had "warped into hyperspace." In fact, 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 a safemode test, which in turn activated a chain reaction response sequence.
The MOC staff immediately scheduled several extra satellite communication passes so they could communicate with the spacecraft more frequently. Then, over the ensuing 24 hours, they methodically worked through a series of tests and command sequences to return the spacecraft to its normal science operation mode. We initially assumed that the GPS receiver had suffered a proton hit in the SAA region, but further analysis suggests that this was not the case. Rather, this anomaly was apparently caused by one of the four accessible GPS satellites being in the wrong position for proper GPS triangulation. The ATC system usually catches situations of this kind and disallows the data; but, this one slipped through the filter.
The spacecraft has returned to normal operations. This incident was not detrimental to the GP-B experimental data. And, once again, the fact that an anomalous event occurred while the spacecraft was flying through the SAA region appears to be a coincidence-or is it?
Please Note: Until further notice, we intend to continue sending out these updates on a weekly basis. Also, from time to time, we may post special reports and special updates, as warranted by mission events.
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NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu#---------------------------------------------
Regards,
LelandJ
