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

The electrical power system is fully functional and is providingadequate power for all operating conditions. Eclipse operation (whenthe spacecraft is in the shadow of the Earth) meets all requirements.There is no evidence of solar array motion that might disturb theexperiment.All other spacecraft subsystems are fully functional. All four gyroshave been checked out and are performing well. They have all beenelectrically suspended in analog mode; digital suspension activities willcommence shortly. The team is in the process of updating data tablesfor the Gyro Suspension System (GSS) to aid in achieving digitalsuspension of the gyros.The spacecraft’s Attitude Control System (ATC) is maintaining astable attitude (relative position in orbit—pitch, yaw and roll). Weexpect to lock onto the Guide Star, IM Pegasi, within a few days, aftercompleting on-orbit re-calibration of the spacecraft’s 16 microthrusters. During thruster re-calibration, it was observed that one ofthe redundant micro thrusters was stuck partially open. It was isolatedand removed from the system, and the flow of helium to the remainingmicro thrusters has been adjusted to compensate for its removal.Overall, one week after launch, it appears that all of the spacecraft’ssubsystems are functioning well—meeting or exceeding missionrequirements, in preparation for beginning the science experiment.7 MAY 2004—MISSION UPDATE: DAY 17As of Mission Day #17, the Gravity Probe B spacecraft continues toperform well, and we are expecting a smooth and successful transitioninto the science phase of the mission.The spacecraft remains in a science mission orbit, within the plane ofthe Guide Star, IM Pegasi. The gyro readout system performancecontinues to exceed expectations, and all four SQUIDs (SuperconductingQuantum Interference Devices) are functional andcalibrated, with very low noise levels. Power and thermal systems meetall of our mission requirements. All spacecraft subsystems continue toperform nominally.All four gyros have been electrically suspended in analog mode, andgyros #1, #2 and #4 are now digitally suspended; we expect gyro #3 totransition from analog to digital suspension shortly.Last weekend, the spacecraft was hit by radiation while passing overthe Earth’s south magnetic pole. This radiation caused data errors inthe spacecraft’s primary (A-side) computer, which exceeded itscapacity for self-correction. Thus, by design, the spacecraftautomatically switched over to the backup (B-side) computer, placedthe spacecraft in a “safe” mode, and put the planned timeline of eventson hold.The automatic switch over from primary to backup computer workedflawlessly. The GP-B mission operations team has since re-booted theprimary computer, restored its data parameters, and then commandedthe spacecraft to switch back to the primary computer, which is onceagain in control. During this incident, the GP-B science instrumentcontinued to function perfectly—as expected—with all four gyrosremaining suspended in their assigned modes.The spacecraft’s Attitude Control System (ATC) is continuing tomaintain a stable attitude (relative position in orbit—pitch, yaw androll). However, the process of locking onto the Guide Star, IM Pegasi,has been delayed a few days by the South Pole radiation incident.14 MAY 2004—MISSION UPDATE: DAY 24As of Day #24 of the mission, all spacecraft subsystems are functioningproperly. The orbit is stable and meets our requirements for nextmonth’s transition into the science phase of the mission, uponcompletion of the spacecraft initialization and orbit checkout.Furthermore, Gravity Probe B has successfully achieved severalimportant milestones over the past week.All four gyroscopes have now been digitally suspended for over aweek. At launch, the gyros were unsuspended. Once on orbit, eachgyro was first suspended in analog mode, which provides coarsecontrol of the gyro’s suspended position within its housing. Analogmode is used primarily as a backup or safe mode for suspending thegyros. Each gyro was then suspended digitally. The digital suspensionmode is computer-controlled; it puts less torque on the gyros thananalog mode and enables their position to be controlled withextremely high precision.At the end of last week, the Gravity Probe B team practiced LowTemperature Bakeout (LTB), in which discs of sintered titanium (verytiny titanium balls, smaller than cake sprinkles) are “warmed up” a fewKelvin, thereby attracting helium molecules to them. This process willremove any remaining helium from the gyro housings after full gyrospin-up. Last week’s practice LTB procedure had the added benefit ofimparting a very small amount of spin-up helium gas to the gyros.Following the practice LTB, the SQUID gyro read-out data revealedthat gyro #1, gyro #3, and gyro #4 were slowly spinning at 0.001, 0.002,and 0.010 Hz, respectively (1 Hz = 60 rpm). Amazingly, the GyroSuspension Systems (GSS) were able to measure gas spin-up forces atthe level of approximately 10 nano-newton (10-8 N). This means thatthe GP-B science team is able to interpret data from gyro spin ratesfour to five orders of magnitude smaller than what was planned for theGP-B science experiment.Earlier this week, the GP-B spacecraft flew “drag-free” around gyro #1,maintaining translation control of the spacecraft to less than 500nanometers. The term, “drag-free,” means that the entire spacecraftliterally floats in its orbit—without any friction or drag—around onethe gyros. Pairs of proportional micro thrusters put out a steady andfinely controlled stream of helium gas, supplied by the dewar, throughits porous plug. Signals from the Gyro Suspension System (GSS)control the output of the micro thrusters, balancing the spacecraftaround the selected gyro. The initial drag-free Control (DFC)checkout lasted 20 minutes, as planned. Then, a two-hour DFC sessionwas tested, during which the spacecraft roll rate was increased andthen returned to its initial rate, maintaining drag-free statusthroughout the test. Achieving DFC indicates that we are on track tomeet the science mission control requirements.Last, but not least, early this week, the Attitude & Translation Controlsystem (ATC) successfully used data from the on-board star sensors topoint the spacecraft towards the guide star, IM Pegasi. This was thefinal step before initiating the dwell scan process, a series ofincreasingly accurate scans with the on-board telescope that enable theATC to lock onto the guide star. Two days ago, the telescope’s shutterwas opened, and a first dwell scan was completed. We are now in thefinal stages of repeating the dwell scan to home in on the guide starand lock onto it. Photo: ATC Engineers with the Attitude Controlsystem before it was placed on the spacecraft, circa May 2001.474 March 2007 Appendix C — Weekly Chronicle of the GP-B Mission