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

operations crew, including several GP-B graduate students who arebeing cross-trained for these duties, will continue to monitor thespacecraft's system status, send commands to it when necessary, andregularly, though much less frequently, download various types of datavia NASA telemetry satellites and ground stations.We will use magnetic torquers—long electromagnets mounted on thespacecraft frame—to slow down the spacecraft's roll rate from 0.7742rpm to 0.5 rpm or less and to control the spacecraft's orientation inorbit. For about two weeks, we will perform a number of tests onvarious electronic systems in the spacecraft. For example, we want tocompare various performance characteristics of the GSS system after ithas “warmed up” to its performance in a cryogenic superconductivestate. Finally, in about a month, we will spin down all four sciencegyros to speeds slow enough that they will not be in danger ofshattering, should they lose suspension and touch their housing walls.At this point, the spacecraft could be used by other scientists to makevarious non-relativitistic measurements. For example, it could be usedfor experiments in geodesy (measuring the shape of Earth'sgravitational field) and aeronomy (measuring atmospheric density).We are in the process of exploring the interest in further use of theGP-B spacecraft for such experiments with a number of scientists.One last note: All of us on the GP-B team are deeply saddened by thehorrific disaster and unfathomable human tragedy that has beenunfolding in Gulf Coast this past week. Our hearts and prayers go outto the countless victims.9 SEPTEMBER 2005—GRAVITY PROBE B MISSIONUPDATEMission Elapsed Time: 507 days (72 weeks/ 16.6 months)—IOC Phase: 129 days (4.2 months)—Science Phase: 352 days (11.6 months)—Final Calibration Phase: 26 daysCurrent Orbit #: 7,482 as of 4:30PM PSTSpacecraft General Health: GoodRoll Rate: Normal at 0.7742 rpm (77.5 seconds per revolution)Gyro Suspension System (GSS): All 4 gyros digitally suspendedDewar Temperature: 1.82 kelvin, holding steadyGlobal Positioning System (GPS) lock: Greater than 96.5%Attitude & Translation Control (ATC): X-axis attitude error: 149.7marcs rmsY-axis error: 179.9 marcs rmsCommand & Data Handling (CDH): B-side (backup) computer incontrolMulti-bit errors (MBE): 0Single-bit errors (SBE): 8 (daily avg.)Gyro #1 Drag-free Status: Backup Drag-free mode (OFF during somecalibration maneuvers)On Mission Day 507, the Gravity Probe B vehicle and payload are ingood health and all subsystems are performing nominally.The helium in the dewar has lasted throughout this past week, andthus we have continued with the calibration tests that we began overthree weeks ago. These tests involve slewing the telescope (andspacecraft) to “visit” stars (both real and virtual) in the neighborhoodaround the guide star, IM Pegasi. In each case, we visit a neighboringlocation for a period of time and then lock back onto IM Pegasi forseveral hours. The purpose of these tests is to calibrate, in detail, thetorques imparted onto the science gyroscopes by purposely misaligningthe telescope (and spacecraft roll axis) from the direction ofthe gyro spin axes.Last weekend, we visited a “virtual star” (location where no starsvisible to the telescope exist) located 0.1 degree in the direction ofneighboring star HD 216635, northwest of IM Pegasi. We remained atthis location for 24 hours and then returned to IM Pegasi for 16 hours.We then repeated this procedure, visiting a location 0.1 degrees in theopposite direction. Then, this past Tuesday we visited a virtual starlocation halfway to the Star HR Pegasi (HD 216672), which is locatedto the west of IM Pegasi. After dwelling in that location for 24 hours,we again returned to IM Pegasi. And, on Thursday, we visited a virtuallocation 0.3 degrees towards HD 216635, remained there for 24 hoursand then returned to IM Pegasi.Gravity Probe B — Post Flight Analysis • Final Report March 2007 515