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

A special room in the GP-B Mission Operations, called the Anomaly Room, was the home of the GP-B AnomalyReview Board (ARB), a select group of senior GP-B team members from Stanford, NASA, and LockheedMartin, who managed the troubleshooting of anomalies and observations. The Anomaly Room, which waslocated across the corridor from the GP-B MOC, contained a set of spacecraft status monitors, communicationsand teleconference equipment, computer and voice hookups, a documentation library, white boards, acomputer projection system, and an oval discussion table.During the flight mission, whenever an anomaly was in the process of being resolved, the Anomaly Room wasstaffed 24 hours a day, 7 days a week; at other times, it was staffed during normal working hours, with teammembers on call. When major anomalous events, such as computer reboots, occurred outside normal workinghours, the Mission Director on duty activated the Anomaly Room and issued a series of pager and cell phonecalls via computer, summoning key staff members on the ARB, along with a selected anomaly team, comprisedof resident engineers and engineering specialists, to come in and work through the issue. The group used atechnique called “fault tree analysis” to evaluate and determine the root cause of unexpected events.The GP-B Safemode Subsystem and anomaly resolution process worked very well throughout the mission. Overthe course of the flight mission, the ARB successfully worked through 193 anomalies/observations. Most ofthese issues (88%) were classified as observations, and about 9% were classified as minor to medium anomalies.But five (3%) were classified as major anomalies, including the B-Side computer switch-over and the stuck-openvalve problems with two of the 16 micro thrusters early in the mission, as well as subsequent computer andsubsystem reboot problems due to solar radiation strikes. In each case, the established anomaly resolutionprocess enabled the team to identify the root causes and provide successful recovery procedures in every case.You can read a detailed description of the GP-B Anomaly Resolution Process in Chapter 5, ManagingAnomalies and Risk . Also, Appendix D, Summary Table of Flight Anomalies contains a table summarizing thecomplete set of anomalies and observations from launch through the end of the post-science calibrations inOctober 2005.14.1.4 Effects of Anomalous Events on the Experimental Results of GP-BWhenever anomalous events occurred on-board the GP-B spacecraft in orbit, such as computer switch-oversand reboots, we received numerous inquiries asking about the effects of these events on the outcome of theGP-B experiment. These events have resulted in a small loss of science data. In and of itself, this loss of data willhave no significant effect on the results of the experiment. If it turns out that some of the lost data wasaccompanied by non-relativistic torques on the gyros (measurable drift in the gyro spin axes caused by forcesother than relativity), we believe that there will probably be a perceptible, but still insignificant effect on theaccuracy of the end results. We really will not be able to quantify the effect of these events until the full dataanalysis is completed early in 2007.We have long anticipated and planned for dealing with possible lapses in the data and non-relativistic torqueson the gyros during the mission. In fact, this subject was explicitly discussed in Dr. Thierry Duhamel's 1984 PhDthesis entitled Contributions to the Error Analysis in the Relativity Gyroscope Experiment. (This was one of 79 Stanforddoctoral dissertations sponsored by GP-B over the past 42 years.) Chapter 4 of Dr. Duhamel's dissertation isentitled “Effect of Interruptions in the Data.” In this chapter, Dr. Duhamel modeled various hypothetical casesof an undetermined amount of gyro spin axis drift due to non-relativistic torques (forces) on the gyros,accompanying data interruption periods of varying lengths. Each of Dr. Duhamel's hypothetical scenariosmodels these effects over a 12-month data collection period, showing the effect of gyro drift and data loss as afunction of when in the mission (month 1, month 2, etc.) the event occurs.In a nutshell, the results of Dr. Duhamel's research showed that lapses of data in which the gyros do notexperience any non-relativistic torques would have no significant effect on the outcome of the experiment.Lapses of data that are accompanied by gyro drifts of unknown size due to non-relativistic forces would have a408 March 2007 Chapter 14 — Data Collection, Processing & Analysis