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

It is said that “an experiment is only as good as its data.” So, one might ask, “How useful are the actual data thatwere collected during the 50-week science phase of the GP-B experiment? Was it sufficient? For Gravity ProbeB, the best gyro precession data were collected when the telescope was precisely pointing at the guide star, whenthe spacecraft was in Guide Star Valid (GSV) mode (that is, when the spacecraft was “In front” of the Earth, witha direct line of sight to the guide star), and when the spacecraft’s orbit was outside the South Atlantic Anomaly(SAA) region, located in the vicinity of the lower tip of South America. Since the Earth eclipses the guide starevery orbit, these optimal conditions occurred only about 54% of the time. Data collected during the remaining46% of the time was still useful to the experiment, but these data are not directly being used to calculate the gyroprecession due to general relativity.The data processing software and team can only process data that they actually received from the spacecraft.There are many reasons why data may not arrive. For example, ground-based experiments can lose data due tofaulty collection techniques, decommutation errors, or inadequate storage methods. Satellites are prone to allthese factors, and additionally, they can lose data because of temporary on-board data storage problems in thesolid state recorder, communication problems or ground station errors. Fortunately, barring anomalies, manyof these factors can be managed to minimize the amount of data this is lost.For example, during ground contacts, streaming of data was re-routed from the solid state recorder to thetransmitter, during which time the solid state recorder was not recording data. During the interval after thesolid state recorder stopped recording, but before the ground station locked up with the transmitter, data waslost. By careful management of operational parameters, such as accurate vehicle position knowledge and groundstation mask angle, data loss during that transition was minimized. Data was also lost whenever the spacecraftchanged its data stream from the forward antenna to the aft antenna. For the GP-B spacecraft, loss of dataduring these antenna-switching periods was typically minimized to less than 40 seconds. These periods couldnot be managed by GP-B, since they were a function of the slew rate of the ground dish and the speed of thevehicle. By scheduling ground contacts during guide star invalid periods, we further minimized science dataloss. Unfortunately, given the geometry of the GP-B orbit with respect to the ground stations, schedulingground passes during Guide Star Invalid (GSI) periods (the period during each orbit when the spacecraft was“behind” the Earth, eclipsing its direct view of the guide star) was not always possible.Management of the solid state recorder itself also mitigated data loss. Some ground station passes resulted in anunacceptably high number of data gaps— three or four is typical and acceptable, but occasionally ground passesgenerated more than 100 gaps. When data gaps were unacceptably high, the solid state memory location writerpointer had to be moved to prevent data from being over written. On these occasions, data processing workedwith the operations team to prevent data loss.For the Gravity Probe B mission, data loss was minimal. Mission specifications mandated that 95% of the dataretrieved by the ground station had to be decommutated and stored in the GP-B L1 database. In fact, 99.62% ofall 32K data, and 97.47% of 32K data collected during GSV periods was recovered and processed. Table 4-9summarizes the data capture statistics.Table 4-9. Summary of Data CaptureSampling PeriodData RecoveredPre-launch Specification 95.00%Total 99.62%Guide Star Valid, outside SAA (“Optimal Science Data”) 97.47%108 March 2007 Chapter 4 — GP-B On Orbit