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

This chapter provides an overview of the collection, processing and analysis of data collected from the GP-Bspacecraft and science instrument. The chapter includes two main sections:1. “Collection and Processing GP-B Data” —Several vignettes excerpted from GP-B Mission News storiesthat were posted to our GP-B Web site and also sent out to over 2,400 GP-B Email Update Listsubscribers during the 17 months of the GP-B flight mission.2. “The GP-B Science Data Analysis Process” —A brief description of the procedures that are currentlybeing used to analyze the science data and determine the experimental results that will be announcedand published in the spring of 2007.14.1 Collection and Processing GP-B DataThis section contains a series of vignettes describing how GP-B data were collected and stored, how safemodesprotected the spacecraft, science instruments and data, how anomalies such as computer memory errors werehandled, and how intentional telescope dither and stellar aberration were used to calibrate the science datasignals received. These vignettes were originally written to answer the question most asked throughout themission by people around the world who have been following GP-B: “Why is it that proton hits to thespacecraft, computer memory errors, and other anomalies and glitches that occurred during the science phaseof the mission will have little, if any effect on the final experimental results?”14.1.1 Data Collection and TelemetryOur GP-B spacecraft has the capability of autonomously collecting data—in real time—from over 9,000 sensors(aka monitors). Onboard the spacecraft there is memory bank, called a solid-state recorder (SSR), which has thecapacity to hold about 15 hours of spacecraft data—both system status data and science data. The spacecraftdoes not communicate directly with the GP-B Mission Operations Center (MOC) here at Stanford. Rather, itcommunicates with a network of NASA telemetry satellites, called TDRSS (Tracking and Data Relay SatelliteSystem), and with NASA ground tracking stations.Figure 14-1. A drawing of the spacecraft communicating with ground tracking stationsMany spacecraft share these NASA telemetry facilities, so during the mission, GP-B had to schedule time tocommunicate with them. These scheduled spacecraft communication sessions are called “passes,” and duringthe flight mission, the GP-B spacecraft typically completed 6-10 TDRSS passes and 4 ground station passes eachday. During these communications passes, commands were relayed to the spacecraft from the MOC, and datawere relayed back via the satellites, ground stations, and NASA data processing facilities. The TDRSS links haveGravity Probe B — Post Flight Analysis • Final Report March 2007 399