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

spacecraft orbited the Earth some 5,000 times while the Earth made one complete orbit around the Sun, the fourgyros spun undisturbed—their spin axes influenced only by the relativistic warping and twisting of spacetime.We kept the telescope pointed at the guide star, and each orbit, we recorded the cumulative size and direction ofthe angle between the gyroscopes' spin axes and the telescope. According to the predictions of Einstein's generaltheory of relativity, over the course of a year, an angle of 6,606 milliarcseconds should have opened up in theplane of the spacecraft's orbit, due to the warping of spacetime by the Earth, and a smaller angle of 39milliarcseconds should have opened up in the direction of Earth's rotation due to the Earth dragging its localspacetime around as it rotates. In reality, what went on behind the scenes in order to obtain these gyro driftangles was a very complex process of data reduction and analysis that is taking the GP-B science team more thana year to bring to completion.14.1.7.1 GP-B Data LevelsThroughout the science phase of the GP-B mission, we continuously collected data during all scheduledtelemetry passes with ground stations and communications satellites, and these telemetered data were stored—in their raw, unaltered form—in a database here at the GP-B Mission Operations Center. This raw data is called“Level 0” data. The GP-B spacecraft is capable of tracking some 10,000 individual values, but we only capturedabout 1/5 of that data. The Level 0 data include a myriad of status information on all spacecraft systems inaddition to the science data, all packed together for efficient telemetry transmission. So, our first data reductiontask was to extract all of the individual data components from the Level 0 data and store them in the databasewith mnemonic identifier tags. These tagged data elements were called “Level 1” data. We then ran a number ofalgorithmic processes on the Level I data to extract ~500 data elements to be used for science data analysis, andthis was called “Level 2 data.” While Level 2 data include information collected during each entire orbit, ourscience team generally only uses information collected during the Guide Star Valid (GSV) portion of each orbitwhen the telescope was locked onto the guide star. We do not use any gyroscope or telescope data collectedduring the Guide Star Invalid (GSI) portion of each orbit—when the spacecraft is behind the Earth, eclipsedfrom a direct view of the guide star—for science data analysis.14.1.7.2 Gaps in the DataAnalyzing the data collected in the GP-B experiment is similar to fitting a curve to a set of data points—themore data points collected, the more accurate the curve. If there were no noise or error in our gyro readouts,and if we had known the exact calibrations of these readouts at the beginning of the experiment, then we wouldneed only two data points—a starting point and an ending point. However, there is noise in the gyro readouts,so the exact readout calibrations had to be determined as part of the data collection and analysis process. Thus,collecting all of the data points in between enables us to determine these unknown variables. In other words, theshape of the data curve itself is just as important as the positions of the starting and ending points.All measurements we collected were time-stamped to an accuracy 0.1 milliseconds. This enables our scienceteam to correlate the data collected from all four gyros. If one gyro's data are not available for a particular timeperiod, such as the first three weeks of the science phase when gyro #4 was still undergoing spin axis alignment,that gyro is simply not included in the analysis for that particular time period. In cases where all science data arelost for an orbit or two, the effect of these small data gaps on the overall experiment is very small. Examples ofsuch data gaps include the March 4, 2005 automatic switch-over from the A-side (main) on-board computer tothe B-side (backup) computer, and a few telemetry ground station problems over the course of the mission.14.1.7.3 Sources of NoiseAnother important point is that the electronic systems on-board the spacecraft do not read out angles. Rather,they read out voltages, and by the time these voltages are telemetered to Earth and received in the sciencedatabase here in the Stanford GP-B Mission Operations Center, they have undergone many conversions andGravity Probe B — Post Flight Analysis • Final Report March 2007 417