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

Figure 11-25. Comparison of estimated photocurrent based on Science slopes andon least square fit slope derived from TRE snapshots11.4.7 Snapshots Comparison to Science SlopesThe telescope signals are processed by two on-board algorithms, which are named ATC Slopes and ScienceSlopes. Both the ATC slopes and the Science slopes are calculated at the rate of 10 per second (for all eightdetectors); the ATC slopes are used in near real time by the attitude control system to point the telescope at theguide star. Typically, the Science slopes are in the telemetry formats at the highest rate of 10 per second, whilethe ATC slopes are at a rate of one per second. Intermittently the raw A/D converter samples, which are at a rateof 2200 samples per second, are also in the telemetry data stream called Snapshots. Snapshots need to bemanually scheduled, and they are telemetered at a maximum rate of one frame of 300 contiguous A/D samplesper ten seconds. There is a 0.3 s offset between vehicle times for Snapshots and Science slopes.The snapshot data have been processed on the ground for a fidelity comparison between the ground-basedSnapshot slope (calculated with a least squares algorithm using the same points as used by the onboard Scienceslope algorithm) to the slope estimated and reported by the Science slope algorithm. Figure 11-25 shows theresult of 267 snapshots compared with Science slopes, where the resulting slopes have been scaled to detectorphotocurrent in fA. This comparison verifies that the on-board Science filter and a simple least square filterproduce similar results with a correlation coefficient of 0.97364. Hence the Kalman filter derived Science slopesdiffer somewhat from the least squares estimate. This difference is attributed to the choice of Kalman filtercoefficients that were optimally derived from a similar comparison early in IOC part of the mission. TheKalman filter provides an optimal estimator for the slope since the charge integrated signal contains bothelectronic white noise and photon/electron shot noise.11.4.8 Telescope and Windows operating temperatureThe temperatures of the window mounts and telescope top plate were monitored continuously until the ECUwas turned off on 28 April 2005. After that date, the temperature was measured once a week.Gravity Probe B — Post Flight Analysis • Final Report March 2007 331