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

Additionally, the warm electronics power dissipation is increased when most of the amplifiers are driven tosaturation. The signal level from the output of the charge locked loop is normally within a few mV of zero withguide star illumination, but increases to nearly -10 V during earth shine. The power dissipation in the resistiveattenuator and the feedback resistors of the amplifiers increases, and it is distributed over much of the analogprinted wiring board (PWB). The board temperature is measured using the temperature output of the REF-02voltage reference mounted on the PWB and appears in the local box temperature monitor. The degree ofheating changes with season, and the disturbance went through a long period during which earth shine wasobserved for nearly the entire guide star invalid period.11.3 Hardware PerformanceHardware performance will be discussed in greater detail with examples, long term plots and a typical look atdaily data.11.3.1 TRE Detector Dark NoiseThe TRE detector dark noise is difficult to measure separately from the effects of pointing ambiguity when theguide star is in the field of view. On May 31, 2005, when the guide star was lost for about three hours, there wasa whole guide star valid interval during which the science slope processing was active and there was no star inthe field of view. This provided 58 minutes of dark detector data in a normal operating environment. The effectsof particle hits were removed, and the standard deviations of the remaining signals were calculated after scalingthe data to fA using the appropriate factor for gain level 9. Each detector sample noise calculation used 33546samples out of a total of 34801 available during the interval, indicating that 1255 samples or 3.6% werediscarded due to particle hits. Using this data, Table 11-2 is a listing of the resulting RMS dark current noise foreach of the eight photo detectors, which are labeled with the detector designations P/M, X/Y, and A/B. A latersection shows the effect on pointing noise as the star brightness varies.Table 11-2. Photo detector dark noiseDetector Dark NoisePXA: 0.297 fAMXA: 0.255 fAPYA: 0.252 fAMYA: 0.257 fAPXB: 0.249 fAMXB: 0.379 fAPYB: 0.264 fAMYB: 0.248 fA11.3.2 Platform Temperature StabilityThe temperatures of the detector platforms are maintained using a computer algorithm with proportional,integral, and derivative (PID) gain coefficients. The temperature controller compares the temperature measuredby the silicon diode sensor on the platform with a commanded set point temperature, usually 72 K. The PIDalgorithm output controls a DAC in the TRE warm electronics that provides the voltage to the heater resistorson the platforms. The platform temperatures are maintained at 72 K with a standard deviation ofapproximately 23 mK.312 March 2007 Chapter 11 — Telescope Readout Subsystem (TRE) Analysis