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

Table 11-1. Gain code change timesA-sideB-sideUTC Time Gain Code UTC Time Gain Code4/20/04 22:30 12 4/20/04 22:30 125/14/04 7:15 11 5/14/04 7:15 113/31/05 21:20 10 3/26/05 7:36 104/4/05 21:24 9 3/29/05 9:10 93/31/05 8:00 115/21/05 8:01 9Gain codes are the commands to select specific taps on a resistive ladder attenuator. Following the attenuatorselector is a large amount of linear gain and a 500 Hz low-pass filter. The net gain between the charge lockedloop and the A/D converter for various gain codes is as follows:• Gain code 9 commands gain value of 307 V/V, or 0.353 relative to gain 12.• Gain code 10 commands gain value of 434 V/V, or 0.499 relative to gain 12.• Gain code 11 commands gain value of 614 V/V, or 0.706 relative to gain 12.• Gain code 12 commands gain value of 870 V/V.11.2.2 PID Temperature Controllers for DMAsThe detector platform temperature for each of the four DMAs is controlled by PID controllers that sense thetemperature and provide commands for the heater voltage. These controllers maintain the platformtemperatures at 72 K. The controllers were used to purposely vary the platform temperatures during a stabilitytest to allow determination of a pointing sensitivity to the platform temperature. Performance results are shownin section “Platform Temperature Stability” on page 312.11.2.3 Signal StrengthsThe currents of the 8 photo detectors ranged from about 6 to 12 fA. Weighting factors are used in the signalprocessing to match the pointing sensitivities from the plus and minus detectors for each axis.11.2.4 Telescope Operation with the Shutter OpenThe GP-B system was designed with the expectation that a shutter would open and close once per orbit toprevent sunlight reflected from the earth from affecting the detectors. Unfortunately, the shutter mechanismincluded a spring loaded mechanism and its operation resulted in un-damped contact with an end stop. Theresulting disturbance gave a small acceleration shock to the gyroscopes, so the decision was made to operatewith the shutter open continuously. Such operation did not cause damage to the detectors, however a fewunexpected features arose. The earthshine resulted in an optical input several thousand times larger than theguide star, which caused significant unbalance in the four-JFET preamplifier circuits and saturation in most ofthe analog amplifiers in the signal chain of the warm electronics. The unbalance of the preamp currentseffectively reduced the power dissipation of the circuit, so in order to maintain the cold platform temperature atthe 72 K operating temperature, the PID controller increases the heater power until the earthshine subsides.This is followed by an under damped recovery transient to the normal steady-state condition observed in a timehistory of the heater voltage engineering monitor. The recovery response time depends on the thermal timeconstant of the DMA and the characteristics of the PID temperature controller. There was observable dampingduring the first 15 minutes of guide star valid, but this did not adversely affect the TRE contributions to thepointing error.Gravity Probe B — Post Flight Analysis • Final Report March 2007 311