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

Because there are only two SQUID brackets inside the Probe (each one holding two gyros) there are also onlytwo boards outside the Probe where the control circuits for the SQUID bracket temperatures are. Each of theseboards has a primary and backup heater and sensor. A PI algorithm controls the heaters and attempts to keepthe temperature of the SRE bracket at its set temperature. There are two modes: Coarse Mode (750 ohms/volt)and Fine Mode (10 ohms/Volt).The specification for temperature control of the SRE is given only in terms of the FLL. It states that, “Thethermal-induced bias drift shall be less than 20 PPM of full scale per K for FLL ranges 1 and 2.” In other words,a 1 K temperature change of the FLL will result in less than a 0.4 mV change in the SQUID readout. This isroughly equivalent to 0.54 marcs, quite large by GPB’s standards. However, the average orbital temperaturevariation of the FLLs under temperature control is only 1mK so the thermal induced bias drift of the SQUIDsignal at orbital frequency is only 0.54 μarc. An astoundingly small effect.8.2.2.3 Verifying QBS Temperature Control SpecificationsAnalyzing a typical 24 hr. period of telemetry data from the monitor TE_QBS_a_GT10P (the QBS temperature)shows that 1mK temperature control is achieved a majority of the time. More specifically: 85% of the time theQBS temperature was within +/- 0.5 mK of the average temperature over the 24 hr period.65% of the time the temperature was within +/-.25 mK of the average temperature over the 24 hr period. 96% ofthe readings were within +/- 1 mK of the average temperature. 99.3% of the readings were within +/-1.5 mK ofthe average temperature.The QBS temperature control loop must overcome temperature fluctuations caused by orbital, annual and rollcycles. The most rapid and drastic change in temperature is caused by an orbital cycle and this prevents the QBSfrom maintaining its specified thermal stability of 1 mK 100% of the time.When the telescope shutter is open while the earth is between the guide star and GPB, light from the sun canreflect off the earth and travel down the telescope, heating the Probe. Areas of the earth covered in ice, snow orclouds reflect more light from the sun than other parts of the earth, so when the telescopes viewing field is overone of these areas, there is a larger increase in temperature of the QBS.The PI control loop cannot respond (change the heater voltage) quickly enough to keep the QBS temperaturewithin the control range when light reflecting off the earth travels down the telescope. Closing the telescopeshutter during Guide Star Invalid periods would solve this problem. However, this was deemed an unacceptablesolution as constantly opening and closing the shutter would cause vibrations in the Probe that could disturbthe gyros. Another possible solution would be to use a PID (Proportional Band, Integral, Derivative) controlloop that would be able to react fast enough to keep the QBS temperature within the desired range. However,this solution is not feasible as it is not possible to change the control loop after launch.8.2.2.4 Verifying SQUID Bracket and SRE Temperature Control SpecificationsThrough telemetry, it was easy to verify that the thermal stability of the SQUID bracket met its specificationsexcept during the time period when sunlight reflected off the earth traveled down the telescope, heating up theQBS. The chart in Figure 8-16 below shows the temperature from of the SQUID Bracket (top), the QBS(middle) and the Cryo Pump, which is not under temperature control but was included to contrast temperaturecontrol against no temperature control (bottom).220 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses