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

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

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Because the specifications for SRE thermal stability only pertained to the temperature of the FLL, they are theonly Bias Temperature Coefficients necessary to verify that the specifications have been met. The average BTCfor the FLLs is 0.52 mV/K. This is slightly higher than the specification of 0.4 mV/K, but with the superbtemperature control of the FLLs the effect on the SQUID readout is minimal.Using these thermal coefficients it is possible to determine the bias of the SQUID readouts caused by thermalfluctuations of the SRE boards. To do this, the temperature variations of each board at orbital and roll frequencywere multiplied by that boards BTC to determine the bias that the thermal variation produced on the SQUIDreadouts. It was found that at roll frequency, the average effect on all the SQUIDS was 6 nV. This correlatesroughly to an angle of 8 μarcs.8.2.2.5 Tangible Comparisons to Understand <strong>GP</strong>B Temperature Control<strong>GP</strong>B is a macro project in terms of depth and importance yet the experiment itself is on a micro level and it canbe difficult to grasp some of extremely small numbers involved in the experiment. The following are a fewallegories that will hopefully enlighten the reader as to how precise temperature control on <strong>GP</strong>B truly is.8.2.2.5.1 QBS ComparisonsThere are two allegories that can be made to demonstrate the excellence of QBS temperature control. The first isby comparing it to the human body and the second is by comparing the temperature inside a room to the QBStemperature and the temperature outside the room to the Cryo Pump, which is near the QBS but not undertemperature control.1. The “nominal” range for an adult’s body temperature is 36.4 to 37.1° C (97.6 to 98.8° F).Control rangeof.7 °C (1.2° F). The QBS temperature control is 700 times more precise!2. Imagine the temperature outside the room you are in ranging from -63° C to 808° C (-81° F to 1486° F).To give you an idea, this ranges from the coldest temperature ever recorded in Canada (breathe hissesand turns to ice crystals) to the temperature of freely burning jet fuel. Now, if the room you are in had thetemperature control of the QBS, it would be a comfortable 24 +/- 0.5° C (75 +/- 0.9° F) 85% of the timeand 24 +/- 1° C (75 +/- 1.8° F) 96.5% of the time.8.2.2.5.2 SRE and SQUID Bracket Comparisons1. The SQUID Bracket temperature control is 140000 times as precise as the human bodies2. The disturbance in the SQUID readout at roll frequency due to the thermal variation of the SQUIDS wasextremely small. To be precise, it was 8 μarcs. To get a better idea of the size of this angle let’s project itonto the moon, which is 384,400 km away. The projection would only be 50 m on the moon. This is 1 /69520 of the moons diameter.8.2.2.6 ConclusionsThe temperature control technology aboard <strong>GP</strong>B is superb. It nearly meets the extremely rigid specificationsand ensures that thermal bias in the experiment will be minimal and easy to eliminate from the results, thusyielding a true test of Einstein’s Universe.222 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses

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