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

Figure 7-18. GSV/GSI rate gyroscope bias offsetWhile the guide star is out of the field of view and the vehicle is nulling its rates using only gyroscope signals, thecontroller tries to null out the pitch and yaw rates. However, as the ARP moves at orbit rate, the projection ofthe roll rate onto the pitch and yaw axes changes over the orbit. If the orientation of the ARP at the end of oneguide star valid period does not match that of the beginning of the next, the vehicle effectively changes roll axesin order to keep the gyroscope signals nulled out. Thus, at the beginning of the next guide star valid period, thetelescope will not be pointed in the same direction, and it is this initial error that must be damped out to centerthe guide star in the telescope.The alignment of the ARP with the telescope axis can be corrected for by setting hardware bias settings. It hasbecome regular maintenance on the vehicle, often multiple times per week, to set the biases so that the ARP islined up correctly at the beginning of guide star valid. Bias errors on the order of one arcsec/sec can cause anincrease in the guide star capture time by as much as 100-500%.7.5.5.5 Effects of Dewar Shell Temperature Variation on ARP motionOne of the challenges of dewar design is to provide adequate mechanical support for the cryogenic region whilemaintaining low thermal conductance between the exterior and the interior. In the case of the GP-B dewar, thisis accomplished by means of twelve low-thermal-conductivity composite strut tubes (PODS), six forward, andsix aft. (See Figure 7-19 and Figure 7-20 for a depiction of the support system.) In the case of GP-B, there is anadditional complicating factor: In order to acquire accurate roll phase and pointing information for the ATCsystem and science, there are two external Attitude Reference Platforms (ARPs) located on opposite sides of thedewar shell, near the Y axis, which provide mounting surfaces for control gyros and star trackers. These ARPsshould maintain a stable angular relationship to the science instrument deep inside the dewar. In order toGravity Probe B — Post Flight Analysis • Final Report March 2007 193