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

7.5.6.1 South Atlantic AnomalyProton activity in the South Atlantic Anomaly (SAA) has provided challenges to the ATC configuration over awide range of areas from the telescope settings to the range of times where we can have the rate notch filterenabled. The telescope settings are affected by the presence of proton hits to the detectors that lead toabnormally large signal readings. The large data values are interpreted by the telescope as a very strong lightsource entering one side of the telescope. The offset of the signal in the telescope field of view looks, to thecontroller, like a mis-centering of the telescope axis with the guide star and causes erroneous control commandsto the thrusters. There is exception handling, a telescope sanity check and a sum magnitude limit in placeintended to filter out these signals, but due to an error in the Ada exception handling compiler, the signals arenot properly filtered. See ATC Slope Exception Handling section below for more details.A new telescope configuration has been implemented to eliminate the exception handling problem, but evenwith this fix in place, the telescope is still essentially blinded for several minutes while passing over the SouthAtlantic Anomaly. While traveling through the center of the SAA, up to 90-100% of the data is filtered out dueto proton activity corruption as shown in Figure 7-21 below. Thus, during this brief period of time, the ATCsystem is essentially controlling without any proportional term data that is generally used to keep the guide starcentered.Figure 7-21. Percentage of data corrupted by proton activity in the South Atlantic AnomalyDuring this period, the controller relies mostly on rate damping to keep from wandering too far off the guidestar. However, as discussed earlier, there is a rate notch filter that strongly attenuates the body roll rate, orinertial signal. It has been deemed too much of a risk to keep the filter on through the strongest parts of theSAA, because the system would be controlling without proportional or inertial rate damping data. Therefore,the notch filter is turned off just before entering the SAA, and the roll rate disturbance from the ARP motion isnot attenuated during passes through the SAA.196 March 2007 Chapter 7 — Attitude & Translation Control Subsystem Analysis