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

Due to the shape of the Earth’s magnetic field and the orbital altitude of GPB, a majority of charged particleinteractions occurred either over the North / South poles or over the South Atlantic Anomaly (SAA).Figure 8-39shows the locations of SBEs aboard GPB during a typical period of the science data collection phase.Figure 8-39. Location of Single Bit Errors during Science Data CollectionThe GPB Flight Software together with Error Detection and Correction (EDAC) circuitry in the CCCA areresponsible for monitoring the effect of this radiation on the memory components of all subsystems. A portionof the overall processing is dedicated to a continuous memory scrub that cycles through all onboard memoryevery 2.5 seconds. This function is able to detect and correct Single-Bit Errors (SBEs) automatically. The EDACcircuitry can also detect Multi-Bit Errors (MBEs)—the corruption of more than one bit within the same 64-bitmemory word—but MBEs must be corrected manually. See section “CCCA Multi-Bit Errors” on page 252 formore information about the autonomous detection and manual correction of MBEs.SBEs occurred at a relatively consistent rate throughout the GPB mission, subject to increases and decreasesdepending on solar activity and variations in the Earth’s magnetic field. Data from typical one month periodsduring science data collection using both the A and B side CCCA are plotted in Figure 8-40 and Figure 8-41.These plots represent common SBE rates during periods of normal solar activity. The CCCA A-side(Figure 8-40) experienced about 8.7 SBEs per day during the period between October 10, 2004 and November10, 2004. The CCCA B-side (Figure 8-41) detected about 7.6 SBEs per day during the period between April 10,2005 and May 10, 2005. In general, a slightly higher SBE rate was observed during operation on the A-sideCCCA as compared to the B-side CCCA.Gravity Probe B — Post Flight Analysis • Final Report March 2007 249