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

Table 13-5. Environmental Testing HighlightsTest Receiver AntennaSurvival Temp High (C) +85 + 85Survival Temp Low (C) -55 -55Operational Temp High (C) +20 +30Operation Temp Low (C) -9 -34Base Pressure (Torr) 3.2x10 -7 3.2x10 -7VibroAcoustic (G rms) 6.1 9.7Perhaps more interesting were the tests accomplished to verify software modifications. In particular, twocompeting effects needed to be investigated: rolling and orbiting.Figure 13-36. Rolling RigStanford University used a WellNav GPS Simulator to show that the software modifications allowed the receiverto acquire and maintain position fixes while in the simulated GP-B Orbit. But because the WellNav simulatorcannot model a rolling, orbiting vehicle, a rotating platform (Figure 13-36) was designed and fabricated. Therig, which can rotate over the entire range of predicted roll rates (0.1 – 2 rpm), showed that the antennaswitching algorithm would work up to 2 rpm, although slower rates yield better results.The receiver calculates the Signal to Noise Ratios (SNRs) for up to six tracked GPS vehicles. The antennaselection algorithm compares the SNR values every 15 seconds. The antenna with the GPS satellite with thehighest SNR is selected as the master antenna. The pattern should exactly repeat at the roll period. However, ifthe algorithm calculation occurs when the antenna with the best visibility is ascending, the next best antennawill erroneously be selected. Figure 13-37 shows the master antenna assignment for one channel at threedifferent roll rates, plotted vs. time. The top plot shows the master antenna switching occurring more or lessperiodically, as expected. Errors increase as the roll rate increases (middle and bottom plots). Nevertheless, andmost importantly, the receiver is able to maintain a lock in all cases.Gravity Probe B — Post Flight Analysis • Final Report March 2007 381