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

Figure 13-38. Histogram of PPS DataA histogram of the data is shown in Figure 13-38. Data for twelve different roll rates was used, in addition to adata set for the static case. Three data points were outside the plus/minus ten microsecond requirement range,99.88% of the data is within a plus/minus five microsecond range. No correlation was discovered between theoutput PPS signal and roll rate.A comprehensive set of tests verified that science telemetry could be time tagged to better than the 10 μsrequirement, and showed that the GPS PPS signal allows telemetry to be reconciled with UTC time withoutintroduction of further measurable time error.Details regarding the time reconciliation method can be found in [5], but an example of challenges overcome isuseful for appreciating the complexity of the GP-B spacecraft as well as its detailed design. In addition toproviding a 10 Hz timing strobe for vehicle hardware, the space flight computer’s master clock drives a 31 bitcounter at 16.368 MHz. Registers capture the value of the counter when a PPS or a 10 Hz strobe pulse isdetected. The values are included in telemetry. Since there is no a priori way of differentiating one PPS signalfrom its neighbor, the GPS receiver telemetry includes transmission time: the time at which the GPS data packetleaves the receiver. Another telemetry mnemonic captures the time the GPS data packet was received by theflight computer. By design the interval is minimized. Figure 13-39 shows the difference between thetransmission and reception time. The figure shows that the interval is always less than 0.6 seconds.Gravity Probe B — Post Flight Analysis • Final Report March 2007 383