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

Figure 8-32. Plot of TDRS AGC vs Temperature over a 20-minute time frameFigure 8-32 above shows the increase in the signal level (better Signal to Noise) as transponder temperatureincreases. The sample above shows an improvement of AGC from –264.0 dBm to -171.0 dBm in a twentyminuteperiod.8.4.3.2 Transponder-A Anomaly MitigationTo mitigate the reduced SNR, the GP-B transponder configuration macros were modified to operate the FwdAntenna connected Transponder-A at the command data rate of 125 bps. This in turn provides an increase of12 dB above the nominal command rate of 2000 bps, which the Aft Transponder’s nominal command rate. Inaddition, the TDRSS supports that can provide high power on the forward link (SSA) are routinely operated inthat mode, providing an additional increase in the link margin (5 dB better than low power). With bothprocedures in place, the command ability and reliability with the use of the forward transponder has not been amajor issue during the GP-B science mission phase.8.4.3.3 Ground Network Communication AnomaliesThe Ground Network has had few problems with the transponder, as the transponder configuration (mode) isdifferent and the GN uplink is of higher power. During the month of April of 2005, the Svalbard Ground Stationwas shut down for Antenna work, so the schedule shifted to the Svalbard commercial site (SKS). In addition,during the same month, Poker Flat Alaska commercial site (PF1) began to be scheduled more frequently.During this time period, the project experienced noisy data from these sites. Because of the compressed datadump, one-minute of data, lost due to noise, results in the loss of one hour and twenty minutes of 32K data.Whenever this kind of problem occurs, the operations team has to reconfigure the Solid State Recorder to repeatthe playback during the next scheduled GN. Although, this was/is a good way to recover the data, it was not the238 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses