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

Figure 8-23. Solar Array Temperatures Variations vs. Gamma Angle8.3.9 Solar Array Power Degradation AnalysisA detailed power analysis was performed by a team of Stanford Analysts supported by the Lockheed Martinpower experts to determine the solar array power degradation over the mission lifetime and compare resultswith power margin calculation performed by the operation team using vehicle telemetry. The power analysismodeled the solar array panels including the panel temperature, solar intensity fluctuations, and the decay ofthe Gallium Arsenide (GaAs) cells. These factors have been added into the decay analysis and several efficiencyfactors have been determined to account for each of these effects. A formula to predict power generation in thecourse of the mission lifetime was developed and from which power values during 2004 and 2005 werecalculated from the vehicle telemetry. A detailed power analysis is available for interested parties.Results from analysis showed a drop of approximately 80 watts between 2005 and 2004, equating to 3.62%degradation in the power production over the mission lifetime (Figure 8-24 depicts the EPS peak power datacomparison between 2004 and 2005). The analysis also showed ample power margin produced by the EPSsystem which would be sufficient to support mission extension, if required.As mentioned, Figure 8-24 displays the peak power produced in 2004 and 2005. Additionally, the differencebetween the two curves was calculated and represents the power degradation of the arrays over the year.Figure 8-25, below, displays only the portion of the data prior to post science calibrations. During thesecalibrations the vehicle’s attitude was purposely adjusted, which in turn changed the perceived gamma angleand likewise the peak power generated. Additionally, the data from 2004 was eliminated which included avehicle anomaly resulting in an unplanned maneuver (gamma angle change) and roll down of the space vehicle.Therefore, Figure 8-25 provides the most consistent comparison of 2004 to 2005 and likewise the most accurateassessment of solar array degradation. Results from this analysis showed a decrease of approximately 86 watts,equating to 3.89% degradation in power production.230 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses