Revised Estimates of Photochemically Deposited Contamination on the Global Positioning System Satellites

摘要

The revised estimates of GPS Block II/II-A lifetime prediction are shown in Table 4. As shown, it is now believed that the Block II-A spacecraft 22-40 can be expected to experience noticeable degradation in power due to photochemical deposition of contaminants on the arrays. The new model predicts less degradation for spacecraft 13-21 than did the original study. Based on the goodness of fit with the data, we have high confidence in the final result. As shown in Table 4, all GPS Block II/II-A spacecraft are expected to meet their 7.5-year lifetime requirement with power to spare. Even the most pessimistic approach, the linear fit to the data, agrees that all spacecraft should exceed their lifetime requirement. This study has verified that photochemically deposited contamination traced to outgassing from the vehicle interior is the likely source of anomalous power loss on the GPS spacecraft. The lessons learned from this study appear to be virtually identical to those quoted in the original work, with one exception. It now seems plausible that contamination could reach the arrays by outgassing through the MLI. This being the case, spacecraft designers must pay attention to the location of even those vents that are covered by MLI.