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EXECUTIVE SUMMARY 10 Block IIR satellite communication crosslinks should be used to the extent possible with the existing crosslink data rate to support on-board satellite health monitoring for improved reliability and availability and in order to permit a more rapid response time by the operational control segment. The Block IIR inter-satellite communication crosslinks should be used to relay integrity information determined through ground-based monitoring. The DOD's more frequent satellite navigation correction update strategy should be fully implemented as soon as possible following the successful test demonstration of its effectiveness. In addition, the current security classification policy should be examined to determine the feasibility of relaxing the 48-hour embargo on the clock and ephemeris parameters to civilian users. If the above recommendations are implemented, the NRC committee believes that the overall GPS performance and reliability will be greatly enhanced and that a stand-alone horizontal accuracy of the basic GPS signal approaching 5 meters (95 percent probability) could be achieved for both civilian and military users. Improvement Implementation Strategy Because of the relatively long life time of GPS satellites (5 to 10 years) and the length of time required to replace the total constellation of 24 satellites, opportunities for introducing enhancements and technology improvements to the system are limited. Figure 1 shows the current plan for satellite replacements. According to the GPS Joint Program Office, current plans for the Block IIF contract include 6 short-term and 45 long-term "sustainment" satellites. As currently planned, the Block IIF satellites will be designed to essentially the same specifications as the Block IIR satellites. The current program and schedule make it possible for another country to put up a technically superior system that uses currently available technology before the United States can do so. Under the current planning and in the absence of a preplanned product improvement (P3I) program, the earliest opportunity for an infusion of new technology in the GPS space segment would be after Block IIF, probably sometime after the year 2020. The NRC committee believes that there are significant improvements that could be made to the system much earlier than post-Block IIF that would not only enhance its performance for civilian and military use but also make it more acceptable and competitive internationally. One method to incorporate technology in an efficient and timely manner is through a P3I program beginning as early as possible in Block IIR. With this type of approach, planned changes and improvements could be intentionally designed into the production of the satellites at specific time intervals. Assuming that the first improvements suggested in this report are incorporated in the later half of the Block IIR satellites, additional funding might be required to incorporate changes for the already completed Block IIR satellites. However, the NRC committee
EXECUTIVE SUMMARY 11 believes that the timely improvement in system performance is adequate justification for the additional cost. In addition to the specific recommendations given in this report, the NRC committee also discussed several enhancements that it believes have particular merit and should be seriously considered for future incorporation. These items are discussed in Chapter 4. Although a few enhancements could be included on the Block IIR spacecraft, especially if a P3I program were implemented, most of the enhancements would have to be incorporated in the Block IIF spacecraft design. Figure 1 Current Plan for Satellite Replacement. (Courtesy of the GPS Joint Program Office)
