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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 117 (4) Schedule denial jamming/spoofing. Tactically, the U.S. military can interrupt denial jamming/ spoofing for short time periods, typically 2 to 3 minutes hourly, to assist those friendly forces in need of C/A-code to reacquire the Y-code. These scheduled times would be short and random to prevent hostile troops from taking advantage of interrupted jamming. Dependence upon this technique will diminish as improved training procedures and time discipline techniques are disseminated into the force structure. (5) Develop and utilize C/A-code selective denial techniques that minimize impact upon friendly L1-only military receivers, such as the Plugger. The L4 selective denial analysis of Appendix J suggests a four-part approach to selective denial of C/A-code on the L1 band: â apply shaped denial jamming combined with spoofing; â use a switchable retrofit bandstop RF front-end filter; and, â improve clock discipline, through operator training. 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 3-6 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 current planning, 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. As noted throughout this chapter, the NRC committee believes that there are significant improvements that could be made to the system 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 preplanned product improvement (P3I) process. With this type of approach, planned changes and improvements could intentionally be 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 believes that the timely improvement in system performance is adequate justification for the additional cost. Recommended improvements to the space segment and the operational control segment are summarized in Tables 3-13 and 3-14.
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 118 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 3-6 Current Plan for Satellite Replacement. (Courtesy of the GPS Joint Program Office)
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 119 Table 3-13 Space Segment Enhancements Proposed Enhancement Satellite Block Implementation Date Benefit of Enhancement Turn SA to zero. Block II/IIA, IIR, IIF Immediately Approximately 30- meter (2 drms) stand-alone accuracy for civil users. Add a new L-band signal (would Block IIR, IIF As soon as possible Approximately 12- meter (2 be usable before the Block IIR drms) stand-alone accuracy for constellation is complete). civil users. Enhanced integrity monitoring. Use inter-satellite crosslinks to Block IIR, IIF As soon as possible Improve overall system reliability relay satellite health information and availability and commands. Use inter-satellite crosslinks to Block IIR, IIF As soon as possible Improve GPS signal integrity for relay ground-based integrity all users. monitoring information and commands.
Table 3-14 Operational Control Segment Enhancements Proposed Enhancement Implementation Date: Benefit of Enhancement Provide more frequent correction updates. As soon as possible Improve stand-alone GPS accuracy for PPS and SPS users (if SA is off and 48-hour embargo is lifted) by reducing combined clock and ephemeris errors by half. Add more monitoring stations. As soon as possible Improve stand-alone GPS accuracy Improve overall system reliability by allowing prompt detection of satellite anomalies. Allow for uninterrupted tracking of all satellites. Improve Kalman Filter and dynamic models. Added to 1995 Operational Control Station Request for Improve accuracy by reducing combined clock and Proposal ephemeris errors with non-partitioned Kalman Filter (15%) and with improved dynamic model (5%). Establish procurement coordination of improved As soon as possible and in conjunction with the 1995 Improve accuracy. Allow for integrity monitoring of C/ monitor station receivers, computers, and software contract award A-code. contracts. Reexamine planned Block IIR operation and compare to Immediately Possibly improve accuracy over planned Block IIR the accuracy advantages gained by incorporating inter- operation. satellite ranging data in the ground-based Kalman Filter and uploading data at some optimal time. PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 120
Proposed Enhancement Implementation Date: Benefit of Enhancement Use Block IIR satellite communication crosslinks to the Block IIR satellites Improved reliability and availability Permits more extent possible with the existing crosslink data rate to rapid response time by the ground station. support on-board satellite health monitoring. Use Block IIR inter-satellite communication crosslinks Block IIR satellites Permits more rapid response time for integrity to relay integrity information determined through monitoring. ground-based monitoring. Permanent backup master control station. Immediately Reduce risk and improve reliability of overall system. Provide simulator to test software and train personnel. Immediately Reduce risk and improve reliability of overall system, improve efficiency of operations. Update the operational control segment software using Should be specified in 1995 Operational Control Station Easier to make modifications to software. Reduces modern software engineering methods in order to permit Request for Proposal. cost and complexity easy and cost-effective updating of the system and to enhance system integrity. This should be specified in the 1995 OCS upgrade request for proposal. PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 121
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 122
