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PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 82 Enable System Modifications that Further Improve Accuracy. If SA is turned to zero, then accuracy is limited by ionospheric errors, clock and ephemeris errors, multipath errors, and receiver noise errors, as Table 3-3 illustrates. As discussed later, technical modifications can reduce these errors. However, with SA set at its current level, any modifications to reduce other errors and improve accuracy will be overwhelmed by the degrading effects of SA. Improved WAAS. When SA dithering of the GPS signals is employed, the DGPS corrections required to circumvent SA accuracy degradation must keep up with the dithering rate. Since WAAS will broadcast its differential corrections as part of the navigation message data carried by a GPS-like L1 signal, a high-data rate for the differential correction is required, which constrains the flexibility of providing additional information on the navigation message. If SA were eliminated, the data rate requirement could be relaxed and more information, such as GPS integrity information and other safety or air traffic control related information, could be sent to the user. As noted above, integrity also would improve if SA were eliminated, However, even if SA were removed, the FAA's integrity and availability requirements would still not be met with the basic GPS. Some type of augmentation, such as WAAS, would still be required. Findings and Recommendations The NRC committee finds that in view of the rapid proliferation of both local and wide-area DGPS systems worldwide and the ease with which local DGPS stations can be deployed, the current effectiveness of SA in deterring precision attack by adversary forces is severely limited and will essentially be ineffective in the near future. The NRC committee also found that effective countermeasures to adversary use of GPS and DGPS are currently inadequate. The NRC committee believes that future military strategy should focus on electronic denial of all useful signals to our enemies, for example, by jamming and spoofing, while improving U.S. military ability to use GPS in a jamming and spoofing environment. The principal shortcoming in this strategy, regardless of the level of SA, is the difficulty military GPS receivers currently have in acquiring the Y-code during periods when the C/A-code is unavailable due to jamming of the L1 signal. The implementation of direct Y-code acquisition capability, as recommended later in this chapter, would provide the optimal solution to this problem. Based on information from receiver manufacturers, the committee believes that the technology for developing direct Y-code receivers is available
PERFORMANCE IMPROVEMENTS TO THE EXISTING GPS CONFIGURATION 83 today. The committee believes that a focused high priority effort by the DOD to develop and deploy direct Y- code user equipment, backed by forceful political will from both the legislative and executive branches, can bring about the desired result in a relatively short period of time. However, in the interim time before direct Y- code receivers are fielded by the military, various operating disciplines also discussed in this chapter, can minimize the impact of L1 C/A-code jamming on the ability to acquire the Y-code. The committee also has taken cognizance of the DOD belief that exploitation of the GPS C/A-code is more likely in the near term than exploitation of DGPS signals. Even if potential adversaries are not taking advantage of DGPS at this time, the NRC committee believes that it would be prudent for the DOD to recognize the potential capability that currently exists. The NRC committee believes that continued reliance on SA as a means of denying precise GPS position location to all non-military users over a wide area is a strategy that will ultimately fail. In addition, the removal of SA and the subsequent increase in accuracy obtainable by civil and commercial GPS users would have substantial benefits, as previously discussed. If the use of SA is eliminated, the NRC committee also expects that the market for GPS receivers and systems would increase substantially, as discussed further in Appendix E. The six most important findings of the NRC committee regarding the impact of SA on the various classes of civilian users and on meeting its intended purpose are (1) The military effectiveness of SA is significantly undermined by the existence and widespread proliferation of DGPS augmentations as well as the potential availability of GLONASS signals. (2) Turning SA to zero would have an immediate positive impact on civil GPS users. Without SA, the use of DGPS would no longer be necessary for many applications. System modifications that would further improve civilian accuracy also would be possible without SA. (3) Deactivation of SA would likely be viewed as a good faith gesture by the civil community and could substantially improve international acceptance and potentially forestall the development of rival satellite navigation systems. Without SA, the committee believes that the number of GPS and DGPS users in North America would increase substantially. 11 (4) It is the opinion of the committee that the military should be able to develop doctrine, establish procedures, and train troops to operate in an L1 jamming environment in less than three years. 11 The analysis by Michael Dyment, Booz · Allen & Hamilton, 1 May 1995, is shown in Appendix E.
