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GPS APPLICATIONS AND REQUIREMENTS 26 â¢ have sufficient jamming-to-signal ratio strength to navigate through the jamming environment successfully; â¢ be able to null out the jamming signal; and/or â¢ have an alternative to GPS for navigating through the jamming environment. The military currently relies on its SA and A-S (Anti-spoofing) security procedures to deny full GPS accuracy to the enemy while maintaining the use of a highly accurate spoof resistant signal. Anti-jam antennas and antenna electronics also are deployed on many weapons systems to provide increased jam resistance, and integrated GPS/inertial navigation systems provide a means of navigating to a target in spite of successful jamming. None of these procedures and technical measures, however, can be considered the final solution to the military's requirement to simultaneously use GPS and deny its use to the enemy. A number of candidate improvements in this regard are presented in the next chapter. Findings The GPS PPS meets most of the military's positioning and navigation accuracy requirements, although some applications require accuracy and integrity that is beyond the capability of the PPS as currently configured. The anti-jamming and anti-spoofing capabilities of military GPS user equipment are critical to successful mission completion in a battlefield environment characterized by both U.S. and enemy spoofers and jammers. GPS AVIATION APPLICATIONS Despite the fact that investigations into the use of satellites for civil aviation applications have been conducted for over 20 years, the concept was not considered financially or technically feasible until the development of GPS.11 Instead, a large number of ground-based radionavigation systems have been relied upon around the world for air navigation services, and ground-based air traffic controllers have utilized radar, voice position reporting, and visual sightings for aircraft surveillance.12 It now appears that a 11 Federal Aviation Administration (FAA), FAA Satellite Navigation Program Master Plan. FAA Research and Development Service, Satellite Program Office (ARD-70), 15 February 1993, p. 2. 12 Existing ground-based radionavigation systems include NDBs (Non-Directional Beacon), VORs (VHF Omni-directional Range), VOR/DMEs (VORs with Distance Measuring Equipment), TACANs (Tactical Air Navigation), and VORTACs (combined VORs and TACANs). Other systems include the Instrument Landing System (ILS), used for precision approach and landing, and Loran-C and Omega, both of which are used for en route navigation. Each of these systems is described in detail in the 1992 Federal Radionavigation Plan.