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GPS APPLICATIONS AND REQUIREMENTS 21 to replace all other operational systems.3 The coalition military forces demonstrated the effective use of GPS for many of these proposed applications during the Persian Gulf War, despite the fact that the GPS constellation consisted of only 16 satellites at the time. This limited three-dimensional coverage of the Persian Gulf region to 18 hours per day. Another limiting factor was the small number of P-code military receivers in the DOD inventory at the time of the conflict. This prompted a National Command Authority decision to turn SA to zero during the war and led to the DOD's purchase of thousands of civilian GPS receivers, which became known as "sluggers".4 In addition to this official procurement, many units and individuals deployed to the Persian Gulf ordered their own GPS receivers directly from vendors and manufacturers.5 Current and Future Applications and Requirements The most common use of GPS during the Persian Gulf War, and perhaps the most critical, was for land navigation. U.S. Army tanks and infantry relied heavily on GPS to avoid getting lost during movements to various destinations in the featureless desert. GPS also was used by coalition forces for en route navigation by aircraft, helicopter search and rescue, marine navigation, and even munitions guidance in the case of the U.S. Navy's Standoff Land Attack Missile (SLAM).6 The use of GPS for precision-guided munitions such as the SLAM will increase in the future. The U.S. military currently has, or is developing, eight additional types of precision land attack weapons that utilize GPS integrated with inertial navigation systems for mid-course guidance.7 Another important GPS application under consideration is the 3 The DOD plans to phase out use of Loran-C and Omega in 1994, Transit in 1996, and land-based navigation aids by 2000, depending on the progress of GPS installation and integration. Civilian use of these systems, however, may continue. Source: Radionavigation System Users Conference held in Washington D.C. on November 9-10, 1993, (unpublished). 4 The "slugger" or Small Lightweight GPS Receiver is a Trimble Navigation TRIMPACK, three-channel receiver that utilizes the L1, C/A-code to provide three-dimensional navigation capability. More than 10,000 receivers were purchased by the DOD from Trimble Navigation and other receiver manufacturers during the Persian Gulf War. 5 Bruce D. Nordwall, "Imagination Only Limit to Military, Commercial Applications for GPS," Aviation Week & Space Technology, 14 October 1991, p. 60. 6 Joseph Wysocki, "GPS and Selective AvailabilityâThe Military Perspective," GPS World, July/August 1991, pp. 38-43. 7 These eight weapons include: the Tomahawk Block III and IV cruise missile; the Tri-Service Stand-Off Attack Missile; the Joint Direct Attack Munition; the Joint Stand-Off Weapon; the GBU-15 precision glide bomb; the AGM-130, a powered version of the GBU-15; and finally, the ATACMS ballistic missile. Source: J.G. Roos, "A Pair of Achilles Heels: How Vulnerable to Jamming are U.S. Precision-Strike Weapons?" Armed Forces Journal International, November 1994, p. 22.