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APPENDIX C 170 U.S. Government-Supported Differential GPS There are currently at least a dozen U.S. federal agencies that operate or plan to operate permanent DGPS networks.46 Three agencies in particular, the FAA (Federal Aviation Administration), the U.S. Coast Guard, and NOAA (National Oceanic and Atmospheric Administration), plan to provide nationwide DGPS services. Each of these three programs is described briefly below. FAA Wide-Area and Local-Area DGPS Concepts The FAA plans to improve the accuracy, integrity, and availability of GPS to levels which support flight operations in the National Airspace System from en route navigation through Category I precision approaches by using a wide-area DGPS concept known as the Wide-Area Augmentation System (WAAS).47 In June 1994, the FAA released an RFP (request-for-proposal) for the WAAS that calls for a ground-based communications network and several geosynchronous satellites to provide nationwide coverage. The ground-based communications network will consist of 24 wide-area reference stations, two wide-area master stations, and two satellite uplink sites. Differential corrections and integrity data derived from the ground-based network, as well as additional ranging data, will be broadcast to users from the geostationary satellites using an "L1-like" signal with a frequency of 1575.42 MHz.48 The RFP calls for the WAAS to be in place by the end of 1997. Local-area DGPS systems are also being considered by the FAA to support landing operations beyond Category I. The airline industry estimates that there are approximately 120 runways in the United States that will require this type of service through the year 46 According to the U.S. General Accounting Office, nine federal agencies either owned and operated, or planned to own and operate permanent differential GPS base stations by fiscal year 1996. They included: the Army Corps of Engineers; the Bureau of Land Management; the Environmental Protection Agency, the Federal Aviation Administration; the Forest Service; the National Oceanic and Atmospheric Administration; the St. Lawrence Seaway Development Corporation; the U.S. Coast Guard; and the U.S. Geological Survey. Source: U.S. General Accounting Office, Global Positioning Technology: Opportunities for Greater Federal Agency Joint Development and Use, GAO/RCED-94-280 (Washington, D.C.: U.S. Government Printing Office, September 1994). At least three additional U.S. federal agencies own and operate permanent DGPS reference stations, including the Department of Defense, the National Aeronautics and Space Administration, and the National Science Foundation. 47 Category I approaches can be flown when the visibility is no less than 0.5 miles (0.8 kilometers), and the ceiling is no lower than 200 feet (61 meters). 48 Federal Aviation Administration, System Operations and Engineering Branch. Wide-Area Augmentation System Request For Proposal, DTFA01-94-R-21474 (Washington, D.C.: U.S. Department of Transportation, 8 June 1994).
