The Global Positioning System: A Shared National Asset (1995)
Chapter: Maritime DGPS Services
reference station position. Each reference site would measure coded and codeless L1 and L2 data. This data would then be sent to the CORS Central Facility, where it can be stored on computer disc. Users could then access this data electronically within one hour after it has been measured, providing post-processed positioning accuracy of 5 to 10 centimeters. All Coast Guard, Army Corps of Engineers, and FAA reference stations that are part of the DGPS services described above are designed to be CORS-compatible. In addition, a recent technical report to the Secretary of Transportation has recommended that all future federally provided DGPS reference stations should comply with the CORS standard.55
State and Local Government DGPS
A number of state and local governments either have established or plan to establish permanent DGPS reference sites. For example, Riverside County, California, has established two continuously operating, permanent DGPS reference stations as part of the Permanent GPS Geodetic Array. This array, whose participants also include federal agencies, state agencies, other local government agencies, and universities, is used primarily for earthquake monitoring and, perhaps, eventually will be used for earthquake prediction. Riverside County engineers and surveyors, however, also use the array for typical day-to-day surveying applications.
Differential Systems Supported by Foreign Governments and International Organizations
Foreign governments and public sector international organizations are actively developing and utilizing differential GPS networks. Several examples designed to support aviation, maritime, and survey/scientific applications are discussed below.
Maritime DGPS Services
Many countries are currently operating, prototyping, or planning maritime DGPS services similar to the U.S. Coast Guard's. The low cost, combined with the absence of any international frequency allocation problems makes these systems practical for all nations. Since most sea coasts and ports have medium-frequency radiobeacons for direction finding, DGPS services can be added quite simply with the purchase and installation of off-the-shelf GPS equipment. The International Association of Lighthouse Authorities (IALA) coordinates the assignment of frequencies and DGPS reference station identifying numbers,
