National Academies Press: OpenBook

The Global Positioning System: A Shared National Asset (1995)

Chapter: Selective Availability

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Suggested Citation:"Selective Availability." National Research Council. 1995. The Global Positioning System: A Shared National Asset. Washington, DC: The National Academies Press. doi: 10.17226/4920.
Page 36

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GPS APPLICATIONS AND REQUIREMENTS 36 longitude) can be directly plotted on a paper nautical chart in the traditional fashion. This procedure often limits the accuracy of a position solution, not because of GPS errors but because of chart errors and plotting errors. More precise charts and plotting methods are therefore required in order to take advantage of the accuracy of GPS and DGPS. The current state of the art in marine navigation is the Electronic Chart Display Information System (ECDIS), which is capable of displaying information from nautical publications, electronic navigational charts, and navigation sensors simultaneously. The International Maritime Organization (IMO) has recently drafted an assembly resolution on performance standards for ECDIS. Final approval of this draft document at the next meeting of the IMO assembly in 1995 would represent the first step in replacing paper nautical charts with computer-generated electronic charts for commercial vessel navigation. Under current IMO regulations, all merchant vessels are required to carry and use up-to-date paper charts. Once ECDIS's built to IMO standards are in use, vessels will not be burdened by a requirement to maintain paper charts and will have superior navigation capabilities in coastal and harbor areas. A key issue for the timely implementation of ECDIS is the availability of digital data for the production of Electronic Navigational Charts. NOAA has begun the process of developing digital databases for electronic nautical charts, but faces serious resource limitations in this endeavor.26 Existing hydrographic surveys are often very old, and must be updated before accurate digital data can be developed from them. At today's rate of progress, NOAA expects that it will take 5 to 10 years to digitize paper charts of U.S. waters.27 Until this task is completed, charts will continue to be a source of marine navigation error that cannot be overcome by the widespread use of GPS. Selective Availability Despite the fact that users who desire accuracy better than 100 meters (2 drms) can now get it from DGPS services such as the U.S. Coast Guard's, SA still has a negative impact on the marine use of GPS. For recreational boaters, who prefer not to spend additional money on DGPS-capable receivers, this is especially true. Loran-C, which is still the most popular marine navigation system, is frequently used by fishermen to return to previously known fishing grounds with an accuracy of 20 to 30 meters. If GPS cannot meet or better this capability, recreational boaters, who could represent a large market for GPS, will be reluctant to embrace it in their operations. SA also has a negative impact on the ability of commercial ocean-going vessels to use GPS as the navigation sensor for automatic piloting equipment. Controlling a ship by 26 NOAA's electronic chartmaking efforts are the focus of an NRC report titled: Charting a Course Into the Digital Era: Guidance for NOAA's Nautical Charting Mission, Marine Board, National Research Council (Washington, D.C.: National Academy Press, 1994). 27 NRC, Minding the Helm, Marine Board, National Research Council (Washington, D.C.: National Academy Press, 1994), pp. 227-228.

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The Global Positioning System (GPS) is a satellite-based navigation system that was originally designed for the U.S. military. However, the number of civilian GPS users now exceeds the military users, and many commercial markets have emerged. This book identifies technical improvements that would enhance military, civilian, and commercial use of the GPS. Several technical improvements are recommended that could be made to enhance the overall system performance.

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