pilots. Others are more automated and rely on the ability of ships to monitor themselves.²⁴ In both cases, GPS and DGPS are used to provide accurate positioning information that is integrated with other positioning, communications, and computing technologies.

The navigational use of GPS has evolved slowly in the maritime community, due in part to the lack of continuous service available from GPS until initial operational capability was declared in December 1993. Commercial shippers are now beginning to equip their vessels with GPS for navigation, however, and in April 1994 the Coast Guard declared that a GPS receiver meets the requirements for carriage of electronic position fixing devices as prescribed under US CFR Title 33, Part 164, section 164.41. The U.S. Coast Guard also has established a DGPS network that will eventually provide coverage to all U.S. coastal areas, ports, harbors, and inland waterways. Some commercial shippers have begun to experiment with DGPS capability as well.

Use of GPS among recreational boaters is becoming widespread. The low cost ($400-$2,000) of marine GPS equipment has made it an attractive alternative to other systems such as Loran-C and Transit. Recreational use of DGPS, however, has been limited to applications, such as yacht racing, that require improved position and velocity information. The additional cost of the beacon receiver used to receive the Coast Guard DGPS corrections has limited the recreational use of DGPS.

Maritime navigational requirements are well documented in the Federal Radionavigation Plan.²⁵ It breaks the marine navigation problem down into several distinct phases that relate to different geographical considerations. These are oceanic, coastal, harbor/harbor approach, and inland waterway. The oceanic and coastal requirements have been derived from the limitations of systems that have been used for these phases of navigation for some time, such as celestial plotting techniques, Loran-C, and Transit, whereas the harbor/harbor approach requirements were developed through research on ship maneuvering and the human factors involved in piloting large commercial vessels. Table 2-5 lists the current GPS requirements for the oceanic, coastal, and harbor/harbor approach phases of navigation.

Because official inland waterway requirements have not yet been adopted, the values shown in Table 2-5 for this phase of navigation should be considered as tentative estimates. Recent field trials of the Coast Guard's DGPS service, however, have demonstrated sufficient accuracy to satisfy the Army Corps of Engineers inland waterway construction requirement of 6 meters (2 drms). It is likely that this same system also could satisfy inland waterway navigation requirements. The Coast Guard's goal for their DGPS service is to

achieve 3-meter (2 drms) accuracy for these operations, and provide the needed integrity and availability for navigation as well.

In contrast to navigation, GPS became practical for many positioning applications as soon as there were a few hours of satellite coverage each day. The Coast Guard, for example, began positioning navigation buoys with DGPS in 1990, when there were only 12 hours of GPS coverage per day. Other applications include the positioning of offshore oil platforms by petroleum companies and hydrographic surveying conducted by the National Oceanic and Atmospheric Administration (NOAA) to develop nautical charts. These users often augment the GPS standard positioning service with DGPS services provided by the Coast Guard or private sector companies.

Requirements for positioning applications are not well documented. Generally, positioning applications strive to achieve the best accuracy possible within a user's practical limitations, which are often related to time and cost. A system that satisfies marine navigation requirements for accuracy often satisfies some marine positioning requirements as well. Many high frequency, very-high frequency, ultra-high frequency and microwave systems have been developed and successfully used over the years to provide high accuracy positioning information in specific geographic areas. With DGPS coming on line and meeting the harbor/harbor approach requirement of 8 meters (2 drms), however, the need for these other systems has waned.

Marine surveillance systems, such as Coast Guard and commercial VTS, require accurate velocity data in addition to accurate positioning information. The continuous broadcast of velocity from each ship in a given VTS coverage area will allow pilots and VTS operators to take evasive action when two or more ships are approaching the same location at a fast closure rate. DGPS currently yields velocity accuracy on the order of 0.1 nautical miles per hour, which is sufficient for this application.