DGPS sets new course in navigation

The Coast Guard turned on its nationwide Differential Global Positioning System network last week, launching what Secretary of Transportation Federico Pena called a "new era in navigation."

The 47 sites in the Coast Guard DGPS network provide complete coverage of all U.S. coastal waters, including Alaska, Hawaii and Puerto Rico. Coast Guard DGPS sites also blanket the Great Lakes, and in cooperation with the Coast Guard, the Army Corps of Engineers has installed six DGPS sites along the Mississippi river, providing coverage to much of the Midwest.

Pena said the DGPS network will provide accuracies at 10 meters or better to some of the 275,000 commercial and recreational vessels already equipped with GPS receivers. This compares with the 100-meter accuracy available to civil users of the 24-satellite GPS constellation built by the Defense Department, which deliberately skews the signal from the satellites to degrade accuracy to nonmilitary users.

Chief Warrant Officer-4 Dick Soucy, of the Coast Guard Electronics Engineering Center, Wildwood, N.J., which did much of the development work for the DGPS network, said accuracies from the system are far better than the "advertised" 10 meters. "We're getting anywhere from .8 meters to 1.5 meters, depending on a number of factors, including ionospheric conditions," Soucy said.

DGPS, Soucy explained, works by installing a reference GPS receiver at a known, accurately surveyed site. This special receiver computes the difference between the surveyed position and the position reported by the satellite and then broadcasts corrections to users equipped with receivers able to pick up signals broadcast by the Coast Guard DGPS network.

Pena said the Coast Guard recycled one of the oldest types of navigation aids—radio beacons, which the service began installing in 1924—to serve as the heart of its DGPS network. Soucy said each DGPS site includes a radio beacon transmitter plus two GPS reference receivers from Ashtech Inc. and two GPS integrity monitors from Trimble Navigation Inc., all rack-mounted inside a weatherproof hut.

The nationwide DGPS network is totally automated and unmanned, with control and monitoring performed by a control station located at the Coast Guard Navigation Center, Alexandria, Va.

Soucy said the center is connected to the individual DGPS sites through an X.25 packet data network on AT&T's FTS 2000 network. The control station uses two Navy Tactical Advanced Computer-3 computers to drive four displays, with one computer acting as a backup.

A West Coast site in Petaluma, Calif., has a similar equipment configuration and can take over management of the network if an outage occurs at the Alexandria site.

Besides improving maritime navigation, Pena said the DGPS network will aid other transportation users. The Army Corps of Engineers funded the Mississippi River chain of stations to provide precise positioning information for dredges operating on that river, Pena said.

Dick Shamberger, a senior rail operations specialist with the Federal Railroad Administration, said the FRA plans to use DGPS stations in the Pacific Northwest to help control trains by improving safety and allowing railroads to operate more high-speed trains on the same stretch of track.

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