Keeping snowplows on the road

Starting this month, the Federal Highway Administration plans to broadcast a high-accuracy signal from space that could serve transportation safety planners, snowplowers and seed sowers alike.

The High Accuracy Nationwide Differential Global Positioning System (HANDGPS) will boost the accuracy of the Nationwide Differential Global Positioning System (NDGPS), which uses a combination of satellite networks and ground stations to determine the positioning of people or objects equipped with GPS receivers.

The current nationwide system is accurate within 1 meter to 3 meters, allowing for mapping and surveying of road segments. The new system will calculate the correction needed to offset errors inherent in NDGPS, thus helping improve its scope — down to the location of a fire hydrant (see box).

"GPS gives you position with varying degrees of accuracy," said David Grace, president and chief executive officer of Nautel Ltd., which is supplying equipment for the test of the new system. "If you can transmit a local correction factor, you increase your accuracy. If you have very accurate positioning, you can probably keep your plow on the highway."

That application is of particular interest to Grace, who lives in Nova Scotia. It also represents just one of several possibilities being explored.

A host of organizations began deploying NDGPS in 1998, including the Federal Highway Administration (FHWA), the Federal Railroad Administration, the Coast Guard, the Office of the Secretary of Transportation, the National Oceanic and Atmospheric Administration, the Air Force and the U.S. Army Corps of Engineers.

So far, about 80 percent of the country has been outfitted with 300-foot towers that receive information from 24 GPS satellites orbiting the Earth, according to James Arnold, a research electronics engineer at FHWA. "They're running quite well [and are] easy to maintain," he said. FHWA is trying to calculate the correction needed to offset errors in the system. However, errors beyond human control occur when solar wind and gravity alter a satellite's position or when atmospheric matter delays the signal. Satellites' clocks also cause inaccuracies.

The errors result in less precise data. "It's not accurate enough for a lot of applications," said David Gorg, a surveying and mapping engineer at the Minnesota Transportation Department. That's why FHWA wants to take the technology to the next level by enabling 3-D positioning at an accuracy of better than 20 centimeters.

The agency, in collaboration with the Coast Guard, the National Geodetic Survey and several other agencies, is implementing HANDGPS for testing at the NDGPS facility in Hagerstown, Md.

FHWA will loan up to four receivers with software to organizations for testing, with the stipulation that information gathered during the test be shared with the agency.

Transportation and safety workers might use HANDGPS to locate manholes, pipes, signs, property lines, construction limits and utilities; to measure skid marks at accident scenes; or for automated lane keeping.

Snowplowers could use the system to navigate through a blizzard, and farmers could use it to monitor yields, applying pesticides and herbicides only as needed.

"If we can provide very high accuracy on a very wide scale, it would reduce the cost of infrastructure, thus enabling applications to occur much faster," Arnold said. And cutting costs is especially attractive for a program that has faced steady opposition from Congress.

If the Hagerstown test is a success, FHWA could look to modify a second tower for the data correction, ultimately creating a corridor.


How It Works

Getting a fix on GPS

The High Accuracy Nationwide Differential Global Positioning System (HANDGPS) is the latest in a series of steps to use satellite technology to gather positioning information.

* GPS uses a network of 24 satellites to triangulate the position of a person or object equipped with a satellite receiver. The information is subject to errors caused when the signal passes through the Earth's atmosphere. GPS is accurate within 10 meters.

* Nationwide Differential GPS (NDGPS) uses ground-based reference stations to improve accuracy. The precise location of a reference station is compared to a code range generated by the signal. That difference is used to calculate and compensate for any errors. NDGPS is accurate within 1 meter to 3 meters.

* HANDGPS measures the signal — instead of its code — and compares it to the location of the reference station. Preliminary studies indicate HANDGPS has an accuracy of 20 centimeters.


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