U.S. deployment of a nationwide system to boost the accuracy of the GPS satellite navigation and positioning system risks being stalled because funds for continued rollout have been removed from the U.S. Department of Transportation’s 2007 budget request.

Unless the funds are reinstated by the U.S. Congress, a service that permits GPS users to benefit from positioning accuracy of less than a meter will be put on hold despite the fact that more than two-thirds of U.S. territory already has been outfitted with the necessary hardware.

The Federal Railroad Administration is the lead agency inside the Department of Transportation for the Nationwide Differential Global Positioning System, or NDGPS.

Len Allen, the railroad authority’s program manager in the office of research and development, said NDGPS backers remain hopeful that the budget cancellation will be reversed during congressional review of the administration’s 2007 federal budget request.

Allen said in a June 6 interview that the NDGPS program already saw a 50-percent budget cut, to $10 million, for 2006. The program is part of the Federal Railroad Administration’s research and development budget.

“Since the program has been zeroed out of the 2007 budget, we are postponing new construction to preserve the remaining funds for operations and maintenance,” Allen said. “We were a bit surprised by the budget decision since NDGPS has been listed as one of the Transportation Department’s priorities.”

The NDGPS budget battle comes even as the U.S. government seeks to persuade other nations of the merits of the GPS system, which in the coming years will be joined by global or regional satellite navigation programs sponsored in Europe, Russia, China, Japan and India.

Differential GPS refers to the network of ground stations — each includes a small equipment hut, two 3-meter-tall towers to received GPS satellite signals and a 60- to 90-meter tower to broadcast data to users — that are being deployed throughout the United States. The program is an expansion of a similar effort managed by the U.S. Coast Guard to facilitate maritime navigation.

NDGPS stations receive GPS satellite signals and broadcast the data to GPS users. These users also receive data directly from the GPS satellites. Allen said 92 percent of the territory of the lower 48 U.S. states already are equipped with one NDGPS station. Nearly two-thirds of this territory is covered by two stations, providing backup in the event of an outage of one. Once finished, the network would feature 125-130 NDGPS stations.

To complete the deployment, the Federal Railroad Administration estimates it would need a budget of $32.1 million per year for three years. After that, the annual NDGPS maintenance budget would be $9.2 million per year.

As is often the case with GPS market studies, estimates of the economic value of the NDGPS system are difficult to verify. The Federal Railroad Administration says that NDGPS-aided signals for precision agriculture can save between $5 and $14 per acre for farmers.

By permitting more-efficient use of railways, NDGPS — used as part of what is called Positive Train Control — could save the railroad industry alone several billion dollars per year, according to Federal Railroad Administration Estimates.

Other uses include surveying of roads, dredging and buoy positioning, and mapping. Using a high-accuracy version of NDGPS, the basic GPS satellite signal that provides positioning accuracy of 5-15 meters can be enhanced to 10-15 centimeters or less. The NDGPS stations also measure water vapor in the atmosphere above the station, helping weather prediction.

Several dozen nations, including most of Western Europe, have adopted DGPS technical standards to deploy similar networks on their territory.

DGPS is a subject rarely evoked publicly in Europe because it would undermine European governments’ argument in favor of a separate satellite navigation system, called Galileo, which is now under construction. Galileo backers compare Galileo’s higher-accuracy signal to GPS without mentioning the advances in DGPS.

NDGPS is separate from the satellite-based GPS augmentations that have been deployed in North America, Europe and Japan, with airline use as one of their primary target markets.

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