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Federal Highway Administration > Publications > Public Roads > Vol. 65 · No. 2 > New Life for Old Transmitters: Converting GWEN to NDGPS

Sept/Oct 2001
Vol. 65 · No. 2

New Life for Old Transmitters: Converting GWEN to NDGPS

by James A. Arnold

The recent conversion of two Ground Wave Emergency Network (GWEN) sites in Annapolis and Hagerstown, Md., to Nationwide Differential Global Positioning System (NDGPS) broadcast stations is the latest chapter in the emerging NDGPS.

Whether mapping the natural resources of the largest state or determining the precise boundaries of a family farm, the practical applications of NDGPS are expanding exponentially. (Two previous Public Roads articles —"A More Precise Sense of Where We Are," Vol. 63, No. 4, January/February 2000, pages 7-13, and "New Applications Make NDGPS More Pervasive," Vol. 64, No. 4, January/February 2001, pages 39-43 — describe the emerging NDGPS and its burgeoning number of civilian uses.)

In a nutshell, this nationwide system of 300-foot- (91-meter-) high broadcast towers is being created to improve the accuracy of the positioning information obtained via radio signals emitted by the Global Positioning System (GPS) satellites traveling in orbit around the Earth.

In 1994, the departments of Transportation, Commerce, and Defense, which were looking for ways to improve the information provided by GPS, recommended the expansion of the Coast Guard's differential global positioning system (DGPS) system. They also noted that the U.S. Air Force, which operated GWEN, planned to decommission the network and its 53 transmitters across the country. In a subsequent field test, it was demonstrated that these transmitters, which operated on frequencies near the Coast Guard DGPS radio-beacon frequencies, could be successfully used to provide navigational information for surface transportation applications. The Department of Transportation (DOT), which was designated in 1996 as the lead agency for all federal civil GPS matters, determined that conversion of the GWEN transmitters would save the Air Force $6 million in decommissioning costs, while decreasing NDGPS deployment costs by $10 million.

photo of crane
Workmen are getting ready to use a crane to pull and replace equipment shelters. Notice the thin tower in the background. (Photo credit: Melissa Winn)

The process of converting existing GWEN sites to NDGPS sites is relatively simple, compared to the engineering feat of developing the system. The state of Maryland owns the land on which the Annapolis and Hagerstown GWEN sites are located. The Air Force leased the land from Maryland, so the lease had to be transferred to DOT. In addition, the leased property needed to be resurveyed and expanded slightly to accommodate additional equipment. The Coast Guard worked with Maryland's Department of Public Works to meet all the requirements. Although governmental approvals for the Annapolis site took about eight months due to unanticipated delays, all approvals for the Hagerstown site were secured within about four weeks from start to finish. The average cost of a conversion is about $300,000 to $400,000 for the survey, equipment, and construction of the site.

There are several major steps to convert a GWEN facility into an NDGPS facility. The first step is to replace the GWEN electronics in the equipment shelter with two racks of GPS equipment. One rack contains a transmitter, and the other has the GPS receivers and communications equipment. This takes place in Portsmouth, Va., at the Coast Guard's Command and Control Engineering Center (C2CEN). Equipment shelters are shipped to C2CEN where they are stripped and refurbished. This process includes replacement of the air-conditioning units with new, more efficient, and environmentally friendly units.

photo of equipment rack
NDGPS equipment rack.

(Photo credit: Melissa Winn)

Once the shelter is ready, it is shipped to the site where the existing equipment shelter is removed and the refurbished shelter is installed. The removed shelter is then shipped back to C2CEN for refurbishment so it is ready for the next site.

Prior to the arrival of the refurbished shelters, the concrete foundations for the two reference station towers are installed along with cable runs to bring the cabling from the equipment hut to the reference station tower.

While the concrete is hardening, the new antenna coupler is installed in the antenna tuning unit at the base of the broadcast tower. The only difficulty with doing this is reconnecting the antenna cables and ensuring that the coupler is secure.

Once the concrete has hardened sufficiently, the reference station masts are installed, the refurbished equipment huts are set in place, and any associated cabling is run from the equipment hut to the reference station masts, and any other wiring that is needed is installed and connected. The reference station masts and the refurbished huts are installed at the same time so that the crane is only called to the site one time.

Miscellaneous tasks that occur as time allows include maintenance on the backup diesel generator, weed control, etc.

When all is ready, the equipment is put through its final check and, if all is fine, begins broadcasting DGPS corrections.

An interagency group of technical people, the NDGPS Policy and Implementation Team, recommends policy for the implementation of the NDGPS system, including establishing priorities for converting GWEN sites to NDGPS.

The practical applications of NDGPS include grand applications and simple ones. For example, the state of Alaska plans to survey the entire state to map out natural resources and roads accurately. To accomplish this by traditional survey methods would cost about $100 million. Alaska transportation officials are convinced that an aerial survey with NDGPS will be more accurate than a land survey and will probably be much less costly.

To survey Alaska — a state with more than 570,000 square miles (almost 1.5 million square kilometers) of land — with NDGPS presents significant challenges: setting up 14 NDGPS sites, grappling with severe weather, getting to remote sites to set up the equipment, providing power to the sites, and coping with permafrost ground conditions in constructing the sites. However, if the funds are available to tackle the task, the result will be an extremely accurate picture of the natural and man-made landscape.

South Carolina has been particularly creative in its use of NDGPS. It is making road surveys more accurate to enhance 911 emergency response times and is using data obtained from NDGPS to update its tax maps.

The service has immediate practical application to the U.S. Department of Agriculture. Various conservation programs provide money to farmers for taking their fields out of crop production and putting them into trees or warm season grasses to reduce runoff into local streams and waterways. Quite often, survey markers have become lost and must be resurveyed. The traditional methods are expensive and can take time. With the NDGPS and a plat of the property, it is relatively easy to relocate the markers and determine the area that is covered under the program. We worked with the Natural Resource Conservation Service on a site in Maryland to demonstrate how easy this process was. Using an NDGPS receiver and having one survey point clearly visible, it was easy to generate a bearing and distance to find the remaining survey markers.

DOT currently has 25 operational NDGPS sites with an additional 55 planned before the end of 2004. In addition, the Coast Guard and the U.S. Army Corps of Engineers already have 55 maritime NDGPS sites, creating a seamless network over most of the country.

Many of the sites are located in rural areas, resulting in fewer neighborhood complaints, little vandalism, and little or no incursion into environmentally sensitive areas. DOT prefers to lease sites from the owners, many of whom are private citizens, so that the properties will stay on the tax rolls when DOT no longer needs them.


James A. Arnold is a research electronics engineer with the Federal Highway Administration's Office of Operations Research and Development. He received his bachelor's degree in electrical engineering from the University of Delaware in 1985 and his master's degree in electrical engineering from the Florida Institute of Technology in 1990. His experience includes the development of military communications systems and commercial communications systems related to intelligent transportation systems; technical evaluation of an integrated GPS for the U.S. Navy; technical management of the 1994 Augmented GPS study completed for the Department of Transportation; network design, spectrum planning, and environmental analysis for NDGPS; and service as the chairman of the State and Local Municipality Subcommittee of the Civil GPS Service Interface Committee (CGSIC). His primary responsibilities at FHWA include radionavigation and wireless communications in support of intelligent transportation systems.

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