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Federal Highway Administration > Publications > Public Roads > Vol. 63· No. 4 > A More Precise Sense of Where We Are

January/February 2000
Vol. 63· No. 4

A More Precise Sense of Where We Are

by James A. Arnold, Rudy Persaud, and David Smallen

NDGPS partner logos.

Across the nation, 91-meter communications towers are sprouting up. Located on grassy patches of 4.5 hectares, these towers look no more imposing than the average radio station tower, but they are part of one of history's most significant information breakthroughs.

The new towers, along with some existing towers, will form the Nationwide Differential Global Positioning System (NDGPS). This new system, which will be operational across the country by 2002, will refine and improve the ability of the existing Global Positioning System (GPS) technology to provide real-time location information. The improvement will be so profound that it will create an ever-increasing number of applications in transportation and other fields.

NDGPS applications benefit aviation.
NDGPS applications benefit trucking.

NDGPS will, for the first time, provide a valuable resource for many surface transportation applications -- especially Intelligent Transportation Systems (ITS) -- that require accuracy more precise than can be provided by the existing system, which is accurate to within 100 meters.

NDGPS applications benefit agriculture.NDGPS is an outstanding example of how different federal agencies with different needs and goals can work together to deploy a new technology with widespread benefits. It also demonstrates how the conversion of technology and facilities from military to civilian use can benefit both parties.

Three federal departments, led by the Department of Transportation (DOT), will construct and implement NDGPS by taking over the decommissioned U.S. Air Force Ground Wave Emergency Network (GWEN) and expanding an existing U.S. Coast Guard differential GPS.

"Nationwide Differential GPS technology will leverage the Department of Defense's investment in GPS and the U.S. Coast Guard's Maritime Differential GPS into the most accurate, most reliable, cost-effective navigation service available for civilian use," said Secretary of Transportation Rodney E. Slater in March 1999. "NDGPS will certainly be the next great transition for our nation's navigational capabilities."

The First GPS Satellite

NDGPS applications benefit shipping.
NDGPS applications benefit railroads.

GPS is a technology that is now more than two decades old. The Air Force launched the first prototype GPS satellite n 1977. This satellite was the forerunner of what became a 24-satellite system, providing location information to he earth's surface and surrounding space. In 1984, the system was made available for civilian use; however, until the deployment of NDGPS, surface transportation uses were limited by the lack of precision.

The existing system has been used for many important purposes, such as helping boaters, hikers, and fliers determine their location, but NDGPS will make possible a multitude of new uses.

The 24 GPS satellites transmit time signals continuously, allowing users equipped with GPS receivers to receive signals to calculate their position, time, and velocity. The civilian GPS signal, known as Standard Positioning Service (SPS), provides guaranteed accuracy within 100 meters.

The SPS accuracy standard is guaranteed 95 percent of the time. It is not 100-percent accurate because of several factors, the most significant of which is the deliberate limiting of the signal's accuracy by the Department of Defense for national security reasons. This random distortion, known as "selective availability," creates an inaccuracy of 100 meters or more.

Other errors can be caused by atmospheric distortion, improper geometric positioning of the satellites, changes in satellite orbits, inaccuracies in the satellite's atomic clock, and receiver inaccuracies. Any of these errors would make GPS ineffective for many surface transportation applications that require an accuracy of better than three meters.

Differential GPS (DGPS) uses the fixed location of a reference station to determine the inaccuracy of the satellite signal. The location derived from the satellite signal is compared to the reference station. That difference or inaccuracy can then be transmitted to non-stationary receivers. By comparing the inaccuracy to the satellite signal, the non-stationary receivers can then accurately determine their location. The closer to the transmitter, the more accurate the determination.

Lightly shaded areas on the map represent areas with DGPS coverage; darker shaded areas indicate the planned coverage to extend the system nationwide.
The single and overlapping light ovals represent the DGPS coverage provided by the Internationl Association of Lighthouse Authorities, Canadian Coast Guard, U.S. Coast Guard, and the U.S. Army Corps of Engineers in March 1999. The overlapping dark ovals indicate the planned coverage to extend the system nationwide.

These fixed locations -- the 91-meter towers -- are the basis for DOT's NDGPS. Single-availability coverage nationwide is expected by Dec. 31, 2001, meaning that the differential signal will be available 99.7 percent of the time. Dual coverage to offer 99.999 percent availability is expected by Dec. 31, 2002. Beyond the increase in availability, dual coverage also offers the ability to turn a station off for maintenance without disrupting service to users.

The Start of NDGPS

The preparations for a GPS that would provide accurate information nationwide began in early 1993 when then-Secretary of Defense Les Aspin and then-Secretary of Transportation Federico Peña chartered a joint task force to recommend civilian uses for GPS. In December 1993, the task force recommended a study of all differential GPS services -- deployed or under development -- to determine the best system.

A system deployed by the U.S. Coast Guard was one of those studied. The Coast Guard began exploring the uses of differential GPS in 1987 as part of a search for a system that would provide accurate navigational information along the coasts and on the Mississippi River and tributaries.

DOT, with the assistance of the departments of Defense and Commerce, took the lead on a study of ways to improve the information provided by GPS. In December 1994, the report of the study team, A Technical Report to the Secretary of Transportation on a National Approach to Augmented GPS Services, recommended a Coast Guard-like system for use in land navigation in those sections of the country that would not be covered by the Coast Guard system. The study team determined that use of the Coast Guard's DGPS Navigation Service Broadcast Standard would satisfy most surface transportation applications.

During the study, DOT learned that the U.S. Air Force planned to decommission GWEN, which provided emergency communications using 53 transmitters across the country. These transmitters operated on frequencies near the Coast Guard DGPS radio-beacon frequencies.

The tennis court demonstration shows the incredible increase in the precision and reliability provided by DGPS in comparison with the earlier GPS.
The tennis court demonstration shows the incredible increase in the precision and reliability provided by DGPS in comparison with the earlier GPS. DGPS is so precise that in the demonstration, you can see that the walker went around the judge's stand on the left center side of the court.

To test the use of the GWEN transmitters in NDGPS, the Air Force loaned the Coast Guard and the Federal Railroad Administration (FRA) a transmitter in Appleton, Wash. With this transmitter, the agencies provided navigational information along the Columbia and Snake rivers and "positive train control" to an FRA test bed. Positive train control allows dynamic spacing of trains based on their weight, location, and control characteristics, such as breaking ability, current speed, and other operational characteristics. Based on the success of the Appleton station test, DOT 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.

On March 28, 1996, President Clinton issued a presidential decision directive, U.S. Global Positioning System Policy, designating DOT as the lead agency for all federal civil GPS matters. The president directed DOT to "develop and implement U.S. Government augmentations to the basic GPS for transportation applications."

The presidential directive followed by only a few weeks the Coast Guard's achievement of initial operational capability for its Maritime DGPS. Full operational capability would follow in 1999.

The Prairie Push

Meanwhile, others were becoming aware of the numerous applications in transportation and other fields that could be gained from DGPS. In 1997, the South Dakota Department of Transportation persuaded Sen. Tim Johnson, D-S.D.; Sen. Tom Daschle, D-S.D.; and Rep. John Thune, R-S.D., of the enormous benefits of NDGPS. Johnson said the benefit-to-cost ratio for NDGPS is 150 to 1 "with future uses for the technology appearing almost limitless."

"The South Dakota Department of Transportation's goal was to map every mile of road in the state of South Dakota to give the state and local governments the ability to develop their communities and allocate important highway funds," said Johnson.

As a result of the efforts of the South Dakota DOT, Johnson and Daschle took the lead in pushing for funding for NDGPS. They introduced legislation that added to the DOT fiscal year (FY) 1998 appropriations bill.

The Johnson-Daschle appropriations amendment authorized the secretary of transportation to establish, operate, and manage NDGPS. The bill appropriated $2.4 million in capital expenses. The FY 1999 appropriations bill provided $5.5 million.

The Final NDGPS

Following the passage of the Johnson-Daschle amendment, DOT agreed with the departments of Commerce and Defense on a plan to deploy NDGPS. The Federal Railroad Administration was designated the lead agency because of its need for an accurate system for positive train control.

The plan includes deployment of 80 low-frequency broadcast stations by the end of 2002, of which 46 will be GWEN transmitters in their existing locations and the other 34 will be relocated GWEN transmitters. The final system will have 125 to 135 sites, which will include the Coast Guard's Maritime DGPS and the Army Corps of Engineers' sites.

The estimated cost of deployment is $36.8 million, and annual operating and maintenance costs are estimated to be $6.9 million.

DOT has projected the benefits from NDGPS with a 15-year life cycle to be $11 billion, of which $8.4 billion would be realized through safety and efficiency improvements for railroads and highways.

ITS Applications

Map showing all the current and projected DGPS sites in the United States.
This map shows all the current and projected DGPS sites in the United States.

A fully deployed NDGPS will operate as an "enabling technology" -- one that allows other technologies to function at a higher level. A significant number of the 30 user services to be provided by ITS depend on navigation systems and require navigation accuracies better than the 100 meters, 95 percent of the time, guaranteed by the GPS Standard Positioning Service. For ITS, the ability to pinpoint a location within three meters and even less than one meter in real-time will make a whole new range of applications possible.

By providing a continuous, standard, accurate location signal, NDGPS will help the ITS program achieve its goal of integration and interoperability in metropolitan and rural areas in a way that the less precise GPS without augmentation could not. It will also aid the Commercial Vehicle Intelligent Transportation Infrastructure Deployment Program to improve the safety and productivity of commercial drivers and vehicles and to reduce the costs associated with commercial vehicle operations. NDGPS also has the potential to be another major step forward in multistate cooperation and corridor development.

ITS applications that will benefit from NDGPS include navigation and route guidance; the management of fleets of vehicles, such as trucks, public transit buses, emergency vehicles, snowplows, and maintenance equipment; emergency notification or mayday services; roadway maintenance; and intelligent vehicle infrastructure.

Using sonar in tandem with DGPS, engineers can develop precise three-dimensional maps of the river, lake, or ocean bottom.
Engineers use sonar to determine the depth of the water adn to indicate possible scour around and below the dam's gates, intakes, baffles, and so on, but it is necessary to use DGPS to know precisely where the boat is when specific sonar readings are recorded. Using sonar and DGPS in tandem, engineers can develop precise three-dimensional maps of the river, lake, or ocean bottom.

Navigation and route guidance systems require the ability to continuously determine the vehicle's location. When equipped with NDGPS, these systems can determine a vehicle's location with respect to intersections and side of a divided highway. With NDGPS, the motorist can be given clear routing instructions that would not otherwise be possible.

GPS has already become a major factor in commercial fleet management, allowing companies to know the real-time location of their trucks, taxis, or limousines. The level of accuracy provided by NDGPS will make pick-up and delivery services even more efficient. Coordination of roadway maintenance and construction in remote rural areas would also be improved and automated with the positioning aid offered by NDGPS.

NDGPS and computer-aided dispatch systems will improve the effectiveness, efficiency, and coordination of emergency response teams, including firemen, paramedics, and police. The accurate location information provided by NDGPS could also reduce the time for responses to emergencies. Systems being tested today are designed to trigger and broadcast mayday messages when an event takes place, such as deployment of an airbag. If the vehicle position can be provided to within 10 meters, even to the correct lane, emergency response crews can be more effectively dispatched to the scene by the quickest route.

It is estimated that widespread use of an integrated vehicle safety system with an NDGPS receiver can save more than 1,000 lives annually on the roads. An alert system can warn drivers about crashes ahead of them, and the communications link can send for help in case of a crash.

Additionally, NDGPS can be used to map crash locations so that dangerous sections of road can be identified and corrected. The National Park Police are already using the Coast Guard DGPS system to document accident sites on the Baltimore-Washington Parkway. When an officer parks a police car at the crash site, the location is automatically placed in the electronic form that is completed for each incident. Each site is then placed on a map, and groupings that indicate potential problem sites are identified.

Mapping NDGPS benefits in the mapping of sediments around locks and dams.

NDGPS can also be a valuable tool for mapping roads and infrastructure. With the improved accuracy, it will soon be possible to map a large area quickly and efficiently and then be able to return to that area to inventory and re-inventory the roadside structures, such as signs and guardrails, in real time. Routine use of NDGPS for this purpose can result in maintenance funds being applied to areas or projects of greatest need. In addition, officials will have a highly accurate digital map of the highway infrastructure.

Accurate maps have many valuable uses. In growing communities, officials with updated maps will be able to revise school bus routes to increase efficiency, resulting in reduced fuel consumption, shorter bus trips for students, and less wear and tear on buses. Emergency response, mail delivery, and other delivery services can also benefit from accurate maps.

There are many other transportation applications using an augmented GPS. It can be used to inventory roadside infrastructure, such as highway signs, milepost markers, rights of way, guardrails, and bridges; to support operations and maintenance; to provide guidance to snowplows in low-visibility situations; to inventory railroad crossings and road centerlines; to track hazardous materials from origin to destination; and to map pavement condition and other traffic data.

State Applications

Many state transportation departments have recognized the value of NDGPS. They are using the technology in many different ways. Some states are using the technology for standard applications. Others are experimenting and testing new uses. Still others are reviewing and evaluating it.

These departments view NDGPS as a means of improving public safety and as a way of increasing efficiency by reducing the person-hours required to collect and process location data.

One of the most widely used applications by transportation agencies is the integration of DGPS with the Geographic Information System. GIS allows the association of data statistics of any kind with a specific geographic location and the displaying of the data on an interactive map. With DGPS, the exact location of each data point can be determined. An example is the use of DGPS to monitor dangerous sections of highway by mapping accident statistics on a GIS map.

The following is a list of some of the current uses of DGPS data in particular states:

  • Vehicle guidance systems for snowplows and other vehicles (Arizona and California).
  • Video-logging of roadside features (Arizona and Kansas).
  • Surveys for roadway design (Colorado, Minnesota, North Dakota, and Wisconsin).
  • A "mayday" system for police and snowplows (Colorado, Kansas, Oregon, and South Dakota).
  • Updating road maps (Colorado, Florida, Louisiana, Minnesota, Nevada, New York, North Dakota, South Dakota, Virginia, and Wisconsin).
  • Creating a database for an interactive map (Florida, Indiana, Michigan, Montana, New York, North Dakota, South Dakota, and Tennessee).
  • Mapping crash sites (Indiana, Montana, New York, North Dakota, and Wisconsin).
  • Mapping of wetlands and other environmental features to determine construction impact (Iowa, Michigan, New York, North Dakota, and Virginia).
  • Pavement management and other maintenance (Kansas, Michigan, Minnesota, Utah, and Virginia).
  • Automatic vehicle location and fleet management (Kansas, Minnesota, New York, North Dakota, and Virginia).
  • Congestion management and planning (Louisiana and South Dakota).
  • Remote vehicle control (Minnesota).

No Limits

As more transportation agencies on the federal, state, and local levels continue to explore uses for NDGPS, there will be an ever-expanding number of applications. Transportation officials will find that a modest investment in the necessary equipment, such as receivers, will pay returns worth many times more.

DGPS is an already proven technology, and many areas already have DGPS at their disposal. The entire country will have it within three years. Forward-looking transportation agencies will not be limited by the system's technological capabilities -- only by institutional and legal barriers.

NDGPS will mean improved safety, efficiency, and mobility, virtually revolutionizing our transportation system for the 21st century.

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 communication systems and of commercial communication systems related to intelligent transportation systems; technical evaluation of an integrated GPS for the US Navy; technical management of the 1994 Augmented GPS study completed for the Department of Transportation; network design, spectrum planning, and environmental analysis for the NDGPS service; 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.

Rudy Persuad is a transportation specialist with the Office of Planning and Engineering for the South Dakota Department of Transportation. He is in charge of the local road system, and he implemented the GPS and GIS program for all roads in South Dakota. He was recently on an assignment with the Federal Highway Administration, conducting a nationwide investigation of GPS applications within state agencies. His experience includes development of a statewide program for GPS/GIS, responsibility for the statewide rural road inventory program, participation on the NDGPS Policy and Implementation Team, coordination of state and local government program support for NDGPS, and service as co-chairman of the State and Local Municipality Subcommittee of the Civil GPS Service Interface Committee (CGSIC).

David Smallen is the president and chief executive officer of David Smallen Associates, a consulting/writing/editing company in Washington, D.C. For 14 years, he served on Capitol Hill, starting as press secretary and then as director of communications for the House Committee on Public Works and Transportation and as senior staff member of the House Subcommittee on Investigations and Oversight. Before that, he was a newspaper and news service reporter. He has a bachelor's degree from Duke University, and he attended the graduate school of journalism at the University of North Carolina.

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