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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: Date: Jan/Feb 2001|
Issue No: Vol. 64 No. 4
Date: Jan/Feb 2001
Imagine that you are driving through Minnesota on a January evening. You know that blizzard conditions have been forecast for the area. Snow is falling, and driver advisories have been posted for the entire region. Are you very nervous? No! Not when Minnesota has equipped their snowplows with Nationwide Differential Global Positioning System (NDGPS) receivers.
With NDGPS, the roads can be kept clear even in the worst conditions. You also know your vehicle is equipped with NDGPS linked to your cell phone to automatically alert emergency service personnel if you are involved in an accident. And you know that each NDGPS reference location also contains precipitable water vapor sensors that have increased the accuracy of the precipitation forecasts from the National Weather Service.
And that's not all of the many advantages of NDGPS. With an accurate forecast from the National Weather Service telling travelers where a blizzard is headed and when it will arrive with NDGPS, travelers may be able to navigate around the blizzard.
Does this imaginary scenario sound too good to be true? It shouldn't because all of these applications of NDGPS are being developed by government scientists, who can envision these and other uses for this new service. It has been called an "enabling technology" because it will allow other technologies to function at improved levels.
In a previous article ("A More Precise Sense of Where We Are," Public Roads, Volume 63, Number 4, January/February 2000, pages 7-13), co-authors David Smallen, Rudy Persaud, and I described the emerging NDGPS. In summary, this nationwide system of 91-meter- (300-foot-) high reference towers is being created to improve the accuracy of the positioning information obtained via radio signals emitted by the 24 Global Positioning System (GPS) satellites traveling in orbit around the Earth.
Dramatic Improvements in Accuracy
While the civilian GPS signal, known as the Standard Positioning Service (SPS), currently provides positioning information for civilian hikers, boaters, users of in-vehicle positioning devices, and others, the accuracy of SPS is limited for various reasons to approximately 25 meters (82 feet). Preliminary work has shown that even a limited NDGPS can improve that accuracy to approximately 1 meter (3 feet). Accuracy is expected to improve to 2 to 20 centimeters (0.8 to eight inches) over the next few years as new correction techniques evolve.
Improvements are possible because the fixed location of an NDGPS tower is compared with the GPS-determined location of the tower and the measured difference between actual location and GPS location can be used as a correction factor. This factor or "signal" is transmitted to mobile receivers in vehicles or in the hands of individuals. These receivers use the corrective signal to adjust the GPS location so that it becomes the more accurate NDGPS location. The closer the receiver is to the transmitter, the more accurate is the correction and, thus, the adjusted location. Also, with more towers, it is possible to perform a dual-tower fixed-location comparison for even more precise accuracy.
The NDGPS installation plan calls for the deployment of 80 low-frequency broadcast stations by the end of 2002. Significant factors have been incorporated into the plan to increase its cost-effectiveness by reusing existing resources. The new NDGPS broadcast stations will be redesigned, decommissioned Ground Wave Emergency Network (GWEN) towers provided to the Department of Transportation (DOT) by the U.S. Air Force. In addition, NDGPS will be sharing the use of the U.S. Coast Guard's Maritime Differential GPS sites and the U.S. Army Corps of Engineer sites. NDGPS is expected to have 125 to 135 involved sites by 2004.
This massive undertaking is possible because of efficient and economical cooperation among the U.S. departments of Transportation, Defense, and Commerce. DOT has the primary coordinating responsibility, and within DOT, the Federal Railroad Administration is leading the development of the emerging NDGPS service. Even though NDGPS at present has not reached its full potential, many applications of this service are evolving.
Improvements in Weather Forecasting
For example, one agency within the Department of Commerce has already discovered a very important use for more accurate positioning. Scientists in Boulder, Colo., at the Forecast Systems Laboratory (FSL) of the National Oceanic and Atmospheric Administration (NOAA) have been investigating the use of NDGPS to improve weather forecasting and climate monitoring. Seth Gutman of the FSL reports that NDGPS data does have a positive effect on weather forecasting vectors.
For some time, FSL has been using GPS data to measure the delays in the transfer of signals from the GPS satellites caused by water vapor in the atmosphere. Although the total amount of water vapor could be measured through calculations, it was not clear that this information helped in weather forecasting. Work with the new NDGPS has provided the ability to measure the slowing of satellite signals by water vapor with unprecedented accuracy.
The improved accuracy has resulted in another application for NDGPS and a new weather observation system at NOAA. According to Gutman, scientists and engineers at FSL are now building the prototype of an operational NDGPS-integrated perceptible water vapor (IPWV) observation system for the National Weather Service. This modified NDGPS system is being built in collaboration with FHWA, the Coast Guard, and the National Geodetic Survey/Continuously Operating Reference Station (NGS/CORS).
The system uses ground-based GPS meteorological observation instruments installed at the NDGPS tower sites. This is a relatively inexpensive system because each site basically consists of a barometer and a thermometer and devices to collect and transmit the GPS and meteorological data in near real time. The NDGPS stations will be providing water vapor-caused, satellite signal-delay data. The connected NGS ground instruments will collect the data and use it to calculate accurate water vapor information. This information will be sent to NOAA via CORS every 30 minutes.
A dozen NDGPS sites are currently in the process of being retrofitted to include the weather observation tools, and FHWA has transferred funds to FSL to support creation of an additional 22 GPS Surface Observation Systems (GSOS).
It already has been shown that including NDGPS data in modern weather-prediction models improves the accuracy of weather forecasts, especially under active weather conditions when it is needed most. NOAA scientists are confident that data collected through the NDGPS stations will be used in the next-generation National Weather Forecasting Model. NOAA views its collaboration with FHWA and other government agencies as a very unusual and beneficial opportunity to use NDGPS data for a low-cost improvement in weather forecasting.
New Applications Emerging Fast
Dave Gorg of the Minnesota DOT reports that a host of new applications of the improved positioning accuracy available through NDGPS are being planned. It is expected that immediate positioning will be routinely accurate to within 5 to 10 centimeters, which is the degree of accuracy that many transportation vehicles and transportation support services need to operate safely and successfully.
Snowplows, in particular, have a very small margin for error when they are operating near guardrails, light standards, and other safety devices or near bridges, overpasses, and public areas. Also, when snowplows are clearing airport runways, it is of the utmost importance that the entire runway is cleared quickly without running into any adjacent safety or signal equipment.
In Minnesota, a computer-driven heads-up display that presents accurate positioning information from NDGPS has been developed. This device flips down much like a sun visor so that it can be used when needed and placed out of the way when visibility is not bad enough to warrant its use. With this display, the snowplow operator can move ahead with confidence even in poor visibility conditions. (For more information about "smart" snowplows, see "Safe Plowing - Applying Intelligent Vehicle Technology" on page 3.)
Mapping roads will be a high-priority function for NDGPS. The description and location of all roads will be more precise with more accurate information about topography and the positions of road safety appurtenances and signage. With better information available from the NDGPS map - better quantitatively and qualitatively - many more applications will be possible.
For example, the highway patrol and other law enforcement agencies will be able to examine and describe vehicle accidents more precisely; highly accurate measures of accident scenes will be possible. Aerial photographs (digital orthophotos), which employ "white targets" on the ground, can be compared with NDGPS exact positioning data to document the history of accidents in selected areas. Such comparisons should reveal clues about accident trends, such as safety hazards and/or possible road design flaws that may contribute to accidents. Both highway safety officials and insurance company representatives would be interested in obtaining and using such information to track and prevent accidents.
An application similar to the snowplow heads-up display is being planned for buses in the Minneapolis-St. Paul area, Dave Gorg reports. Major highways through the cities are built with a 10-foot (3.05-meter) emergency lane. The buses are 9 feet (2.75 meters) wide. The plan is to allow the buses to use the emergency lane as a bus-only express lane during rush hours. But with the small margin for error - one foot (0.3 meters) - the driver will need the assistance of an NDGPS positioning map on the heads-up display.
Other NDGPS applications being planned today in Minnesota are well worth mentioning and may stimulate more ideas in other states tomorrow. For instance, utility companies are interested in mapping their underground transmission lines more accurately. With NDGPS, it should be possible to map these within one-foot accuracy. Is this important? Ask someone who has seen a natural gas line explode when the line was ruptured by drilling or digging because, as a result of inaccurate mapping, no one knew the line was there. Also, utilities and state DOTs need accurate right-of-way maps to facilitate the installation of fiber-optic and similar communication transmission lines.
The Many Possibilities of Precise Vehicle Positioning
Perhaps one of the most far-reaching applications involves knowing the exact location of vehicles. With NDGPS, it will be possible to avoid airplane collisions on the ground because a constantly updated map of all objects, including vehicles and safety equipment as well as airplanes, will be available.
Historical records of where vehicles were located at particular times will also be possible. This may mean that trucking companies can explicitly document their operations within a state - how many trucks, for how long, at which weights, and on which highways. This could affect road-use taxes.
In addition, other service vehicles, such as paving machines, bulldozers, fire engines, and ambulances, could use the heads-up display to guide and speed their efforts.
NDGPS could be a boon to farmers also. Farmers may have many applications for NDGPS as a tool for mapping their crop plantings, fertilizing, irrigating, harvesting, and field use, as well as being a guide for their tractors and other farm machinery.
|This snowplow, which is jointly operated by the Intelligent Vehicles Laboratory of the University of Minnesota's ITS Institute and the Minnesota Department of Transportation, is presently being used in experiments to determine the design details of driver assistive technology used for operating vehicles under low-visibility conditions. The design includes high-accuracy DGPS, digital maps, radar, and an innovative heads-up display.|
The best news is that the NDGPS modernization program is proceeding rapidly. As the new data messages are developed, the accuracy of positioning information improves. True "ground zero" measures can be reached with 20-centimeter (8-inch) accuracy, down from the current three meters (10 feet). The implications of this change are as dramatic as the 1-to-15 ratio in improved accuracy.
The new system will be tested in Maryland later this spring. Funds have been made available to begin research/testing soon in Hagerstown, Md., with the emphasis of the work to be on "service applications," including some of the possibilities previously mentioned.
In short, NDGPS technology will soon be available for everyone to use and benefit from in a host of ways never before possible. The number of ways that NDGPS will be employed is as great as the imaginations of those who consider new applications.
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 commercial communication 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.