New Applications Make Nationwide Differential Global Positioning System More Pervasive

Public Roads, Jan, 2001 by James A. Arnold

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 65, 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.