United States Department of Transportation
i
Nationwide Differential Global Positioning System Program Fact Sheet
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2002-01-01
[PDF-604.66 KB]
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Abstract:Currently, the Global Positioning System (GPS) service offers only 4- to 20-m navigational accuracy. For many land transportation uses, this accuracy is insufficient. NDGPS offers a 1- to 3-m radio-navigational service that meets the needs of many more transportation users. NDGPS improves the accuracy, availability, and integrity of the GPS by constantly monitoring and broadcasting corrections to the GPS service. This is accomplished through a network of ground facilities called reference stations. Differential GPS 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 nonstationary receivers. By comparing the inaccuracy to the satellite signal, the nonstationary receivers can then accurately determine their location. The closer to the transmitter, the more accurate the determination. Using current techniques, this correction is most accurate near the NDGPS facilities (approximately 1 m) and degrades up to 3 m at the edge of the coverage area, which is up to 402 kilometers (km) away. While determining this correction, the NDGPS facilities are also monitoring GPS for anomalous behavior. When this behavior is identified, the NDGPS facilities will broadcast a warning to users not to use that satellite. This warning occurs within 5 seconds of identifying the error. This notification, called integrity, can be thought of as a user's ability to trust the system's output.
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