Nationwide Differential Global Positioning System
Program Fact Sheet
P U R P O S E
The purpose of the Nationwide Differential Global Positioning System
(NDGPS) is to provide accurate positioning and location information
to travelers, emergency response units, and other customers. The system
provides 1- to 3-meter (m) navigation accuracy. This will improve
collision notification systems, enable cooperative vehicle-highway
collision-avoidance systems, and provide more accurate in-vehicle
route guidance systems.
D E S C R I P T I O N
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 users ability to trust the systems output.
F E A T U R E S
- Low Frequency Broadcast: The NDGPS service employs a low
radio frequency broadcast technique. This technique has been used
for many years in both maritime and aviation applications to ensure
coverage throughout a large geographic area and through obstructions
between the broadcast site and the users equipment.
- Dual Frequency Receivers: The NDGPS service uses dual frequency
GPS receivers to generate accurate and stable corrections. The 95
percent value is better than 1 m at the broadcast site.
- Integrated Precipitable Water Vapor: The National Oceanic
and Atmospheric Administrations Forecast System Laboratory
in Boulder, CO, is measuring precipitable water vapor in the atmosphere.
GPS receivers installed for the primary purpose of performing basic
measurements and generating corrections are used to measure satellite
signal delay. Data taken from these measurements is then correlated
with the precipitable water vapor present in the atmosphere and
data taken from other equipment installed at each NDGPS broadcast
site to measure temperature, barometric pressure, humidity. This
data is fed into the National Weather Services forecast models,
providing improved short-term weather forecast for all users, including
transportation departments.
- Tectonic Plate Monitoring: Each NDGPS site incorporates
accurate survey reference station antennas. Working with the University
Navstar Consortium, NDGPS provides long-term monitoring of the tectonic
plates. Academia, Industry, and Government use this information
to determine the drift rate of the continental plates.
S P E C I A L U S E S
- Many State departments of transportation use NDGPS to map their
transportation infrastructure. For instance, surveying even small
sections of road using traditional techniques used to require a
number of days; a complete road inventory could take years. With
NDGPS, the same road segment can be surveyed and all pertinent data
recorded in a matter of hours.
- NDGPS is capable of mapping wetlands and locating endangered and
threatened species. Offering unparalleled accuracy, even in rough
terrain and forests, NDGPS allows biologists and other investigators
to monitor these species more effectively.
- NDGPS also has been used by police agencies to identify accident
locations, and NDGPS reference stations are being used to monitor
plate tectonics (the movements of the Earths crustal plates).
A C C O M P L I S H M E N T S
Current Activities
FHWA, in cooperation with other Federal, State, and local organizations,
is deploying sites across the country. Please refer to the U.S. Coast
Guards Web page (http://www.navcen.uscg.gov/ADO/DgpsSelectStatus.asp)
for specific coverage and deployment information.
F U T U R E A C T I V I T I E S
NDGPS meets many users needs today; however,
greater accuracy will benefit more users in the future. To make NDGPS
more accurate, researchers are currently developing an enhancement
to NDGPS. The future system is called High Accuracy-NDGPS, and it
will obtain navigation accuracy at the centimeter level. With the
nationwide availability of 10-centimeter navigation accuracy or better,
collision avoidance, lane keeping, and other applications will become
available on a widespread basis, saving countless lives every year.
A test version of the High Accuracy-NDGPS is being
made available at the Hagerstown, MD, NDGPS facility. The signal will
be available for approximately 1 year and can be received by anyone
with appropriate receiver equipment operating within approximately
322 km of the Hagerstown facility.
P A R T N E R S
The NDGPS program is implemented jointly with the Department
of Transportations Federal Highway Administration, Federal Railroad
Administration, and Office of the Secretary of Transportation; the
Department of Homeland Securitys U.S. Coast Guard; the Department
of Commerces National Geodetic Survey and Forecast Systems Laboratory;
and the Department of Defenses Air Force and Army Corps of Engineers.
FHWA-RD-02-072
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