High Accuracy-Nationwide Differential Global Positioning
System Program Fact Sheet
P U R P O S E
The High Accuracy-Nationwide Differential
Global Positioning System (HA-NDGPS) program provides the capability
to broadcast corrections to the Global Positioning System (GPS) over
long ranges to achieve a better than 10 centimeter (cm) (95 percent)
accuracy throughout the coverage area. HA-NDGPS is currently undergoing
a research and development phase. The signal is available for test
purposes from Hagerstown, MD, and soon Hawk Run, PA. Application of
this technology will provide advanced safety features for transportation,
including lane departure warning, intersection collision warnings,
and railroad track defect alerts. It also could be used for economic
enhancements such as precision container tracking and automated highway
lane striping.
B A C K G R O U N D
Currently, the GPS service offers a 4-
to 20-meter (m) navigational accuracy. For many land transportation
uses, this accuracy is insufficient. The Nationwide Differential Global
Positioning System (NDGPS) offers a 1- to 3-m radio-navigational service
that meets the needs of many more transportation users.
H A - N D G P S D E S C R I P T I O N
Because greater precision is needed to support many of the safety enhancements
envisioned for the future, the U.S. Department of Transportation, in
conjunction with the Interagency GPS Executive Board, is supporting
the development of HA-NDGPS to provide 10 cm horizontal and 20 cm vertical
(95 percent) corrections to users. HA-NDGPS uses the infrastructure
employed by the NDGPS service to broadcast these corrections. The addition
of a diplexer and transmitter allow the existing infrastructure to broadcast
the additional signal, keeping implementation costs very low. Additionally,
the signal will be monitored to ensure it is providing the accuracy
needed to meet safety-of-life applications.
F E A T U R E S
- Low Frequency Broadcast: The HA-NDGPS service employs a
low radio frequency broadcast technique that has been used for many
years in both maritime and aviation radio navigation systems to
ensure coverage throughout a large geographic area and through obstructions
between the broadcast site and the user’s equipment. As a nonline-of-sight
communication link, it is very robust in urban and rural terrain
where higher frequency broadcasts are blocked and become unusable.
- Quad-Frequency Receivers: The HA-NDGPS service uses quad-frequency
receivers to enable interpolation between broadcast sites. This
allows for faster code and carrier phase resolution, enabling dynamic
navigation solutions in seconds rather than minutes. The accuracy
throughout the coverage area is better than 10 cm horizontal (95
percent).
- Improved Atmospheric Corrections: During average (quiet)
atmospheric conditions, the changes the ionosphere over the Continental
United States, and the changes in temperature, pressure, and moisture
in the lower atmosphere, are usually small. Under these circumstances,
existing methods to correct for excess signal delays caused by the
ionosphere and troposphere over short to moderately long baselines
work reasonably well. During significant space and tropospheric
weather events, however, the changes that are seen in the ionosphere
and troposphere can vary greatly, leading to rapid changes and large
errors in GPS accuracy. Research is underway to better model and
predict these changes and will be incorporated into the HA-NDGPS
research program.
A C C O M P L I S H M E N T S
Current Activities
FHWA currently is involved in the testing and implementation of
this proposed system. Additional information will be available at
http://www.tfhrc.gov/its/ndgps/index.htm.
Future Activities
A test version of the HA-NDGPS is being
made available at the Hagerstown, MD, and Hawk Run, PA, NDGPS facilities.
The signal will be available for approximately 1 year and can be received
by anyone with appropriate receiver equipment operating within approximately
322 kilometers of the Hagerstown and Hawk Run facilities. Application
developers are encouraged to learn more about this testing and to
determine the ability of the technology to meet safety application
requirements.
P A R T N E R S
The HA-NDGPS program is implemented through funding made available
from the Interagency GPS Executive Board and jointly with the U.S.
Department of Transportation’s Federal Highway Administration;
the Department of Homeland Security’s U.S. Coast Guard; and the
U.S. Department of Commerce’s National Geodetic Survey and Forecast
Systems Laboratory.
FHWA-RD-03-039
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