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Publication Number: FHWA-HRT-05-034
Date: July 2005
When the Department of Defense (DoD) originally developed the Global Positioning System (GPS), it was a military system. GPS quickly became a tool for both military and civilian applications worldwide. Initially, DoD intentionally degraded GPS signals. To compensate, civilian engineers placed equipment at specific sites to determine intentional errors. These errors were then broadcast to users who would use them to correct their measurements. Because these corrections did not compromise security, Differential GPS (DGPS), as these corrections came to be known, flourished with little opposition from DoD. Because of this improved accuracy and greater reliability, the U.S. Coast Guard developed a version of DGPS for use in the marine surface environment.
The success of this system encouraged other government agencies to make such capabilities available in other parts of the country, particularly in the West and Midwest. This national extension is known as the Nationwide DGPS (NDGPS). The NDGPS is advertised as a 1- to 3-m system with 99.97 percent availability. The new vision, High Accuracy-NDGPS (HA-NDGPS), is designed to broadcast additional information from the same NDGPS network using a new carrier frequency to achieve fixed or moving centimeter (cm)- decimeter (dm)-level accuracies while maintaining as much integrity as possible.
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. The addition of a diplexer and transmitter to the existing HA-NDGPS stations allows the existing infrastructure to broadcast the additional signal, keeping implementation costs low. In addition, the signal will be monitored to ensure it provides the accuracy needed to meet safety-of-life applications.
The HA-NDGPS program is implemented through funding made available from the Interagency GPS Executive Board. Participating agencies include the U.S. Department of Transportation's Federal Highway Administration and Federal Railroad 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.
In 2001 and 2002, HA-NDGPS was successfully demonstrated at the Hagerstown, MD site. All the target objectives were met and exceeded. The initial effort report, "Support of the System Test and Analysis Program for the NDGPS Modernization Program," was published July 12, 2002.
This report on Phase II activities documents the continuation of that effort with a new set of objectives. In general, Phase II objectives included signal analysis, data collection, data analysis, operational convenience and ease, multiple reference stations, integrity, format translation, bandwidth conservation, improved HA-NDGPS facilities and communication, and examples.
Phase II of the HA-NDGPS research and development project began March 14, 2003, with a meeting to schedule tasks, set priorities, and establish a plan to accomplish the government objectives. The government assignment was to secure spectrum approval; obtain demodulator receivers and antennas; provide hardware modifications to the Hagerstown, MD, and Hawk Run, PA, USCG NDGPS reference stations; install and operate the contractor's software at these sites; and manage the HA-NDGPS project.
Various segments of the real-time navigation systems for the robust positioning over long baselines were developed by a contractor, which had eight tasks:
This report follows the order of the remaining tasks (after Task 1) listed in this introduction.
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Keywords: Global Positioning Systems, GPS, Nationwide Differential Global Positioning System, NDGPS, Carrier Phase, long baseline, multiple reference station