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High Accuracy-Nationwide Differential Global Positioning System Test and Analysis: Phase II Report

Chapter 1-Introduction

Background

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:

• Task 1-Meet with other demonstration participants, communicate progress, and document findings. Throughout the year, the participants maintained weekly and often daily correspondence on the test and evaluation, development, and documentation. This final report completes this task.
• Task 2-Collect, process, and report information from the Hawk Run and Hagerstown stations. There were five developmental subtasks:
  • Pretest installation, configure a new modulator program, and update GRIM^TM (GPS Receiver Interface Module) with transmission control protocol/Internet protocol (TCP/IP)at Hagerstown and Hawk Run.
  • Implement modulator feedback to GRIM.
  • Implement Restricted Active Mode (R.A.M.) at Hagerstown and Hawk Run.
  • Gather broadcast GPS data from Hagerstown and Hawk Run simultaneously and process the information at an in-between user site.
  • Describe the algorithm used to combine independent solutions from Hagerstown and Hawk Run.
• Task 3-Develop and implement a prebroadcast integrity algorithm. There were three subtasks:
  • Detect errors in the GPS constellations and its broadcasts.
  • Discuss methods of inserting integrity information into the data stream.
  • Describe possible techniques and algorithms.
• Task 4-Develop interface software for multiple brands of GPS receivers. There was one subtask:
  • Convert the demodulated output format into Radio Technical Committee for Maritime Services Special Committee 104 (RTCM-SC104) type 18/19 compatible with existing user GPS equipment and deliver the source code.
• Task 5-Rewrite the modulator interface. There were two subtasks:
  • Rewrite the existing modulator interface to implement a remote control capability through a TCP/IP interface and deliver the source code.
  • Add limited commands to control GPS receiver parameters such as data rate and elevation mask and deliver the source code.
• Task 6-Evaluate a low-baud-rate message such as 100 baud to 300 baud and user processing based on a HA-NDGPS data rate less than once every epoch.
• Task 7-Collect observations on one or more highways in real-time kinematik (RTK) mode to demonstrate driver analysis. This task had two subtasks:
  • Collect data for mapping one or more highway segments.
  • Compare the driver's control of the vehicle.
• Task 8-Consider studying noise levels and possible noise reduction at the Hagerstown site. This task had two subtasks:
  • Locate a GPS antenna high above possible nearby multipath sources and compare the cleanliness of the measurements there with measurements from current NDGPS equipment.
  • Use identical models of the same antenna at both the reference site and a nearby user site to gather observations and fix the integers to establish the level of site cleanliness.

This report follows the order of the remaining tasks (after Task 1) listed in this introduction.

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