Gene Hall

U. S. Coast Guard

The United States Coast Guard provides a Differential Global Positioning System (DGPS) service for the Harbor and Harbor Approach (HHA) phase of marine navigation. DGPS technology is the first to economically offer geodetic accuracy meeting the Federal planning requirement of sub10 meters for harbor and harbor approach navigation. The DGPS service coverage area includes the coastal United States, Great Lakes, Puerto Rico, and most of Alaska and Hawaii. This DGPS service is available to the public navigator as an all-weather navigation sensor to supplement traditional visual, radar, and depth sounding techniques.

The design process for the United States Coast Guard's DGPS service began with efforts to define system operational requirements. The goal of these requirements was to ensure the same level of user integrity provided by present Coast Guard electronic navigation aids (Loran-C and Omega). Refinement of operational requirements by risk analysis of specific harbor navigation scenarios was then conducted. The final system architecture evolved to meet the defined requirements under traditional restraints of current technology, present and future economics, and the flexibility to adapt to future requirements.

The operational doctrine to define DGPS service parameters and the service management infrastructure has been developed. The DGPS operations phase has begun. This paper provides a brief history on the evolution of DGPS and describes the operation of the DGPS service including technical information and broadcast site specifications.

DISCLAIMER- The views expressed herein are those of the author and are not to be construed as official or reflecting the views of the Commandant or of the U.S. Coast Guard.

The U.S. Coast Guard is mandated by Federal law (14 USC 81) to implement, maintain, and operate electronic navigation aids that meet maritime needs of the U.S. armed forces and/or U.S. commerce. The U.S. Coast Guard's expertise in enhancing maritime safety through the utilization of radio (electronic) navigation services dates to 1921 with the first operational radiobeacons. In the last two decades, the U.S. Department of Defense (DOD) has led technology from terrestrial to space-based radionavigation systems, first with TRANSIT, and then the prototype NAVSTAR Global Position System (GPS).

In 1987, the U.S. Coast Guard Research and Development Center in Groton, Connecticut, began conducting research and testing of differential techniques to enhance GPS accuracy. Simply stated, the differential technique involves installing navigation equipment at a precisely known location. The equipment receives the GPS signal and compares the position solution from the received signal to its known location. The result of this comparison is then generated in the form of a correction message and sent to local users via radiobeacon broadcast. The received correction is applied by the user's GPS equipment to reduce the system position error, thereby improving the user's absolute accuracy. This effort was coordinated through the Special Committee (SC) 104 created by the Radio Technical Commission for Maritime Services (RTCM).

The differential effort was driven by the search for a system with the capability to meet the accuracy requirement for Harbor/Harbor Approach navigation as had been defined in the Federal Radionavigation Plan (FRP). The FRP identifies that accuracy on the order of less than 10 meters (2drms)¹ is required for the HHA phase of navigation [FRP 94]. The FRP also states requirement for the Coastal and Ocean phases for maritime navigation which have respectively been satisfied with Loran-C and Omega services.

In 1989, the U.S. Coast Guard modified the existing marine radiobeacon located at Montauk Point, New York to broadcast differential corrections in the RTCM SC-104 format. The Montauk Point field tests demonstrated that Minimum Shift Keying (MSK)² modulation of an existing radiobeacon signal was effective in transmission

Front Matter (R1-R6)

Contents (R7-R10)

I Workshop Summary (1-2)

Executive Summary (3-5)

1 Introduction (6-9)

2 Networks and Data Sources (10-13)

3 Remote Sensing of the Atmosphere (14-17)

4 Dynamic Positioning and Navigation (18-20)

5 Static Positioning (21-22)

II Workshop Proceedings (23-24)

6 Civilian GPS Planning and Policy Making in the Federal Government (25-39)

7 Networks and Data Sources (40-109)

8 Remote Sensing of the Atmosphere (110-163)

9 Dynamic Positioning and Navigation (164-200)

10 Static Positioning (201-226)

Appendices (227-227)

A Statement of Task (228-228)

B Biographical Sketches of Steering Committee Members (229-230)

C Workshop Attendees (231-233)

D Working Group Participants (234-235)

E Workshop Agenda (236-239)