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USCG DGPS Navigation Service

The U.S. Coast Guard (USCG) is working to provide Differential Global Po sitioning System (DGPS) service for harbor and harbor approach navigation by 1996. DGPS technology is the first to economically offer geodetic accuracy meeting the federal planning requirement of 26.2 to 65.6 ft. (eight to 20 m) for harbor and harbor approach navigation.

The DGPS service coverage area is to include the coastal U.S., Great Lakes, Puerto Rico and most of Alaska and Hawaii. This DGPS service will be 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 USCG'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 USCG 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 three traditional constraints: current technology; present and future economics; and maximum flexibility to adapt for future requirements.

The final design step is nearing completion. The operational doctrine to define DGPS service parameters and to service management infrastructure has been developed. The DGPS implementation phase has begun. This paper provides a brief history on the evolution of DGPS and discusses the implementation phase with an update on current status, future goals and actions being taken to achieve these goals.

Background The USCG is mandated by federal law to implement, maintain and operate electronic navigation aids that meet maritime needs of the U.S. armed forces and/or U.S. commerce. The USCG'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 DeTRANSIT, and then the prototype NAVSTAR Global Positioning System (GPS).

In 1987, the USCG Research and Development Center in Groton, Conn, 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 (HHA) navigation, as had been defined in the Federal Radionavigation Plan (FRP). The FRP identifies that accuracy of 8 to 20 meters is required for the HHA phase of navigation. The FRP also states requirements for the Coastal and Ocean Phases for maritime navigation, which have respectively been satisfied with the Loran-C and Omega services.

In 1989, the USCG modified the existing marine radiobeacon located at Montauk Point, N.Y. to broadcast differential corrections in the RTCM SC-104 format. The Montauk Point field tests demonstrated that Minimum Ship Keying (MSK) modulation of an existing radiobeacon signal was effective in transmission of RTCM SC-104 format corrections. The MSK modulation technique could be used with no adverse effect on the automatic direction finding receivers of traditional marine radiobeacon users. Important to both the U.S. Coast Guard and the public, MSK technology is economical to implement at existing radiobeacons and within user receivers. By January 1990, the RTCM published the SC-104 formats version 2.0 document. With a formal U.S. industry differential GPS correction format and the initial radiobeacon broadcast success, Montauk Point began the first continuous public U.S. DGPS broadcast on August 15,1990. This transmission marked the beginning of the USCG transition from DGPS research and development towards implementation of a U.S. maritime DGPS service.

DGPS Architecture The USCG gained Congressional funding to implement a maritime DGPS service. The functional elements of the USCG DGPS Navigation Services include: • Reference Station. Precisely located GPS receiving equipment with computer to calculate corrections based on comparison of satellite navigation message to known location.

• Broadcast Site. A marineradiobeacon providing correction GPS receiver capable of applying termine if the correction broadcast data link to DGPS service users. differential corrections. The cor- w a s tolerance.

Integrity Monitor. Precisely lo- rected GPS position would be com- * Control btation bite for huservice element. Also, DGPS service performance data archiving and processing is accomplished here. • Communication Network. Provides connectivity between sites for passing performance data and control commands. DGPS user equipment consists of two interfaced receivers with a display — a radiobeacon receiver capable of MSK demodulation, and a GPS receiver capable of applying differential corrections from the radiobeacon receiver.

Status Report At the time of this writing, 10 prototype reference stations comprised the USCG DGPS service architecture. These 10 sites are transmitting GPS satellite corrections while being monitored by the USCG for evaluation purposes. The data collected is being used to analyze broadcast signal characteristics to ensure station availability, accuracy and reliability requirements are met. Exact numbers showing percentages for each requirement are not available, as all sites are undergoing maintenance, upgrade and hardware installation. However, each site has performed quite well during on-air periods. The RTCM committee has updated the broadcast format standard and we are now using version 2.1. Version 2.2 is in progress. The RTCM committee also provided the Reference Station Integrity Monitor (RSIM) standard for the communication interface between the Reference Station, Integrity Monitor, and Control Station. • Integrity Monitor. The software needed to provide system integrity by monitoring RTCM correction broadcasts and communicating that information back to the Control Station has been written by the USCG R&D Center. Testing and evaluation began with the installation of the monitor equipment at the USCG Electronics Engineering Center (EECEN) in Wildwood, N.J. in June 1994. Integrity monitor hardware was installed at the Northeast test sites last September, and alarm messages were relayed to the control station in Alexandria, Va.

• Control Stations. The first version of the Control Station software was installed in the Eastern U.S. Control Station. The Control Station is undergoing testing and performing well while connected to the integrity monitor equipment at the N.E. test sites.

• Communication Network. The USCG is using the federal government FTS2000 AT&T X.25 Packet Switched Service as the data link between the Control Stations and each DGPS site. This service is in place and meets all data communication requirements.

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