of RTCM SC-104 format corrections. The MSK modulation technique could be utilized 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 format version 2.0 document. With a formal U.S. industry differential GPS correction standard and the initial radiobeacon broadcast success, Montauk Point began the first continuous public U.S. DGPS broadcast on August 15, 1990. This transmission marks the beginning of the U.S. Coast Guard transition from DGPS research and development towards implementation of a U.S. maritime differential GPS service.

The DGPS service architecture is shown in Figure 1. The functional elements of the U.S. Coast Guard DGPS Navigation Service include:

• Reference Station - Precisely located GPS receiving equipment which calculates satellite range corrections based on a comparison of the satellite navigation message to its known location.

• Integrity Monitor - Precisely located GPS receiver and MSK radiobeacon receiver which applies differential corrections. The corrected position is compared to its known location to determine if the correction broadcast from the Reference Station is in tolerance.

• Broadcast Site - A marine radiobeacon transmitting correction data in the 285 to 325 kHz band.

• Control Station - Site for human centralized control of the DGPS service elements. DGPS performance data processing and archiving is accomplished here. The East Coast Control Station is located at the USCG Navigation Center in Alexandria, Virginia. The West Coast Control Station is located at the Navigation Center Detachment in Petaluma, California. Both sites are manned 24 hours per day.

• Communicative Network - An X.25 packet-switched service providing connectivity between broadcast sites and control stations.

• DGPS User Equipment - Consists of two interfaced receivers with a display; a radiobeacon receiver for MSK demodulation and a GPS receiver capable of applying differential corrections.

GPS correction data based on NAD-83 coordinates is provided for both real-time and post processing applications. Real-time correction data is broadcast to the user via radiobeacon only for satellites at an elevation angle of 7.5 degrees or higher through use of the type 9-3 message in the RTCM SC-104 format. The official GPS coverage provided is based on elevation angles of ten degrees or higher. Satellites at elevation angles lower than 7.5 degrees are adversely affected by spatial decorrelation, multipath, and minimal processing time between acquisition and actual use. Corrections for a maximum of nine satellites will be broadcast. If more than nine satellites are above 7.5 degree elevation angle, a situation which occurs less than one percent of the time, then corrections are broadcast for the nine satellites with the highest elevation angles [USCG Broadcast Standard].

The latency of this information is determined by the baud rate at which it is transmitted. There are 210 bits in a type 9-3 message (three satellites corrected) including the message header. Therefore, at 100 baud the latency is 2.1 seconds. Naturally, this time is cut in half when transmitting at 200 baud. In reality, latencies on the order of 2-5 seconds are realized depending on the number of satellites in use. Other factors contributing to latency include partial decoding techniques, parity checking, and the receiver's internal processing.

GPS satellites data consisting of CA code, P1 and P2 Range, and L1 and L2 Carrier Phase information is collected every 30 seconds by the National Geodetic Survey (NGS) from both Reference Stations at each broadcast site. NGS processes the data and makes it available to the public for post processing applications. A

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)