Encl. (1) to COMDTINST 16577.2
USCG DGPS CONOP
APPENDIX C: DGPS DEVELOPMENT HISTORY
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In the last three 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 Positioning System (GPS). GPS, as with previous DOD radionavigation
systems, was designed to meet military mission requirements with little consideration for civilian
applications. However, as prototype GPS satellites were placed in orbit, innovative civil users
found economical applications for the available GPS signals. Perceiving the growing demand,
industry developed and produced GPS receivers tailored to emerging civil market applications.
DOD requested the U.S. Department of Transportation (DOT) assume the lead in civil GPS
matters. In February 1989, DOT assigned the U.S. Coast Guard as the lead agency in providing a
civil GPS interface. The U.S. Coast Guard's assignment to the civil interface role followed the
natural evolution of past U.S. radionavigation systems. The U.S. Coast Guard had been involved in
the investigation of potential GPS civil use since the late 1970s and was therefore well prepared for
this interface initiative.
During this same period, the U.S. Coast Guard Research and Development (R & D) Center in
Groton, Connecticut, had been conducting research and testing of differential techniques to enhance
Loran-C and Omega accuracy. 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 an accuracy of 8
to 20 meters (2 drms) is required for the HHA phase of navigation.
As the DOD development of GPS evolved, and the U.S. Coast Guard gained knowledge from civil
application research, GPS appeared as the natural progression of differential technology
application. The GPS Standard Positioning Service (SPS) that DOD makes available for civil users
provides predictable accuracy to 100 meters (2 drms). Military GPS user's Precise Positioning
Service (PPS) provides an accuracy of 22 meters (2 drms). Both SPS and PPS service falls short of
the Federal Radionavigation Plan's HHA requirement. However, the application of differential
technology to GPS does promise to provide the required accuracy improvement. As early as 1983,
GPS receiver manufacturers and the U.S. Department of Transportation, with the U.S. Coast Guard
as a participant, began work to develop a standard differential GPS correction message format. This
effort was coordinated through the Special Committee (SC) 104 created by the Radio Technical
Commission for Maritime Services (RTCM). In 1987 the U.S. Coast Guard R & D Center
demonstrated that differential corrections received by user equipment inside the coverage area of
the correction broadcast improved GPS SPS to a predictable accuracy of 10 meters (2 drms). The R
& D Center used a VHF data link to broadcast corrections conforming to a draft RTCM SC-104
format at a 50 bits per second data rate. With the promising capability of DGPS, the HHA accuracy
level requirement was verified by a U.S. DOT study of the navigation of vessels over 30,000 dead
weight tons in the restricted waters of the Great Lakes/St. Lawrence Seaway.
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