ETL 1110-1-184
01 Oct 1998
carrier, and provides accuracy of 100 meters.
b. For many applications, absolute positioning does not
provide sufficient accuracy. Differential GPS (DGPS) is a
technique which can provide relative positioning with an accuracy
of a few meters to a few millimeters depending on the DGPS method
used. DGPS utilizing code measurements can provide a relative
accuracy of a few meters. DGPS utilizing carrier phase
measurements can provide a relative accuracy of a few
centimeters. DGPS requires two or more GPS receivers to be
recording measurements simultaneously. With two stations
recording observations at the same time, GPS processing software
can reduce or eliminate "common errors". If one of the stations
is a survey control point, DGPS will determine a baseline between
the stations and effectively establish the position of the other
receiver in the same reference system as the survey control
point.
Discussion.
6.
a. USCG DGPS Radiobeacon System. One function of the USCG is
to provide aids to navigation in all navigable waterways. In the
past, Loran-C and Omega systems were used as the primary
positioning tools for marine navigation. Today, the USCG is
making use of the full coverage from GPS for a more accurate
positioning tool for marine navigation. Utilizing DGPS and
marine radiobeacon technology, the USCG has designed a real-time
positioning system for the coastal areas and Great Lakes regions
of the U.S. The USCG has also partnered with USACE to expand this
coverage to inland waterways.
(1) General. The system consists of a series of GPS
reference stations with know coordinate values based on the North
American Datum of 1983 (NAD83) datum. GPS C/A-code pseudorange
corrections are computed based on these known coordinate values
and transmitted via a marine radiobeacon. A user with a marine
radiobeacon receiver and a GPS receiver with the ability to
accept and apply pseudorange corrections can obtain a relative
accuracy of 0.5-3 meters. This accuracy is dependent on many
factors including the design and quality of the user's GPS
receiver, distance from the reference station, and the satellite
(2) Coverage. The system was designed to cover all harbor
areas, Great Lakes region and some inland waterways. Each site
has a coverage area between 150 to 300 miles, depending on the
transmitter power, terrain, and signal interference. In most
areas, this provides overlapping coverage. Currently the system
covers all U.S. coastal harbor areas, the Mississippi and part of
2
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