Differential GPS - Understanding GPS: Principles and Applications - page 429

Variable length
data message

Preamble Reserved Message length Cyclic redundancy check

11010011 6 bits 10 bits 24 bits

0

−

1,023 bytes

Figure 8.23 RTCM SC-104 version 3.0 message frame.

The initial release of version 3.0 includes 13 message types, designed primarily
to support RTK applications using GPS or GLONASS. These message types provide
the reference station's pseudorange and carrier-phase measurements for L1 or
L1/L2, as well as a wealth of auxiliary information including precise station coordi-
nates, receiver configuration, and antenna characteristics. Later versions will
include a number of new message types, at least eight of which have been proposed,
as well as a number of legacy message types to support both RTK and conventional
differential GNSS.

8.6 Examples

8.6.1 Code Based
8.6.1.1 MDGPS/NDGPS
In the late 1980s, the USCG began the development of a MDGPS system to satisfy
maritime navigation requirements in the United States. In 1989, a radiobeacon
located on Montauk Point, New York, was modified to broadcast DGPS correc-
tions in the RTCM SC-104 message format. By February 1997, 54 radiobeacons
had been modified to provide DGPS correction coverage for most U.S. coastal areas
and inland waterways, and the MDGPS service was declared to have achieved full
operational capability. That same year, a decision was made to expand radiobeacon
DGPS coverage throughout the United States. This program, referred to as Nation-
wide DGPS (NDGPS), is supported by a partnership of U.S. agencies including the
USCG, the U.S. Air Force Air Combat Command, the Federal Railroad Administra-
tion, the Federal Highway Administration, the National Oceanic and Atmospheric
Administration (NOAA), the U.S. Army Corps of Engineers, and the Office of the
Secretary of Transportation [43]. At the time of this writing, 84 of 136 proposed
sites were operational, providing nearly complete coverage over the United States
with two or more sites visible to users in many locations. This section provides a
short description of the MDGPS/NDGPS systems.

Network Design
The network architecture for MDGPS/NDGPS is shown in Figure 8.24 [44]. These
systems essentially utilize the code-based LADGPS techniques described in Section
8.3.1. The network includes reference stations (RSs) to monitor GPS and generate
differential corrections. Each reference station consists of two GPS receivers for
redundancy. Integrity monitors (IMs) are collocated with the RSs. All of the equip-
ment is generally installed in unmanned equipment sheds with a backup power
source (e.g., batteries or a generator). Each IM includes another pair of GPS receiv-
ers and radiobeacon receivers to monitor the corrections that the site is itself broad-