Differential GPS - Understanding GPS: Principles and Applications - page 431

band at sea because of the excellent conductivity of salt water. This led to a decision
to use type 9 SC-104 messages rather than type 1 to broadcast pseudorange and
range-rate corrections. The use of type 9 messages provides more frequent pream-
bles for user equipment to resynchronize following a strong impulse. The standards
for MDGPS/NDGPS support data rates of 50, 100, or 200 bps.
A large antenna is required to broadcast efficiently at MF because of the large
wavelength (1 km). Most of the MDGPS sites use converted radiobeacon broadcast
towers with heights ranging from 90-150 feet. The NDGPS expansion began with
the conversion of 47 obsolete U.S. Air Force low-frequency ground wave emergency
network (GWEN) sites in one of the largest military-to-civilian reutilization pro-
jects. The GWEN sites were already equipped with 299-foot antennas. A minimum
field strength of 75 V/m for a 100-bps transmission is specified within each trans-

mitter's coverage volume [45], which is typically on the order of 250 nmi.

µ

Performance
The specified accuracy of the MDGPS/NDGPS systems is 10m, 2 drms, within cov-
erage areas [44]. Typical accuracies are much better, typically 1-3m. An often-used
general rule is 1-m accuracy at the base of a transmitter with errors growing by 1m
per 150 km of separation [43]. The specified availability is 99.9% for dual-cover-
age areas and 99.7% for areas with single coverage, based on a 1-month average
per site and discounting GPS anomalies [44]. Current coverage is shown in Figure
8.25.

International Harmonization
International standards for maritime DGPS systems, fully compatible with MDGPS
and NDGPS, have been developed by the International Maritime Organization. At
the time of this writing, radiobeacon-based DGPS services had been deployed in
over 30 nations.

Dual coverage
Single coverage
New single coverage
Maritime USCG/ACOE sites
Operating NDGPS sites
Funded sites
Planned NDGPS sites

Figure 8.25 MDGPS/NDGPS coverage. (Courtesy of U.S. Coast Guard.)