Differential GPS - Understanding GPS: Principles and Applications - page 430

GPS satellite

Observables

Observables

RS/IM

Frame relay
network

To other sites

RS = Reference station
IM = Integrity monitor

Control Station

Figure 8.24 MDGPS/NDGPS network architecture. (Courtesy of U.S. Coast Guard.)

casting. The IMs compute their positions using GPS and the differential corrections
and compare their computed positions with their known (surveyed) positions. If the
position exceeds a preset tolerance, problem satellites are expunged from the differ-
ential correction calculation and the user is notified that the satellite is “unhealthy”
or the site is shut down to guarantee accurate and reliable information.
A frame relay network is used so that two central control stations, which are
manned 24 hours a day, 7 days a week, can monitor the status of all the sites. One
control station is in Alexandria, Virginia; the other is in Petaluma, California. Per-
sonnel at the control stations, upon observing an equipment failure, can switch in
redundant hardware or dispatch a maintenance crew if necessary.

Data Link
Each RS/IM broadcasts digital DGPS corrections in the RTCM SC-104 message for-
mat. Version 2.1 message types 3, 5, 6, 7, 9, and 16 are currently supported [45].
These message types are retained in the later RTCM SC-104 version 2.3 standard,
described in Section 8.5.1. The digital data is broadcast in the 285-325-kHz
medium frequency (MF) band, which is allocated internationally for radiobeacons.
A digital modulation technique referred to as minimum shift keying (MSK) (e.g., see
[46]) is employed either directly on the radiobeacon center frequencies or on a
subcarrier. The use of a subcarrier was originally motivated by the desire to not
interfere with direction-finding receivers that employed existing radiobeacon signals
[47]. At present, all marine radiobeacons in the United States that are not used for
MDGPS have been decommissioned, so backwards compatibility is no longer an
issue. Impulsive noise due, for example, to lightning strikes, is prevalent in the MF