ETL 1110-1-184
01 Oct 1998
the data is 2.1 seconds for a site transmitting at 100 baud. For
stations transmitting at 200 baud, the latency would be half,
1.05 seconds. The user can expect a latency of 2-5 seconds for
all of the corrections for a group of satellites observed at the
reference station to reach them. A correction can be considered
valid for a period of 10-15 seconds from generation (the USCG
limit is 30 seconds). Using corrections beyond this period of
time, especially for positioning of a moving platform, may cause
spikes in the positional results.
d. Availability of DGPS Corrections and Signal.
The
system was designed for and operated to maintain a broadcast
availability (i.e., transmitting healthy pseudorange corrections)
that exceeds 99.7 percent (in designed coverage areas) assuming a
healthy and complete GPS constellation. The signal availability,
in most areas, will be higher due to the overlap of broadcast
stations.
A-2. Integrity of the System. The USCG monitors each site
within the entire system for problems or errors 24 hours a day.
Each site is equipped with two integrity monitors (i.e. a GPS
receiver with a MSK radiobeacon) that are mounted over known
positions. The integrity monitors receive the pseudorange
corrections from that site and compute a position. The computed
or corrected position is compared to the known location to
determine if the corrections are within the expected tolerance.
The corrected positions calculated by the integrity monitors are
sent via phone lines to the control monitoring stations. For the
stations east of the Mississippi River, this information is sent
to USCG's NAVCEN in Alexandria, Virginia. Sites west of the
Mississippi River send their corrected positions to the NAVCEN
Detachment in Petaluma, California. Users are notified via the
type 16 message of any problems with a radiobeacon site within 10
seconds of an out-of-tolerance condition.
A-3. Coverage. The system was designed to cover all harbors and
harbor approach areas and other critical waterways for which USCG
provides aids to navigation. Each site has a coverage area
between 150 to 300 miles, depending on the transmitter power,
terrain, and signal interference. Since the sites utilize an
Omni-directional transmitting antenna, some areas have
overlapping coverage.
Currently the system covers all U.S.
coastal harbor areas, the Mississippi and part of the Missouri
and Ohio Rivers, and the Great Lakes Region. Additional areas
within the Midwest U.S. are still under consideration. A map of
the coverage area can be found at the NAVCEN web site under the
DGPS section.
A-4.
User Requirements and Equipment.
To receive and apply the
A-2
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