NorthStar 961X Operation And Reference Manual page 57

Chapter 2 - Introducing the 961
These differential corrections are generated by fixed reference
Differential GPS
stations, and then broadcast by marine radiobeacons over a
corrections
relatively limited area—usually a maximum broadcast radius of
250 miles in any direction—to differential-capable GPS (DGPS)
receivers. (The exact geographic positions of these fixed refer-
ence stations are used to calculate the position corrections,
which are based on GPS data received from the earth-orbiting
GPS satellites.) These corrections are broadcast to the DGPS
receivers, which convert this data into extremely precise posi-
tion, heading, and speed data. Referred to as beacons, the
transmitters are situated at various locations along the U.S.
coastline, Great Lakes, and major rivers.
What does all this mean? Using DGPS, the 961's high-perfor-
mance 12-channel GPS receiver and optional two-channel
internal differential receiver work together to provide position
accuracy of one to three meters 50 percent of the time, and under
four meters 95 percent of the time. (Remember: one meter
equals just over one yard!)
Differential corrections are invaluable when navigating in
such narrow or congested areas as ports and waterways, or
wherever you require extreme accuracy to locate a precise
area (retrieving lobster traps, returning to a favorite diving
spot, and so on.)
The 961 displays lat/lon coordinates to thousandths of min-
utes (approximately six feet). When using a radar plotter or
paper plotter, DGPS corrections dramatically improve the
steadiness of your displayed plot line.
Monitoring GPS signal integrity
Besides providing highly accurate position data, DGPS moni-
tors the integrity of the received GPS signals. Integrity moni-
toring helps detect a satellite problem in seconds, compared to
hours using uncorrected GPS. How? The integrity monitor tells
the DGPS receiver to automatically stop using any unhealthy
satellites.
961 Operations and Reference Manual Rev. D Page 2-15