8120
Installation and Operation Manual
31
GPS worldwide navigation
The US Government operates the GPS
system. Twenty-four satellites orbit the earth
and broadcast position and time signals. The
positions of these satellites are constantly
changing. The GPS receiver analyses the
signals from the closest satellites and
calculates exactly where it is on earth. This is
called the GPS position.
The accuracy of the GPS position is typically
better than 10 m (33 ft) for 95% of the time.
A GPS antenna can receive signals from the
GPS satellites when it is almost anywhere on
earth.
DGPS
A DGPS system uses correction signals
to remove some of the errors in the GPS
position. The Instrument can use one of two
types of DGPS system:
WAAS and EGNOS DGPS
WAAS and EGNOS are two satellite based
DGPS systems. The correction signals are
broadcast by satellites and are received by
the Instrument’s standard GPS antenna.
The accuracy of the corrected GPS
position is typically better than 5 m (15 ft)
for 95% of the time.
WAAS covers all of the USA and most of
Canada. EGNOS will cover most of Western
Europe when it becomes operational.
8 Satellites
Differential beacon DGPS
Differential beacons are land based radio
transmitters that broadcast correction
signals that can be received by a special
receiver on the boat. Differential beacons
are usually only installed near ports and
important waterways, and each beacon
has a limited range. The accuracy of the
corrected GPS position is typically better
than 2 to 5 m (6 to 16 ft).
GPS receiver
Navman GPS units have a sensitive
12-channel receiver, which tracks signals from
all GPS satellites visible above the horizon
and uses measurements from all satellites
more than 5° above the horizon to calculate
the position.
Each time a GPS receiver is turned on, it
normally takes about 50 seconds before
it outputs the first position. Under some
circumstances it will take up to two minutes
or longer.
