14 TRACKER950 CHARTPLOTTER Installation and Operation Manual
GPS and DGPS
The US Government operates the GPS system.
There are twenty-four satellites orbiting the earth
that broadcast position and time signals. The
positions of these satellites are constantly
changing. Your GPS receiver analyses the signals
from the closest satellites and calculates exactly
where it is on earth. This is called the GPS position.
All GPS positions have some errors, caused by
small variations in the positions of the satellites
and by atmospheric effects. The accuracy of the
GPS position is typically better than 10 m (33 feet)
for 95% of the time.
To make GPS positions more accurate, a system
of land-based transmitters has been developed,
called differential beacons. These transmit
correction signals that a receiver can use to correct
for most of the errors in the GPS position. Such a
receiver is called a differential receiver and the
corrected position is called the DGPS position. The
accuracy of the DGPS position is typically 2 to 5 m
(6 to 15 feet), depending on the distance to the
A GPS antenna can receive signals from the GPS
satellites when it is almost anywhere on earth.
However, differential beacons are usually only
installed near ports or important waterways and
each beacon only has a limited range, typically
less than 400 km (220 nm).
3 Satellite Status screen
GPS and DGPS antennas
The NAVMAN GPS antenna gives a GPS position.
The unit has a high-gain antenna and a sensitive 12-
channel receiver. It tracks signals from all satellites
visible above the horizon and uses measurements
from all satellites more than 5° above the horizon to
calculate the position.
The NAVMAN differential receiver has a unique
‘H-Field’ toroidal antenna for high sensitivity and
excellent immunity from electrical noise and
which requires no grounding. The receiver has
two channels and automatically selects the
strongest differential beacon signal to use.
The NAVMAN DGPS antenna has both a GPS
receiver and a differential receiver and
automatically applies the differential corrections to
the GPS position and gives a DGPS position.
Each time a NAVMAN GPS antenna is turned on,
it normally takes about 50 seconds before it outputs
the first position. If it has not been turned on for
several months or if it has been moved more than
1000 km (550 nm) since it was last turned off then
it will take two minutes or longer.
The TRACKER950 must be connected to:
a GPS antenna
or a DGPS antenna, which is a combined
GPS and differential receiver
or a GPS antenna and a separate differential
3-1 GPS and DGPS worldwide navigation
The satellite status screen displays the signal strengths of the visible GPS satellites, their current positions
and information about the accuracy of the calculated position.