NorthStar Navigation Multi-Function Unit M84 - Manual (Page 33)

Northstar M121/M84 Installation and Operation Manual 32
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
Northstar 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.