56
Technology Overview
3004149-0001 Revision B
Technology Overview
GPS
The Global Positioning System (GPS) uses 24 orbital satellites to
determine the position of the Terminal anywhere on the globe.
Obtaining a GPS Fix
In normal operation, a GPS receiver, such as that built in to the
terminal, needs to be able to receive signals from at least four
satellites so that it can then calculate a latitude, a longitude and
an altitude – this position fix is referred to as a 3-dimensional or
3-D fix. If only three GPS satellites can be seen by the GPS
receiver, then the last available altitude measurement is assumed
and the GPS receiver calculates a position fix based on latitude
and longitude only. This simpler position fix is referred to as a 2-
dimensional or 2-D fix and is quicker and easier to obtain than a
3-D fix, but may be less accurate.
The GPS receiver may take between a few seconds and a few
minutes to obtain a GPS fix, depending on how frequently the
GPS receiver is being used. The frequency of use determines the
how quickly the GPS Terminal is able to start.
Hot start
if the GPS receiver is being used frequently, (that
is, in the last two hours), it is regularly updated with data
from the GPS satellites, and so only takes a few seconds to
obtain a GPS fix after being switched on.
Warm start
if a GPS receiver has not been used for more
than two hours then it may take up to 45 seconds to obtain a
GPS fix.
Cold start
if the GPS receiver has not been used for some
time or is 300 km or more from where it was last used, it can
take a few minutes to obtain a valid position fix.
The time taken to obtain a valid GPS fix can also be affected by
the visibility that the GPS receiver has of the GPS satellites. The
GPS system is relatively tolerant of atmospheric conditions such
as heavy cloud or rainfall. However, physical blockages, such as
tall buildings or terrain can significantly degrade the ability of
the GPS receiver to obtain a fix. For this reason, ensure that the
GPS receiver has a clear view of as much open sky as possible.
