Technology Overview; Gps; Obtaining A Gps Fix - Hughes 9450 Series User Manual

Technology Overview

GPS

Obtaining a GPS Fix

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Technology Overview
44
3004128-0001 Revision C
The Global Positioning System (GPS) uses 24 orbital satellites to
determine the position of the Terminal anywhere on the globe.
In normal operation, a GPS receiver, such as that built in to the
Tracking Antenna, 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 will 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 as long as 15 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.