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ECHNOLOGY
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The Global Positioning System (GPS) uses 24 orbital satellites to determine the position of the terminal anywhere
on the globe.
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In normal operation, a GPS receiver, such as that built into 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 three-dimensional or 3-D fix. If only three GPS satellites can be seen by the GPS receiver, 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 two-dimensional or 2-D fix, which 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.
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Hot start: If the GPS receiver is being used frequently, (that is, in the last 2 hours), it is regularly
updated with data from the GPS satellites, and thus only takes a few seconds to obtain a GPS fix after
being switched on.
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Warm start: If a GPS receiver has not been used for more than 2 hours, it will take up to 45 seconds
to obtain a GPS fix.
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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.
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BGAN uses the accurate position and timing information obtained from GPS to help ensure efficient registration of
a BGAN Terminal with the BGAN network.
Following successful registration and providing the terminal is left switched on and remains stationary, the GPS is
no longer needed. Periodically, the BGAN terminal contacts the BGAN network to inform the network that it is
still switched on. In addition, the BGAN network periodically checks each terminal for activity, and if the terminal
has not automatically contacted the BGAN network as described above, the terminal will be deregistered from the
network.
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VERVIEW
Hughes 9350 BGAN Terminal User Guide
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