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T
ECHNOLOGY
GPS
The Global Positioning System (GPS) uses 24 orbital satellites to determine the position of the
Terminal anywhere on the globe.
O
GPS F
BTAINING A
IX
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 20 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.
GPS
BGAN R
AND
EGISTRATION
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, then the Terminal will be de-registered from
the network.
32
O
VERVIEW
Hughes 9350 BGAN Terminal User Guide
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