AVL – GPS Terminals
8 Nov 2010
GPS systems come in many different configurations and can communicate the GPS
related data and other telemetry (door open/close, engine temperature/oil pressure,
etc. and message data in many different ways. The typical vehicle based GPS “black
box” contains the following:
A GPS receiver and related components, which must receive signals from a minimum
of 3 geo-stationary satellites (4 if altitude data is required) simultaneously. An
antenna must be positioned on the vehicle so it has a clear “line of sight” to the sky.
A processor and memory to manipulate the GPS data and provide for other external
inputs and outputs (e.g. telemetry and/or message data), along with temporary
storage of the data.
Components for communicating the stored data from the “black box” which can be as
simple as a serial data connector to plug in a device (a Palm PDA, or other computer)
to various wireless communications methods. Wireless communications employ a
modem (a device that converts digital data to modulated radio frequency data and
back) and a transmitter, with a connected antenna. Wireless communications can
either be one-way, a transmit only unit, or two-way, a transmit and receive
(transceiver) unit. Depending on the one or two-way ability, the functionality of the
system is greatly impacted.
Support items such as a power supply/converter, one or more printed circuit boards
to mount the three major components to, various leads and connectors and an
enclosure – the black box, in or on which all is mounted and connected.
Today’s GPS receivers can handle from 8 to 66 simultaneous signals, with more
signals improving the accuracy of the position fix. There are generally not significant
differences between GPS receivers from reputable mobile vehicle tracking system
manufacturers.
The type, capacity and functionality of the processor and memory and associated
programming of the units “firmware”, vary a great deal. As in any computer system
it will take a manufacturer several “versions” along with several thousand installed
units to reach stability and a reasonably robust feature set. An example of a key
feature is downloadable “geo-fencing” and “route segments” via the wireless
communications link – this feature is important in route management applications,
especially where the routes change with some degree of frequency.
