User guide

Rev 1 Oct.13 185
9
9: GPS Configuration
Most AirLink devices are equipped with a Global Positioning System
receiver (GPS) to ascertain its position and track the movements of a
vehicle or other devices which move. The AirLink device relays the
information of its location as well as other data for use with tracking
applications.
The AirLink ES440 does not support GPS.
GPS Overview
The Global Positioning System (GPS) is a satellite navigation system
used for determining a location and providing a highly accurate time
reference.
GPS consists of a “constellation” of 32 satellites in 6 orbital planes.
Each satellite circles the Earth twice every day at an altitude of
20,278 kilometers (12,600 miles). Each satellite is equipped with an
atomic clock and constantly broadcasts the time, according to its own
clock, along with administrative information including the orbital
elements of its motion, as determined by ground-based
observatories.
A GPS receiver, such as the AirLink device, requires signals from four
or more satellites and performs Time Difference of Arrival (TDoA)
calculations in order to determine its own latitude, longitude, and
elevation.
The GPS data can then be transmitted to a server with a tracking
application to compile information about location, movement rates,
and other pertinent data.
Note: Depending on the location of the satellites in relation to the device’s
location and how many signals are being received, the AirLink device may
encounter “GPS drift”, a phenomenon whereby a stationary device is
reported as moving by the GPS system. This “drift” is within the location toler-
ances of the GPS system, but the device may appear to be moving, based
on continuous GPS calculations.
Common Uses for GPS
• Driver navigation—The AirLink device provides real time GPS
data via the serial or Ethernet port to a local application,
including applications that provide mapping and navigation
support.
• Automatic Vehicle Location (AVL)—The AirLink device provides
real time GPS data to the server that tracks the location and
other variables of the vehicle or asset.