GF82.61-P-3007QMJ Global Positioning System(GPS) Position Finding, Function

GF82.61-P-3007QMJ Global Positioning System(GPS) Position Finding, Function
- with CODE (498) Japan version







The global positioning system (GPS) is a satellite-supported localizing system set up by the American Department of Defense. The GPS satellites permanently transmit time and position data. 4 satellites are needed for the first position calculation, then 3 receivable satellites are sufficient.

The GPS position finding is, in particular, for journeys outside the digitized map area and after transportation of the vehicle (e.g. train, ferry and towing) irreplaceable. Starting GPS position finding again (following interruption of the satellite reception) may take several minutes (for initial startup of the device up to 20 min).

GPS position finding, function
The navigation processor of the COMAND operating, display and controller unit (A40/3) calculates the vehicle's location from the satellite signals. The actual calculation is essentially based on the measurement of the runtimes of the signals transmitted synchronously by the various satellites. An important requirement here is the synchronicity of the clocks for the satellites and the navigation processor. To this end the receiver is equipped with an automatic time correction, which in turn is made possible with the aid of the satellite signals. When the navigation processor determines that basic position finding (compound position finding) is less precise than GPS position finding, the currently calculated position is corrected with the aid of the GPS data. In addition to this the sequence of locations calculated by the GPS is used to determine the vehicle's route.

The location is recalculated approx. 50 times per minute by the GPS system.

Since GPS operates in the microwave-range (GHz) and the signals are weak, reception interferences can occur as the result of the following events:
^ Atmospheric interference
e.g. bad weather, water vapor, fog
^ Multipath reception due to the reflection of the signals
e.g. from walls of buildings
^ Signal shading
e.g. in built-up areas, tunnels, through high buildings, through trees

GPS reception therefore necessitates a clear view of the sky. Positioning accuracy is approx. 100 m.