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

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

MODELS 203 up to 23.4.04
- with CODE (352a) COMAND operating and display system
- with CODE (498) Japanese version







Position finding by Global positioning system (GPS)

GPS is a satellite-supported localizing system set up by the American Department of Defense. It is a toll-free service available throughout the world. Presently several satellites are located in various different orbits. The GPS satellites constantly transmit time and position data. 4 satellites are needed for the first position calculation, then 3 receivable satellites are sufficient.

GPS position finding is irreplaceable for journeys outside the digitized map area and after vehicle transport (e.g. train or ferry journeys and towing). A new GPS position (after interruption of the satellite reception) can take several minutes ( up to 20 minutes when the equipment is first set up).

GPS position finding, function
The navigation processor of the COMAND operating, display and control unit (A40/3) uses the satellite signals received and amplified from the GPS antenna (A2/23) to calculate the vehicle position. The actual calculation is essentially based on the measurement of the travel times of the signals sent synchronously by the various satellites. Here, an important prerequisite is that the satellite and receiver clocks are synchronized . For this purpose, the receiver has an automatic time correction feature which again is possible with the aid of the satellite signals. When the navigation processor determines that the basic position finding (dead reckoning) is less precise than GPS position finding, the current calculated position is corrected with the aid of the GPS data.
In addition, the vehicle's course of travel is determined with the aid of the sequence of the locations calculated with GPS.

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

As GPS operates in the microwave (GHz) range and the signals are weak, the following events may cause reception interference:
- Atmospheric interference, e.g. weather, water vapor, fog
- Multiple reception resulting from reflection of signals, e.g. from building walls
- Signal shadowing e.g. in built-up areas, tunnels, tall buildings, trees

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