Global positioning system (GPS) position finding, function - GF82.61-P-3007BA
MODEL 215, 220 with CODE (352a) COMAND operating and display system with CODE (494a) USA version
Global positioning system position finding
The global positioning system (GPS) is a satellite supported position finding system established by the US Defense Department.
It is a toll-free service available throughout the world. Currently several satellites are on different orbits of the earth. 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 particularly valuable when driving outside of the digitized map area and after vehicle has been transported (e.g. train, ferry or 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).
In vehicles as of 1.9.03, the navigation processor (N41/1) is installed as a separate control unit on the left-hand side of the trunk.
GPS position finding, function
The navigation processor calculates the location of the vehicle and the accuracy of positioning from the satellite signals received and amplified by the GPS antenna (A2/23) (model 215) or (A2/49a2) (model 220).
Actual calculation is based primarily on measurement of the transmission times of the signals transmitted synchronously by the various satellites. An important prerequisite here is that the satellite clocks and receiver clocks are synchronized.
For this purpose, the receiver has an automatic time correction feature which again is made possible with the aid of the satellite signals. When the navigation processor determines that basic location finding is less precise than GPS location finding, the currently 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.
Every minute, approximately 50 GPS recalculations of the location take place.
Possible GPS signal reception interference
As GPS operates in the GHz range (microwaves) and the signals are weak, reception interference may occur due to:
- Atmospheric interference, e.g. weather, water vapor, fog
- Multiple reception from reflection of signals, e.g. from building walls
- Signal shadows, e.g. in built-up areas, tunnels, high buildings, trees
GPS reception therefore requires a clear view of the sky. The accuracy of GPS location finding is approx. 100 m or better.
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