Basic position finding, function - GF82.61-P-3006R

MODELS 203.0 /2 /7 up to 23.4.04 with CODE (352a) COMAND operating and display system with CODE (491) U.S. version 

Position finding (dead reckoning) 

Function 

Basic location finding (dead reckoning) is the basic function of continuous location finding. It is continuously performed by the navigation processor of the COMAND operating, display and control unit (A40/3) when ignition is switched on (circuit 15 ON) and it is independent of the Compact Disk (CD) card. After switching off the ignition (circuit 15 OFF) the current position is stored.

The following information is calculated with the aid of basic position finding (dead reckoning):

• Distance traveled
• Direction of travel (forward or reverse)
• Vehicle orientation

Basic position finding (dead reckoning) is performed every 1 s.

Distance traveled 

The navigation processor receives the wheel speed signals from the ESP and BAS control unit (N47-5) via the Controller Area Network bus Class C (engine compartment) (CAN C). The evaluation of the wheel speed signals reveals the mileage covered.

Direction of travel 

The signal for reversing, which is sent by the electronic ignition switch control unit (N73) via the Controller Area Network bus Class B (interior) (CAN B), enables the navigation processor to determine the direction of travel (forwards or backwards).

Vehicle orientation 

From the data of the yaw rate sensor and the sequence of the locations calculated with the global positioning system (GPS), the navigation processor calculates the orientation (compass direction) of the vehicle.

The difference in the calculated mileage and the mileage actually covered between 2 turnoff points is used to automatically recalibrate the tire circumference (necessary due to tire wear).

Position finding errors during basic location finding (dead reckoning) 

Rolling backwards without the transmission being shifted into reverse gear is evaluated as moving forwards and can lead to temporary errors in location finding. Rolling with the ignition switched off (circuit 15 OFF) or transport of the vehicle also makes the position finding result erroneous.

In both cases, this deviation is automatically corrected by the navigation processor on receipt of GPS data.

Calibration 

Wheel calibration is necessary following a tire change. To this end, it is necessary to enter the tire size and the coding as to whether new tires (depth of tread approx. 5 to 10 mm) or used tires (depth of tread approx. 3 to 5 mm) have been fitted. The entry is accomplished with Star Diagnosis or directly at the COMAND operating, display and control unit (A40/3). A special calibration drive is not required.

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