Basic Location Finding

Basic location finding (dead reckoning) is the basic function of the continuous location finding feature. It is accomplished continuously by the navigation processor (A40/3n2) when the ignition is switched on and is independent of the CD-ROM map. After switching off the ignition the current position is stored for a delayed shut-off period.

The following information is calculated with the aid of the basic location finding feature:
- Distance traveled
- Direction of motion (forward or backward) and
- Direction in which vehicle is pointing.


NOTE: Basic location finding is accomplished approx. once each second.


Distance traveled
The navigation processor (A40/3n2) receives the signals from the speed sensors on the undriven wheels over discrete loads (not CAN) from the traction system control module (N74).
- Left front axle VSS sensor (L6/1)
- Right front axle VSS sensor (L6/2)


NOTE: On the (J) version, only the signal from L6/2 is fed to the navigation processor (A40/3n2).


Evaluation of the wheel speed signals provides the distance traveled.

Direction of motion
The backup signal (from CAN passenger compartment bus) allows the navigation processor (A40/3n2) to recognize the direction of motion (forward or backward).

Vehicle alignment
The navigation processor (A40/3n2) calculates the direction in which the vehicle is pointing (compass direction) from the difference in the signals from the speed sensors (except (J)), the data from the gyrosensor and the sequence of locations calculated by GPS.

The difference between the calculated and actual distance traveled between two turning points is used automatically for recalibration of the tire circumference (required due to tire wear).

Location finding errors with basic location finding
Rolling backward without the transmission shifting into reverse is evaluated as forward motion and can lead to an error in location finding.

Rolling with the ignition switched off or transport of the vehicle results in a location finding error.

However, the calculated position is automatically corrected by the navigation processor (A40/3n2) when GPS data is received.

Calibration
Calibration is required following tire change. This requires entry of the tire size, e.g.:

- 225/060/016

and coding (except (J)) whether
- new tires (tread depth approx. 5-10 mm), or
- used tires (tread depth approx. 3-5 mm)

are installed. Entry is possible with Star diagnosis. A special calibration drive is not required.