Continuous location finding, function

General

The term "Continuous location finding" means continuous calculation of the current position of the vehicle by the APS navigation processor (A85). Continuous location finding is always active when the ignition is switched on.

Precise determination of the vehicle location is possible only with a combination of three different types of location findings.

We differentiate between:

 Basic location finding (dead reckoning)

 Map-supported location finding (map matching)

 Global positioning system (GPS) location finding.

While the data required is loaded by the system after switching on the ignition, only the basic location finding feature is available.

After the data is loaded, the basic location finding (dead reckoning), map-supported location finding (map matching) and Global positioning system (GPS) location finding, all run parallel to one another.

When the ignition is switched off, all important location finding parameters are stored.

-----------------------------------------------------------------------------------------

Basic location finding (dead reckoning)

Basic location finding (dead reckoning) is accomplished continuously by the APS navigation processor (A85) when the ignition is switched on. After switching off the ignition, the current position is stored in a non-volatile memory.

In the basic location finding (dead reckoning) mode, the APS navigation processor (A85) calculates the current location using:

 the distance travelled,

 the direction of motion (forward, backward) and

 the vehicle direction.

 Basic location finding is accomplished approx. once per second.

 Calculation of the distance travelled

The APS navigation processor (A85) receives signals from the speed sensors (B81, B82) on the undriven wheels via the ABS control module (A2).

The distance travelled is calculated from these signals and the stored position data.

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

-----------------------------------------------------------------------------------------

 Following a tire change it is necessary to recalibrate the wheels.

 Calculation of direction of motion

The direction of motion (forward, backward) is determined from the signal from the backup light switch (S19/S75).

 Calculation of vehicle direction

The vehicle direction (compass direction) is calculated by the APS navigation processor (A85) from the speed difference between the right rear speed sensor and left rear speed sensor (B81, B82), the data from the gyrosensor and the sequence of the locations calculated using GPS.

 Location finding errors with basic location finding (dead reckoning).

Rolling backward without the transmission shifted into reverse is evaluated as driving forward and can lead to incorrect location determination.

Rolling without the ignition switched on or transport of the vehicle can also lead to incorrect determination of the location.

The incorrectly calculated position is automatically corrected by the APS navigation processor (A85) when global positioning system (GPS) data is received.

-----------------------------------------------------------------------------------------

Map-supported location finding (map matching)

The position of the vehicle is continuously compared with the data from the CD-ROM by the navigation processor (A85). If the vehicle is located in the digitalized area of the CD-ROM map, up to 10 alternative routes are calculated. Attributes such as one-way streets, turning prohibitions, street classes etc., are taken into consideration in this calculation.

Any deviation can be corrected by comparison of the vehicle position calculated with the aid of dead reckoning and the possible position on the digitalized CD-ROM map.

If the vehicle route deviates from the calculated route, the position for basic location finding is corrected as required. Map-supported location finding ( map matching) is possible only in the digitalized map area.

Global positioning system (GPS) location finding

The global positioning system (GPS) is a satellite-supported location finding system.

25 satellites are located in 6 different earth orbits. The GPS procedure is based on measurement of the differences in the transmission times of the signals transmitted synchronously from various satellites.

-----------------------------------------------------------------------------------------

These signals contain time, orbit and identification data from the GPS satellites.

The position of the receiver can be calculated only when data from at least 4 satellites is available.

GPS location finding is particularly important outside of the digitalized map area and following vehicle transport (e.g. by train, ferry or towing vehicle).

The data (location coordinate) transmitted by the GPS satellites is received by the global positioning system antenna (W35/W36) and amplified.

The received data is transferred from the GPS control module to the APS navigation processor (A85), where the position of the vehicle is calculated.

The APS navigation processor (A85) calculates the location using the location coordinates and other information (such as map data from the CD-ROM).

The GPS location coordinates are used for support of the continuous location finding function.

 GPS is used primarily along street routes and for determining the starting location

The location finding accuracy is 100 m or better.

-----------------------------------------------------------------------------------------

 Problems receiving GPS signals

Since GPS signals are in the Giga Hz range (microwaves) and the signals are weak, reception interference can result from

 atmospheric interferences (weather, water vapor)

 multiple reception from reflection of the signals, e.g. from building walls

 signal shadows, e.g. in areas with high buildings or tunnels.

-----------------------------------------------------------------------------------------

Wheel recalibration

Calibration of the wheels provide the APS navigation processor (A85) with data on the condition of the tires.

Wheel recalibration is required following a wheel change or replacement of the APS navigation processor (A85).

For this purpose, it is necessary to enter the type of tire (e.g. 195/070/015) on the APS radio and navigation operating unit (A9) or with the aid of the hand-held tester.

In addition, a code is required indicating whether the tires are new or used.

New tires : Tread depth 7-10 mm

Used tires : Tread depth 3-7 mm

The diameter of used tires is calculated using a test path, which must be entered on the radio and navigation operating unit (A9) previously.

APS navigation processor (A85) counts the electrical impulses transmitted by the speed sensors (via ABS control module) over this test path. The length of the test path and the number of electrical impulses allows calculation of the tire diameter.

Continuous tire wear can be recognized by the APS system by the fact that the vehicle arrives at a street at which it is to turn increasingly later than calculated.

Continuous tire wear is therefore taken into consideration automatically by evaluation of the speed sensor signals.