NAVIGATION SYSTEM (Description): DESCRIPTION AND OPERATION

NAVIGATION SYSTEM

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DESCRIPTION AND OPERATION

Description

General
The navigation system provides audio and visual route guidance to help the driver reach a selected destination. The 
system is an optional fit consisting of a Traffic Pro navigation computer and antenna, manufactured by Harman/
Becker Automotive Systems, which are fitted in place of the In Car Entertainment (ICE) head unit and antenna. 
Compact Disc (CD) and radio functions are incorporated into the navigation computer.

The navigation system allows the driver to choose between the shortest and fastest routes between the vehicle's 
current position and a selected destination, and to select a stopover point in the journey and a route that avoids 
motorways, ferries and toll roads. Directions to Points Of Interest (POI) e.g. airports, hospitals, petrol stations etc, 
either local, national or in another country, can also be selected. A traffic jam function enables the driver to request 
diversion instructions, around an obstructed part of the selected route, during the journey. A Traffic Management 
Control (TMC) function, currently only available in some European countries, monitors traffic broadcasts and 
automatically selects an alternative route during the journey if the original route is effected by a traffic jam, accident 
or road works etc.

The position of the vehicle is determined by the navigation computer using a combination of vehicle sensor inputs and 
radio signals from the 24 Global Positioning System (GPS) satellites orbiting the earth. The position of the vehicle is 
then plotted on a digitised map, loaded into the navigation computer from a CD-ROM, to determine the journey route 
and provide the route guidance.

The GPS satellite signals are used for initial determination of the vehicle's position and periodic position updates. The 
vehicle sensor inputs are used to monitor the vehicle's direction of travel and distance travelled between position 
updates from the GPS satellite signals. The vehicle sensor inputs consist of:

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A vehicle speed signal from the ABS ECU, to monitor the distance travelled and for automatic volume control.

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A reverse gear signal from the selector and inhibitor switch of the automatic gearbox, or reverse gear switch of 
the manual gearbox, to enable the navigation computer to differentiate between forward and rearward movement 
of the vehicle.

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A gyro in the navigation computer, to monitor changes of direction, i.e. steering inputs.

The signal from each GPS satellite contains information about satellite position, almanac data and time (almanac data 
is the current status of the satellite). Signals from between five and 11 of the GPS satellites can be received at a given 
point on the earth's surface at any one time. The number and quality of separate GPS satellite signals received also 
varies with vehicle location. In hilly or tree lined areas, built up areas with tall buildings, multi-storey car parks, 
garages, tunnels, bridges and during heavy rain/thunderstorms, signal reception of some or all of the GPS satellites 
will be poor or non existent.

A minimum of three separate GPS satellite signals are required for the navigation computer to calculate a three 
dimensional (3D) positional fix. When only two signals are being received, the navigation computer will calculate a 
less accurate two dimensional (2D) positional fix. The more widely dispersed that the GPS satellites are, the more 
accurate the positional fix. The navigation computer can store information from a maximum of 12 GPS satellites at 
any one time. When more than three signals are stored, the navigation computer selects the three most widely 
dispersed signals for the position calculation.

GPS Antenna and diplexer unit
The GPS antenna is installed at the rear of the roof on the centreline. A diplexer unit on the underside of the GPS 
antenna amplifies the radio signals received from the GPS satellites and transmits them through separate dedicated 
co-axial cable to the navigation computer for processing.