Navigation system (Description): 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: l A vehicle speed signal from the ABS ECU, to monitor the distance travelled and for automatic volume control. l 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. l 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.