Introduction To The Global Positioning System (GPS)

WARNING: This page is about a different car, the 2006 Land Rover Range Rover. However, it is still accessible from the selected car via links, so may be relevant.

The system used to calculate the current position of the vehicle is called the global positioning system (GPS). The system utilizes satellites which are owned by the United States Department of Defense (DoD). A total of 24 satellites circular orbit the earth every 12 hours at a height of 20, 000 km (12500 miles), and between 5 and 11 of these satellites can be seen from a single point at any given time. The orbits are tilted to the earth's equator by 55 degrees to ensure coverage of polar regions. Each satellite transmits radio signals to provide information about the satellite position i.e. latitude, longitude, altitude, almanac data and an accurate time signal generated by an on-board atomic clock. Each satellite contains four atomic clocks

The vehicle needs to receive data from at least four different satellites to give a three dimensional fix on its current position.

As the vehicle moves, this information is continually being updated. The computer determines which satellites are 'visible' to the system and their current position and relationship to each other. Using this information the computer can account for positional deviations of the satellites and compensate to enhance the accuracy of the navigation system.

The global positioning system (GPS) signal is also known as the Precision Positioning Signal (PPS).

PPS predictable accuracy is :

22 meters horizontal accuracy
27.7 meters vertical accuracy
200 nanosecond time accuracy.

The system uses almanac data to determine the vehicles position. Almanac data is the current status of the 24 satellites which orbit the earth. The computer determines which satellites are 'visible' to the system and their current position and relationship to each other. Using this information the computer can account for positional deviations of the satellites and compensate to enhance the accuracy of the navigation system. The navigation system requires the almanac data from at least four different satellites to calculate a three dimensional 'fix' on its location. As the vehicle moves the computer continually up dates this information so that at all times the computer knows the precise location of the vehicle.