419-07 Navigation System: Description And Operation

Navigation System  

INTRODUCTION TO THE GLOBAL POSITIONING SYSTEM (GPS) 

The system used to calculate the current position of the vehicle is called the GPS (global positioning system) . The 
system utilises satellites which are owned by the United States Department of Defence (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 GPS signal is also known as the Precision positioning signal (PPS). This is now the only GPS signal available, since 
the Selective positioning signal (a deliberately degraded signal) was turned of on the 2nd May 2000. The ensures that 
everyone can now use the higher accuracy available with PPS. 
 
SPS predictable accuracy was: 
 

100 metres horizontal accuracy  
156 metres vertical accuracy  
340 nanoseconds time accuracy.  

PPS predictable accuracy is : 
 

22 metres horizontal accuracy  
27.7 metres vertical accuracy  
200 nanoseconds time accuracy.  

Each navigation system provides computer generated audible and visual route guidance information to enable the driver 
to reach a desired destination. The system allows the driver to choose the desired route using minor or major roads or 
motorways and the quickest or shortest route. Directions to hospitals, museums, monuments and hotels are also 
available. The computer uses map information stored on a CD-ROM to determine the best route for the journey and 
provide the driver with details of directions and approaching junctions. 
 
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. 
 

NAVIGATION SYSTEM 

Navigation System Component Location

Published : May 14, 2004