A satellite navigation or sat nav system consists of a receiver and a system of satellites that, when combined with a computerized map, can pinpoint an individual or a vehicle's location to within 30 feet.  Most sat navs use the United States NAVSTAR Global Positioning System (GPS), which is made up of around 30 US military satellites orbiting the earth.  They constantly beam high frequency radio signals to earth. Each satellite contains an electronic clock, accurate to 1 second in 138 million years.  

The signals, which travel at the speed of light, are intercepted by sat navs where a small electronic receiver determines its location (longitude, latitude, and altitude) to within a few metres, by working out how far away the satellite is using the transmitted time signals.  A sat nav will use information from at least 4 of the satellites, in order to pinpoint its position as accurately as possible.  Using two satellites the sat nav could calculate two possible positions, so a third satellite is needed to calculate where the position intersects in 1 location, and the fourth satellite is used to confirm the position and correct any discrepancies.

The sat nav then superimposes its position onto a map.  Digital mapping databases like MultiNet mean your sat nav can give you turn-by-turn directions, road signage, intersections, positioning centrelines for drivable roads and streets. More than 50,000 global data sources provide MultiNet with a continuous stream of updates.  Nowadays mobile mapping vehicles are used to map new areas, with cameras, lasers and scanners to record the environment. The information collected is analysed and used to update the database. 

In the UK the road system changes by about 15% per year, due to new roads, road layouts, speed limit changes and the re-designation of house numbers.

Sat navs are now equipped to tell you about traffic problems.  This data comes from a variety of different sources, in the UK this includes traffic monitoring systems, emergency services and even motorists phone calls to radio stations. 

Nowadays smartphones can be used as navigation systems, with additional information coming from cellular masts.

In addition to the United States GPS, the Russians have GLONASS, China is expanding its regional Beidou navigation system into the global Compass navigation system by 2020. The European Union's Galileo positioning system is  currently being deployed and is scheduled to be fully operational by 2019. France, India and Japan are also in the process of developing regional navigation systems.

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