Satellite Navigation

Satellite Navigation Information

Vehicle satellite navigation systems use the Global Positioning System (GPS) which consists of a network of over 24 GPS satellites orbiting the Earth. These satellites transmit radio signals, allowing GPS receivers to determine the receiver's location, speed and direction.

History of GPS

The US Department of Defence developed the system which is officially called NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System). The first experimental satellite was launched in 1978 and GPS has since become an important tool for navigation, map making and land surveying.

GPS is maintained by the US Air Force at an estimated annual cost of $400 million, however it is provided free of charge to civilians as a "public good".

In late 2005, the first of the "next generation" satellites was launched, offering several new capabilities including a second civilian GPS signal known as LC2. There are plans to increase the number of these satellites in the future and to add a third and then fourth civilian signal.

How GPS Works

In principal, a GPS receiver calculates its position by measuring the distance between three GPS satellites and itself. This is done by measuring the time delay between transmission and reception of each of the three radio signals as each travels at a known speed.

The signals also carry information about the satellites' location. By determining the position of and distance to at least three satellites the receiver can compute it's position.

In practice, because GPS receivers do not have perfectly accurate clocks, they track one or more additional satellites to correct their clock errors.

How Vehicle Satellite Navigation Systems Work

Vehicle sat nav systems use a combination of GPS data and a map database. The GPS data is used to locate the user on a road in the systems map database. The system can then give directions to other locations in it's map database.

In some systems "dead reckoning" is used by collecting information from sensors attached to the vehicle. This enables the system to be more accurate in the event of signal loss or if a vehicle is traveling through a tunnel for example.

Most systems include both visual and aural information and many include speed camera location, traffic congestion information and alternative routes.

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