GPS (Global Positioning System) is the way to locate a receiver by satellite in three dimensional space. GPS is one of the greatest inventions nowadays, to know why it is so important, we need to look into how GPS works.
A satellite network is set around the Earth to send out signals, which is similar with normal radio signal and strong enough to be received by the aerial. The GPS reciever gets signals from the aerial, the signal is so specific that GPS receiver can identify the specific satellite and signal duration.
With different signals, the GPS software can triangulate the position of the receiver. GPS needs at least three satellites for triangulation, requires data on the position of satellites and needs to calculate the travel time that signal takes to reach the destination.
In the real world, error is unavoidable, generally it is impossible to guarantee that the clocks are all synchronized. As the satellite contains highly accurate atomic clock, it is not the satellite itself that causes the error.
To make the price acceptable to the consumers, it is not realistic to install expensive atomic clock in every GPS. The solution to solve this problem is to use a fourth satellite to do cross check, which informs GPS software of the error and provides additional values for reference.
Mapping is very important for GPS as it is useless if you just know where is device is, the GPS needs map data to work out the routes. Mapping data elevates the price of GPS, and it must be updated instantly.
There are several kinds of maps for different users to meet different needs. Road users requires mapping data with detailed road network information, hiking GPS users may want map with good description of surroundings like rivers, mountains, lakes and so on. Marine users need full information on the sea, channels, etc.
There are many choices of GPS available for consumers. Car GPS is used in car for drivers to find the shortest or fastest way. Handheld GPS like Garmin stores a map inside, it is portable and waterproof for outdoor application. Besides, there is also a kind of GPS whose receiver is attached to a PDA to work.