1 Answer

Global Positioning System works by the reciever in your mobile,satnav or Onboard GPS picking up signals from several satelites in line of sight and calculating its position from them.There are between 24 and 32 satelites in mid earth orbit which should mean that at a particular time and place on earth there will be at least 6 satelites in line of sight to recieve a signal from.Although normally you only require 3 points of reference to triangulate a position a GPS reciever needs 4 or more.The satelite sends a signal containing it's exact position and time when the signal was sent.Due to the distances involved and the difference in gravity between the surface and orbit which effects the rate of time to a minute degree the reciever needs a 4th or 5th reference of time to compensate for any time errors.A 1000th of a second error can put your position out by up to 1000 yards or more.