How GPS Works?
Any any instant of time, there are at least 4 GPS satellites in line of sight to a receiver on the earth. Each of these GPS satellites send information about its position and the current time to the GPS receiver at fixed regular instants of time. These information are transmitted to the receiver in the form of signal which are then intercepted by the receiver devices. These signal are radio signals that travels with the speed of light. The distance between a GPS receiver and the satellite is calculated by finding the difference between the time the signal was send from GPS satellite and the time the GPS receiver received the signal.
Once the receiver receives the signal from at least three satellites, the receiver then points its location using trilateration process. A GPS requires at least 3 satellites to calculate 2-D position(latitude and longitude on a map). In this case, the GPS receiver assumes that it is located at mean sea level. However, it requires at least 4 satellites to find receivers 3-D position(latitude, longitude and altitude).