GPS tracking explained(1)

How does real-time GPS tracking work? 

GPS tracking everywhere?
GPS tracking is not always possible. Global Positioning System (GPS) indicates that the system can be used everywhere on Earth, on land, at sea and in the air. A GPS device receives signals from the GPS satellites, high in the sky. If it receives strong enough signals from three or more different satellites, it can calculate its position. As these signals are very weak, in some circumstances it can be difficult to receive even three different signals. This is the case in towns with high buildings and relatively narrow streets or under trees with thick foliage. Under these conditions real-time GPS tracking could be troublesome. GPS tracking inside buildings is seldom possible.

A GPS device does NOT send any signal, not to the GPS satellites, nor anywhere else. It can only receive. So, if we want to know where the GPS device is, we will need a second technology to send this information to us. For this we often use a cellular network. This means that a GPS tracking device must at least contain a GPS receiver and a cellular phone modem.