GPS tracking everywhere?
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
ot 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.

And when there is no cell phone coverage?
Great surfaces of our planet are covered by cell phone towers. But most surfaces are not. We only have to think of the oceans and the deserts. Even in these regions GPS tracking can be done, but the cell phone modem has to be replaced by a satellite modem. This way the position information can be sent over a communication satellite network (not the GPS satellites!). Advantage: In principle these kind of GPS tracking devices can operate everywhere. Disadvantages: These devices are more sophisticated and bigger than GPS tracking devices with a cell phone modem. The devices AND the service are much more expensive.