- The Global Positioning System is the world's only functioning satellite-based navigational system. It consists of a network of 2-3 dozen satellites. While the Russians, European Union, India, and China are all working on competing systems, none are functional.
- A GPS receiver monitors microwave signals from the orbiting satellites. These signals contain the time of transmission and the position of the satellite. The receiver analyzes these signals by using the travel time of the signals and positions of the transmitting satellites to trilaterate its position. At least four signals are necessary for a good position lock using the GPS system.
- Since the GPS signals are relatively weak, many forms of electromagnetic interference can disrupt one or more of the necessary microwave transmissions. In nature, solar flares and other magnetic events can disrupt the signals. Metal parts in automobiles, such as the frame plus the wires of a defroster, can combine to form a Faraday Cage that blocks out the signals. Finally, there are man-made jamming devices.
