- The Global Positioning System (GPS) is the sole functioning satellite-based navigational system in operation. It is made up of a network of 2-3 dozen satellites. While the Russians, European Union, India, and China all have plans for competing systems, none are functional.
- A handheld GPS device is a navigation tool for pinpointing location. Many also have directional features for guiding a person to a particular place. They operate on the same principles as all other GPS-based devices. The device receives microwave signals from the orbiting satellites. These signals contain information about the time of transmission and the position of the satellite. The device analyzes these signals by using the travel time of the signals and positions of the transmitting satellites, and trilaterates its position. At least four signals are necessary for a good position lock using the GPS system.
- Commercial GPS devices that are available to the public are typically designed to be less accurate than their military counterparts, to prevent their use in creating homemade precision bombs. Also, since GPS signals are relatively weak, many forms of electromagnetic interference can disrupt one or more of the necessary microwave transmissions, limiting or crippling the effectiveness of a handheld GPS. In nature, solar flares and other magnetic events can disrupt the signals. The metal parts in automobiles, such as the frame plus the wires of a defroster, can combine to form a Farraday Cage that blocks out the signals.
