The Global Positioning System (GPS) is a satellite-based navigation system consisting of a network of 24 orbiting satellites. These satellites are approximately 20,180 kilometres above Earth's surface and make two complete orbits in less than 24 hours. They are built to last about 10 years, and replacements are constantly being built and launched into orbit. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Accuracy of GPS data depends on the positioning of the satellites. Ideally, satellites should be located at wide angles relative to each other. This is why GPS receivers are generally better at calculating accurate information in open areas, rather than in areas with obstructions like tall buildings or trees. The satellites are spaced so that a receiver can always be in contact with at least four of them from anywhere on Earth. The speed of radio waves is constant and known, allowing the receiver to calculate the distance to each satellite based on the time it took for the signals to arrive. The GPS system is maintained by the United States government and is freely accessible to anyone with a GPS receiver. The system was originally developed for military use, but wide-ranging applications now benefit civilians around the globe.
