The Global Positioning System (GPS) is a global navigation satellite system developed by the United States Department of Defense . It is the only fully functional global navigation satellite system in the world, can be used freely, and is often used by civilians for navigation purposes. It uses a number of satellites (between 24 and 32) that transmit precise radiowave signals, which allow GPS receivers to determine their current location, the time, and their velocity. Its official name is NAVSTAR GPS. A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth, each satellite transmits messages containing the time the message was sent and precise orbital information. The receiver measures the transit time of each message and computes the distance to each satellite. Geometric trilateration is used to combine these distances with the location of the satellites to determine the receiver's location. The position is displayed, perhaps with a moving map display or latitude and longitude; elevation information may be included. Many GPS units also show derived information such as direction and speed, calculated from position changes. Nowadays there are many brand of car and handheld GPS systems available on the market, amongst the most popular: TomTom, Garmin, Navman.