A car navigation system must correctly detect the current location and indicate it on the map display. Two systems are available for detecting the current location: the independent (estimation) navigation system in which the vehicle itself detects its current location, and the GPS (satellite) navigation system that uses GPS signals.
The independent navigation system detects the current location by calculating the relative position of the vehicle in accordance with the vehicle’s driven track, which is obtained from an onboard gyro sensor and vehicle speed sensor. The independent navigation system is unable to detect and absolute position. Therefore, corrections must be made if the vehicle is moved on a ferry or the like.
This system uses the radio wave signals transmitted by GPS satellites to detect the current absolute position of the vehicle. It calculates the arrival time of the radio waves from three or more satellites to detect the absolute position. GPS system can detect the current absolute position as long as it can receive radio wave signals; thus, it can be used throughout the world.
GPS satellites were launched by the United States Department of Defense for military purposes. They transmit two types of codes: the high-accuracy P (protected) codes for military use and the low-accuracy C/A (cause/access) codes for civilian use. Car navigation systems use the C/A codes. A GPS satellite was first launched in 1978. Currently there are 24 satellites that are circling the earth along 6 orbits, at an altitude of 21,000 km. They are arranged in such a way that 5 or more are visible at any time around the world. These GPS satellites circle the earth about every 12 hours.