The Global Positioning System (GPS) consists of a constellation of 24 or more satellites that transmit navigation signals in two frequency bands called L1 (centered at 1575.42 MHz) and L2 (centered at 1227.6 MHz). The signals are synchronised to within about 10 nanoseconds of a composite time frame known as GPS System Time. The satellites also transmit messages that accurately describe their orbital path in space. GPS users with unobstructed visibility can receive up to 12 of these signals, each one coming from a different direction. The GPS receiver measures the arrival time of each signal. With a minimum of four signals, the receiver can calculate its three physical position coordinates (for example, latitude, longitude, and altitude) plus a precise correction for its internal clock. In general, the more signals used, the better the result.