A satellite-based navigation system comprising a constellation of 24 Navstar satellites launched by the United States Department of Defense from 1978 to 1994.The satellites are in multiple medium-earth orbital (MEO) paths at altitudes of approximately 11,000 miles, and are positioned such that signals from six of them can be received by a GPS terminal at virtually any point on the Earth's surface at any time. The satellites constantly broadcast timing signals based on atomic clocks that are accurate to within three nanoseconds.The signals are broadcast on two frequencies -- the L1 civilian signal and the L2 military signal.The civilian signal is transmitted in the UHF band at 1575.42 MHz, which requires line-of-sight (LOS).The terminals receive those signals and correlate them based on their knowledge of the satellites' positions, adjusting for propagation delay. Assuming that three signals are received, the terminal can determine its two-dimensional position within a few meters of the exact longitude (x) and latitude (y) and. Signals from four satellites allow determination of its three-dimensional position, including altitude (z). See also LOS, MEO, propagation delay, and satellite.