The first satellites used to aid navigation were operated by the military, particularly by nuclear submarines, which would have to know their exact position before they could fire their nuclear missiles. In the late 1960s navigation satellites became available for commercial use. The US Transit series pioneered this, and remained operational into the 1990s. The Russians have a similar system, called Tsikada. The satellites orbit at about 1,000 km, like the Transits. A more advanced satellite navigation system is now being operated by US Navstar satellites, orbiting at some 19,000 km. Called the global positioning system, it comprises some 24 satellites, which carry atomic clocks. Aircraft, ships and even cars can tune into a satellite and receive data that will pinpoint their position to within a few metres. The Russians have an equivalent system called Glonass, also orbiting at 19,000 km.
