A total of 18 satellites, in different orbits, so that at least four are visible at any time from all points on the Earth’s surface. They give a locational accuracy of less than 2 cm on baselines of 1000 km. It is planned to use them for measuring the motions of small blocks of the Earth’s crust, e.g. opposite sides of a fault system, microplates, etc. (The satellites also have obvious military applications.)