Etalon-1 and Etalon-2 are geodetic satellites. They are designed to determine, with high accuracy, terrestrial reference frame and earth rotation parameters, to improve the gravity field, and to improve the gravitational constant. Each satellite is a high-density passive laser reflector in a very stable medium Earth orbit (MEO). References Geodetic satellites Laser ranging satellites Russian space probes Earth observation satellites of the Soviet Union Spacecraft launched in 1989