GFZ-1 (GeoForschungsZentrum) is a passive geodetic satellite consisting of a 21-centimeter sphere covered by 60 retroreflectors; it was designed to study changes in Earth's rotational characteristics and variations in its gravitational field. GFZ-1 was the lowest-flying geodynamic satellite to be ranged by lasers and led to a significant improvement in modeling of Earth's gravity. As it decayed, its orbital motion was also used to calculate atmospheric densities. GFZ-1 was the first satellite designed and funded by the GeoForschungsZentrum Potsdam, Germany. Built and launched by Russia, it was transported to Mir aboard a Progress supply craft and from there placed into low Earth orbit.