Etalon are a Russian family (Etalon-1, Etalon-2) of passive geodetic satellites dedicated to satellite laser ranging. Etalon-1 was the first geodynamic satellite launched by the former Soviet Union. ... The Etalon spacecraft were launched in 1989 in conjunction with a pair of GLObal'naya NAvigatisionnay Sputnikovaya Sistema (GLONASS) satellites. The mission objectives were to determine a high accuracy terrestrial reference frame and earth rotation parameters, to improve the gravity field, and to improve the gravitational constant.
[Summary provided by NASA.]