ETALON is a passive satellite launched on January 10th, 1989 in the frame of Global Navigation Satellite System development.

The satellite was designed to serve as a reference of an ideal-shaped man made object natural motion and operate in the same orbits as GLONASS satellites. Its prismatic reflectors allow it to relay the signals coming from ground quanto-optical range meters.

Observing changes in ETALON’s flight caused by the gravitational field and outer space environment in general made it possible to draw up a reliable model of navigation satellites motion, aiding in the refinement of GLONASS user positioning.

In May that same year another ETALON craft was launched into orbit. Despite their expected lifetime being 5 years, both satellites are still carrying out their mission to this day, helping Russian and international specialists study the changes in the Earth's gravitational field and calculate the effect it has on various spacecraft.