The SENTINEL-3 Laser Retro Reflector (LRR) is a passive device that acts as a target for laser tracking measurements performed by dedicated ground stations. It is mounted on the Earth panel of the satellite and consists of a hemispherical array of seven corner cubes, each reflecting back, and in the same direction, any incoming laser pulse from the stations. The hemispherical arrangement ensures that at least one cube is visible from any station that has the satellite in view. In this way, each ground station is able to determine the range to the satellite with an accuracy of the order of a few millimetres, by measuring the propagation delay of a laser pulse from the station to the LRR and back.
Figure 1 LRR Instrument (Credit: ESA)