Envisat is equipped with a Laser Retroreflector (LRR), a passive optical instrument operating in the infra-red, to permit ranging of the satellite by the use of Laser Tracking Stations, and therefore the accurate determination of its height. These measurements allow:
Thus, the operating principle of the LRR is to measure on the ground the time of a round trip of laser pulses reflected from an array of corner cubes mounted on the Earth-facing side of the satellite. The corner cubes ensure that the laser beam is reflected back parallel to the incident beam. The detailed design of the cubes includes a compensation for the aberration of the laser beam by the velocity of the satellite: the satellite moves almost 40 metres between the emission and reception of the laser pulse from the SLR station, and this is compensated by slight nonparallelism of the reflected beam.
The corner-cubes, made of the highest quality fused silica, work in the visible spectrum at two specified wavelengths (l=694 nm and l=532 nm). The cornercubes are symmetrically mounted on a hemispherical housing with one nadir-looking corner-cube in the centre, surrounded by an angled ring of eight cornercubes. This will allow laser ranging in the field of view angles of 360° in azimuth and 60° elevation around the perpendicular to the satellite’s -Zs Earth panel. The mass of the LRR is 2 kg.
The LRR was developed by AEROSPATIALE.