The LRA is a totally passive reflector designed to reflect laser pulses back to their point of origin on Earth. It is used for calibrating the satellite’s Precise Orbit Determination system.
LRA ground station.
The LRA (Laser Retroreflector Array) is an array of mirrors that provides a target for laser tracking measurements from the ground. By analysing the round-trip time of the laser beam, we can locate very precisely where the satellite is on its orbit. The LRA is used to calibrate the other location systems on the satellite (DORIS, GPSP) with a very high degree of precision.
Laser Retroreflector diagram
The LRA is a passive instrument that acts as a reference target for laser tracking measurements performed by ground stations. Laser tracking data are analysed to calculate the satellite’s altitude to within a few millimetres. However, the small number of ground stations and the sensitivity of laser beams to weather conditions makes it impossible to track the satellite continuously. This is why other onboard location systems are needed.
The retroreflectors are placed on the nadir side of the satellite. The totally passive unit consists of nine quartz corner cubes arrayed as a truncated cone, with one cube in the centre and the others arranged azimuthally around the cone. This arrangement will allow laser ranging at field-of-view angles of 360 degrees in azimuth and 60 degrees in elevation around the perpendicular. The retroreflectors are optimised for a wavelength of 532 nanometres (green), offering a field of view of about 100 degrees.