The LRA or Laser Retroreflector Array (29 kg) reflects laser signals from a network of 10 to 15 ground laser tracking stations (Satellite Laser Ranging stations, SLR) to provide tracking data for precise orbit determination and the altimeter bias calibration.

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(Crédits ATSC)
The LRA is an array of mirrors that provide 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 with a very high degree of precision.

Principle

 

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Laser Retroreflector diagram LRA station in action
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 make it impossible to track the satellite continuously. This is why other onboard location systems are needed.

 

Technical data

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 allows 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. Topex/Poseidon’s LRA was built by ITE Inc. under contract to NASA’s Goddard Space Flight Center.

Further information on Instrument Description: LRA (NASA/JPL website), and ILRS (International Laser Ranging Service)