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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:

  • calibration of the Radar Altimeter altitude measurements with an error of 10 cm
  • improvement of the satellite orbit determination ensuring the production of a global ephemeris accurate to better than 1-2 m for the radial component


The LRR is a passive device which is used as a reflector by ground-based SLR stations using high-power pulsed lasers. In the case of Envisat, tracking using the LRR is principally accomplished by the International Laser Ranging Service (ILRS). The ILRS provides tracking for the satellite from its global network of laser ranging stations. Laser stations fire short laser bursts at Envisat and time the interval before the pulse is reflected back. These ILRS stations are relatively few, but because their position is very accurately known, they provide a set of independent reference measurements of Envisat's position, which contribute to the satellite's precise orbit determination.

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.

Instrument characteristics:
 
Wavelength: 350-800 nm optimised for 532 nm 
Efficiency: less or equal than 0.15 end-of-life 
Reflection Coefficient: less or equal than 0.80 end-of-life 
Field of view: elev. half-cone angle 60 degrees; azimuth 360 degrees 
Diameter: more or equal than 20 cm 

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The LRR was developed by AEROSPATIALE.