The LRR 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 where the satellite is on its orbit.

Function

A laser retroreflector is attached to a mount on the nadir panel close to the RA-2 antenna. It has two functions:

  • support-to-satellite ranging,
  • RA-2 altitude calibration.

The LRR is a passive device which is used as a reflector by ground-based SLR (Satellite Laser Ranging) stations using high-power pulsed lasers. In the case of Envisat, tracking using the LRR is mainly performed 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 positions are very accurately known, they provide a set of independent reference measurements of Envisat’s position, which contribute to the satellite’s precise orbit determination.

Principle

The operating principle of the LRR is, therefore, to measure on the ground the return trip time 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 caused by the satellite’s velocity: the satellite moves almost 40 metres between the emission and reception of the laser pulse from the SLR station, and this is compensated for by slight nonparallelism of the reflected beam.

EnviSat LRR. (Credits ESA)

 

The corner cubes are made of the highest-quality fused silica and work in the visible spectrum. Their performance is optimised at the two wavelengths (694 nm and 532 nm) commonly used in SLR stations. The corner cubes are symmetrically-mounted on a hemispherical surface with one nadir-pointing corner cube in the centre, surrounded by an angled ring of eight corner cubes. This allows laser ranging in the field of view angles of 360° in azimuth and 60° in elevation around the perpendicular to the satellite’s Earth-facing panel (positioned on the axis at point -Zs with respect to the satellite’s centre of mass). The mass of the LRR is 2 kg.

The design is identical to the very successful reflectors used on ERS-1 and ERS-2.

 

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

Further information on the LRR instrument (ESA website)