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Instrument: LRA : Laser Retroreflector Array
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Associated Platforms

Spectral/Frequency Information
Wavelength Keyword: Visible
Number Channels: 1 Channel
Spectral/Frequency Coverage/Range: 532 nm (primary)

Wavelength Keyword: Near Infrared
Number Channels: 1 Channel
Spectral/Frequency Coverage/Range: 1064 nm (secondary)

Related Data Sets
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The Laser Retroreflector Array (LRA) is used to calibrate the other location systems on the satellite with a very high degree of precision. The ability to determine a satellite’s precise position on orbit is critical in interpreting altimetry data used for measuring ocean surface topography. LRA is a totally passive reflector designed to reflect laser pulses back to their point of origin on Earth. It consists of nine 39 millimeter quartz corner-cube reflectors arrayed on a circular structure on the satellite’s nadir side (facing Earth). A corner-cube reflector is a special type of mirror that always reflects an incoming light beam back in the direction from which it came. The retroreflectors are optimized for a wavelength of 532 nm (green), providing a field of view of about 100ư. The eight equally spaced peripheral cubes are oriented at 50 degrees with respect to NADIR. The array structure is aluminum alloy and the corner cubes are constrained to allow for the differential thermal expansion of the structure and the quartz corner cubes.

The LRA is an array of mirrors onboard the satellite that provides a target for laser-tracking measurements from ground stations. By analyzing the round-trip time of the laser beam, it is possible to determine precisely where the satellite is on its orbit. The laser-tracking data are analyzed to calculate the satellite’s altitude to within a few mm; however, because there are a small number (10–15) of ground stations and the laser beams are sensitive to weather conditions, it is not possible to track the satellite continuously using the LRA alone. That is why other location systems are needed on board the satellite.

Key LRA Facts
Heritage: TOPEX/Poseidon
Function: Laser-tracking targets
Configuration: 9 corner cubes: 1 nadir looking, 8 arrayed azimuthally in truncated cone
FOV: 110ư w/1.5 arcsec dihedral angle per cube
Dimensions: Each cube is 163-mm diameter ư 66-mm height
Mass: 0. 8 kg
Duty Cycle: 100%
Thermal Operating Range: -65ư to 95ư C

Online Resources

Instrument Logistics
Instrument Start Date: 2002-01-15
Instrument Owner: USA/NASA

Instrument Start Date: 1992-09-23
Instrument Stop Date: 2005-10-09