Laser Retroreflector Array (LRA)

Description

The Laser Retroreflector Array (LRA) is designed to use reflected laser light from orbiting spacecraft laser (typically a laser altimeter or light detection and ranging - lidar) to precisely determine the location of the lander, as a fiducial marker, and the distance to that point on the lunar surface with respect to the orbiter. The retroreflectors reflect any light striking them directly back to the source. They can be tracked by an orbiting laser altimeter or lidar from a few hundred kilometers.

The LRA consists of eight circular 1.27-cm diameter corner-cube retroreflectors mounted on a 5.11 cm diameter, 1.65 cm high hemispherical gold-painted platform. Each of the eight retroreflectors points in a slightly different direction, and each has a maximum useful light incidence angle of about +-20 degrees. Four retroreflectors are evenly distributed in a ring on the hemisphere 20 degrees from zenith, and four in a ring 40 degrees from zenith. They are made from Suprasil quartz, index of refraction is 1.46. The equivalent optical cross section is 10,000 to 100,000 square meters, but is still not enough to use Earth-based laser ranging. Total mass of the LRA is 20 grams, it requires no power.

Note that laser altimeters are not designed to be used to range to the LRAs, they have very small footprints and would only "hit" an LRA on occasional passes. Lidars with wider footprints are more efficient for actual ranging from orbiters. Also note that if the range to an orbiter can be precisely determined (e.g. from a larger LRA on the orbiter that can be ranged to from Earth), an accurate distance from Earth to the LRA on the lunar surface can be obtained. Measurements from multiple LRAs at different points on the Moon can be used to create an accurate geodetic network.

The Intuitive Machines 1 Odysseus lander touched down on the Moon on February 22, but tipped over and came to rest tilted at 30 degrees to the horizontal. It is believed the LRA is still in a position where it will be able to return laser transmissions from orbiting spacecraft.

Image credit: NASA-GSFC