Retro-reflector corner cubes have been used for decades as back reflecting targets for laser and lidar alignment and precision range calibration measurements. Each corner cube consists of three mirrors placed in alignment as a corner cube and has the unique capability of reflecting any incoming laser beam back upon itself. As an example, an array of corner-cubes has been placed on the moon by Apollo astronauts for precision lidar measurements of the earth-moon distance (to within a cm) and on satellites for ground-to-satellite navigation. However, most retro-arrays have been bulky, heavy, and the optical surfaces can be short lived in the harsh environment of the moon. Toward solving these limitations, a group at NASA Goddard along with MIT, USRA, and KBR have developed a unique retro-reflector array specifically for the lunar environment with diffraction-limited optical performance. They have made a small 5-cm dome-shaped structure with 8 corner cubes (1.25 cm diameter) placed upon the dome’s surface. Each corner cube retro reflects light from an incidence angle of +/- 20 degrees, so that a strong laser/lidar return is obtained from any incidence angle from at least one if not more of the 8 corner cubes. A major challenge overcome by their design was consideration of the lunar extreme temperature fluctuations (85 K to 385 K), high radiation levels (solar protons), and expected vibrations of earth-launch (26 g). They have made extensive environmental tests and are confident that the corner cube array will have more than a 10-year lifetime (decades) on the open lunar surface. Of importance in their design is that the corner cube material is quartz Suprasil, which does not degrade or darken due to the high radiation levels seen on the lunar surface, total-internal-reflection retro-reflectors without any optical coatings are used for simplicity, and the bonding agent used for the cubes has passed extreme vacuum and radiation tests. The retro-reflector arrays may be placed in four or five different locations, for example spread out over a 1 km × 1 km range, so that precision triangulation can be used for navigation and landing of a spacecraft. They also can be used singularly for laser ranging using a lidar system to determine a lander position on the lunar surface. Lidar Equation analysis indicates that the retro-reflecting arrays should be detected by the NASA Commercial Lunar Payload Service program lidar at ranges out to about 300 km.
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