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The Apollo 15 Lunar Laser Ranging RetroReflector
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The Apollo 15 Lunar Laser Ranging RetroReflector

November 29, 2012
Finding your position on the Moon can be tricky. Since the 17th century, the Moon's coordinates have been measured in what is now known as the “mean Earth/polar axis system,” or ME system for short. This is a latitude and longitude system where the mean (average) direction of the Earth over time defines the 0° longitude, or prime meridian, and the average direction of the Moon’s polar axis defines 90° north and south latitude. However, in order to effectively use such a system, it is necessary to assign latitude and longitude to a set of points on the Moon. This is where the LRRR arrays come in. Over the years since they were emplaced on the Moon, lunar laser ranging (LLR) observations of these arrays from several sites on the Earth have been processed and the relative positions of these arrays determined to the centimeter level. These relative coordinates have been converted to absolute latitude and longitude coordinates according to the above definition of the ME system, and thus are now the points on the Moon whose positions are most accurately known. However, for the general purpose of locating things on the Moon, knowing the positions of only four points is not particularly useful. Therefore an important step in being able to use the data being returned from the LRO mission, as well as from previous U. S. and international missions, is to connect those datasets to the LLR coordinates. For LRO, a first step will be to determine the coordinates of LROC images and digital elevation models (DEMs) covering the LRRR sites. A second step will be to compare the positions of LRO Lunar Orbiter Laser Altimeter (LOLA) data to these images and DEMs in order to make sure that the global LOLA dataset is connected to these coordinates. The final step will be to assure that all other LROC and eventually even past U.S. and international mission data are in the same LLR coordinate frame. This will assure that lunar datasets will be in the same coordinate frame and that such data can be used together, consistently, for human exploration planning and associated science activities. This image shows Goddard's Laser Ranging Facility directing a laser (green beam) toward the LRO spacecraft in orbit around the moon (white disk). The moon has been deliberately over-exposed to show the laser. Credit: Tom Zagwodzki/Goddard Space Flight Center