History of Laser Ranging and MLRS

The Birth of Lunar Laser Ranging

In the 1950’s, a small group of students and researchers, working at Princeton University under Robert H. Dicke, probably first gave substance to the concept of what would become the technique of optical laser ranging [Alley, 1972]. In an attempt to probe the fundamentals of gravity, they suggested that powerful, pulsed searchlights on the Earth be used to illuminate optical corner retroreflectors placed upon an orbiting artificial Earth satellite. One could then precisely analyze the orbital characteristics of that satellite’s motion by photographing its position against the background of fixed stars. The invention of the laser, with its precise wavelength and beam divergence characteristics, coupled with the technique of Q-Switching to produce laser pulse lengths on the order of only a few nanoseconds, caused a re-thinking of the artificial satellite photography experiment and ushered in the era of optical laser ranging. Procedures similar to those which were performed with microwave radars were upgraded to provide optical range measurements that had remarkable precision and accuracy. The initial such laser-to-target-and-return time-of-flight experiments were made at the NASA Goddard Space Flight Center in Greenbelt, Maryland in the mid-1960’s. Degnan [1993] has recently reviewed U. S. artificial satellite ranging efforts.

The concept of receiving laser light echoes from the lunar surface proceeded more or less in parallel with the artificial satellite experiments. However, the spreading of a beam of outgoing laser light as it interacted with, and was reflected by, the moon’s rough topography made ultra-precise distance determinations, as was done with artificial satellites, an impossibility. A number of such lunar experiments had been performed in the early 1960’s, both at the Massachusetts Institute of Technology and in the former Soviet Union, but with little success. A later, more refined concept recommended the deployment of a corner retroreflector package on the lunar surface as a part of one of the unmanned, soft-landing Surveyor missions. This was never brought to fruition, however. It was only in the late 1960’s, with the birth of the NASA Apollo project for landing an astronaut safely on the moon, that the concept of laser ranging to a lunar surface corner retroreflector package became a reality. The first deployment of such a package on the lunar surface took place during the Apollo 11 mission in the summer of 1969 and lunar laser ranging (LLR) became a reality [Bender et al., 1973]. Additional retroreflector packages were landed on the lunar surface by NASA during the Apollo 14 and Apollo 15 missions. Two French-built retroreflector packages were soft-landed on the lunar surface by Soviet landers [Barker et al., 1975].

For historical completeness, it should be mentioned that the very first lunar laser ranging observations of the Apollo 11 retroreflector package were made with the 3.1-m telescope at Lick Observatory [Faller et al., 1969]. However, the ranging system at Lick was designed solely for quick acquisition and confirmation, rather than for an extended program. In those very early days, successful lunar laser range measurements were also reported by the Air Force Cambridge Research Laboratories Lunar Ranging Observatory in Arizona [AFCRL, 1969] the Pic du Midi Observatory in France [Calame et al., 1970] and the Tokyo Astronomical Observatory [Kozai, 1972]. Over the past almost 30 years, lunar laser ranging has also been accomplished by stations in Maui, the former Soviet Union, Australia, and Germany. A new lunar capable station is being built by researchers in Italy. However, the only stations to produce these observations in a routine and continuous way are the McDonald station in the United States and the CERGA station in France. A paper describing the early efforts of the CERGA station can be found elsewhere in this volume [Veillet et al., 1993].

McDonald Observatory and LLR

The emplacement of suitable targets upon the Moon’s surface is only part of the task to be performed to accomplish LLR. In order to complete the experiment, suitable observing stations have to be present on the surface of the Earth. Such a station needs to have, in addition to a satisfactory optical telescope to both transmit the outgoing beam and gather in the few lunar reflected photons, a powerful laser, an accurate timing system, and a fast computer. These all have to be coordinated into a smoothly functioning unit and be staffed with a team of skilled personnel. Since, at the time of the NASA Apollo program, neither the time nor the money existed for the construction of such a dedicated station from the ground up, it was necessary to assess a number of presently existing optical observatories to see if at least a nucleus of suitable instrumentation could be had at an already existing site. Although such a station was eventually planned for a facility on top of Mount Haleakala on the island of Maui in the Hawaiian chain, very late in the planning stages for the Apollo 11 mission it was learned that logistical changes at the Hawaii site would make it impossible to install the necessary equipment and modifications, as well as bring everything up to operational status, in time for the Apollo 11 landing, planned for the summer of 1969.

It was at this time that Harlan J. Smith, Director of the McDonald Observatory, located in west Texas, near Fort Davis, was approached by the LURE (Lunar Ranging Experiment) team. The new 2.7-meter McDonald reflecting telescope, funded largely by NASA for a major planetary observation program, had just become operational and a commitment to long-term LLR activities was a distinct possibility. In March of 1969, C. O. Alley and D. G. Currie, from the University of Maryland, met with R. G. Tull, of the McDonald Observatory staff, to look at the feasibility of such a project being carried out at McDonald. The present record tells us that the experiment was a magnificent success in that McDonald Observatory had become the premiere LLR station of the 1970’s and early 1980’s [Silverberg, 1974]. The 2.7-m system, using a Korad ruby laser system, routinely produced LLR normal point data with an accuracy in the range 10-15 cm [Abbot et al., 1973; Shelus et al., 1975; Mulholland et al., 1975].

After almost 16 years of continuous LLR operations at McDonald Observatory, the 2.7-m laser ranging system was de-commissioned and was superseded by a dedicated 0.76-m system [Shelus, 1985]. This new station is capable of ranging to artificial satellites as well as to the Moon. Using many of the plans and most of the equipment that was to be a part of a previously planned mobile LLR system, the MLRS was built to satisfy the following objectives: 1) provide for a continuing program of LLR observations at McDonald Observatory, taking advantage of the experience gained from more than a decade and a half of lunar observations, without requiring access to the 2.7m telescope; 2) take advantage of 15 years of progress in laser, timing, electronics, and computer technology to build a much more accurate station; 3) reduce the cost of McDonald LLR activity with a more highly automated system; 4) provide a permanent, first-quality artificial satellite laser ranging station at a good-weather site in the southwest U. S., sharing costs with the LLR facility; and 5) provide both lunar and artificial satellite observations close to one node of the National Geodetic Survey/International Radio Interferometric Surveying network (NGS/IRIS), permitting efficient comparisons between the laser and radio-interferometric techniques. The MLRS was built around a frequency-doubled neodymium-YAG laser and now produces LLR data approaching 1 cm normal point accuracy. This completely new station was originally positioned in the saddle between Mt. Locke and Mt. Fowlkes and preliminary lunar observations began in the summer of 1983. However, the MLRS saddle site proved to be problematical in several ways. Atmospheric seeing became an immediate problem and some uncertainties with the stability of the telescope’s concrete support pad arose. A new site on top of Mt. Fowlkes was developed. The MLRS was moved to its present site in February 1988 and has operated at that site until its closure after a significant lightning strike in September 2019.


Abbot, R. I., P. J. Shelus, J. D. Mulholland, and E. C. Silverberg, Laser observations of the Moon: identification and construction of normal points for 1969-1971, Astron. Jour., 78, 784-793, 1973.

Air Force Cambridge Research Laboratories, Bull. Geodesique, 94, 443-444, 1969.

Alley, C. O., Story of the development of the Apollo 11 laser ranging retro-reflector experiment, in Adventures in Experimental Physics, edited by B. Maglich, pp. 132-149, 1972.

Barker, E. S., O. Calame, J. D. Mulholland, and P. J. Shelus, Improved coordinates for Lunakhod 2 based on laser observations from McDonald Observatory, Space Research, XV, 71-74, 1975.

Bender, P. L., D. G. Currie, R. H. Dicke, D. H. Eckhardt, J. E. Faller, W. M. Kaula, J. D. Mulholland, H. H. Plotkin, S. K. Poultney, E. C. Silverberg, D. T. Wilkinson, J. G. Williams, and C. O. Alley, The lunar laser ranging experiment, Science, 182, 229-238, 1973.

Calame, O., M.-J. Fillol, G. Guérault, R. Muller, A. Orszag, J.-C. Pourny, J. Rösch, and Y. de Valence, Premiers échos lumineux sur la lune obtenus par le télémètre du Pic du Midi, Comptes Rendus Acad. Sci. Paris, Ser. B, 270, 1637-1640, 1970.

Degnan, J. J., Millimeter accuracy satellite laser ranging: a review, in Contributions of Space Geodesy to Geodynamics: Technology, AGU Geodynamics Series, Vol. 25, edited by D. E. Smith and D. L. Turcotte, pp. 133-162, 1993.

Faller, J. E., I. Winer, W. Carrion, T. S. Johnson, P. Spadin, L. Robinson, E. J. Wampler, and D. Wieber, Laser beam directed at the lunar retro-reflector array: observations of the first returns, Science, 166, 99-102, 1969.

Kozai, Y., Lunar laser ranging experiments in Japan, Space Research, XII 211-217, 1972.

Mulholland, J. D., P. J. Shelus, and E. C. Silverberg, Laser observations of the Moon: normal points for 1973, Astron. Jour., 80, 1087-1093, 1975.

Shelus, P. J., J. D. Mulholland, and E. C. Silverberg, Laser observations of the Moon: normal points for 1972, Astron. Jour., 80, 154-161, 1975.

Shelus, P. J., MLRS: a lunar/artificial satellite laser ranging facility at the McDonald Observatory, IEEE Trans. on Geosci. and Rem. Sens., GE-234, 385-390, 1985.

Silverberg, E. C., Operation and performance of a lunar laser ranging station, Appl. Opt., 13, 565-574, 1974.

Veillet, C., J. F. Mangin, J. E. Chabaudie, C. Dumoulin, D. Feraudy, and J. M. Torre, Lunar laser ranging at CERGA for the ruby period (1981-1986), in Contributions of Space Geodesy to Geodynamics: Technology, AGU Geodynamics Series, Vol. 25, edited by D. E. Smith and D. L. Turcotte, pp. 133-162, 1993.


Updated by R. Ricklefs from material by P. Shelus.