Apollo 11 Laser Ranging Retro-Reflector: Initial Measurements from the McDonald Observatory

Lunar Laser Ranging: A Continuing Legacy of the Apollo Program

On 21 July 1969, during the first manned lunar mission, Apollo 11, the first retroreflector array was placed on the moon, enabling highly accurate measurements of the Earthmoon separation by means of laser ranging. Lunar laser ranging (LLR) turns the Earthmoon system into a laboratory for a broad range of investigations, including astronomy, lunar science, gravitational physics, geodesy, and geodynamics. Contributions from LLR include the three-orders-of-magnitude improvement in accuracy in the lunar ephemeris, a several-orders-of-magnitude improvement in the measurement of the variations in the moon's rotation, and the verification of the principle of equivalence for massive bodies with unprecedented accuracy. Lunar laser ranging analysis has provided measurements of the Earth's precession, the moon's tidal acceleration, and lunar rotational dissipation. These scientific results, current technological developments, and prospects for the future are discussed here.

The Lunar Laser Ranging Experiment

The lunar ranging measurements now being made at the McDonald Observatory have an accuracy of 1 nsec in round-trip travel time. This corresponds to 15 cm in the one-way distance. The use of lasers with pulse-lengths of less than 1 nsec is expected to give an accuracy of 2 to 3 cm in the next few years. A new station is under construction in Hawaii, and additional stations in other countries are either in operation or under development. It is hoped that these stations will form the basis for a worldwide network to determine polar motion and earth rotation on a regular basis, and will assist in providing information about movement of the tectonic plates making up the earth's surface. Several mobile lunar ranging stations with telescopes having diameters of 1.0 m or less could, in the future, greatly extend the information obtainable about motions within and between the tectonic plates. The data obtained so far by the McDonald Observatory have been used to generate a new lunar ephemeris based on direct numerical integration of the equations of motion for the moon and planets. With this ephemeris, the range to the three Apollo retro-reflectors can be fit to an accuracy of 5 m by adjusting the differences in moments of inertia of the moon about its principal axes, the selenocentric coordinates of the reflectors, and the McDonald longitude. The accuracy of fitting the results is limited currently by errors of the order of an arc second in the angular orientation of the moon, as derived from the best available theory of how the moon rotates in response to the torques acting on it. Both a new calculation of the moon's orientation as a function of time based on direct numerical integration of the torque equations and a new analytic theory of the moon's orientation are expected to be available soon, and to improve considerably the accuracy of fitting the data. The accuracy already achieved routinely in lunar laser ranging represents a hundredfold improvement over any previously available knowledge of the distance to points on the lunar surface. Already, extremely complex structure has been observed in the lunar rotation and significant improvement has been achieved in our knowledge of lunar orbit. The selenocentric coordinates of the retroreflectors give improved reference points for use in lunar mapping, and new information on the lunar mass distribution has been obtained. Beyond the applications discussed in this article, however, the history of science shows many cases of previously unknown, phenomena discovered as a consequence of major improvements in the accuracy of measurements. It will be interesting to see whether this once again proves the case as we acquire an extended series of lunar distance observations with decimetric and then centimetric accuracy.

Transportable lunar-ranging system

Ruby lasers in large, fixed installations are being used by groups in France, Japan, the Soviet Union, and the U.S. for measuring distances to retroreflectors on the moon. This paper describes a transportable transmitting unit that can be installed at any astronomical observatory where a large telescope is available to detect the received signal. The transmitter consists of a high radiance, frequency-doubled, neodymium-glass laser and a coudé optical system of moderate size. It has been installed, but is not yet in operation, at the Agassiz Observatory, Harvard, Massachusetts.

Optimum signal processing for distance measurement with lasers

The measurement accuracy of laser telemeters have been analyzed, taking into account the statistical properties of the propagation, detection, and measure process. For an arbitrary waveform of the light intensity modulation envelope, the optimum filter response and the resulting accuracy are found. As special cases, the optical radar and the sine wave modulation techniques have been considered, and their accuracy and optimum mode of operation are evaluated and discussed.

Direct Measurement of the Quantum Counting Efficiency of RCA C31000E/F Photomultipliers at 6328 A

Direct measurement of the total quantum counting efficiency of the RCA C31000E/F shows that it is about 25 +/- 22% lower than the photocathode quantum efficiency in optimized commercial base units. Measurements with an EMI 9558 give similar results and with an Amperex 56TVP give similar although more erratic results due to its higher sensitivity to magnetic fields. The measurement techniques and conditions are described in detail. Diagnostic studies such as the question of dependence of counting efficiency on signal or background count rate are also presented.

Laser Ranging Retro-Reflector: Continuing Measurements and Expected Results

After successful acquisition in August of reflected ruby laser pulses from the Apollo 11 laser ranging retro-reflector (LRRR) with the telescopes at the Lick and McDonald observatories, repeated measurements of the round-trip travel time of light have been made from the McDonald Observatory in September with an equivalent range precision of +/-2.5 meters. These acquisition period observations demonstrated the performance of the LRRR through lunar night and during sunlit conditions on the moon. Instrumentation activated at the McDonald Observatory in October has yielded a precision of +/-0.3 meter, and improvement to +/-0.15 meter is expected shortly. Continued monitoring of the changes in the earth-moon distance as measured by the round-trip travel time of light from suitably distributed earth stations is expected to contribute to our knowledge of the earth-moon system.