Apache Point Observatory Lunar Laser-ranging Operation

By using a large telescope at a site with good atmospheric seeing, the APOLLO collaboration gets much stronger reflections than any existing facilities.

APOLLO records approximately one returned laser photon per pulse, as opposed to the roughly 0.01 photon-per-pulse average experienced by previous LLR facilities.

High precision Lunar Laser Ranging (LLR) started soon after the Apollo 11 astronauts left the first retroreflector on the Moon.

[4][5] For the first few years of the Lunar Laser Ranging Experiment, the distance between the observatory and the reflectors could be measured to an accuracy of about 25 cm.

Then McDonald Observatory built a special purpose system (MLRS) just for ranging, and achieved accuracies of roughly 3 cm in the mid-to-late 1980s.

In the early 1990s a French LLR system at the Observatoire de la Côte d’Azur (OCA) started operation, with similar precision.

The APOLLO laser has been operational since October 2005, and routinely achieves millimeter-level range accuracy between the Earth and the Moon.

[6] The goal of APOLLO is to push LLR into the millimeter range precision, which then translates directly into an order-of-magnitude improvement in the determination of fundamental physics parameters.

Specifically, assuming improvements of a factor of ten over prior measurements,[7][8] APOLLO will test: The Weak Equivalence Principle says that all objects fall the same way in a gravity field, no matter what they are made of.

This would be an incredible coincidence since WEP and SEP depend on very different and arbitrary properties – the exact composition of the Earth and the Moon, and their self-energies.

But this unlikely case cannot be completely ruled out until either other solar system bodies are measured to similar precision, or laboratory experiments reduce the bounds on WEP violations alone.

APOLLO is based on measuring the time-of-flight of a short-pulse laser reflected from a distant target—in this case the retroreflector arrays on the Moon.

This effect may seem small, but it is not only measurable, it forms the largest unknown in finding the range, since there is no way to tell which corner cube reflected each photon.

The biggest array, the 0.6 m2 Apollo 15 reflector, can have a corner-to-corner range spread of ≈ 1.2 sin (10°) m, or 210 mm, or about 1.4 ns of round-trip time.

This explains why a much larger system is needed to improve the existing measurements—the pre-APOLLO 2 cm RMS range precision required only about 10 photons, even at the worst-case orientation of the retroreflector array.

Compared to McDonald Observatory ranging station, the Apache Point telescope has a factor of 20 greater light-collecting area.

APOLLO should therefore gain about 20 (from the bigger telescope) × 25 (for better seeing) = 500 × in return signal strength over MLRS, and additional factor of 25 in signal-to-noise (from fewer stray photons interfering with the desired ones).

Likewise APOLLO should get a signal about 50 times stronger than the OCA LLR facility, which has a 1.5 m telescope and seeing of about 3 arcsec.

[2] Any laser ranging station, APOLLO included, measures the transit time, and hence the distance, from the telescope to the reflector(s).

[18] This instrument is capable of sensing vertical displacements as small as 0.1 mm, by measuring the change in gravity as the observatory moves closer to or farther from the Earth's center.

In April 2010, the APOLLO team announced that, with the aid of photos from the Lunar Reconnaissance Orbiter, they had found the long-lost Lunokhod 1 rover and had received returns from its laser retroreflector.

[21][22] By the fall of 2010, the location of the rover had been trilaterated (using range measurements from different points in the Earth's rotation and the Moon's libration) to about a centimeter.

The location near the limb of the moon, combined with the ability to range the rover even when it is in sunlight, promises to be particularly useful for determining aspects of the Earth-Moon system.

The cause was suspected to be due to dust on the arrays, leading to temperature gradients, distorting the returned beam.

[28] However, careful record-keeping allowed the old data to be reanalyzed in light of the new understanding of the clock's variations and most of the accuracy recovered.

APOLLO is collaboration between: University of California, San Diego (Tom Murphy Principal investigator), University of Washington, Harvard, Jet Propulsion Laboratory, Lincoln Laboratory, Northwest Analysis, Apache Point Observatory, and Humboldt State.

APOLLO shooting a laser at the Moon. The laser pulse is reflected from the retroreflectors on the Moon (see below) and returned to the telescope. The round-trip time tells the distance to the Moon to great accuracy. In this picture the Moon is very over-exposed, needed to make the laser beam visible.
Apollo 15 Lunar Ranging Retro-Reflector (LRRR). The small circles are corner cubes , which reflect light directly back in the direction from which it came.
Graph of returned photons. The X axis is the time into the experiment, during which the telescope emits 20 pulses per second towards the Moon. The experiment waits until just before each pulse is expected back, then opens the "gate", during which returned light can be detected. Each return is plotted on the Y axis as one dot, based on how far into the gate it arrived. The entire Y axis corresponds to about 18 meters in range. The black lines shows a large fraction of all detected photons are returned from an object at a very specific distance.