Lunar Laser Ranging (LLR) has a distinguished history [1, 2] dating back to the placement of
retroreflector arrays on the lunar surface by the Apollo 11 astronauts. Additional reflectors were
left by the Apollo 14 andAp ollo 15 astronauts, andt wo French-built reflector arrays were placed
on the Moon by the Soviet Luna 17 andLuna 21 missions. Early range precisions hoveredaround
25 cm from 1970–1975, followedb y a periodof 12–16 cm precision until 1984, after which the
McDonald Laser Ranging system (MLRS) drove the precision into the 3 cm regime in the mid-tolate
1980s. MLRS has continued to deliver 2–3 cm lunar range data to the present, and was joined
in the early 1990s by a French LLR system at the Observatoire de la Cˆota d’Azur (OCA) with
comparable range precision.
APOLLO will utilize an off-the-shelf Q-switched, mode-locked Nd:YAG laser operating at a wavelength
of 532 nm . This laser has a pulse-width of 120 ps FWHM (full-width at half-max), a
pulse energy of 115 mJ, anda repetition rate of 20 Hz. Though our ultimate goal is 6 ps timing
precision, as notedb efore, the libration effect on the retroreflector arrays spreads the return pulse
to widths typically around300 ps, andp otentially exceeding 1 ns.
The optical system for APOLLO, shown in Fig. 1, consists of transmit andreceiv e paths, both
sharing the same path through the telescope. A negative lens collimates light from the telescope,
or alternatively, expands the collimated laser beam to fill the telescope aperture.
The transmit path includes a variable beam
expander, and otherwise consists of simple steering mirrors. The receive path, with its photonsensitive
detector at a reimaged focal plane, will be carefully protected from stray laser light. The
transmit/receive switch provides most of the protection, and baffling plus spatial filtering helps
eliminate remaining light
0.5 nm bandpass filter at the entrance to the receiver prevents
the bulk of the high backgroundphotons from the moon or daylight sky from entering the baffled
enclosure. The rejectedligh t can be usedto feedan off-bandCCD imager for purposes of acquisition
The APOLLO system, like other laser ranging systems, is basedon measuring the time-of-flight of
a short-pulse laser reflectedoff a distant target—in this case the retroreflector arrays on the lunar
surface. One-millimeter range determination corresponds to a mere 6 ps of round-trip travel time.
Recognizing that the moon undergoes apparent librations—orientation variations due to a constant
rotation rate but an elliptical, inclinedorbit—of up to 10 deg in magnitude, one quickly sees that the
passive array of retroreflectors spans a finite, measurable range of distances from the transmit point.