Acta Polytechnica


doi:10.14311/AP.2013.53.0746
Acta Polytechnica 53(Supplement):746–749, 2013 © Czech Technical University in Prague, 2013

available online at http://ojs.cvut.cz/ojs/index.php/ap

MOONLIGHT: A NEW LUNAR LASER RANGING
RETROREFLECTOR AND THE LUNAR GEODETIC PRECESSION

M. Martinia,∗, S. Dell’Agnelloa, D. Currieb, G.O. Delle Monachea,
R. Vittoric, S. Berardia, A. Bonia, C. Cantonea, E. Cioccia, C. Lopsa,
M. Garattinia, M. Maielloa, G. Patrizia, M. Tibuzzia, N. Intagliettaa,

J. Chandlerd, G. Biancoe

a INFN–LNF, via E. Fermi, 40 Frascati (Rome)
b University of Maryland (UMD), College Park, College Park MD 20742 and NASA Lunar Science Institute,
USA

c Aeronautica Militare Italiana (AMI), Viale dell’Universitá 4, 00185 Rome, ASI, INFN–LNF, Italy, and
ESA-HSOU

d Harvard-Smithsonian Center for Astrophysics (CfA), 60 Garden Street, Cambridge, MA 02138, USA
e Agenzia Spaziale Italiana (ASI), Centro di Geodesia Spaziale “G. Colombo” (CGS), Localitá Terlecchia, P.O.
Box ADP, 75100 Matera, Italy

∗ corresponding author: Manuele.Martini@lnf.infn.it

Abstract. Since the 1970s Lunar Laser Ranging (LLR) to the Apollo Cube Corner Retroreflector
(CCR) arrays (developed by the University of Maryland, UMD) supplied almost all significant tests
of General Relativity (Alley et al., 1970; Chang et al., 1971; Bender et al.,1973): possible changes in
the gravitational constant, gravitational self-energy, weak equivalence principle, geodetic precession,
inverse-square force-law. The LNF group, in fact, has just completed a new measurement of the lunar
geodetic precession with Apollo array, with accuracy of 9 × 10−3, comparable to the best measurement
to date. LLR has also provided significant information on the composition and origin of the moon.
This is the only Apollo experiment still in operation. In the 1970s Apollo LLR arrays contributed
a negligible fraction of the ranging error budget. Since the ranging capabilities of ground stations
improved by more than two orders of magnitude, now, because of the lunar librations, Apollo CCR
arrays dominate the error budget. With the project MoonLIGHT (Moon Laser Instrumentation for
General relativity High-accuracy Tests), in 2006 INFN–LNF joined UMD in the development and
test of a new-generation LLR payload made by a single, large CCR (100 mm diameter) unaffected
by the effect of librations. With MoonLIGHT CCRs the accuracy of the measurement of the lunar
geodetic precession can be improved up to a factor 100 compared to Apollo arrays. From a technological
point of view, INFN–LNF built and is operating a new experimental apparatus (Satellite/lunar laser
ranging Characterization Facility, SCF) and created a new industry-standard test procedure (SCF-Test)
to characterize and model the detailed thermal behavior and the optical performance of CCRs in
accurately laboratory-simulated space conditions, for industrial and scientific applications. Our key
experimental innovation is the concurrent measurement and modeling of the optical Far Field Diffraction
Pattern (FFDP) and the temperature distribution of retroreflector payloads under thermal conditions
produced with a close-match solar simulator. The apparatus includes infrared cameras for non-invasive
thermometry, thermal control and real-time payload movement to simulate satellite orientation on
orbit with respect to solar illumination and laser interrogation beams. These capabilities provide:
unique pre-launch performance validation of the space segment of LLR/SLR (Satellite Laser Ranging);
retroreflector design optimization to maximize ranging efficiency and signal-to-noise conditions in
daylight. Results of the SCF-Test of our CCR payload will be presented. Negotiations are underway to
propose our payload and SCF-Test services for precision gravity and lunar science measurements with
next robotic lunar landing missions. In particular, a scientific collaboration agreement was signed on
Jan. 30, 2012, by D. Currie, S. Dell’Agnello and the Japanese PI team of the LLR instrument of the
proposed SELENE-2 mission by JAXA (Registered with INFN Protocol n. 0000242-03/Feb/2012). The
agreement foresees that, under no exchange of funds, the Japanese single, large, hollow LLR reflector
will be SCF-Tested and that MoonLIGHT will be considered as backup instrument.

Keywords: lunar laser ranging, space test, cube corner reflector, laser technology, test of geodetic
precession.

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vol. 53 supplement/2013 MoonLIGHT: a New Lunar Laser Ranging Retroreflector

Figure 1. The SCF cryostat.

1. Introduction
Lunar laser ranging (LLR) is mainly used to conduct
high-precision measurements of ranges between a laser
station on Earth and a Corner Cube Retroreflector
(CCR) on the lunar surface. Over the years, LLR
has benefited from a number of improvements both
in observing technology and data modeling, which led
to the current accuracy of postfit residuals of ∼ 2 cm.
Nowadays LLR is a primary technique to study

the Earth–Moon system and is very important for
gravitational physics, geodesy, and studies of the lunar
interior.
Since 1969 LLR has supplied a lot of tests of Gen-

eral Relativity (GR): it has evaluated the Geodetic
Precession [12], probed the weak and strong equiv-
alence principle, determined the Parametrized Post
Newtonian (PPN) parameter β and γ, addressed the
time change of the gravitational constant (G) and
1/r2 deviations. LLR has also provided important in-
formation on the composition and origin of the Moon
through measurement of its rotations and tides.

Future lunar missions will expand this broad scien-
tific program. Initially, the Apollo arrays contributed
a negligible portion of the LLR error budget. To-
day the ranging accuracy of ground stations has im-
proved by more than two orders of magnitude: the
new APOLLO1 station at Apache Point, USA, is capa-
ble of mm level range measurements; MLRO (Matera
Laser Ranging Observatory), at the ASI (Agenzia
Spaziale Italiana) Space Geodesy Center in Matera,
Italy, has restarted LR operations. Now, because of
lunar librations, the Apollo arrays dominate the LLR
error budget, which is a few cm.

1.1. The Satellite/lunar laser ranging
Characterization Facility (SCF)

In 2004, INFN started the operation to build the
Satellite/lunar laser ranging Characterization Facility
(SCF) in Frascati. The main purpose of this apparatus
is the thermal and optical characterization of CCR
arrays in simulated space conditions. A schematic
view of the SCF is shown in Fig. 1.

The size of the steel cryostat is approximately
2 m length by 0.9 m diameter. The inner copper

1Apache Point Observatory Lunar Laser-ranging Operation

shield is painted with the Aeroglaze Z306 black paint
(0.95 emissivity and low out-gassing properties) and
is kept at T = 77 K with liquid nitrogen. When the
SCF is cold, the vacuum is typically in the 10−6 mbar
range. Two distinct positioning systems at the top
of the cryostat (one for rototranslation movements
in the plane of the prototype and one for spherical
rotations and tilts) hold and move the prototype in
front of the Earth infrared Simulator (ES, inside the
SCF), the Solar Simulator (SS), the infrared camera
and laser, all located outside the SCF. The SS beam
enters through a quartz window (∼ 37 cm diameter,
∼ 4 cm thickness), transparent to the solar radiation
up to 3000 nm. A side Germanium window at 45° with
respect to the SS beam allows for the acquisition of
thermograms of the Laser Retroreflector Array (LRA)
with an IR digital camera (both during the ES/SS
illumination and the laser interrogation phases).

1.2. The MoonLIGHT – ILN experiment
In 2006, INFN proposed the MoonLIGHT – ILN
(Moon Laser Instrumentation for General Relativity
High accuracy Tests – International Lunar Network)
technological experiment [5, 8], which has the goal
of reducing the error contribution of LLR payloads
by more than two orders of magnitude. In Tab. 1,
the possible improvements in the measurement of
gravitational parameters achievable through reaching
the ranging accuracy of 1 mm or even 0.1 mm are
reported [14–16].

After the building of the best station, APOLLO [11],
in New Mexico, the main uncertainty is due to the
multi-CCR arrays. The Apollo arrays now contribute
a significant portion of the ranging errors. This is due
to the lunar librations, which move the Apollo arrays,
since that have dimensions of 1 square meter, for the
Apollo 15, and half square meter, for the Apollo 11
and 14. In this paragraph we will describe the Moon-
LIGHT/LLRRA21 payload to improving gravity tests
and lunar science measurements [3, 6]. This project
is the result of the collaboration of two teams: the
LLRRA21 team in the USA, led by Douglas Currie
of the University of Maryland, and the Italian one
led by INFN–LNF. We are exploring improvements
of both the instrumentation and the modeling of the
CCR. The main problem that affects the Apollo ar-
rays is the lunar librations, in longitude, that results
from the eccentricity of the Moons orbit around Earth.
During the lunar phase, 27 days, the Moons rotation
alternatively leads and lags its orbital position, of
about 8°. Due to this phenomenon the Apollo arrays
are moved so that one corner of the array is more
distant than the opposite corner by several centime-
ters. Because the libration tilt, the arrays increase
the dimension of the pulse coming back to the Earth
(Fig. 2). The broadening of the pulse will be greater
proportionally to the array physical dimensions and
to the Moon–Earth distance increase.

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Manuele Martini et al. Acta Polytechnica

Gravitational 1st generation 2nd generation 2nd generation Time
measurement LLR accuracy LLR accuracy LLR accuracy scale

(∼ cm) (1 mm) (0.1 mm)
EP < 1.4 × 10−13 10−14 10−15 few years
SEP < 4.4 × 10−4 3 × 10−5 3 × 10−6 few years
β < 1.1 × 10−4 10−5 10−6 few years

(Ġ/G) < 9 × 10−13 5 × 10−14 5 × 10−15 ∼ 5 years
Geodetic 6.4 × 10−3 6.4 × 10−4 6.4 × 10−5 few years
precession

(1/r2) Deviation < 3 × 10−11 10−12 10−13 ∼ 10 years

Table 1. Narrowing of parameter bounds due to gains in the accuracy of ranging measurements by one or two
orders of magnitude, (reaching accuracy of 1 mm or even 0.1 mm).

Figure 2. Comparison between 1st and 2nd generation
LRAs. The librations tilt the arrays on the left, but
the single big CCRs are unaffected, on the right. So
we have single short pulses coming back using the
MoonLIGHT payloads.

2. Analysis of lunar laser
ranging data

In order to analyze LLR data we used the PEP soft-
ware, developed by the CfA, by I. Shapiro et al. start-
ing from 1970s. PEP was designed not only to gener-
ate ephemerides of the Planets and Moon, but also to
compare model with observations. One of the early
uses of this software was the first measurement of the
geodetic precession of the Moon [12].

The PEP software has enabled constraints on devi-
ations from standard GR physics. Here we show the
first determination of the relative deviation from the
value expected in GR of the geodetic precession, KGP.
We have used all the data available to us from Apollo
CCR arrays: Apollo 11, Apollo 14 and Apollo 15.

The value obtained using only old stations (Grasse,
McDonald and MLR2) vs. the value obtained using
the APOLLO station is shown in Tab. 2.

This preliminary measurements are to be compared
with the best result published by JPL [13] obtained
using a completely different software package. On the
contrary, after the original 2 % KGP measurement by
CfA in 1988, the use of PEP for LLR has been resumed

Station Value obtained value
in GR

OLD stations (9 ± 9) × 10−3 0
APOLLO station (−9.6 ± 9.6) × 10−3 0

Table 2. Value obtained for the geodetic precession.

only since a few years, and it is still undergoing the
necessary modernization and optimization.

3. Conclusions
The analysis of existing LLR data with PEP is making
good progress, thanks to the important collaboration
with CfA, as shown with the preliminary measurement
of the geodetic precession (de Sitter effect) with an
accuracy at 1 % level.

In the future we are going to deepen our knowledge
about data and software in order to better estimate the
KGP uncertainty and other GR parameters. A possi-
ble way to improve the precision of LLR measurements
is to have lunar station to range not only to the Moon,
but also to satellites around the Earth and primarily
to LAGEOS, thus improving station intercalibration.

Acknowledgements
References
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[3] Boni, A. et al, ‘World-first SCF-Test of the
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Retroreflector’, in Proc. 17th International Workshop
on Laser Ranging, Bad Kötzting, Germany, May 2011.

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[4] Chang, R. F., Alley, C. O., Currie, D. G., Faller, J. E.,
Space Research XII, Vol. 1, p. 247–259 00/1971; Optical
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[5] D. Currie, S. Dell’Agnello, G. Delle Monache, Acta
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[8] S. Dell’Agnello, D. Currie, G. Delle Monache et al.,
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[11] T. W. Murphy et al, Publications of The
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[17] A scientific concept for the LGN has been developed
by the International Lunar Network (see
http://iln.arc.nasa.gov/). See Core Instrument and
Communications Working Group Final Reports.

Discussion
Jim Beall — Do you have a planned date for deploying
the retroreflector on the Moon?
Manuele Martini — There are several scientific agree-
ments for opportunities for robotic mission on the lunar
surface that will deploy MoonLIGHT CCRs, but there
is no firm approved planned date for deploying CCR on
lunar surface.

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http://iln.arc.nasa.gov/

	Acta Polytechnica 53(Supplement):746–749, 2013
	1 Introduction
	1.1 The Satellite/lunar laser ranging Characterization Facility (SCF)
	1.2 The MoonLIGHT – ILN experiment

	2 Analysis of lunar laser ranging data
	3 Conclusions
	Acknowledgements
	References