ETRUSCO-GMES and MoonLIGHT-2

G. Bellettini (Ass.), G. Bianco (Ass.), A. Boni (AR), C. Cantone (Art. 36),E. Ciocci (PhD),S. Dell’Agnello, G.O. Delle Monache, N. Intaglietta (Tecn.),C. Lops (AR), M. Maiello (Ass.),R. March (Ass.), M. Martini (PhD), C. Mondaini (Bors.),G. Patrizi (AR), L. Porcelli (AR),

L. Salvatori (Tecn.), R. Tauraso (Ass.), M. Tibuzzi (Tecn.), P. Tuscano (Tecn.), R. Vittori (Ass.)

1 Introduction

The SCF Lab (Satellite/lunar/GNSS laser ranging/altimetry and Cube/microsat CharacterizationFacilities Laboratory) is a specialized infrastructure, unique worldwide, dedicated to design, char-acterization and modeling of the space segment of Satellite Laser Ranging (SLR), Lunar LaserRanging (LLR) and Planetary Laser Ranging and Altimetry (PLRA) for industrial and scientificapplications. We developed advanced laser retroreflectors for solar system exploration, geodesy andfor precision tests of General Relativity (GR) and new gravitational physics. Our key experimentalinnovation is the concurrent measurement and modeling of the optical Far Field Diffraction Pat-tern (FFDP) and the temperature distribution of the SLR/LLR payload of retroreflectors underthermal conditions produced with a close-match solar simulator. The primary goal of these inno-vative tools is to provide critical design and diagnostic capabilities for SLR to Galileo and otherGNSS (Global Navigation Satellite System) constellations. The implementation of new retroreflec-tors designs being studied will be helpful to improve GNSS orbits, increasing, this way, accuracy,stability, and distribution of the International Terrestrial Reference Frame (ITRF), in order toprovide a better definition of the geocenter (origin) and the scale (length unit). The SCF is alsoactively used to develop, validate and optimize 2nd generation LLR arrays for precision tests ofGR with the MoonLIGHT-2 (Moon Laser Instrumentation for General relativity High-accuracyTests Phase 2) project. Laser ranging and laser reflectors throughout the solar system are alsoused to develop new fundamental gravity physics models and study the experimental constraints tothese models. Starting from 2004 INFN invested resources and manpower to build and operate theSCF Lab in Frascati, near Rome, dedicated to the characterization of the thermal properties andthe laser ranging response of laser retroreflector arrays (LRAs) of CCRs (Cube Corner Retroreflec-tors) in space conditions accurately simulated in the laboratory (SCF-Test). The SCF Lab consistsof two OGSE (Optical Ground Support Equipment) called SCF (property of INFN) and SCF-G(which doubles our metrology capabilities for applications to GNSS, property of INFN and ASI).A schematic view of the cryostats is shown in Fig.:1. We have tested a large variety of CCRs, fromLAGEOS (Laser GEOdynamics Satellite) to Apollo and GNSS LRAs.

2 APPLICATIONS TO GALILEO AND OTHER GNSS

With SCF-Testing of old-generation, Al back-coated retroreflectors, deployed on satellites of GPS,GIOVE and GLONASS (prior to GLONASS-115) we have de-qualified the use of Al back-coatingfor GNSS. From 2010 to 2014 INFN performed an extensive SCF-Test campaign under ESA con-tract, on Galileo IOV retroreflectors. These are fused silica uncoated CCRs, arranged in planararrays of 84 units on four Galileo IOV (In Orbit Validation) satellites.

Measurements on the Galileo IOV retroreflector prototype during the simulated orbit (Fig.:2)have demonstrated that the choice of uncoated retroreflectors has improved significantly the opticalresponse of GNSS LRAs. These results will be published soon, after having been authorized by ESAand Galileo (in progress). With the ETRUSCO and the ETRUSCO-2 (Extra Terrestrial Rangingto Unified Satellite COnstellations) RD programof INFN and ASI, we designed and characterizeda full size GNSS Retroreflector Array (GRA), with application to Galileo and future GNSS.TheGRA is a planar array with CCRs mounted on an aluminum base. On the array there are 55

Figure 1: SCF and SCF-G cryostats with GNSS LRAs inside.

CCRs; each one of them is a solid uncoated retroreflector, made of Suprasil 1, with a circularfront face of 33 mm diameter. The analysis of thermal and optical simulations are fundamental totest the designed prototype in conditions impossible to realize in laboratory. We realized a finiteelement and thermal model of the CCR based on the characteristics of the GRA retroreflector.The excellent and unsurpassed GRA optical performance measured inside the SCF Lab is shownin Fig.:3 and Fig.:4 shows the results of ourthermal modeling of the measurement along the GCOorbit reported in Fig.:3.

2.1 Earth Observation

We are developing a midsize LRA suited for LEO and EO constellations like ESA Sentinels and,in general, for the space segment of Copernicus (European Flagship space program, also part ofHORIZON2020). One model is shown in Fig.:5, which is one co-developed and co-studied by INFNand the Italian Ministry of Defense and the Italian Ministry of Foreign Affairs and InternationalCooperation (high-relevance Italy-USA bilateral project AUGUSTUS-2014, Absolute crust, Glacierand iceberg Georeferencing with Unified Sar, optical, gnss laser observations by Italy and USa-2014). CORA and/or its appropriately adapted versions are suited for deployment as Phobos ANDDeimos laser Retroreflector Arrays (PANDORAs).

3 TESTS OF GENERAL RELATIVITY WITH LLR

LLR provides accurate measurements of the lunar orbit through a high-precision measurementof ranges between a laser station on the Earth and the Apollo CCRs on the lunar surface. Fordecades LLR has provided the best tests to validate Einsteins theory of General Relativity withmeasurements of the weak and strong equivalence principle, the Parameterized Post Newtonian(PPN) Parameter β, the time change of the Gravitational Constant, the Geodetic Precession(KGP ) and 1

r2 deviations. Over the years, LLR has benefited from improvements in both observingtechnology and data modeling, which led to the current 2cm precision. Unfortunately, the currentgeometry of the CCR array deployed on the Moon significantly limits further improvements. Themain problem that affects the Apollo CCR array is caused by the lunar librations in longitude thatresult from the eccentricity of the Moons orbit around the Earth. Because of this phenomenon,one corner of the Apollo arrays is more distant from the Earth than the opposite corner by severalcentimeters, broadening the pulse coming back to the Earth. Together with University of Maryland(PI of Apollo LRAs), we developed a new CCR design which is not affected by lunar librations orregolith motion as indicated by Fig.:7. Figure 6 shows GR tests from (column 3, cm accuracy);best-case expected improvements for several MoonLIGHTs of 1mm and 0.1mm accuracy deployedon best locations (poles/limbs), observed by stations with ideal performance and orbit softwarecapable of reaching 0.1mm, that is, capable of exploiting the ultimate improved accuracy provided

Figure 2: FFDP intensity of GRA during GCO test compared with the nominal performance inair and isothermal conditions: the GRA developed by INFN and ASI shows, on average, no per-formance degradation under GCO SCF-Test.

Figure 3: The GCO, GNSS Critical Orbit, for Galileo.

Figure 4: Temperature trend of CCR’s front face and tip along the orbit, showing that the axialthermal gradient of the GRA CCR is very small, thus it ensures negligible FFDP perturbations.

Figure 5: Model of CORA, COpernicus laser Retroreflector Array.

Figure 6: The expected improvements on the GR measurements with MoonLIGHT are shown intable, together with their measurement time scale.

by the new MoonLIGHT CCRs. The parameter under estimation is shown in the first column;the timescale indicates the approximate data campaign length for achieving the scientific goal andin the last two columns are shown the estimation of the parameters using the accuracy of 1 mmand 0.1 mm. In order to test GR with LLR, we use the Planetary Ephemeris Program (PEP)developed by the Center for Astrophysics (CfA) during 1960s. This software is designed not onlyto generate ephemerides of planets and Moon, but also to compare models with observations. Oneof the first applications of the PEP software was the first measurement of the geodetic precessionof the Moon. We have performed a preliminary analysis of real LLR data from station and dummyobservations using MoonLIGHTtype CCRs. The parameters considered are the variation of thegravitational constant G, the Nordtvedt parameter η, the geodetic precession KGP and the PPNparameter β. The original design of MoonLIGHT-2 had a sunshade (Fig.:8) that was designedto block the direct sun into the CCR for most of the lunar day. It also reduced the exposure todust that could accumulate on the front surface of the CCR, over the very-long term, reducing thereturn signal.

4 GR Test results from LLR

During 2014-2015, the SCF Lab in collaboration with the CfA (Center for Astrophysics) runneda number of numerical simulations using PEP (Planetary Ephemeris Program) to develop andoptimize the MoonLIGHT-2 design in time for its launch and deployment. PEP is a FORTRANsoftware package, developed by I. Shapiro at CfA since 1970 and includes a detailed mathematicalmodel of the solar system, with the masses of all solar system bodies and a large number of ad-justable parameters. With PEP we can also include the position of different Earth laser stationslike APOLLO (Apache Point Observatory Lunar Laser-ranging Operation, USA) or CERGA (Cen-tre d’Etudes et de Recherches Geodynamiques et Astronomiques, France) useful for simulations.The main GR tests which are now being done in collaboration with CfA are the KGP, β, η andG

Gand for these simulations we used both real and dummy data from the real Apollo CCR in

addition with the dummy data from MoonLIGHT-2. We run two different GR simulations usingthe Apollo CCR array and MoonLIGHT-2 CCR. Simulation n.1: In the first simulation, we useddummy data for MoonLIGHT-2 from the APOLLO ground Station and real data from the Apollo

CCR computing the values of KGP, β and G

G. We use two different Moon site for MoonLIGHT-2,

first simulating only one MoonLIGHT-2 on one of the two sites separately and then simulatingtwo MoonLIGHT-2 simultaneously. With this simulation we studied if the GR measurements with

Figure 7: Concept of the 2nd generation of Lunar Laser Ranging

Figure 8: Design concept.

MoonLIGHT-2 will be depended from the lunar position, and if we can determine an optimal de-ployment site. We refer to Table 9 for details about simulation parameters used. The results of the

Figure 9: Details about the best deployment site simulation with PEP.

Simulation n.1: Deployment site are presented in Table 10. The data using only one MoonLIGHT-2 (the third column) gives similar results for both the Moon site showing that the GR simulationswith MoonLIGHT-2 are poorly dependent from the Moon site position. The worst positions forthis kind of simulation, where we use only few spot for MoonLIGHT-2 with few data collected, areat the poles since the array will not be always visible from Ground Station because of the Moonlibration. In addition, the current PEP version does not allow us to considerate automatically thelibration. The fourth column shows that in the case of low precision in the assessment of the geom-etry of the CCR array (for this simulation we use a precision of 2.5mm for the dummy data), we donot have any improvement in the GRs test even when adding additional CCR array to the ApolloCCR. It is important to underline that in this simulation the Apollo data became predominantrespect to MoonLIGHT-2 because the low MoonLIGHT-2 precision data and the few spot used forMoonLIGHT-2. For this reason, we obtain that MoonLIGHT-2 is spot independent. In order tobetter understand the MoonLIGHT-2 performance we make another set of simulations, Simulationn.2. Simulation n.2: In the second simulation, we used only dummy data from the Apollo and the

Figure 10: Results of Simulation n.1: MoonLIGHT-2 best deployment site. All the results areexpressed in terms of variation from constant value.

MoonLIGHT-2 CCR for four different ground stations: APOLLO, CERGA, MLRS (McDonaldLaser Ranging Station, USA) and MLRO (Matera Laser Ranging Observatory, Italy). For theMoonLIGHT-2 sites, we chose realistic deployment sites, the ones for the mission MoonExpress,Astrobotic and Israel. We simulated data until 2030 and use two types of MoonLIGHT-2 design,with and without sunshade. In design with a sunshade the MoonLIGHT-2 reflectors were shieldedand therefore available whenever conditions were suitable at the observation site. In the casewithout sunshade, the MoonLIGHT-2 reflectors were unavailable when illuminated, reducing theamount of collectable data. Also for this simulation, we computed the values of KGP, β, η and

G

G. We refer to Table 11 for parameters used. The results of Simulation n.2: Optimal design and

Figure 11: Details about the optimal design and GR expected improvement simulation with PEP.

GR expected improvement are presented in Table 12. The GR tests with the sunshade show aslightly better accuracy compared to the case without the sunshade. The improvement is due tothe longer data acquisition interval available for the design with sunshade. We think that thisimprovement doesnt affect significantly the GR results and the minor decrease in performance inthe absence of a sunshade is compensated by the optimization of the MoonLIGHT-2 weight for itsdeployment. Comparing the expected GR results at 2030 in Table 12 with the actual best accuracyin GR test we can see the expected improvement in GR test accuracy using the new generationlunar retroreflector MoonLIGHT-2 CCR with the Apollo CCR.

Figure 13, Figure 14 and Figure 15 show the accuracy improvement until 2030 of β, G

Gand KGP.

For these simulations, the accuracy improvement in the GR tests using the new generation lunarretroreflectors is about one/half order of magnitude. All the results of this simulation show anaccuracy improvement in the GR simulation results using MoonLIGHT-2 together with Apollo.However, results GR test accuracy shown in Table 12 is still lesser to the expected improvementpredicted. However for a better data analysis we must considerate few points:

• The simulations use only a minimal number of MoonLIGHTs deployed in non-optimal loca-tions (neither the poles, nor the limbs). In addition we use the current and non-upgraded laserstation hardware neither the improvements in their data taking. Many of this improvementsare already planned (like for LLR stations at ASI-Matera).

• We do not consider none of the new LLR stations that will be operative in the next few years(like the one in South Africa).

• The simulations does not include the updates, optimizations and improvements of any currentorbit software (PEP) with which the current version supports a total LLR error budget onthe order of a few cm. The updates will have to be implemented, and that will be possibleto implement, as the LLR instrumental accuracy will improve thanks to the progressivedeployment of MoonLIGHT CCRs on the Moon and thanks to the progressive upgrades ofthe LLR ground stations and/or additions of new LLR ground stations. Future softwareupdates will allow us also a better estimation of libration. Summarazing the expected resultsshows the best improvement case while the simulation results in Table 12 we carried outshows the pessimistic case.

Figure 12: Results of Simulation n.2: Optimal design and GR expected improvement. Be careful,all the results are expressed in terms of variation from constant value.

Figure 13: β improvement during a long time simulation using MoonLIGHT-2 with Apollo CCR.

Figure 14: G

Gimprovement during a long time simulation using MoonLIGHT-2 with Apollo CCR.

Figure 15: KGP improvement during a long time simulation using MoonLIGHT-2 with ApolloCCR.

5 Conclusion

Although Apollo retroreflectors will continue to operate and provide new science results, theirgeometry is now limiting the precision of the single photoelectron returns. The next generationretroreflector, MoonLIGHT-2, will support improvements in ranging precision, by one order ofmagnitude, depending on the method of deployment. With the preliminary simulations describedin this work we show that:

• The GR tests with MoonLIGHT-2 will be not dependent from the MoonLIGHT-2 deploymentsite with the exceptions of the poles (because of the lunar libration, the array is not alwaysvisible from Earth).

• There are not great differences in the GR tests using a MoonLIGHT-2 design with or withoutSunshade. So we choose the design without the sunshade that will provide an importantweight optimization (about 1kg) with similar results in GR tests.

• The expected improvement in the GR with MoonLIGHT-2 is about one/half order of mag-nitude during the 10 years of analysis for most GR tests. The improvements shown in thesimulations represent the most pessimist case where we do not considerate the LLR stationupgrade or any software update and only few MoonLIGHT deployed in non-optimal locations.

In addition we are working at the thermal and mechanical design of MoonLIGHT-2 package,conducting a number of experimental tests in the SCF Lab and simulations in order to define andoptimize the CCR optical and thermal behavior on the Moon. The results of the tests, althoughare only preliminary, has provided us useful data for the characterization. Other experimentaltests will be conducted in the SCF Lab as other GR simulations. All of this will allow us to fulfillthe ultimate scientific objective of MoonLIGHT-2: provide constraints on the theories that areproposed to determine the properties of Dark Matter and Dark Energy, and other gravitationaltheories. The improved precision that MoonLIGHT-2 will provide us once it will be deployed (inadequate number) on the Moon will be useful to identify the theoretical directions that will furtherthe development of an understanding of these mysterious phenomena.

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