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Since 1969 Lunar Laser Ranging (LLR) to the Apollo Cube Corner Reflector (CCR) arrays has supplied several significant tests of gravity: Geodetic Precession, the Strong and Weak Equivalence Principle (SEP, WEP), the Parametrized Post Newtonian (PPN) parameter β, the time change of the Gravitational constant (G), 1/r2 deviations and new gravitationa...
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... general concept of the second generation of LLR is to consider a number (notionally eight) of large single Cube Corner Retroreflectors (CCRs). Each of these will produce a light echo that, with a single photoelec- tron detection system such as the current APOLLO system, can be used to improve the ranging beyond the limit of accuracy determined by the librational effects of current arrays and the laser pulse length. When single CCRs are used, the return is unaffected by the libration, that is, there is no increased widening of the FWHM caused by the librational effects and by the CCR itself. In this way, an accuracy improvement on the time of flight measurement of 1 ns (roundtrip) will be obtained. If two such single reflectors are deployed with a relative distance of tens of meters, their return of light will be recorded separately and can be recognized by comparison with the nominal orbit of the Moon and the rotational parameters of the Earth [6]. This idea is illustrated schematically in Fig. 1. ...
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... Since the turn of the century, more efficient, single, large retroreflectors have been proposed by various international teams to replace the large Apollo 15 array on future lunar-landing missions [14][15][16][17][18][19]. Whereas each of the 300 38 mm-diameter retroreflectors in the Apollo 15 array currently contributes about 0.33% to the overall signal seen by the ground stations due to the wide divergence angles of their reflected beams, no dihedral angle is required for small-diameter reflectors (<150 mm for coated and <100 mm for uncoated and hollow reflectors). ...
... However, the retro produces a "hot spot" in the return, which can be isolated spatially via a pinhole in the receiver FOV or, better yet, a multipixel SPAD (Single-Photon Avalanche Diode) array, which can detect the "hot spot" during target acquisition and drive it to the central pixel of the array in order to maximize the retro signal strength while simultaneously assigning the vast majority of the surface photons to other pixels in the array. Locating the Earth-facing retro near the perimeter of the lunar disk would drive the cosθ term in Equation (19) close to zero. The Southern lunar pole region might be a good choice since it is believed to be a potential source of water for future manned missions. ...
The present paper discusses the basics of retroreflector theory and the manner in which they are combined in arrays to service the laser tracking of artificial satellites and the Moon. We begin with a discussion of the relative advantages and disadvantages of solid versus hollow cube corners and the functional dependence of their optical cross-sections and far-field patterns on cube diameter. Because of velocity aberration effects, the design of an array for a particular space mission depends on many factors, including the desired range accuracy and the satellite’s orbital altitude, velocity, and pass geometry relative to the tracking station. This generally requires the individual retroreflectors in the array to be “spoiled” by perturbing one or more of the 90-degree angles that define a perfect cube corner, or alternatively, by adding a curved surface to a hollow cube. In order to obtain adequate return signal strengths from all points along the satellite path, the rotational orientation of the retroreflectors within the array may need to be varied or “clocked”. Possible approaches to developing millimeter-accuracy arrays with both large cross-sections and ultrashort satellite signatures are discussed, as are new designs proposed to replace aging reflectors on the Moon. Finally, we briefly discuss methods for laser ranging beyond the Moon.
... The advantages of 10 cm lunar CCRs were discussed by Martini et al. (2012), Garattini et al. (2013), Currie et al. (2013), and Ciocci et al. (2017). The first 10 cm CCRs are now being prepared for delivery to the Moon. ...
The Lunar Laser Ranging (LLR) retroreflector arrays have been on the Moon for half a century. During that time, the laser range uncertainty has improved by a factor of 100. Consequently, the science results have also improved by orders of magnitude. New retroreflectors are scheduled to go to the Moon on Commercial Lander Payload Services missions and the Lunar Geophysical Network mission. The new retroreflectors are single 10 cm corner cube retroreflectors that will not spread the laser pulse during reflection like the existing arrays do. Due to the orbital and Earth rotational speeds, there is a velocity aberration of 0.″8–1.″5 for existing stations. Larger corner cubes require attention to ensure that the spread of possible velocity aberration displacements is optimally contained within the diffraction pattern. The diffraction pattern can be changed by making one or more of the rear dihedral angles slightly different from 90°. Improvements in the equipment at the LLR stations and improvements in the data analysis software are also desirable. Future possibilities are described.
... The performance in the IR is also very favorable for new monolithic CCRs, which are smaller but have a better ranging precision. A sparse network of large single corner cubes that could be resolved by a 100 ps laser pulse system would greatly improve the precision of LLR Garattini et al., 2013). The use of the IR wavelength also increases the number of stations that could enter the LLR community. ...
Abstract Based on a fully passive space segment, the lunar laser ranging experiment is the last of the Apollo Lunar Surface Experiments Package to operate. Observations from the Grasse lunar laser ranging station have been made on a daily basis since the first echoes obtained in 1981. In this paper, first, we review the principle and the technical aspects of lunar laser ranging. We then give a brief summary of the progress made at the Grasse laser ranging facility (Observatoire de la Côte d'Azur, Calern Plateau on the French Riviera) since the first echoes. The current performance, driven by the use of an infrared wavelength laser, is presented in the last section for the year 2018.
Context. Differential Lunar Laser Ranging (DLLR), which is planned to be conducted at Table Mountain Observatory (TMO) of Jet Propulsion Laboratory (JPL) in the future, is a novel technique for tracking to the Moon. This technique has the potential to determine the orientation, rotation, and interior of the Moon much more accurately if the expected high accuracy of about 30 μm can be achieved.
Aims. We focus on the benefit for the related parameters when only DLLR data with a short time span are available in the beginning.
Methods. A short DLLR time series is not enough to provide an accurate lunar orbit, which has a negative effect on parameter estimation. Fortunately, Lunar Laser Ranging (LLR) has been collecting data for a very long time span, which can be used to compensate this DLLR disadvantage. The combination of LLR data (over more than 50 yr) and simulated DLLR data over a relatively short time span (e.g., 5 or 10 yr) is used in different cases which include changing reflector baselines and extending data time span, along with adding more stations and “new” reflectors.
Results. The results show that the estimated accuracies of the parameters related to the lunar orientation, rotation, and interior can be improved by about 5–100 times by simply adding 5-yr DLLR data in the combination. With LLR, further enhancing the parameter determination can be achieved by choosing appropriate reflector baselines. By investigating different scenarios of reflector baselines based on the present five reflectors on the Moon, we find that two crossing baselines with larger lengths offer the greatest advantage. A longer data time span is more helpful, rather than having more stations involved in the measurement within a shorter time span, assuming the amount of data in these two cases is the same. Furthermore, we evaluated the preferred position of an assumed new reflector.
Context. To obtain more details about the lunar interior, a station at Table Mountain Observatory of JPL will enable a new measurement of lunar laser ranging (LLR), known as differential lunar laser ranging (DLLR). It will provide a novel type of observable, namely, the lunar range difference, which is the difference of two consecutive ranges obtained via a single station swiftly switching between two or more lunar reflectors. This previously unavailable observation will have a very high level of accuracy (about 30 μm), mainly resulting from a reduction in the Earth’s atmospheric error. In addition to the intended improvements for the lunar part, it is expected to contribute to improved relativity tests, for instance, the equivalence principle (EP).
Aims. This paper focuses on the simulation and investigation of the characteristics of DLLR.
Methods. Using simulated DLLR data, we analyzed and compared the parameter sensitivity, correlation, and accuracy obtained by DLLR with those attained by LLR.
Results. The DLLR measurement maintains almost the same sensitivity to certain parameters (called group A) as that of LLR, such as the lunar orientation parameters. For other parameters (called group B), such as station coordinates, it is shown to be less sensitive. However, owing to its extraordinary measurement accuracy, it not only retains nearly the same level of accuracy of group B as LLR, but it also improves the estimation of group A significantly (with the exception of reflector coordinates, due to the DLLR measuring mode). Also, DLLR increases the correlations among the reflectors and between stations and reflectors caused by its constellation. Additionally, we compared different switching intervals with respect to sensitivity and correlation. Large switching intervals are more beneficial for group B and the decorrelation of stations and reflectors. Furthermore, DLLR enhances the accuracy of EP tests.
