The lunar retroreflectors used in past analysis (excluding Lunokhod 1 as it was only recently rediscovered) all lie within 26 degrees latitude of the lunar equator, and the most useful ones within 24 degrees longitude of the sub-Earth meridian [77, 17] as shown in Figure 4 and Table 1. This clustering is sub-optimal, particularly with respect to measuring the lunar librations. In addition, the active LLR ground stations do not cover a large range of latitudes, further weakening the geometric strength of the observations. Additional observatories could improve the situation somewhat, but Mt. Stromlo in Australia is the only station capable of ranging to the Moon not situated at similar northern latitudes. Unfortunately, Mt. Stromlo has not been active in ranging to the Moon.
The frequency and quality of observations also varies greatly with the power of the laser employed and other characteristics of the facility. Most ranging over the recent past has occurred between three ground stations (MLRS, OCA, and Apache Point) and one reflector (Apollo 15). The solar noise background and other issues make ranging to some reflectors possible only around the quarter-moon phase for most stations other than Apache Point, which has very good distribution among the reflectors. APOLLO is capable of ranging during all lunar phases, but it must share time on its 3.5 meter telescope with other programs.
Improvements in the geometric coverage, both on Earth and on the Moon, will have a direct impact on the science gained through LLR. Studies of the structure and composition of the interior require measurements of the lunar librations, while tests of GR require the position of the lunar center of mass. In all, six degrees of freedom are required to constrain the geometry of the Earth-Moon system (in addition to Earth orientation). A single ranging station and reflector is insufficient to accurately determine all six degrees of freedom, even given the rotation of the Earth with respect to the Moon. The addition of one or more reflectors and one or more additional ranging stations in the Earth’s southern hemisphere would strengthen the geometric coverage and increase the sensitivity to lunar motion by as much as a factor of 4 in some degrees of freedom at the same level of ranging precision . The rediscovery of Lunokhod 1 will also greatly improve the geometric coverage, and consequently, the science return.
Satellite Laser Ranging (SLR) began in 1964 at NASA’s Goddard Space Flight Center. Since then it has grown into a global effort, represented by the International Laser Ranging Service (ILRS) . The ILRS includes ranging to Earth-orbiting artificial satellites, ranging to the lunar reflectors, and is actively working toward supporting asynchronous planetary transponder ranging. The current SLR network consists of over 40 stations worldwide, funded and operated by research organizations, universities, and foreign governments (see Figure 5). It varies widely in its designs for tracking mounts, telescopes, receivers, timing electronics, and laser transmitters.
The development of NASA’s Next Generation Satellite Laser Ranging (NGSLR) system has moved system operation from the regime of high laser energy/low repetition rate to high repetition rate single photon detection [21, 33]. This technique uses receive photons more efficiently and because of the higher return rate, minimizes acquisition time and enables closed loop tracking. The current laser in use (300 picosecond pulsewidth) limits single shot measurement accuracy to 2 to 3 cm but, because of the high return rate, normal point data can be reduced to the millimeter level. As many as 12 NGSLR stations are expected to be built and deployed around the world under NASA’s Earth Science Program in the coming decade. The first of these systems is now operational on the 40 cm telescope at NASA’s Goddard Space Flight Center. It is currently ranging to targets at altitudes ranging from Low Earth Orbiting (LEO) satellites to LAGEOS, as well as up-link ranging to the Lunar Reconnaissance Orbiter (LRO) .
The development of this new network of SLR stations provides a potential opportunity to expand the number of LLR stations. To be lunar capable, the SLR stations would need to be upgraded with higher power lasers or, alternatively, new high cross section retroreflectors and/or laser transponders would need to be put on the Moon. Taking advantage of the existing SLR infrastructure is a very compelling way to increase both the spatial and temporal LLR coverage at minimal cost, and would ensure continuous availability of LLR data through the indefinite future as it would not rely on unique facilities and individual investigators continuing operations.
Living Rev. Relativity 13, (2010), 7
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