Abstract
We present the results of the first series of successful two-way laser ranging experiments from a ground station, the Lunar Laser Ranging (LLR) station in Grasse, France, to a spacecraft at lunar distance, the Lunar Reconnaissance Orbiter (LRO). A 15 × 18 × 5 cm, 650-g array of twelve 32-mm diameter solid corner cubes is mounted on its anti-nadir deck. Ranging to this small retro-reflector array onboard a lunar orbiter from a ground station was a challenge compared to ranging to larger lunar surface retro-reflectors. Grasse measured 67 returns in two 6-min sessions on September 4, 2018. Clear returns were also recorded during two additional sessions on August 23–24, 2019 for which active slewing by LRO was performed to bring the array in view of the station. The measured echos yielded range residuals less than 3 cm (two-way time-of-flight RMS < 180 ps) relative to the reconstructed LRO trajectory. This experiment provides a new method of verifying theories of dust accumulation over decades on the lunar surface. It also showed that the use of similar arrays onboard future lunar landers and orbiters can support LLR lunar science goals, particularly with landing sites near the lunar limbs and poles, which would have better sensitivity to lunar orientation.
Highlights
For 50 years, Lunar Laser Ranging to surface laser retroreflector arrays (LRAs) by lasers at Earth stations has provided important geodetic data constraining the lunar ephemeris and orientation, with implications for both lunar interior properties and for astrophysics and fundamental physics.Technological improvements to the station equipments, such as detectors, lasers, and telescopes, have improved the accuracy of the ranging measurements from tens of centimeters to sub-cm (Murphy 2013; Courde et al 2017b; Müller et al 2019)
We report on a two-way laser ranging experiment at lunar distance much akin to Lunar Laser Ranging (LLR) except the target is a small retro-reflector array onboard the Lunar Reconnaissance Orbiter (LRO) spacecraft, < 1/10th the area of Mazarico et al Earth, Planets and Space (2020) 72:113 the Apollo 11 array and 1/30th the mass if counting the support structure
Several successful two-way laser ranging observation sessions were made from the Grasse station, but not enough at this time to robustly assess the link margin to the LRO array and quantify the relative loss of the surface arrays from dust accumulation over 5 decades
Summary
For 50 years, Lunar Laser Ranging to surface laser retroreflector arrays (LRAs) by lasers at Earth stations has provided important geodetic data constraining the lunar ephemeris and orientation, with implications for both lunar interior properties and for astrophysics and fundamental physics. Several successful two-way laser ranging observation sessions were made from the Grasse station, but not enough at this time to robustly assess the link margin to the LRO array and quantify the relative loss of the surface arrays from dust accumulation over 5 decades. Beyond their use to assess the relative return strength of the surface reflectors, 2-way laser ranging observations to LRO could provide geodetic range information on the LRO trajectory At this time, with only a few successful passes spread over a year, they are too sparse to be able to support orbit determination efforts. The RMS values are ~ 166 ± 35 ps, which corresponds to 2.50 ± 0.54 cm in one-way range
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