The first Lunar Laser Meeting (LLR 2023) will take place on 14-15 sept 2023 in Grasse.
Lunar Laser ranging has provided observations since the early days of laser ranging. Despite of a number of notable achievements, we have to note that LLR has comparatively little visibility among all the techniques of space geodesy. This has most likely to do with the detrimental fact that the group of users and producers of observations are rather disconnected from each other.
Therefore the first LLR meeting wishes to bring both the producers, the users and the science behind lunar laser ranging together in order to join our efforts and resources, to define goals and enhance the visibility for a LLR community and last but not least to identify synergies and to plan for the future. In other words, we wish to encourage every group to help and form a visible global community
In December 2020, the project celebrated the news of the capsule returning to Earth. But behind the scenes, we were working on another experiment. The Hayabusa2 laser altimeter (LIDAR) that was used to measure the terrain at asteroid Ryugu was about to be used to measure the distance from the Earth.
Recently, products that can measure distance using a laser have become widespread and can be bought by individuals. However, it is not easy to measure the distance to a far away object such as a satellite or spacecraft. For artificial satellites that orbit at distances of hundreds to tens of thousands of kilometers, and also on the Moon’s surface, reflective prisms, known as a corner-cube reflectors are installed. The distance can be obtained by emitting a pulsed laser from a telescope on the ground and measuring the time required for a round trip as the light is reflected from the corner-cube reflectors. However, when an object such as a spacecraft is further than the Moon, the reflected light becomes too weak to use this method. Instead, it is necessary to emit lasers from both the ground and the spacecraft. There are very few successful examples in the world of such a distance measurement that use this method.
The Hayabusa2 LIDAR emitted laser light alone when around asteroid Ryugu and received reflections from objects at a distance of several tens of kilometers. But in this experiment, we set the laser to emit light only when a laser signal is received from the outside. We then emitted laser light from the ground station telescope on Earth directed at Hayabusa2, and accurately measured the time when the laser light left the ground station, and the time when the laser arrived from Hayabusa2.
Since the field of view for the telescope and the beam spread angle of the laser are very narrow for both the Hayabusa2 LIDAR and the ground station, the orbit determination and attitude control of Hayabusa2 and the direction of the ground-based telescope needed to be extremely accurate. It is like matching a needle point to a needle point.
Figure 1/ Different types of transmission / reception systems. Left: near Ryugu. Right: After the Earth swing-by
This experiment was also conducted during the 2015 Earth swing-by. At that time, the one-way (outward) transmission from the Stromlo ground station in Australia was successful, but the return signal could not be confirmed. In 2020, ground stations in Grasse, France and Wettzell, Germany, also joined the ones in Koganai, Japan and Stromlo, Australia. As the signal from Hayabusa2 is very weak, a large telescope is required. For Hayabusa2, the stations also needed to be able to observe in the infrared. The experiment continued from December 7 to 23, the day after the spacecraft’s closest approach to the Earth.
Figure 2: The international observation network.
Because of the previous experiment, the establishment of the outbound route was successful from the first day. But as expected, the return route was not easy. The signal was weak, the weather was changing from moment to moment, the required accuracy of the orbit and attitude of the spacecraft was high, all in addition to the intense noise due to the daytime observation. Even if adjustments are made at the ground station, there can be a delay before the light reciprocates (4 seconds on the first day, 45 seconds on the last day). This all made the experiment difficult.
Furthermore, the experiment was frustrating since the amount of onboard delay (time difference between receiving and transmitting the laser) on the spacecraft side fluctuates so that the success or failure of the return trip is completely unknown at the time of the observation. Only after getting the telemetry data from Hayabusa2 and comparing the signal launch time from the ground station with the arrival time at the spacecraft were we finally able to know the amount of onboard delay. In the initial plan, the team members including the author were scheduled to go to the overseas stations, but as this was not possible, the online conference system to connect with Sagamihara was used and added to the frustration!
On the third day of the experiment, December 9, 2020, while making fine adjustments to the experiment, we succeeded in observing the return trip at the Grasse station. In reality, we had no clues to the success on that day. The next day we saw some maybes and the day after that, we were almost convinced. This graph shows the results of the round-trip time measured at the Grasse station, after applying the onboard delay. The concentration of points at -0.5 microseconds is the return signal from Hayabusa2. The many other points are noise from sunlight. Since Hayabusa2 was headed towards the Sun after the approach to Earth (Sun and Hayabusa2 had an elongation of 30°), the observations were during the daytime and we had to wrestle with a large amount of noise. The ground stations were asked to reduce noise as much as possible without eliminating the signal.
Figure 3: Two-way range observation at the Grasse station on December 9, 2020. Time is in UTC.
When weather permitted, the outbound route from Koganei / Stromlo / Grasse could be established nearly continuously. The return trip was confirmed at Grasse on December 9 (one-way distance 1.4 million km) and December 21 (one-way distance 6 million km). The Wettzell station was unable to make observations due to equipment preparation and weather conditions.
We would like to express our sincere gratitude to the members of the four stations who made preparations during the lockdown and took observations for Hayabusa2 before Christmas. Thank you. Even when there was heavy snow, everyone at the Grasse station commuted to work on the rough roads and found the sunny days to observe.
Everyone who worked with the 1.5m telescope at the Grasse ground station for this experiment.
During that period of three weeks, the spacecraft became steadily more distant from the Earth and difficult to observe, the weather changed and some of the ground station systems malfunctioned. On the other hand, for the author who was participating in a deep space mission for the first time, it was an exciting series of experiences at the Sagamihara campus each day. Under the leadership of PI Mizuno, the people of JAXA, NAOJ, the National Institute of Information and Communications Technology, Chiba Institute of Technology, Hokkaido University, the University of Occupational and Environmental Heath and the Oshima National College of Technology did everything from the preparation through to the operation perfectly. I feel this is what gave us our results.
We hope that this experiment can be recognised as one of the achievements of Hayabusa2 and will contribute to high-precision navigation in deep space in the future.
Hayabusa2 Laser Altimeter Science Team Contact:
Toshimichi Otsubo (Professor, Graduate School of Social Sciences, Hitotsubashi University)
Télémétrie laser MéO : Record de distance pour un lien laser synchrone deux voies 5 février 2021
Des expériences de télémétrie laser sur Hayabusa2 ont été menées du 7 au 23 décembre 2020 en collaboration avec la JAXA et les stations de télémétrie laser de l’Observatoire de la Côte d’Azur (OCA), de Wettzell (Allemagne), de Mont Stromlo (Australie) et de Koganei (Japon). Elles ont permis d’établir un record de distance pour un lien laser synchrone deux voies entre le satellite Hayabusa2 de la JAXA et la station de télémétrie laser MéO de l’OCA-CNRS
Ces expériences de démonstration avaient pour objectif d’établir des liens lasers synchrones bidirectionnels pour la mesure de distance Terre-satellite en profitant de l’altimètre LIDAR embarqué sur le satellite.
A deux occasions, la station MéO de l’OCA a réussi à établir ces liens laser records : Une première fois le 09 décembre 2020 à une distance de 1 million de km et une seconde fois le 21 décembre 2020 à une distance de 6 millions de km.
Ces résultats s’inscrivent dans la continuité des précédentes démonstrations de liens laser deux-voies comme en 2005 avec la sonde Messenger à 24 millions de km mais dans ce cas en asynchrone ou bien même plus récemment, sur la sonde Lunar Reconnaissance Orbiter à 0.385 millions de km sur un panneau de retro-réflecteurs ayant une surface inférieure à une feuille A4 et encore une fois avec la station MéO de l’OCA-CNRS.
Ces résultats exceptionnels sont un premier exemple d’un lien laser avec échos sur un transpondeur entre un explorateur planétaire et une station sol. L’autre prouesse est que ces démonstrations ont eu lieu en journée à 30° du soleil, ce qui est un véritable défi pour les instrumentations mises en œuvres fonctionnant en régime de simple-photon. Ces succès tiennent pour une grande partie au savoir-faire et au développement de la station MéO de l’OCA-CNRS dans le cadre de ses activités courantes de télémétrie laser lune. Ces données ont aussi permis de faire une mesure détaillée du champ de vision du LIDAR embarqué sur Hayabusa2
Cette collaboration fructueuse entre des instituts étrangers apportent d’importantes connaissances technologiques pour les futurs missions d’exploration profonde de l’espace et ouvrent la voie à des multiples applications comme les communications sol-espace par lien laser, la déterminations précise d’orbite, la synchronisation d’horloge à des distances interplanétaires et enfin les tests de physique fondamental.
Premier lien laser aller-retour avec un orbiteur lunaire 21 septembre 2020
La mesure de la distance Terre-Lune par télémétrie laser est réalisée depuis presque 40 ans par la station de télémétrie laser lunaire (LLR) à Grasse. L’équipe du laboratoire Géoazur-Observatoire de la Côte d’Azur qui opère la station est à l’origine de développements expérimentaux permettant l’exploitation d’une longueur d’onde infrarouge, ce qui a permis de décupler les capacités de la télémétrie lunaire en termes de quantité de données et de couverture du cycle lunaire. Ces mesures nous renseignent sur la structure interne de Lune, en mettant en évidence la présence d’un noyau liquide de 300 km de diamètre, et permettent de réaliser des tests de physique fondamentale comme le test de l’invariance de Lorentz.
Grâce à ce savoir-faire, la station de Grasse a réalisé les premières mesures de distance aller-retour sur l’orbiteur Lunar Reconnaissance Orbiter (LRO) de la NASA qui dispose d’un petit réseau de rétro-réflecteurs de dimensions 15×18cm. La faible dimension de ce panneau, en comparaison avec les réflecteurs déposés à la surface de la Lune par les missions Apollo et Lunokhod, rend l’exercice bien plus difficile. Afin d’optimiser le pointage des tirs lasers de la station de Grasse vers le LRO, l’équipe POLAC du Syrte-Observatoire de Paris a réalisé des prévisions très fines de l’orbite de la sonde.
Le 4 septembre 2018, Grasse a mesuré 67 retours en deux sessions de 6 minutes. Des retours clairs ont également été enregistrés lors de deux sessions supplémentaires les 23 et 24 août 2019 pour lesquelles la rotation active du LRO a été effectuée par la NASA afin que les réflecteurs soient en vue de la station. Les échos mesurés ont donné des résidus sur la mesure de distance inférieurs à 3 cm par rapport à la trajectoire LRO reconstruite. Cette expérience fournit une nouvelle méthode pour vérifier la possible accumulation de poussière sur les réflecteurs de la surface lunaire, ceux du LRO n’étant pas soumis à ce phénomène. En savoir plus
First two-way laser ranging to a lunar orbiter: infrared observations from the Grasse station to LRO’s retro-reflector array – Earth Planets Space 72, 113 Erwan Mazarico, Xiaoli Sun, Jean-Marie Torre, Clément Courde, Julien Chabé, Mourad Aimar, Hervé Mariey, Nicolas Maurice, Michael K. Barker, Dandan Mao, Daniel R. Cremons, Sébastien Bouquillon, Teddy Carlucci, Vishnu Viswanathan, Frank G. Lemoine, Adrien Bourgoin, Pierre Exertier, Gregory A. Neumann, Maria T. Zuber & David E. Smith https://doi.org/10.1186/s40623-020-01243-w Illustration : Vue rapprochée du réseau de rétro-réflecteurs monté sur la sonde LRO
