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Overview

Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR) use short-pulse lasers and state-of-the-art optical receivers and timing electronics to measure the two-way time of flight (and hence distance) from ground stations to retroreflector arrays on Earth orbiting satellites and the Moon. Scientific products derived using SLR and LLR data include precise geocentric positions and motions of ground stations, satellite orbits, components of Earth’s gravity field and their temporal variations, Earth Orientation Parameters (EOP), precise lunar ephemerides and information about the internal structure of the Moon. Laser ranging systems are already measuring the one-way distance to remote optical receivers in space and can perform very accurate time transfer between sites far apart. Laser ranging activities are organized under the International Laser Ranging Service (ILRS) which provides global satellite and lunar laser ranging data and their derived data products to support research in geodesy, geophysics, Lunar science, and fundamental constants. This includes data products that are fundamental to the International Terrestrial Reference Frame (ITRF), which is established and maintained by the International Earth Rotation and Reference Systems Service (IERS). The ILRS develops the necessary global standards/specifications for laser ranging activities and encourages international adherence to its conventions.

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LAGEOS (LAser GEOdynamics Satellite-1), tracked by SLR since May 10, 1976. (Credit: NASA)

Highlights

Lunar Laser Ranging Stations of the International Laser Ranging Service (ILRS) range to NGLR-1 Release Date: 04/11/2025

The Next Generational Lunar Retroreflector (NGLR-1) was successfully delivered to the surface of the Moon on the Blue Ghost Lunar Lander in Mare Crisium, a large lunar basin at 17.0°N, 59.1°E in the northeast quadrant of the lunar nearside on March 2, 2025 at 08:34 UTC. NGLR-1 was procured by the University of Maryland (College, Park, Maryland, USA), with D. Currie (UMD) as Principal Investigator. A pre-flight NGLR prototype was contributed by the INFN (Istituto Nazionale di Fisica Nucleare) - Frascati National Labs, Italy, long-time partner of this UMD development [Currie et al., 2024]. The Blue Ghost Lander was delivered to the lunar surface under the auspices of the NASA CLPS (Commercial Lunar Payload Services) initiative.

The Next Generation Lunar Retroreflector (NGLR). Credit. D. Currie (University of Maryland, College Park, MD, USA).

The NGLR-1 reflects very short laser pulses from these Earth-based Lunar Laser Ranging (LLR) Observatories. The laser pulse transit time to the Moon and back will be used to accurately determine the distance between the terrestrial station and NGLR on the surface of the Moon. NGLR will improve upon the Apollo and Luna LLR results by providing sub-millimeter range measurements due to its unique location and smaller target signature.

Three LLR stations of the ILRS have confirmed that they have successfully ranged to NGLR-1: Grasse, France (at both the green and infrared wavelengths); Wettzell, Germany (at the infrared wavelength); and the Apache Point Observatory in New Mexico, U.S.A (at the green wavelength). [See Battat et al. (2023); Eckl et al. (2023); Chabé et al. (2020) for descriptions of the Apache Point, Wettzell and Grasse LLR systems and their data].

The initial position of NGLR-1 resulted in timing offsets during ranging on the order of 600 ns. After refined lander positions were provided by the Blue Ghost Lander/Firefly team, and by the Lunar Reconnaissance Orbiter Camera (LROC) on NASA’s Lunar Reconnaissance Orbiter (LRO), the timing offsets were reduced to about 60 ns. The ILRS Paris Observatory Lunar Analysis Center (POLAC) provided the NGLR-1 pointing predictions that were used by the LLR observatories.

The ILRS stations will continue to range to NGLR-1 along with the other retroreflectors on the lunar surface, enriching the more-than-50 year data set of LLR observations [c.f. see Müller et al., 2019]. Initially the goal will be to characterize the performance of NGLR-1 compared to the experience with the Apollo and Luna retroreflectors. NGLR-1 will also become a ranging target for NASA’s Lunar Orbiter Laser Altimeter (LOLA), as LOLA continues to observe different lunar laser reflectors on the surface from lunar orbit.

Congratulations to the NGLR-1 team, and Congratulations to the ILRS LLR Observatories for providing the first observations of NGLR-1!

Location of Lunar Laser Retroreflector Arrays on the Moon overlain on a map of lunar topography provided by NASA’s Lunar Orbiter Laser Altimeter (LOLA). Credit: NASA and the Lunar Reconnaissance Orbiter (LRO)/LOLA.

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Recent News

ILRS Data Files for SLR processing have been updated - The ILRS Analysis Standing Committee (ASC) has released updates to the following files:

(1) Data Handling File
(2) Eccentricity Files
(3) SLRF2020 Positions & Velocities

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Cancellation of ILRS Technical Workshop in Arequipa, Peru - Due to circumstances beyond their control, the University of San Agustin (UNSA) will not be able to host the 2025 International Workshop on Laser Ranging at Arequipa in October 2025. The ILRS thanks the UNSA team for all their efforts planning and preparing for hosting the workshop. The ILRS is considering the option of holding a virtual meeting this year (2025) if there is sufficient interest, support for the organization, and approval by the ILRS Governing Board. The ILRS Central Bureau welcomes your thoughts regarding holding a potential virtual ILRS workshop as well as offers to help organize the meeting along with the Central Bureau. The next in-person ILRS workshop will be in Buenos Aires, Argentina in 2026.

ILRS Central Bureau surveys stations on the ILRS Tracking Priority List - In March 2025, the ILRS Central Bureau (CB) conducted a survey of the ILRS stations to try and understand better how stations used the ILRS Priority List. Currently, the ILRS priority list is constructed to according to satellite orbit parameters as well as according to special project needs. The satellite tracking priority is generally arranged by altitude such that the satellites at the lowest altitude (and thus the shortest satellite passes) have the highest tracking priority. While logical, this mode paradoxically places the geodetic ITRF satellites (i.e. LAGEOS-1, LAGEOS-2, LARES-2) at the apparently high tracking priorities of 24, 25 & 18 respectively. In order to provide information that could inform a redesign of how tracking priorities are presented and communicated to the stations, the ILRS CB asked a series of short questions in a short survey. The survey was designed to be anonymous and left the possibility for survey participants to provide more information and other comments.

The ILRS station tracking priority survey results can be found here.

The decommissioned geodetic satellite WESTPAC will be returned to the ILRS target list with a low priority, not resulting in a significant burden to station scheduling or impact on other priority targets. In tests led by the ILRS Network and Engineering Standing Committeee (NESC), ILRS stations have demonstrated that with good predictions Westpac can be tracked routinely during the day and night, without a significant impact on the station tracking schedules or at the expense of other ILRS targets.

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