The ILRS: approaching 20 years and planning for the future

Satellite Laser Ranging (SLR) is currently the unique technique to determine station coordinates, Earth Orientation Parameters (EOPs) and Stokes coefficients of the Earth’s gravity field from one observation equation with a high accuracy. These parameters form the so-called ‘three pillars’ (Plag and Pearlman, 2009) of the Global Geodetic Observing System (GGOS). In its function as an official analysis center of the International Laser Ranging Service (ILRS) (Pearlman et al., 2002), DGFI is developing and maintaining software to process SLR observations called ‘DGFI Orbit and Geodetic parameter estimation Software’ (DOGS). The software is used to analyze SLR observations and to compute multi-satellite solutions. In this study, up to 10 satellites (ETALON1/2, LAGEOS1/2, STELLA, STARLETTE, AJISAI, LARETS, LARES and BLITS) with different orbit characteristics (e.g., inclination and altitude) are combined. The relative weighting of the satellites is done using a variance component estimation. The diverse orbits allow to decrease the correlation of parameters such as gravity field coefficients (GFCs) and EOPs. Beside the Earth’s gravity field (weekly GFCs with degree and order ≤ 6 and monthly GFCs with degree and order ≤ 20) and rotation (terrestrial pole coordinates, UT1-UTC extrapolated with LOD), 3-D station coordinates are estimated weekly or monthly. Different combined solutions are compared to LAGEOS-only solutions. If LARES is combined with LAGEOS1/2, a significant improvement in the GFCs, the EOP and the TRF can be achieved. If more satellites are combined, the variation of the pole coordinates w.r.t. the IERS 08 C04 time series can be reduced up to 56%. Furthermore, systematics in LOD are nearly completely eliminated. The variation of the station coordinates w.r.t. SLRF2008 (http://www.ilrs.gsfc.nasa.gov/science/awg/SLRF2008.html, 2013) can be reduced by about 30%.

The International Laser Ranging Service: current status and future developments

<p>The Global Geodetic Observing System (GGOS) is the contribution of Geodesy to the observation and monitoring of the Earth System. Geodesy is the science of determining and representing the shape of the Earth, its gravity field and its rotation as a function of time. A core element to reach this goal are stable and consistent geodetic reference frames, which provide the fundamental layer for the determination of time-dependent coordinates of points or objects, and for describing the motion of the Earth in space. Traditionally, geodetic reference frames have been used for surveying, mapping, and space-based positioning and navigation. With modern instrumentation and analytical techniques, Geodesy is now capable of detecting time variations ranging from large and secular scales to very small and transient deformations with increasing spatial and temporal resolution, high accuracy, and decreasing latency. GGOS has been working closely with components of International Association of Geodesy (IAG) to provide consistent and openly available observations of the spatial and temporal changes of the shape and gravity field of the Earth, as well as the temporal variations of the Earth’s rotation. These efforts make available a global picture of the surface kinematics of our planet, including the ocean, ice cover, continental water, and land surfaces, as well as estimates of mass anomalies, mass transport, and mass exchange in the System Earth. Surface kinematics and mass transport together are the key to global mass balance determination, and are an important contribution to understanding the energy budget of our planet. In order to play its vital role, GGOS has following missions; a) to provide the observations needed to monitor, map, and understand changes in the Earth’s shape, rotation, and mass distribution, b) to provide the global geodetic frame of reference that is the fundamental backbone for measuring and consistently interpreting key global change processes and for many other scientific and societal applications, c) to benefit science and society by providing the foundation upon which advances in Earth and planetary system science and applications are built. For the mission, GGOS works tighter with components of the IAG, more specifically, IAG Services, IAG Commissions and IAG Inter-Commission Committees. The IAG Services provide the infrastructure and products on which all contributions of GGOS are based, and the IAG Commissions and IAG Inter-Commission Committees provide expertise and support to address key scientific issues within GGOS. Together with the IAG components, GGOS provides the fundamental infrastructure underpinning Earth sciences and their applications.</p>

The SCF_Lab (Satellite/lunar/GNSS laser ranging/altimetry and Cube/microsat Characterization Facilities Laboratory) is a brand new ISO 7 Clean Room, dedicated to design, characterization and modelling of the space segment of Satellite Laser Ranging (SLR), Lunar Laser Ranging (LLR) and Planetary Laser Ranging and Altimetry (PLRA) for industrial and scientific applications. The role of SCF_Lab is fundamental to study and characterize cube corner retroreflectors (CCRs) necessary for laser ranging. CCRs are special “mirrors” that reflect light in the same direction it came from as it was a normal flat mirror but always orthogonal to the beam no matter where the beam actually came from. It happens thanks to a triple rebound of photons, over the three faces of the corner. In SCF_Lab, thanks to an innovative thermal-vacuum-optical test (SCF-Test), we characterize and model the thermal behaviour and the optical performances of CCRs. The laboratory procedures are approved by the international organization that coordinates all laser ranging activities, the International Laser Ranging Service (ILRS). Our key experimental innovation is the concurrent measurement and modelling of the optical Far Field Diffraction Pattern (FFDP) and the temperature distribution of the SLR/LLR payload of retroreflectors under thermal conditions produced with a solar simulator. The primary target is to provide critical design and diagnostic capabilities for SLR to Galileo and other GNSS (Global Navigation Satellite System) constellations. New retroreflectors designs will be fundamental to improve GNSS orbits, increasing, this manner, accuracy, stability, and distribution of the International Terrestrial Reference Frame (ITRF), in order to provide a more precise definition of the geocenter (origin) and the scale (length unit). The SCF_Lab is also developing, validating and optimizing a next-generation LLR arrays for precision tests of GR with the MoonLIGHT-2 (Moon Laser Instrumentation for General relativity High-accuracy Tests - Phase 2) experiment.

The Global Geodetic Observing System (GGOS) is a collaborative contribution of the global Geodesy community to the observation and monitoring of the Earth System. Geodesy is the science of determining the shape of the Earth, its gravity field, and its rotation as functions of time. Essential to reach this goal are stable and consistent geodetic reference frames, which provide the fundamental layer for the determination of time-dependent coordinates of points or objects, and for describing the motion of the Earth in space. With modern instrumentation and analytical techniques, Geodesy is capable of detecting time variations ranging from large and secular scales to very small and transient deformations – all with increasing spatial and temporal resolution, high accuracy, and decreasing latency. The geodetic observational and analysis infrastructures as well as the high-quality geodetic products provide the foundation upon which advances in Earth and planetary system sciences and applications are built. In this way, GGOS endeavors to facilitate and enable production and sharing of the Earth observations needed to monitor, map, and understand changes in the Earth’s shape, rotation, and mass distribution. GGOS also advocates the global geodetic frame of reference as the fundamental backbone for measuring and consistently interpreting global change processes as well as the essential geospatial infrastructure to ensure a homogeneous and sustainable development worldwide.GGOS closely works with its parent organization, the International Association of Geodesy (IAG), to keep these fundamental geodetic contributions sustainable. The IAG Services provide the infrastructure and products on which all contributions of GGOS are based, and the IAG Commissions and IAG Inter-Commission Committees provide expertise and support to address key scientific issues within GGOS. Additionally, GGOS supports the IAG by strengthening external and interdisciplinary relations and contributions to the broader geospatial information community, including relevant United Nations groups, in particular, the UN Committee of Experts on Global Geospatial Information Management (GGIM), its Subcommittee on Geodesy, and the new UN Global Geodetic Centre of Excellence (scheduled to commence operations in 2023). The main contribution of GGOS in this regard is to support actions and initiatives to communicate the value of Geodesy to society as well as to help to understand and solve complex issues facing the global geodesy community. Towards this objective, GGOS have developed a comprehensive Geodesy portal (https://ggos.org/) including detailed descriptions of geodetic observations (https://ggos.org/obs/) and products (https://ggos.org/products/), and various outreach tools such as short videos to explain the roles and importance of Geodesy to non-geodesists.

The BeiDou-3 Navigation Satellite System (BDS-3) generates navigation messages from a regional ground tracking network and inter-satellite links, relying on predicted station coordinates and Earth rotation parameters (ERPs). These procedures, however, may lead to orientation errors in the broadcast ephemeris, termed constellation rotation errors. Traditionally, these errors are monitored through Helmert transformation analysis comparing broadcast and post-processed orbits, a method that highlights discrepancies between the global navigation satellite system (GNSS) terrestrial reference frame (TRF) and the International TRF (ITRF) but suffers from a two-week evaluation delay. This study proposes a near real-time approach for estimating constellation rotation errors using high-precision satellite laser ranging (SLR) data, representing the difference between GNSS TRF and ITRF through the International laser ranging service TRF. The impact of SLR data quantity and distribution on rotation estimation accuracy is investigated, and the effectiveness of this method on BDS-3 orbits with ERP-induced orientation deficiencies is validated. Results show that the root mean square of broadcast orbit errors and the orbit user ranging error reduce from 0.8/0.7/0.2 m to 0.1/0.1/0.1 m, respectively. This demonstrates the method’s capability for real-time monitoring and correcting orientation errors within BDS-3 navigation messages.

Precise Orbit Determination (POD) is an integral part for analyzing measurements from space geodetic techniques such as Satellite Laser Ranging (SLR) and Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS). In the last two decades, POD based on GPS data has furthermore been established as one of the standard techniques to derive trajectories of satellites in the low Earth orbit (LEO) with highest accuracy. Since the launch of dedicated gravity missions, GPS sensors are not only used as a key tracking system for LEO POD, but also for extracting the long wavelength part of the Earth’s gravity field (together with SLR to spherical satellites). This chapter introduces SLR and GNSS measurements collected by the terrestrial networks of the International Laser Ranging Service (ILRS) and the International GNSS Service (IGS) as the observational basis for the realization of a terrestrial reference frame from satellite data. On this foundation, the basic equations and mathematical methods of orbit determination are introduced and extensively discussed. Pseudo-stochastic orbit modeling techniques are eventually presented as a general and efficient concept to determine satellite trajectories of highest quality even in presence of deficient force models, covering the full range between dynamic and purely kinematic solutions. Selected results from the application of the discussed orbit determination techniques are highlighted for GPS LEO data. Special emphasis is also put to present orbit determination in the context of more general orbit determination problems, where satellite trajectories are simultaneously determined with other parameters encompassing (at maximum) all pillars of geodesy, i.e., the shape, rotation, and gravity field of the Earth.

In order to deal with space debris objects, the precise and accurate knowledge of their positions is of fundamental importance. Past studies have shown that the accuracy of an orbit determination process depends on the length of the observed arc, the number of observations, the geometry of the observation (object-observer relative geometry), the observables used and their accuracy. The use of one observable, especially if the measurements stem from one single site, brings limits in the achievable accuracy when estimating an orbit. One possible way to improve the results of an orbit determination process is to utilize different kind of observables at the same time. The main aim of this paper is to study the benefits of fusing laser ranges and angular measurements in an orbit determination process for space debris. We will treat the cases of the two highly populated orbital regions: LEO and GEO. In particular, with the use of only real measurements, we will show the improvements achievable using laser ranges in a typical space debris observation scenario. We will highlight the differences in the results obtained with merged measurements and short observation arcs, w.r.t. the classical angle-only orbit determination. Particular attention is dedicated to understand the main contributions of each observables, of the number of observations and their distribution, on the estimated parameters. The results shown are obtained using real angular/laser measurements provided by sensors of the Swiss Optical Ground Station and Geodynamics Observatory Zimmerwald owned by the Astronomical Institute of the University of Bern (AIUB) and in some cases using also real ranges from the International Laser Ranging Service (ILRS) stations.

The Optical Wide-field patroL-Network (OWL-Net) is a global optical network for Space Situational Awareness in Korea. The primary operational goal of the OWL-Net is to track Low Earth Orbit (LEO) satellites operated by Korea and to monitor the Geostationary Earth Orbit (GEO) region near the Korean peninsula. To obtain dense measurements on LEO tracking, the chopper system was adopted in the OWL-Net’s back-end system. Dozens of angle-only measurements can be obtained for a single shot with the observation mode for LEO tracking. In previous work, the reduction process of the LEO tracking data was presented, along with the mechanical specification of the back-end system of the OWL-Net. In this research, we describe an integrity assessment method of time-position matching and verification of results from real observations of LEO satellites. The change rate of the angle of each streak in the shot was checked to assess the results of the matching process. The time error due to the chopper rotation motion was corrected after re-matching of time and position. The corrected measurements were compared with the simulated observation data, which were taken from the Consolidated Prediction File from the International Laser Ranging Service. The comparison results are presented in the In-track and Cross-track frame.

Satellite Laser Ranging (SLR) is an established technique providing very accurate position measurements of satellites in Earth orbit. However, despite decades of development, it remains a complex and expensive technology, which impedes its further growth to new applications and users. The miniSLR implements a complete SLR system within a small, transportable enclosure. Through this design, costs of ownership can be reduced significantly, and the process of establishing a new SLR site is greatly simplified. A number of novel technical solutions have been implemented to achieve a good laser ranging performance despite the small size and simplified design. Data from the initial six months of test operation have been used to generate a first estimation of the system performance. The data include measurements to many of the important SLR satellites, such as Lageos, Etalon and most of the geodetic and Earth observation missions in LEO. It is shown that the miniSLR achieves sub-centimetre accuracy, comparable with conventional SLR systems. The miniSLR is an engineering station in the International Laser Ranging Service and supplies data to the community. Continuous efforts are undertaken to further improve the system operation and stability.

GETEMME (Gravity, Einstein’s Theory, and Exploration of the Martian Moons’ Environment), a mission which is being proposed in ESA’s Cosmic Vision program, shall be launched for Mars on a Soyuz Fregat in 2020. The spacecraft will initially rendezvous with Phobos and Deimos in order to carry out a comprehensive mapping and characterization of the two satellites and to deploy passive Laser retro-reflectors on their surfaces. In the second stage of the mission, the spacecraft will be transferred into a lower 1500-km Mars orbit, to carry out routine Laser range measurements to the reflectors on Phobos and Deimos. Also, asynchronous two-way Laser ranging measurements between the spacecraft and stations of the ILRS (International Laser Ranging Service) on Earth are foreseen. An onboard accelerometer will ensure a high accuracy for the spacecraft orbit determination. The inversion of all range and accelerometer data will allow us to determine or improve dramatically on a host of dynamic parameters of the Martian satellite system. From the complex motion and rotation of Phobos and Deimos we will obtain clues on internal structures and the origins of the satellites. Also, crucial data on the time-varying gravity field of Mars related to climate variation and internal structure will be obtained. Ranging measurements will also be essential to improve on several parameters in fundamental physics, such as the Post-Newtonian parameter β as well as time-rate changes of the gravitational constant and the Lense-Thirring effect. Measurements by GETEMME will firmly embed Mars and its satellites into the Solar System reference frame.

Research activities of Polish research groups in a period of 2015–2019 on reference frames and reference networks are reviewed and summarised in this paper. The summary contains the results concerning the implementation of latest resolutions on reference systems of the International Union of Geodesy and Geophysics and the International Union of Astronomy with special emphasis on the changes in the Astronomical Almanac of the Institute of Geodesy and Cartography, Warsaw. It further presents the status of the implementation of the European Terrestrial Reference System 1989 (ETRS89) in Poland, monitoring the terrestrial reference frame, operational work of GNSS permanent IGS/EPN stations in Poland, operational work of the laser ranging station in Poland of the International Laser Ranging Service (ILRS), active GNSS station network for the realization of ETRS89 in Poland, validation of recent ETRS89 realization, expressed in ETRF2000 in Poland, and maintenance of the vertical control in Poland (PL-KRON86-NH). Extensive research activities are observed in the field of maintenance and modernization of gravity control not only in Poland, but also in Sweden and in Denmark, as well as establishment of gravity control in Ireland based on absolute gravity survey. The magnetic control in Poland was also regularly maintained. The bibliography of the related works is given in references.

The International Terrestrial Reference Frame (ITRF), the Earth Orientation Parameter (EOP) time series, and the International Celestial Reference Frame are obtained separately and may thus present inconsistencies. To solve this problem, a first step has been made with the latest ITRF realization (ITRF2005), which has been computed, for the first time, with consistent EOP time series. Another approach to better understand this issue is to directly estimate, in the same process, station positions and EOP time series, from all the space-geodetic measurements.

Terrestrial reference frame requirements within GGOS perspective

Geodesy plays a fundamental role in observing and understanding Earth system processes, yet its societal relevance often remains under-recognized outside the specialist community. To address this gap, the Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) has expanded its science communication activities during the recent years to make geodetic concepts, products, and techniques accessible to diverse audiences. This contribution presents an integrated communication strategy combining digital platforms, visual storytelling, and community-driven initiatives.A central element is the renewed GGOS and IAG web platform https://geodesy.science, which provides an easy understandable introduction to geodesy as well as clear, non-technical explanations of observation techniques, products, and real-world applications.Complementing this, a growing series of multilingual short films (https://www.youtube.com/@iag-ggos) communicates the importance of geodesy for monitoring climate change, natural hazards, sea-level rise, and global reference frames. These videos have reached broad international audiences and are frequently used in public outreach events such as open-day exhibitions.The newest initiative is the Geodesy Cartoons project https://geodesy.science/cartoon , which communicates complex geodetic topics through approachable, story-driven visual narratives. The associated Geodesy Cartoon Competition actively involves the international geodetic community in co-creating educational illustrations. This participatory approach fosters shared ownership, stimulates creativity, and supports the development of communication material usable across research, teaching, and outreach.Together, these multimedia tools illustrate how geodesy contributes to society’s daily life and decision-making. This presentation reflects on successes and challenges in designing accessible content, coordinating contributions across the global geodesy community, and evaluating engagement through online analytics and feedback. By sharing insights from these ongoing initiatives, we aim to contribute to a broader discussion on effective communication of Earth and space sciences and to strengthen connections between geodesy and the wider public.