The objective of the future Global Geodetic Observing System (GGOS) to realize the terrestrial reference system (TRS) with 1 mm accuracy and 0.1 mm/yr long-term stability remains challenging when the four space geodetic techniques – GNSS, SLR, VLBI, and DORIS – are processed independently in the traditional, technique-specific manner. Key challenges arise from the sparse and highly inhomogeneous global distribution of co-location sites used to tie the individual solutions together, as well as from the treatment of technique-specific calibration parameters, such as GNSS antenna phase center offsets and SLR range biases. In this presentation, we present work carried out by the ESA/ESOC Navigation Support Office on the joint processing of GNSS and SLR at the observation level. Our approach – fittingly referred to as COOL (“COmbination at the Observation Level”) – incorporates the primary geodetic ILRS targets LAGEOS-1, LAGEOS-2, LARES-2, Etalon-1, and Etalon-2, and makes use of space ties provided by the Sentinel and Galileo satellites to directly link the two geodetic techniques. Particular attention is given to the Galileo transmit antenna z-offsets and the numerous SLR range biases, which require careful treatment, as they are known to directly influence the scale of the reference frame solution. The primary motivation for this work is to ensure full readiness for the future ESA GENESIS mission, which aims to establish a highly accurate, next-generation International Terrestrial Reference Frame (ITRF) through the use of all four space geodetic techniques, including DORIS and VLBI, on a single platform and the exploitation of the space ties between them.

Differential Code Biases (DCBs) are systematic errors, or biases, between two Global Navigation Satellite System (GNSS) code observations at the same or different frequencies. Knowledge of DCBs is required for positioning of GNSS receivers, extracting ionosphere total electron content (TEC), and other applications. Proper knowledge of DCBs is crucial to many navigation applications but also non-navigation applications such as ionospheric analysis and time transfer. The multitude of new signals offered by modernized and new GNSS constellations, requires a comprehensive multi-GNSS bias product. And because of the multitude of GNSS signals it has become more practical to provide the DCBs, differential signal biases, as observable specific biases, OSBs.At the Navigation Support Office at ESA/ESOC we have for many years relied on the bias products (DCB and OSB) coming from the CODE analysis centre. But in the fast-developing Multi-GNSS landscape it became clear that we needed to have to capability to generate our own independent bias product. For our different GNSS projects we therefore have developed a process to generate our own OSB product with the ambition to have DCBs for all existing MGNSS signals, as long as the signals are tracked by the stations in the large IGS station network. And rather than monthly biases as typically provided and used within the IGS, we have moved to what we call a “Bias Reference Frame” (BREF). The absolute value is still determined by a zero-mean condition at a certain date. But from that point in time the biases are kept stable unless a clear jump in the satellite bias is, automatically, detected. The detectability is at the 0.25ns for well observed biases. The DCB version of the product is publicly available on our Navigation Office website. The OSB version is still under development and testing.In developing this product, we found some very interesting features of the biases when looking at the biases of the so called “interoperable” signals, which we will present and discuss. We demonstrate the importance of this product for the analysis of the Sentinel 6A data which tracks Galileo and an interesting mix of GPS signals which makes it hard to process the GPS data with the standard IGS products, in particular for PPP-AR.

Accurate, low-latency UT1–UTC estimates are essential for monitoring Earth’s highly variable rotation and for real-time applications ranging from GNSS to lunar and deep-space missions. One-hour VLBI Intensive sessions, typically conducted with two stations on long east–west baselines, have the primary goal of providing rapid UT1–UTC estimates. Due to the short session length and the limited network geometry, only a few geodetic parameters can be estimated, while all remaining Earth orientation parameters, as well as station and source coordinates, must be fixed to their a priori values. Previous simulation studies have shown that UT1-UTC sensitivity does not depend solely on the east-west extension and baseline length, but also on the orientation of the baseline, making, for example, equatorial baselines, despite their east-west geometry, suboptimal for determining UT1-UTC. Thus, renewed interest in establishing a European Intensive capability motivated the investigation of the potential for a regional VGOS Intensive network, including NYALE13N (Norway), RAEGSMAR (Portugal), and WETTZ13S (Germany). While earlier concepts for European Intensives did not mature operationally, the recently released improved error models for simulating the troposphere, which is the primary source of error in VLBI, provide a more realistic approximation of performance, as they reflect location- and time-dependent conditions. In this study, we simulate candidate Intensive configurations and quantify their potential for determining the highly variable parameter UT1-UTC through simulations.

For the second yearly update of the 2020 realization of the International Terrestrial Reference Frame ITRF2020, i.e., ITRF2020-u2024, the International DORIS Service (IDS) submitted to the International Earth Rotation and Reference Systems Service (IERS) a set of 209 weekly solution files including station coordinates and Earth Orientation Parameters, covering the time period from 2021.0 to 2025.0. Three of the five IDS Analysis Centers (ACs) contributing to the new IDS combined solution delivered new series. These new series have been derived using new models and processing strategies for almost all of the current DORIS missions: Cryosat-2, HY2C, HY2D, Jason-3, Saral, Sentinel-3A, Sentinel-3B, and Sentinel-6A. In this paper, we present both the IDS AC and the IDS 25 combined solutions released for the ITRF2020-u2024. Then, we evaluate these series in terms of scale, geocenter and station positions with respect to (w.r.t.) ITRF2020-u2023 and in terms of Earth Orientation Parameters (EOPs) w.r.t. the IERS C04 series. The evaluation shows that, over the four-year time span, the IDS 25 scale values lie in a range of ± 2.5 mm with a trend of -0.10 mm/yr . Thanks to an improved modeling of the surface forces on the satellites, especially on Sentinel-6A, compared to the first IDS contribution to the ITRF2020, the new IDS 25 combined solution depicts smaller annual and 118-day signals in the geocenter coordinate time series. The evaluation also showed that the IDS 25 has an internal position consistency in North-East-Up components better than 6 mm per component. The EOPs from the IDS 25 combination were compared with the IERS C04 time series and the RMS agreement between these two time series is about 200 μ as for the x- and y- pole coordinates.

Abstract The goal set by the Global Geodetic Observing System – realising the terrestrial reference system with an accuracy of 1 mm and a long-term stability of 0.1 mm/yr – appears difficult, if not impossible, to achieve with the current independent processing of the key contributing space techniques. Although combining the independent contributions from GNSS, SLR, DORIS and VLBI demonstrated its contribution to precise orbit determination improvement and global parameter estimation strengthening, systematic differences between the techniques are clearly visible in the scale and origin of the respective reference frame contributions. An additional challenge is the limited number of geodetic sites that provide co-location of the different techniques, as well as the highly inhomogeneous global distribution of such sites. One approach to improve the quality of the Terrestrial Reference Frame realisation is the simultaneous processing of the data of these four space geodetic techniques at the observation level. A second approach is to make use of co-location in space by placing multiple techniques on a single satellite. The Genesis mission, a component of the ESA’s FutureNAV program, aims to provide such highly accurate in-space co-location of the four techniques. We, the Navigation Support Office at the European Space Operations Centre of the European Space Agency, are pursuing both approaches; COmbination of the different techniques at the Observation Level (COOL), and the use of space ties offered by the Genesis mission. We are currently working with GNSS and SLR data and are using the Sentinel satellites as surrogates for Genesis, which is scheduled for launch in 2028. For instance, the Sentinel-6A satellite co-locates three of the four space geodetic techniques – VLBI is not available – and is therefore an ideal target to prepare our software and processing approach for both COOL and Genesis. Sentinel-6A offers the additional benefit of tracking not only GPS but also Galileo GNSS signals. Considering the subtleties on inter-technique combinations, our main aims are to gradually build up, test and validate our COOL solutions to ensure that the combined solution geodetic products outperform the single-technique solutions. In this paper, we present the status of our processing capabilities, some initial results, and conclude with an outlook of the work that remains to be done.

In anticipation of the first update of the 2020 realization of the International Terrestrial Reference Frame (ITRF2020), the International DORIS Service (IDS) Combination Center is participating in the estimation of DORIS stations positions/velocities as well as Earth Orientation Parameters (EOPs), using DORIS data. These computations are based on the latest weekly multi-satellite series from all four IDS Analysis Centers and two IDS Associated Analysis Centers, from January 2021 to December 2023. The primary objectives of this study are to analyze the DORIS contribution to this first update of the ITRF2020 in terms of: (1) geocenter and scale solutions, (2) station positions and week-to-week repeatability, and (3) Earth Orientation Parameters (EOPs). Comparisons with the IDS 19 series time extension (contributing to ITRF2020) will highlight the benefits of the new models, including the latest DORIS missions (e.g. HY-2C, HY-2D, Sentinel-6A MF), and the addition of two new IDS contributors. Additionally, this study will assess the impact of new strategies designed to mitigate perturbations caused by the South Atlantic Anomaly (SAA) on certain DORIS missions.

Predicting Earth Orientation Parameters (EOP) is crucial for precise positioning and navigation both on the Earth’s surface and in space. In recent years, many approaches have been developed to forecast EOP, incorporating observed EOP as well as information on the effective angular momentum (EAM) derived from numerical models of the atmosphere, oceans, and land-surface dynamics. The Second Earth Orientation Parameters Prediction Comparison Campaign (2nd EOP PCC) aimed to comprehensively evaluate EOP forecasts from many international participants and identify the most promising prediction methodologies. This paper presents the validation results of predictions for universal time and length-of-day variations submitted during the 2nd EOP PCC, providing an assessment of their accuracy and reliability. We conduct a detailed evaluation of all valid forecasts using the IERS 14 C04 solution provided by the International Earth Rotation and Reference Systems Service (IERS) as a reference and mean absolute error as the quality measure. Our analysis demonstrates that approaches based on machine learning or the combination of least squares and autoregression, with the use of EAM information as an additional input, provide the highest prediction accuracy for both investigated parameters. Utilizing precise EAM data and forecasts emerges as a pivotal factor in enhancing forecasting accuracy. Although several methods show some potential to outperform the IERS forecasts, the current standard predictions disseminated by IERS are highly reliable and can be fully recommended for operational purposes.

AbstractGrowing interest in Earth Orientation Parameters (EOP) resulted in various approaches to the EOP prediction algorithms, as well as in the exploitation of distinct input data, including the observed EOP values from various operational data centers and modeled effective angular momentum functions. Considering these developments and recently emerged new methodologies, the Second Earth Orientation Parameters Prediction Comparison Campaign (2nd EOP PCC) was pursued in 2021–2022. The campaign was led by Centrum Badań Kosmicznych Polskiej Akademii Nauk in cooperation with Deutsches GeoForschungsZentrum and under the auspices of the International Earth Rotation and Reference Systems Service. This paper provides the analysis and evaluation of the polar motion predictions submitted during the 2nd EOP PCC with the prediction horizons between 10 and 30 days. Our analysis shows that predictions are highly reliable with only a few occasional discrepancies identified in the submitted files. We demonstrate the accuracy of EOP predictions by (a) calculating the mean absolute error relative to polar motion observations from September 2021 through December 2022 and (b) assessing the stability of the predictions in time. The analysis shows unequal results for the x and y components of polar motion (PMx and PMy, respectively). Predictions of PMy are usually more accurate and have a smaller spread across all submitted files when compared to PMx. We present an analysis of similarity between the participants to indicate what methods and input data give comparable output. We also prepared the ranking of prediction methods for polar motion summarizing the achievements of the campaign.

The accurate knowledge of the Earth’s orientation and rotation in space is essential for a broad variety of scientific and societal applications. Among others, these include global positioning, near-Earth and deep-space navigation, the realisation of precise reference and time systems as well as studies of geodynamics and global change phenomena. In this paper, we present a refined strategy for processing and combining Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) observations at the normal equation level and formulate recommendations for a consistent processing of the space-geodetic input data. Based on the developed strategy, we determine final and rapid Earth rotation parameter (ERP) solutions with low latency that also serve as the basis for a subsequent prediction of ERPs involving effective angular momentum data. Realising final ERPs on an accuracy level comparable to the final ERP benchmark solutions IERS 14C04 and JPL COMB2018, our strategy allows to enhance the consistency between final, rapid and predicted ERPs in terms of RMS differences by up to 50% compared to existing solutions. The findings of the study thus support the ambitious goals of the Global Geodetic Observing System (GGOS) in providing highly accurate and consistent time series of geodetic parameters for science and applications.

For the realization of the 2020 International Terrestrial Reference Frame (ITRF2020), the International DORIS Service delivered to the International Earth Rotation and Reference Systems Service (IERS) a set of 1456 weekly solution files from 1993.0 to 2021.0 including station coordinates and Earth orientation parameters (EOPs). The data come from fourteen DORIS satellites: TOPEX/Poseidon, SPOT-2, SPOT-3, SPOT-4, SPOT-5, Envisat, Jason-1, Jason-2, Cryosat-2, Saral, HY-2A, Jason-3, Sentinel-3A and Sentinel-3B. In their processing, the four analysis centers which contributed to the DORIS combined solution used the latest time variable gravity models, the new mean pole and diurnal-subdiurnal tidal EOP models recommended by IERS. In addition, all the analysis centers included in their processing precise SPOT-5 solar panel angle values and quaternions for, at least, the Jason satellites. Furthermore, a new Alcatel phase center variation model was implemented for the ITRF2020 processing. The main objective of this study is to present the combination process and to analyze the impact of the new modeling on the performance of the new combined solution. Comparisons with the IDS contribution to ITRF2014 show that i) the application of the new phase center variations for the Alcatel DORIS ground antennas in the data processing combined with the gradual replacement over time of the Alcatel by Starec antennas implies a scale drift from 1993.0 to 2002.5 and ii) thanks to a better modeling of the surface forces on the satellites, the new combined solution shows smaller annual and 118-day signals in the geocenter. A new DORIS terrestrial reference frame was computed to evaluate the intrinsic quality of the new combined solution. That evaluation shows that over almost the full time span the intrinsic IDS scale values lie in a range of ±5 mm. After mid-2008, the new DORIS reference frame has an internal position consistency in North-East-Up better than 7.5 mm.

With global mean sea level rising because of climate change, Copernicus Sentinel-6 is the radar altimetry reference mission to extend the legacy of sea-surface height measurements until at least 2030. The satellite carries a Poseidon-4 radar altimeter and a microwave radiometer. The analysis of the altimeter data relies on highly-accurate knowledge of the orbital position, in particular in the radial component, with errors below 1.5 cm. For this reason, Sentinel-6 carries several instruments, e.g., Laser Retroreflector Array, Doppler Orbitography and Radiopositioning Integrated By Satellite (DORIS) and also a dedicated GNSS receiver for generation of data, which allows to perform Precise Orbit Determination (POD) with the highest possible accuracy. For this reason, the GNSS receiver and respective GNSS observations is of particular interest for this paper because it is the first time that a high-quality dual frequency Galileo/GPS receiver is flown on-board of a Sentinel satellite. ESA’s Navigation Support Office (NavSO), located at the European Space Operations Centre (ESOC) in Darmstadt, Germany is providing an independent Precise Orbit Determination solution for all Sentinel satellites. As part of this activity, the office has been operationally computing and delivering the POD products, including the Sentine-6 multiGNSS-based precise orbits since its launch in November 2020. This presentation will describe the processing techniques adopted at ESOC, with a particular focus on the POD aspects and the related processing of the Galileo and GPS observations and their interoperability. The latest results and their validation are addressed in this paper.

In the context of the realization of the next International Terrestrial Reference Frame (ITRF2020), the International DORIS Service (IDS) is involved in the estimation of DORIS station positions/velocities as well as Earth orientation parameters from DORIS data. Thus, the 4 IDS Analysis Centers have re-analyzed all the DORIS observations from the fifteen DORIS satellites from January 1993 to December 2020.0. The primary objective of this study is to analyze the DORIS contribution to ITRF2020 in terms of (1) geocenter and scale solutions; (2) station positions and week-to-week repeatability; (3) Earth orientation parameters; (4) a cumulative position and velocity solution. Comparisons with the IDS contribution to ITRF2014 will address the benefits of the new antenna models, new models, including improved methods to handle non-conservative force model error on the Jason satellites, as well as the addition of data (compared to ITRF2014) from the latest DORIS missions (e.g. Jason-3, Sentinel-3A/B) in the IDS combination.

In 2018 we celebrated 25 years of development of radar altimetry, and the progress achieved by this methodology in the fields of global and coastal oceanography, hydrology, geodesy and cryospheric sciences. Many symbolic major events have celebrated these developments, e.g., in Venice, Italy, the 15th (2006) and 20th (2012) years of progress and more recently, in 2018, in Ponta Delgada, Portugal, 25 Years of Progress in Radar Altimetry. On this latter occasion it was decided to collect contributions of scientists, engineers and managers involved in the worldwide altimetry community to depict the state of altimetry and propose recommendations for the altimetry of the future. This paper summarizes contributions and recommendations that were collected and provides guidance for future mission design, research activities, and sustainable operational radar altimetry data exploitation. Recommendations provided are fundamental for optimizing further scientific and operational advances of oceanographic observations by altimetry, including requirements for spatial and temporal resolution of altimetric measurements, their accuracy and continuity. There are also new challenges and new openings mentioned in the paper that are particularly crucial for observations at higher latitudes, for coastal oceanography, for cryospheric studies and for hydrology.Thepaperstarts with a general introduction followed by a section on Earth System Science including Ocean Dynamics, Sea Level, the Coastal Ocean, Hydrology, the Cryosphere and Polar Oceans and the ‘‘Green ” Ocean, extending the frontier from biogeochemistry to marine ecology. Applications are described in a subsequent section, which covers Operational Oceanography, Weather, Hurricane Wave and Wind Forecasting, Climate projection. Instruments’ development and satellite missions’ evolutions are described in a fourth section. A fifth section covers the key observations that altimeters provide and their potential complements, from other Earth observation measurements to in situ data. Section 6 identifies the data and methods and provides some accuracy and resolution requirements for the wet tropospheric correction, the orbit and other geodetic requirements, the Mean Sea Surface, Geoid and Mean Dynamic Topography, Calibration and Validation, data accuracy, data access and handling (including the DUACS system). Section 7 brings a transversal view on scales, integration, artificial intelligence, and capacity building (education and training). Section 8 reviews the programmatic issues followed by a conclusion. (cid:1) 2021 COSPAR. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/ by-nc-nd/4.0/).

The importance of an accurate global geodetic reference frame and associated Earth orientation parameters is undisputed and has been recognised by the UN resolution 69/266. Given the importance of global geodetic references, ESA is actively contributing to the IAG services: IGS, ILRS, IDS and the contribution to the IVS is in preparation. ESA’s activities can be divided into four main areas: the operation of Ground Infrastructure (ESTRACK, EGON, …), the establishment and improvement of inter-technique ties, the operation of a scientific data archive (GSSC) and the generation of geodetic products and services. This presentation will focus on the activities performed by the Navigation Support Office. The Navigation Support Office at ESA/ESOC is responsible for providing the Geodetic Reference Frame for all ESA missions and is also the Consortium Coordinator of the Galileo Geodetic Service Provider (GGSP) that generates the Galileo Geodetic Reference Frame (GTRF). Within its responsibilities, the Navigation Support Office is continuously working on improving the consistency of its geodetic products. The possibility to perform a Combination On the Observation Level (CoOL) for all geodetic observations is an excellent tool to identify inconsistencies. Over the recent years, significant improvements have been implemented in the data processing in order to enhance the consistency of the delivered products. As the status of the inter-technique ties remains a limiting factor in this context, ESA is currently investigating the possibility of using space ties, e.g. combining GNSS, SLR, DORIS and VLBI in space. This presentation will give an overview of the geodetic products and services generated by ESA’s Navigation Support Office and outline the associated processing setup. In particular, it will report on the analysis performed to improve the consistency of the results provided by the different observation techniques and outline the recent improvements and ongoing activities.

The availability of highly accurate Earth Orientation Parameters (EOPs) in near real time is of major importance for any type of positioning and navigation applications on Earth, Sea, Air and also in Space. This is equally true for all ESA missions and the EU space programs Galileo, EGNOS and Copernicus.To ensure operational capability, ESA’s Navigation Support Office developed independent EOP products and services.The EOPs are estimated based on a rigorous combination of the ESA’s contributions to the International Association of Geodesy (IAG) that are used as an input for the generation of the International Earth Rotation Service (IERS) products. For the ESA/ESOC EOP products, the individual parameters are combined on normal equation level and propagated with the contribution of model-based predicted Effective Angular Momentum (EAM) functions.The ESA/ESOC’s EOP product generation is currently running in pre-operational mode.This presentation will provide a high-level overview of the methodology and the status of ESA’s EOP products and services. In this context, the accuracy achieved in the test operations and the roadmap for the publication of ESA’s EOP products and services will be outlined.

The availability of highly accurate, up-to-date Earth Orientation Parameters is of major importance for all positioning and navigation applications on Earth, Sea, Air and also in Space. This is equally true for ESA missions and the EU space programs, e.g. Galileo, EGNOS and Copernicus. In the frame of its responsibility to provide the Geodetic reference for ESA missions, ESA’s Navigation Support Office at ESOC is already contributing to the realisation of the International Terrestrial Reference Frame (ITRF) and the combined Earth Orientation Parameters provided by the International Earth Rotation Service (IERS). The contribution is realised through individual contributions to international services such as the International GNSS Service (IGS), the International Laser Ranging Services (ILRS), the International DORIS Service (IDS), the International Earth Rotation Service (IERS) and in the future also to the International VLBI Service (IVS). For the combination and the long-term predictions of the Earth orientation products ESA is still relying on the International Earth Rotation Service (IERS). Over the past years, ESA repeatedly experienced problems with outdated or missing predictions of the Earth orientation parameters (Bulletin A). Considering the importance of up-to-date Earth orientation parameters, the dependence on a single source outside Europe is considered a risk for European industry, for ESA missions and for EU programmes. For this reason, ESA initiated in 2017 a study with the target to develop independent ESA Earth Orientation parameter products. This study, executed by a consortium led by the Deutsches Geodätisches Forschungsinstitut (DGFI-TUM), is expected to finish in the course of this year. In this presentation we will give an overview of ESAs up-to-date reference products and discuss their quality. It will outline the combination approach and discuss the way forward to an fully operational provision of the ESA Earth Orientation Parameter products.

For the previous ITRF calls for participation ESOC reprocessed the historic data from the IDS, IGS, and ILRS. Our three solutions were computed with a single software package (NAPEOS), running on the same machine and using, as far as possible, identical settings. Any systematic differences between the technique dependent reference frame solutions must therefore be caused by the techniques themselves, and not because of model differences or errors. Our three technique dependent solutions gave us a good understanding of the technique dependent effects, helping us to improve our models. At ESOC we have now made a significant step forward by including all satellite geodetic techniques (SLR, DORIS and GNSS) into one solution. This allows us to combine the ILRS, IDS and IGS reference frames by using “space ties”. Of course these space ties are not perfectly known but they still allow for a rigorous combination of the different reference frames. Furthermore, and very important for the GNSS technique, they allow for the direct estimation of the GNSS satellite transmitter phase centre offset. We solve not only for integer ambiguities of the GPS satellites but also for those of the LEO satellites, which is also providing GPS phase observations on two frequencies. Our poster presents an overview of this multi-technique combination approach at observation level (COOL). We have included all observations provided by the following satellites in a single parameter estimation process: GNSS, JASON, SPOT, Sentinels, GRACE, LAGEOS and Etalon satellites. We demonstrate the benefits of such a rigorous approach compared to processing the various space geodetic techniques separately.

ESA and NASA conducted a joint Galileo/GPS space receiver experiment on-board the International Space Station (ISS). The objectives (Enderle 2017) of the joint project were to demonstrate the robustness of a combined Galileo/GPS waveform uploaded to NASA hardware already operating in the challenging space environment - the SCaN (Space Communications and Navigation) software defined radio (SDR) testbed (FPGA) - on-board the ISS. These activities data included the analysis of the Galileo/GPS signal and on-board Position/Velocity/Time (PVT) performance, processing of the Galileo/GPS raw data (code- and carrier phase) for Precise Orbit Determination (POD), and validate the added value of a space-borne dual GNSS receiver compared to a single-system GNSS receiver operating under the same conditions. This paper will provide a general overview of the Galileo/GPS experiment – called GARISS - on-board the ISS, describe design, test and validation and also the operations of the experiment. Further, the various analysis conducted in the context of this joint project and also the results obtained will be presented with a focus on the (Precise) Orbit Determination results.