We welcome contributions that highlight new determinations and analyses of Earth Orientation Parameters (EOP), including combinations of different geodetic and astrometric observational techniques for deriving UT1/length-of-day variations and polar motion. We welcome discussions of EOP solutions in conjunction with a consistent determination of terrestrial and celestial frames. We are interested in the latest achievements in EOP forecasting, especially reports exploring the potential of innovative techniques, such as machine learning, in improving forecast accuracy.
We invite contributions on the dynamical links between Earth rotation, geophysical fluids, and other geodetic quantities, such as the Earth gravity field or surface deformation, and of explanations for the physical excitations of Earth rotation. Besides tidal influences from outside the Earth, the principal causes for variable EOP appear to be related to angular momentum exchange from motions and mass redistribution of the fluid portions of the planet.
We welcome contributions about the relationship between EOP variability and the variability in fluids due to climate effects or global change. Forecasts of these impacts are important especially for the operational determination of EOP, and the effort to improve predictions is an important topic.
We are interested in the progress in the theory of Earth rotation. We seek contributions that are consistent internally with the accurate observations at the mm-level, to meet the requirements of the Global Geodetic Observing System and respond to IAG 2019 Res. 5 and IAU 2021 Res. B2, as well as those derived from the research of current IAU/IAG/IERS working groups on these topics. We also welcome contributions on the variability and excitation of the rotation of other planetary bodies.
Orals: Mon, 4 May, 08:30–10:15 | Room -2.20
The El Niño–Southern Oscillation (ENSO) is the dominant mode of variability in the atmosphere—ocean system. It is characterised by circulation anomalies in the tropical Pacific that affects other regions of the planet through teleconnections. Studies over the last decades have demonstrated that ENSO exerts a strong control on zonal atmospheric angular momentum and consequently changes in length-of-day, but a comparable effect on polar motion remains to be quantified. Here, we test the hypothesis that part of the ENSO imprint on polar motion excitation is embedded in oceanic angular momentum (OAM) changes.
To this end, we analyse output from four CMIP6 (Coupled Model Intercomparison Project 6) climate models using lagged regression analysis, with a particular focus on monthly ocean bottom pressure (pb) changes over a 165-year period. The regression of the pb fields against each model’s ENSO index reveals prominent anomalies in the Bellingshausen Basin and a large-scale bipolar pattern between the Pacific and Indian oceans, which is also evident in satellite gravimetry data. The OAM changes implied by these pb anomalies excite polar motion primarily along 90°E, showing amplitudes of ±4 mas during recently observed El Niño/La Niña events (e.g., 1997/98, 2006/07, 2009/10). After removal of known geophysical fluid effects, the ENSO-related oceanic excitation accounts for ~40%–50% of the variance in observed polar motion excitation. However, as these fluctuations co-occur with other broadband (oceanic) excitation signals, polar motion observations may provide only limited insight into the variability of ENSO itself.
How to cite: Börger, L., Schindelegger, M., and Dobslaw, H.: ENSO-driven oceanic excitation of polar motion, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4777, https://doi.org/10.5194/egusphere-egu26-4777, 2026.
Polar motion (PM) is a fundamental geodetic observable reflecting the global mass redistribution within the Earth system. As a major component of Earth mass changes, the variation of Terrestrial Water Storage (TWS) plays a crucial role in exciting PM, particularly on seasonal to decadal timescales. With the intensification of global climate change in recent years, understanding the coupling between TWS variability and PM has become increasingly important, especially under the influence of large-scale climate fluctuations like El Niño-Southern Oscillation (ENSO). However, quantifying these processes via traditional excitation functions remains challenging due to highly non-linear feedbacks between hydrological signals and Earth rotation.
In this study, we employ a Gated Recurrent Unit (GRU) machine learning framework to perform ablation studies, integrating HAM derived from the LSDM model as physical constraints to isolate the hydrological contribution to PM variability. We investigate the variability of TWS excitation on PM from 2014 to 2023, a period encompassing the 2015-2016 extreme El Niño and the 2020-2023 triple-dip La Niña. Under neutral conditions, the inclusion of HAM significantly reduces the PM prediction Mean Absolute Error (MAE) by 27.7% in and 49.8% in , primarily by eliminating the recurrent bimodal error structure observed in the non-HAM solution. Spatiotemporal analysis revels that as boreal spring transits to summer, the distribution of errors coincide with coherent seasonal soil moisture depletion across mid-latitude Eurasia and North America (NA). This widespread mass deficit generates east-westward excitation vector consistent with the observed bias in the non-HAM solution, confirming that mid-latitude hydrological redistribution is the primary driver of seasonal PM excitation.
However, the contribution of HAM to PM excitatiin exhibits strong phase-dependence characteristic, which is pronounced during the ENSO developing phases and diminish significantly in the mature phases. During the peak of the 2015-2016 El Niño and the termination of the 2022 La Niña, the HAM-induced improvement in sharply degrades to negligible (2.27%) or even negative values (-17.46%). We identify a dipole cancellation mechanism responsible for this degradation. Extreme ENSO events induce opposing precipitation anomalies in NA and East Asia. The conflicting excitation vectors neutralize the dominant hydrological signal, causing the HAM vector to lose its directionality and decoupling the hydrological signal from the linear logic of the prediction model. Our findings reveal that strong climate disturbances can disrupt conventional hydrological excitation patterns through spatial dipole cancellations. Our study not only quantifies the variable impact of TWS on Earth rotation but also highlights the necessity of considering non-linear climate-hydrology interactions in high-precision geodetic modeling.
How to cite: Tang, Y., Zhang, K., Li, X., Fu, Y., and Yuan, Y.: Variability of Hydrological Excitation on Polar Motion Under Extreme ENSO Climate Conditions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12356, https://doi.org/10.5194/egusphere-egu26-12356, 2026.
Main nonsecular polar motion can be constructed from the current information of its excitation. Since early stages of Earth rotation study, several different sources were suggested as the cause of polar motion. Annual wobble was rather early suspected and confirmed to be driven by Earth's fluid sphere excitation. Chandler wobble was also suspected to be mainly excited by atmosphere and ocean, however, the clear proof was not done until recently. Using a key relation in the frequency domain, polar motion (annual wobble and Chandler wobble) can be directly composed from known information of atmospheric/oceanic/hydrologicexcitations. Similarly, the excited Chandler wobble due to the largest earthquakes (M>8) of the last few decades is found over 30 cm in its maximum diameter. Variation of Chandler wobble amplitude can be approximately explained.
How to cite: Yi, Y. and Na, S.-H.: Direct Confirmation of Chandler Wobble Source, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-15661, https://doi.org/10.5194/egusphere-egu26-15661, 2026.
Earth Orientation Parameters (EOP) are used for a broad variety of studies in science as well as in daily live applications, e.g. satellite positioning and navigation. The services of the IAG are generating EOP products in an operational mode based on the space-geodetic techniques GNSS, VLBI, SLR and DORIS. Apart from the EOP products derived from one single technique, the International Earth Rotation and Reference Systems Service (IERS) is responsible for generating combined EOPs. Under the auspice of the IERS, there are combined EOPs with short latency (“rapid”) generated at US Naval Observatory, as well as “final” combined EOP series with a latency of about one month generated at Observatoire de Paris.
We will review the EOP products generated operationally by the IERS and the technique-specific services regarding their availability, latency, completeness, and methods applied during the generation process. Additionally, we will investigate the quality of the existing EOP series and the consistency by conducting cross-comparisons.
A special focus will be put on the VLBI-based EOP products as VLBI is the only space-geodetic technique that can provide also UT1-UTC and corrections to the nutation model. However, not all available VLBI-only EOP products are entering the combined IERS EOP series yet.
As a summary of our investigations, we will identify areas of potential improvements for increasing the availability, quality, consistency and reliability of the EOP products to satisfy the users’ needs.
How to cite: Thaller, D., Klemm, L., Raut, S., Stamatakos, N., Byram, S., and Bizouard, C.: Review of operational series of Earth Orientation Parameters: Availability, consistency, quality, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9382, https://doi.org/10.5194/egusphere-egu26-9382, 2026.
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.
How to cite: Kern, L., Böhm, S., Böhm, J., Bruni, S., Otten, M., and Schönemann, E.: Revisiting European VLBI Intensives for Rapid UT1-UTC Determination, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6565, https://doi.org/10.5194/egusphere-egu26-6565, 2026.
Ring lasers are now resolving the rate of rotation of the Earth with 8 significant digits. Technically they constitute a Sagnac interferometer, where a traveling wave resonator, circumscribing an arbitrary contour, defines the optical frequency of two counter-propagating resonant laser beams. Subtle non-reciprocal effects on the laser beam however, cause a variable bias, which reduces the long-term stability. Over the last two years, we have improved the performance of the G ring laser to the point, that we obtain long-term stable conditions over more than a year. Advances in the modeling of the non-linear behavior of the laser excitation process as well as some small but significant improvements in the operation of the laser gyroscope are taking us now right to the point, where the periodic part of the Length of Day variation of the Earth rotation can be recovered. Furthermore, we also extract the precession and nutation motion of the earth itself from the data as well. This corresponds to a rotation signal of 50 seconds of arc per year. It is the first time that this has been achieved by an inertial sensing technique. A laser gyroscope is a local sensor, but we extract a global quantity from it. How accurate are these measurements and where are the persisting error sources? This talk outlines the current state of the art of inertial rotation sensing in the geosciences and its remaining challenges. Furthermore, we discuss promising ways for a further enhanced sensor stability.
How to cite: Schreiber, K. U., Kodet, J., Hugentobler, U., and Klügel, T.: How accurate is Inertial Earth Rotation Sensing utilizing Large Ring Lasers , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6692, https://doi.org/10.5194/egusphere-egu26-6692, 2026.
In 2006, the IAU adopted a standard precession theory, the IAU2006 model, in which the time variation of the Earth's dynamical flattening J2 has an important contribution to the precession rate in longitude. However, a linear J2 trend, which was valid at that time, is no longer a good approximation and may limit the accuracy of the theory. In this work, we use the most recent satellite laser ranging (SLR) data to model the Earth's J2 long-term variation with a parabola. It was then implemented in calculating the polynomial expressions for precession quantities with a method similar to the IAU2006 approach.
The new precession solution, named IAU2006J2, is checked against high accurate VLBI data over 45 years. It is clearly more consistent with observations: the overall difference between the observed and modeled positions decreases by about 20%, and most of the curvature signals in the CPO series are reduced. Besides the basic precession parameters, the full set of precession-nutation quantities (X, Y, s, EO ...) compatible with the IAU2006J2 model are developed, for both classical and CIO-based transformation from the GCRS to the ITRS. Considering its significant improvement, we propose that the IAU2006J2 precession model be considered in the update of the IERS Conventions which is currently in progress.
How to cite: Liu, J.-C., Huang, C.-L., and Yao, J.: The IAU 2006 precession quantities with an improved Earth’s J2 long-term variation, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-4314, https://doi.org/10.5194/egusphere-egu26-4314, 2026.
The precession-nutation (PN) angles give the location of the so-called celestial intermediate pole, an axis defined so that it has no short period harmonic components in the space frame. Their variations are the largest among the Earth orientation parameters and can be quite well approximated by the conventional PN models IAU2006 and IAU2000, respectively, since the magnitude of the deviations or Celestial pole offsets (CPO) has a WRMS ranging typically around 200-300 micro arcseconds.
Regarding precession, it has become clear from the work reported by different research groups that revising the rates and offsets of the observed CPO is an urgent need to reduce the WRMS of all kinds of CPO time series. Therefore, it must be one of the issues considered in the ongoing update of the IERS Conventions Chapter 5. In practice, this correction can be implemented independently of revising a part of the theory, namely the values of some second-order components of IAU2006 which affects the estimated ellipticity Hd and thus indirectly the nutations amplitudes of IAU2000 to a non-negligible extent. As the linear model for the J₂ variation adopted in the development of IAU2006 is no longer valid, the challenge of updating the precession theory to a more realistic model arose, which would require modifying coefficients beyond the linear ones. Liu and Huang (2025) have published such an update of IAU2006 model named as IAU20006_J2, which allows a larger WRMS reduction and helps to reduce the observed upwards curvature of dX in recent years according to the assessment performed so far.
Regarding nutations, there is strong evidence in favour of no longer neglecting the non-rigid contributions that have been ignored so far in the planetary ones. The simplest way to implement this is to use the available corrections arising from an analytical solution, which can be enhanced with around five empirical corrections to increase the WRMS reduction. However, replacing the whole block of rigid planetary nutations with non-rigid ones would is also an option not more difficult to implement but offering better consistency.
As for the lunisolar nutations, it has been shown that the direct fit of corrections to the amplitudes of a few periods allows reducing the WRMS of VLBI solutions in a significantly larger amount than other approaches such as indirect fits of selected basic earth parameters, which suffers from the incomplete derivation of certain higher-order theoretical and geophysical corrections.
Based on the tests performed so far, applying all the previous corrections to the current PN models would enable the definition of modified CPOs, with a noticeable lesser WRMS across the entire VLBI determined series. Finally, using convenient free core nutation (FCN) models would largely reduce their yet unexplained or unmodelled variability.
Acknowledgments.- This work has been partially supported by the Spanish projects PID2020-119383GB-I00 funded by Ministerio de Ciencia e Innovación and SEJIGENT/2021/001 funded by Generalitat Valenciana
How to cite: Ferrándiz, J. M., Escapa, A., Karbon, M., and Belda, S.: On the corrections to precession and nutation models that can be implemented to reduce their uncertainties at the short term, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-19341, https://doi.org/10.5194/egusphere-egu26-19341, 2026.
In response to the requirements for GGOS Earth, an international effort is currently underway to revise Earth rotation theory and models involvend in EOP determination to achieve the necessary mas accuracy level. This work, conducted by Joint Working Group IAG 3.1, is divided into a Core of Theory component and an Observations component. While the theoretical team is responsible for developing an extended Earth rotation model through the recomputation of the precession-nutation solution and the refinement of nutation and polar motion separation for enhanced Free Core Nutation (FCN) modeling, this presentation details the outcomes of the observational validation phase. The analysis involves a systematic fit of these emerging models to VLBI data, supported by independent validation using SLR and GNSS orbit determination. This multi-technique approach allows for the quantification of residual signals, the verification of theoretical consistency, and the confirmation that new formulations are correctly implemented within operational analysis software. These validation results serve as a critical contribution to the evaluation and eventual adoption of the revised models for the upcoming IERS Conventions update, guaranteeing that future geodetic standards remain both theoretically robust and empirically sound. We acknowledge the essential contributions of all collaborators involved in this progress.
How to cite: Karbon, M. and Iag-Iau, J.: Observational Validation of Enhanced Earth Rotation Models for the Next IERS Conventions Update, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-18206, https://doi.org/10.5194/egusphere-egu26-18206, 2026.
The conservation of Earth’s total angular momentum provides a fundamental constraint linking geodetic observations of Earth rotation to geophysical excitation processes. The closure of effective angular momentum (EAM) budgets remains incomplete, particularly across temporal scales, due to remaining model and data uncertainties that complicate the consistent capture of mass- and motion-related contributions from geophysical forcing. In this study, we investigate the closure of EAM budgets by systematically combining atmospheric (AAM), oceanic (OAM), and hydrological (HAM) angular momentum estimates and comparing them to geodetic angular momentum (GAM) derived from the latest EOP 20 C04 time series of the IERS.
Our analyses focus on the GRACE/-FO era (2002–2022) using daily temporal sampling, with extensions to earlier epochs (back to 1970) for selected datasets. GAM estimates are computed from polar motion and length-of-day observations using Chandler wobble deconvolution and tidal corrections. Available to us are EAM from various atmospheric reanalyses, land surface models and monthly-mean GRACE/-FO gravity fields. A particular focus is placed on a small ensemble of OAM estimates from (i) MPIOM under different atmospheric forcings, (ii) DEBOT (a simple single-layer model) forced with MERRA-2, and (iii) several ECCO (a data-constrained ocean state estimate) realizations. All EAM data are processed through detrending, offset removal, and explicit separation of seasonal, interannual, and higher-frequency variability using harmonic fitting and Butterworth filtering.
Frequency spectra analyses reveal substantial discrepancies among the OAM estimates, particularly at sub-monthly and interannual time scales, suggesting that uncertainties in oceanic angular momentum and its underlying models represent a major limitation for the EAM budget closure. Our ongoing work aims to quantify the impact of different excitation combinations, temporal scales, and mass versus motion terms on residual budget misclosure.
How to cite: Stumpe, L., Börger, L., Schindelegger, M., Dill, R., and Dobslaw, H.: Effective Angular Momentum Budget Misclosure Across Time Scales: Highlighting the Role of Oceanic Uncertainties, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6693, https://doi.org/10.5194/egusphere-egu26-6693, 2026.
With gradual stepwise reduction in rigidity of the Earth model, the three fluid Love numbers of the Earth were determined as; (hf, kf , lf ) = (1.935, 0.935, 1.07) for PREM and (hf, kf , lf ) = (1.937, 0.937, 1.07) for ak135-F Earth models. Also analytical evaluation of the Earth’s secular Love number as well as fluid Love number, originally defined by Munk and MacDonald, were made by using updated Earth’s physical property and minor enhancement in the formulation. The permanent tide of the Earth was calculated as follows: the vertical displacement is 0.1924m at the equator and -0.3801m at the pole, while the horizontal displacement is 0.317m at the mid latitude.
How to cite: Na, S.-H. and Yi, Y.: Asymptotic Determination of Fluid Love Numbers (hf , kf , lf ) of the Earth and Advance in Their Analytical Determination, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8799, https://doi.org/10.5194/egusphere-egu26-8799, 2026.
Accurate and low-latency Earth Orientation Parameters (EOP) are essential for precise transformations between terrestrial and celestial reference frames, supporting satellite navigation, space missions, and geodetic and astronomical applications. Since official IERS EOP products are available with inherent delays of hours to days, robust short-term EOP prediction remains a critical operational requirement.
This contribution presents recent operational and research developments at the Federal Agency for Cartography and Geodesy (BKG) in cooperation with the University of Alicante (UA), focusing on machine-learning (ML) and deep-learning (DL) approaches for EOP prediction that exploit effective angular momentum (EAM) forecasts from GFZ as physically motivated input parameters. The prediction framework is driven by a comprehensive set of technique-specific (VLBI, GNSS, SLR) and multi-technique combined EOP products generated at BKG, complemented by the official IERS EOP reference series for training, validation, and benchmarking. The approach builds on BKG’s established hybrid prediction system, in which deterministic signals are modeled using Singular spectrum analysis and least squares, while stochastic variability is traditionally captured via autoregressive and Copula-based analysis models. In the proposed framework, ML/DL architectures, such as multi-task networks for polar motion and dUT1 prediction and convolutional models for short-term LOD forecasting are employed to replace or augment the stochastic component, without imposing explicit physical constraints within the learning process. Results demonstrate that combining EAM-based predictors with BKG’s technique-specific and multi-technique EOP products leads to systematic improvements in short-term (1–10 day) prediction accuracy compared to purely data-driven baselines. The EAM-based ML/DL framework has been under operational testing at BKG since early 2025 and represents a significant step toward an operational ML-supported EOP prediction service, with ongoing work addressing full EOP integration and impact assessment on VLBI analysis and satellite orbit determination.
How to cite: Modiri, S., Thaller, D., Belda, S., Kehm, A., Klemm, L., König, D., Bachmann, S., Raut, S., and Flohrer, C.: Advances in Operational and Research Earth Orientation Parameters Prediction at BKG: Hybrid and Physics-Informed Approaches, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12551, https://doi.org/10.5194/egusphere-egu26-12551, 2026.
We present updated estimates of Basic Earth Parameters (BEP) from VLBI Celestial Pole Offset (CPO) time series spanning 1980-2025 using ensemble Markov Chain Monte-Carlo Bayesian inversion. Building upon Koot et al. (2008), we employ enhanced sampling algorithms and incorporate recent advances in ocean tidal modeling (Cheng and Bizouard, 2025). Key improvements include: (1) implementation of piece-wise cubic spline modeling for Free Core Nutation (FCN) amplitude variations, which significantly reduces multimodality in MCMC sampling compared to linear modeling; (2) integration of updated Ocean Tidal Angular Momentum (OTAM) values from FES 2014 ocean tidal atlas (Lyard et al., 2021) without the empirical 0.7 scaling factor previously applied; and (3) utilization of five diverse CPO series from different analysis centers spanning up to 45 years of observations.
Our results show good consistency across different CPO series, with estimated dynamical ellipticity values at the edge of the 1σ range of MHB 2000. Notable findings include a larger absolute value for the imaginary part of the core-mantle boundary coupling constant (Im(KCMB)), approaching the 2σ boundary of Mathews et al. (2002), which may reflect contributions from multiple coupling mechanisms, including topographic coupling through the "form drag" effect caused by wave interactions with irregular boundaries (Rekier et al., 2025). The real part of the inner core boundary coupling constant (Re(KICB)) is approximately half the MHB 2000 value, potentially indicating the need to revisit hydrostatic assumptions for the inner core given recent seismic evidence of viscous deformation. Compliance estimates suggest that frequency extrapolation methods from seismic to nutation bands require revision. The enhanced FCN free mode modeling successfully captures amplitude variations that differ from empirical models, particularly after 2000, though the physical interpretation of these differences requires further investigation.
The systematic discrepancies across multiple parameters suggest that the current nutation theory needs substantial updates to incorporate more realistic models of core-mantle coupling and inner core behavior.
How to cite: Cheng, Y., Dehant, V., Rivoldini, A., Rekier, J., and Bizouard, C.: Basic Earth Parameters from VLBI observations using Bayesian inversions in the time domain: updated insights of the Earth's interior, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-12620, https://doi.org/10.5194/egusphere-egu26-12620, 2026.
ERP (Earth Rotation Parameters) are essential parameters for the transformation between the terrestrial reference frame and the celestial reference frame, playing a crucial role in fields such as timekeeping, satellite orbit determination, and deep space exploration. Numerous scholars have focused on achieving high-precision ERP prediction. Currently, the primary method for evaluating ERP prediction accuracy is to use the C04 series published by the IERS as a reference and measure performance by comparing Mean Absolute Error (MAE). This approach is overly simplistic and heavily dependent on the C04 series. In this study, ERP prediction data from Bulletin A, finals.daily, and three participants from the 2nd EOP PCC (Earth Orientation Parameters Prediction Comparison Campaign) are evaluated based on satellite orbit determination accuracy. The evaluation results indicate that: 1. The PMX (Polar Motion X) term in Bulletin A files exhibits significant deviations over time, which is the primary factor affecting orbital accuracy. 2. The predicted values in finals.daily files meet the requirements for satellite orbit determination. 3. For the first-day prediction accuracy, the ranking is ID 136 > ID 101 > ID 117, while for the seventh-day prediction accuracy, the ranking shifts to ID 101 > ID 136 > ID 117.
How to cite: Nan, K., Ferrandiz, J., Palazon, S., Karbon, M., and Yang, X.: Accuracy Evaluation of ERP Prediction Based on Satellite Orbit Determination, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-13830, https://doi.org/10.5194/egusphere-egu26-13830, 2026.
Ocean angular momentum (OAM) is a key geophysical quantity linking ocean dynamics with Earth rotation, as it constitutes a major excitation mechanism of polar motion and length-of-day variations. OAM variability reflects changes in both the mass distribution of the ocean and the velocity field of ocean currents. Although several operational products provide OAM estimates, most of them rely heavily on numerical ocean circulation models, which may introduce model-dependent uncertainties.
Geostrophic currents (GC), which arise from the balance between the Coriolis force and the horizontal pressure gradient, dominate large-scale ocean circulation and therefore play a central role in OAM variability. Recent advances in satellite geodesy now enable the estimation of GC from observations that are largely independent of dynamical ocean models. In particular, GC can be derived by combining Sea Surface Height from satellite altimetry, an independent geoid obtained from satellite gravity missions, and temperature and salinity profiles, allowing the reconstruction of global geostrophic velocity fields at different depths.
In this study, these satellite-based GC fields, with a spatial resolution of 0.25° × 0.25°, are used to compute OAM and its temporal variability. The resulting OAM series are compared with several existing OAM products commonly used in Earth rotation studies. The proposed approach is expected to provide more robust and geophysically consistent OAM estimates, since the satellite-derived GC show improved agreement with in situ current observations compared to model-based products. This work therefore strengthens the connection between satellite gravimetry, ocean dynamics, and Earth rotation research.
Acknowledgements: This work was primarily supported by the Spanish national project PID2021-122142OB-I00 (MCIN/AEI/10.13039/501100011033), and additionally by the EU Horizon Europe project SEA4FUTURE (Grant Agreement No. 101212647).
How to cite: Vigo, M. I., Vargas-Alemañy, J. A., García-García, D., Ferrándiz, J. M., and Wang, M.: Satellite-Based Geostrophic Currents for Improved Ocean Angular Momentum Estimates , EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23230, https://doi.org/10.5194/egusphere-egu26-23230, 2026.
Variations in Earth’s rotation result from a range of geophysical processes, including gravitational forcing by celestial bodies and mass redistribution within the atmosphere, oceans, hydrosphere, and cryosphere. The contributions of these processes to variability in the planet’s rotational motion are commonly quantified using four components of effective angular momentum: atmospheric (AAM), oceanic (OAM), hydrological (HAM), and cryospheric (CAM).
Hydrological angular momentum (HAM) describes the excitation of polar motion (PM) and length-of-day (LOD) variations caused by mass redistribution within the continental hydrosphere and can be estimated from global hydrological models, satellite-derived gravity field solutions, or climate model outputs. In this study, we reassess the mass-related excitation of PM by deriving the equatorial components (χ₁ and χ₂) of HAM from temporal variations in the C21 and S21 geopotential coefficients obtained from a new class of hybrid gravity field solutions. These solutions replace the conventional spherical harmonic representation with empirical orthogonal functions (EOFs) derived from Gravity Recovery and Climate Experiment (GRACE) data and fitted to Satellite Laser Ranging (SLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) observations, enabling the construction of an extended time series spanning from 1984 to the present.
The resulting HAM time series are compared with estimates from global hydrological models and validated against the hydrological signal in geodetic angular momentum (GAO). The consistency between HAM and GAO is evaluated across multiple frequency bands, with particular emphasis on variability at periods longer than three years. In this low-frequency range, correlations between HAM derived from the hybrid solutions and GAO reach values of up to 0.9, indicating that hydrological signals inferred from temporal variations of the Earth’s gravity field account for a substantial fraction of the observed long-term PM excitation..
Typically, the agreement between GAO and HAM time series is analysed by comparing the χ₁ and χ₂ components separately. Here, we perform the analysis along the direction of maximum correlation, providing a more robust and physically meaningful assessment of the agreement between HAM and GAO.
These findings highlight the importance of gravimetry-based HAM for interpreting PM variability across multiple time scales and extend earlier GRACE- and model-based investigations of hydrological PM excitation. In addition, this study provides the first long-term HAM estimates derived from hybrid SLR+DORIS gravity solutions spanning the period from 1984 to the present.
How to cite: Nastula, J., Śliwińska-Bronowicz, J., Wińska, M., and Partyka, A.: Long-period hydrological polar motion excitation based on C21 and S21 coefficients from hybrid SLR+DORIS gravity solutions, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-9738, https://doi.org/10.5194/egusphere-egu26-9738, 2026.
The main part of the precession/nutation of the Earth is due to its gravitational interaction with the Moon and the Sun. Such interaction can be characterized by the Earth geopotential that depends on the orbital ephemerides of the Moon and the Sun. In turn, the ephemerides can be expressed in terms of the so-called fundamental arguments that comprise: the mean anomaly of the Moon (l); the mean anomaly of the Sun (l’); the mean argument of latitude of the Moon (F); the mean elongation of the Moon from the Sun (D); and the mean longitude of the Moon’s mean ascending node (Ω) —to shorten, Delaunay arguments.
Common theoretical developments, for example, those based on the Hamiltonian formalism (e.g., Kinoshita 1977 or Escapa et al. 2017), or practical evaluation of the nutation series (e.g., IERS Conventions 2010, sec 5.7.1) assume that Delaunay arguments can be approximated as linear in time. However, strictly speaking this is not the case (e.g., Simon et al. 1994), the arguments being polynomials in time of fourth degree.
In view of the current demands on Earth rotation determination (about 1mm on the Earth surface); the guidelines of IAG 2019 and IAU 2021 resolutions; and the terms of reference of the IAU / IAG Joint Working Group on Consistent Improvement of the Earth rotation Theory (CIERT), it is necessary to assess the accuracy of such approximation both because the mandatory consistent development of the models, and also because its potential numerical relevance.
In this communication, within the Hamiltonian framework, we will derive the contributions to the nutations due to the non-linear time evolution of Delaunay arguments, comparing them with the common linear case. We will also discuss the practical implications from the point of view of the standards and the operational use of the nutation series.
Acknowledgments.- This work has been partially supported by the Spanish projects PID2020-119383GB-I00 funded by Ministerio de Ciencia e Innovación and SEJIGENT/2021/001 funded by Generalitat Valenciana.
How to cite: Escapa, A., Baenas, T., Karbon, M., Belda, S., and Ferrándiz, J. M.: Contribution of the non-linear time evolution of Delaunay arguments to the Earth nutation series, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14096, https://doi.org/10.5194/egusphere-egu26-14096, 2026.
Observations from space geodesy provide the fundamental basis for determining the Earth’s rotation and orientation in space, which are essential for both geophysical interpretation and a wide range of operational applications. Variations in Earth rotation, commonly described through Earth Orientation Parameters (EOPs), reflect complex interactions between the solid Earth, oceans, atmosphere, and core. These parameters are therefore central to studies of Earth system dynamics as well as to precise positioning, navigation, and satellite orbit determination. Within the framework of the Global Geodetic Observing System (GGOS), EOPs are recognized as key Essential Geodetic Variables (EGVs), with stringent requirements on accuracy, temporal resolution, and, in particular, latency. Meeting these requirements necessitates robust and reliable EOP prediction capabilities over short- and medium-term time scales.
The Space Geodesy Group at the University of Alicante, in collaboration with the Geodesy Group of the Spanish National Geographic Institute (IGN), has long-standing experience in Earth rotation theory, EOP modeling, and prediction. This expertise has been consolidated through active participation in the Second Earth Orientation Parameters Prediction Comparison Campaign (2nd EOP PCC), carried out from September 2021 to December 2022. Building on these developments, the two institutions are progressing toward the establishment of the first Spanish–Portuguese Geodetic Prediction Center, with a primary focus on the operational forecasting of EOPs. Beyond EOP prediction, the center is envisioned as a platform for future expansion toward closely related Earth rotation and geodetic products, including Earth angular momentum functions, station coordinate time series, and atmospheric, oceanic, and ionospheric parameters relevant to Earth rotation studies.
How to cite: Belda, S., Karbon, M., Del Nido, L. D., Modiri, S., Azcue, E., Rodríguez, J. C., Escapa, A., and Ferrándiz, J. M.: Progress Toward the Creation of a New Geodetic Prediction Center for Earth Rotation Products, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-14593, https://doi.org/10.5194/egusphere-egu26-14593, 2026.
Length of Day (LOD) describes variations in the duration of a single Earth rotation relative to the standard 24 hours. It is an important Earth Orientation Parameter (EOP) linking the International Celestial Reference Frame (ICRF) and the International Terrestrial Reference Frame (ITRF). For space geodetic satellite techniques (GNSS, SLR, and DORIS), the estimation of LOD is highly correlated with the precession of the satellite orbital ascending node, which is largely driven by the even low-degree spherical harmonic coefficients of the Earth gravity field (i.e. Earth flattening) and is also sensitive to orbit modeling deficiencies, such as out-of-plane empirical accelerations or solar radiation pressure (SRP).
In the case of SLR and DORIS, LOD estimation benefits from combining observations from multiple satellites with clear different orbital inclinations. Due to the different inclinations of the various satellites, SLR- and DORIS-derived LOD estimates are less correlated with other parameters which results in less biased LOD values. For the GNSS technique, GPS, Galileo and BeiDou constellations share the same orbital inclination of about 55 degrees, while GLONASS and QZSS employs an orbital inclination of 65 and 43 degrees, respectively. Given this small varying range of orbital inclination, modelling deficiencies lead to biased GNSS-based LOD estimates. Up to now, this was not handled, or a long-term constant (constellation-independent) bias was determined and applied at NEQ level.
In this presentation, we evaluate various LOD solutions computed from different satellites (including different satellite blocks, orbital planes, and constellations) and different SRP models.
How to cite: Bloßfeld, M., Seitz, F., Duan, B., Hugentobler, U., and Klug, J.: Satellite-/block-/plane- and constellation-specific GNSS LOD biases, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-23270, https://doi.org/10.5194/egusphere-egu26-23270, 2026.
