The technical article "Ship-based GNSS Contribution to Tsunami Warning in Europe and the Mediterranean" by James Foster and Bruce Enki Oscar Thomas, published in zfv – Journal of Geodesy, Geoinformation and Land Management, has been awarded the Geodesy Prize of the Nico Rüpke Foundation.

The Chairman of the Nico Rüpke Foundation, Prof. Erich Kanngieser, and the lead editor of zfv, Prof. Jürgen Müller, presented the award to the article’s authors, James Foster and Bruce Enki Oscar Thomas.

B. Raineri(1), A. Reinhold(1) , S.Glaser(1)

(1) University of Bonn, Faculty of Agricultural, Nutritional and Engineering Sciences, Institute of Geodesy and Geoinformation, Space Geodesy Group, Bonn, Germany

Für eine Vielzahl an ingenieur- und geowissenschaftlichen Anwendungen, beispielsweise im Bereich der Positionierung und Navigation, ist eine hochpräzise Realisierung eines Referenzsystems von entscheidender Bedeutung. Insbesondere die Überwachung und Erforschung der dynamischen Prozesse im System Erde wie Verschiebungen durch Plattentektonik, Eismassenveränderungen, oder der Anstieg des Meeresspiegels benötigen eine hochpräzise Referenz, um genau und zuverlässig quantifiziert werden zu können.

Aktuell existieren drei verschiedene Realisierungen des Internationalen Terrestrischen Referenzsystems, die alle auf denselben Eingabedaten basieren, sich jedoch in Bezug auf das Stationsnetzwerk, die Kombinationsstrategien und die Gültigkeitsintervalle unterscheiden. Der Internationale Terrestrische Referenzrahmen (ITRF), welcher am IGN (Institut national de l'information géographique et forestière) in Paris gerechnet wird, stellt die offizielle Lösung dar. Weitere Realisierungen werden am Deutschen Geodätischen Forschungsinstitut (DGFI) der Technischen Universität München (TUM) und am Jet Propulsion Laboratory (JPL) der NASA (National Aeronautics and Space Administration) erstellt.  

In dieser Arbeit werden die drei Realisierungen ITRF2020, DTRF2020 und JTRF2020 miteinander verglichen. Das übergeordnete Ziel besteht darin, besser bewerten zu können, welche der drei Realisierungen sich für welche geowissenschaftliche Anwendungen am besten eignet. Für die Langzeitbeobachtung des Meeresspiegels ist beispielsweise eine hohe Langzeitstabilität des Referenzrahmens mit geringen Diskontinuitäten entscheidend. Kurzzeitige tektonische Bewegungen hingegen erfordern eine hohe zeitliche Auflösung und eine dichte Stationsabdeckung. 

Dazu werden auf Lösungsebene Unterschiede in den Kombinationsstrategien sowie im Stationsnetz untersucht. Differenzen in den Stationskoordinaten und -geschwindigkeiten zwischen den Realisierungen können Aufschluss über Unterschiede in den Kombinationsstrategien sowie in der Auswahl und Stabilität des Stationsnetzes geben. Für die Differenzbildung werden nur Stationen berücksichtigt, die in beiden Realisierungen enthalten sind und deren Gültigkeitsintervalle sich überlappen. Der Vergleich erfolgt dabei zu einer gemeinsamen Referenzepoche. Darüber hinaus werden signifikante Geschwindigkeitsunterschiede an ausgewählten Kollokationsstationen (> 0,1 mm/Jahr) innerhalb jedes Referenzrahmens analysiert. Zusätzlich können die Gesamtanzahl der Stationen, die Beobachtungszeiträume sowie die Häufigkeit und die möglichen Ursachen von Diskontinuitäten mit den Koordinatendifferenzen in Verbindung gesetzt werden.

The increasing demands on the accuracy of the International Terrestrial Reference Frame (ITRF), which have been set by the Global Geodetic Observing System at one millimeter and 0.1 mm/year stability, require the consideration of previously neglected influencing factors. These include non-tidal loading caused by atmospheric, oceanic, and hydrological mass redistributions, which can deform the Earth's crust by several millimeters to centimeters.

This work analyzes the impact of such deformations on GNSS time series using models from three different institutions. The evaluation is based on GNSS time series from the Nevada Geodetic Laboratory for 32 stations worldwide over a period of six years.

Special focus is placed on modeling hydrology, as these models show the greatest differences and hydrology causes the largest deformations of the Earth’s crust in most regions.

The results show that suitable correction models can improve the GNSS time series. However, the modeling of hydrology should be further refined and the results validated by additional stations and longer time periods.

Co-authors: Lukas Jendges und Susanne Glaser

Local-Ties are vectors realizing the missing physical connection between space geodetic techniques such as Satellite Laser Ranging (SLR), Global Navigation Satellite System (GNSS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), or Very Long Baseline Interferometry (VLBI). The vectors are a key component in the combination procedure of these four space geodetic techniques to realize a global geodetic reference system. Local-ties defined between the reference points of the space geodetic techniques are observable by precise terrestrial observations in a local reference frame with submillimeter accuracy at co-location stations operating at least two different techniques. To integrate local-ties into the combination procedure, the vectors must be provided in a global reference frame in SINEX format. The required transformation parameters are often derived from homologous points whose global coordinates were determined by GNSS. Thus, the orientation of a transformed local-tie depends on the GNSS precision, which sometimes leads to large discrepancies in the combination procedure. In order to improve the orientation of the local-ties at the Onsala Space Observatory, astrogeodetic observations were performed to determine the local deflection of the vertical and the azimuth and were considered in the data analysis for the first time. The astrogeodetic observations were carried out using a conventional TS60 total station and the Total Station Astrogeodetic Control System (TSACS). The results fit well with the official Earth gravity models such as the Swedish SWEN17 or the global EGM2008.

In a scientific context, our world does not adhere to a true Cartesian framework: plumb lines deviate rather than run parallel, and a potential surface—defined by a water body—fails to conform to a geometric plane.

Although this fact is known, the distances and angles measured by a total station are transformed into Cartesian coordinates to establish local topocentric networks. This method is applicable for project areas of up to several hundred meters. However, for larger project areas, traditional geodesy employs corrections to the measurements: heights determined by trigonometric methods include a correction term for Earth's curvature (in addition to refraction), and horizontal distances measured at varying elevations are adjusted to a reference surface.

The procedures previously described are typically viewed as adjustments to measurements; nonetheless, these refined measurements establish a novel quasi-Cartesian coordinate system. Within this framework, horizontal coordinates are determined on the reference surface while height is measured relative to this curved surface. Although coordinate lines remain orthogonal in this new curvilinear coordinate system, the horizontal lines are not straight, and the vertical lines are no longer parallel.

Another effect of the non-Cartesian world poses unique challenges for aligning terrestrial scans. If, on the one hand, the vertical axes of the scanners are assumed parallel, the point clouds scanned from various positions do not align precisely. On the other hand, if the point clouds are aligned, the scanner axes become tilted.

The author advises implementing the aforementioned corrections on each terrestrial scan prior to merging the point clouds. By applying the Earth's curvature correction to the true Cartesian vertical coordinates and performing reduction to the reference level on the true Cartesian horizontal coordinates of individual scans, transforming them into this quasi-Cartesian curvilinear coordinate system. These transformed scans can be seamlessly co-registered and tranformed to map projection coordinates through a 2D Helmert transformation.

The precise determination of physical heights remains a major challenge in geodesy. Advances in optical clocks offer new opportunities to determine height differences by utilizing relativistic effects. Specifically, the gravitational redshift effect can be used to calculate height differences between distant locations.

The research project "TIME" (Clock Metrology: A Novel Approach to TIME in Geodesy) explores how physical heights can be determined using time transfer instead of traditional frequency comparisons. An optical time transfer link is simulated between the geodetic observatories in Wettzell and Potsdam via the Atomic Clock Ensemble in Space (ACES). Additionally, a fiber-optic connection to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig is considered, enabling the use of microwave and laser links. To further expand the network, connections to Grasse, France, and Borówiec, Poland, are also included in the simulation. The project aims to simulate height differences with an accuracy of 20 cm or better over extended integration times.

Currently, the focus is on the analysis of time intervals, particularly those of a few days. These shorter time periods present a particular challenge in terms of modeling and correcting clock and link errors. The impact of these errors is evaluated through various simulations. Additionally, the advantages of common view and non-common view methods for height determination in relation to these integration times are investigated.

We acknowledge the support of the German Research Foundation (DFG) – Project-ID 490990195 – FOR 5456.

Abstract:                                       

Global Navigation Satellite Systems (GNSS) have been demonstrated in numerous studies as a practical technique for monitoring ionospheric responses induced by seismic activity, such as earthquakes, tsunamis, and volcanic eruptions. One of the key parameters frequently used for monitoring Coseismic Ionospheric Disturbances (CID) is Total Electron Content (TEC), which can be calculated from dual-frequency GNSS observations. However, the overlap of ionospheric TEC anomalies caused by both seismic and non-seismic factors remains an uncertainty in CID interpretation. Therefore, this study utilizes high-rate GNSS data to determine whether statistically significant differences in ionospheric anomalies can be identified, which may help to distinguish CID from other influencing factors. For a comprehensive assessment, we examine GNSS data across a range of sampling rates, from a standard rate (one observation every 30 seconds) up to a high rate of 50 Hz (50 observations per second). We then apply a combination of sine wave analyses to quantify the parameters of GNSS-TEC variations. Our preliminary results reveal that GNSS-TEC signals fluctuate at an average frequency of ~0.12 Hz, which falls within the infrasound band of atmospheric acoustic waves, a range proven to be generated by seismic events. These ionospheric TEC disturbances typically occur around ten minutes after the mainshock of large seismic events, consistent with the speed of acoustic wave propagation from the Earth's surface to the ionosphere. Additionally, using high-rate Global Positioning System (GPS) data, we detect ionospheric TEC disturbances in the Extremely Low Frequency (ELF) band, ranging from ~80 Hz to ~200 Hz. These frequencies are indicative of plasmaspheric hiss waves, which are often generated in the plasmasphere, a region of plasma extending from the ionized upper atmosphere to ~2 to ~6 Earth radii. This distance aligns well with the GPS satellite altitude, which orbits at ~20200 km above the Earth's surface, enabling it to explain why GNSS signals can capture this wave band. Although further work is needed to fully address the challenges associated with wave resonances and their influence on signal interpretation, our findings have provided convincing evidence to enhance understanding of the causal mechanisms behind ionospheric anomalies in general.

Keywords: GNSS, GNSS-TEC disturbances, Seismic activity, Infrasound waves, Plasmaspheric hiss waves

Authors: Nhung Le^1,2*, Yuri Shprits^1,3, Xingzhi Lyu¹, Naofumi Takamatsu^1,4, Kłos Anna⁵, Harald Schuh^1,6

¹GFZ Helmholtz Centre for Geosciences, Potsdam, Germany
²Hanoi University of Natural Resources and Environment, Vietnam
³Universität Potsdam, Germany 4Geospatial Information Authority of Japan
⁵Military University of Technology, Poland
⁶Technische Universität Berlin

Corresponding: nhung@gfz.de