Search

An objective of the International Height Reference System (IHRS) and its realization (the IHRF) is to support the monitoring and analysis of Earth’s system changes. The more accurate the IHRS/IHRF is, the more phenomena can be identified and modelled. Thus, the IHRS/IHRF must provide vertical coordinates and their changes with time as accurately as possible. As many global change phenomena occur at different scales, the global frame should be extended to regional and local levels to guarantee consistency in the observation, detection, and modelling of their effects. From this perspective, an operational infrastructure is needed to ensure the long-term sustainability of the IHRS/IHRF. In this contribution, we present a proposal to establish such as infrastructure within the International Association of Geodesy (IAG) and its International Gravity Field Service (IGFS). Some aspects to be considered are:- Improvements in the IHRS definition and realization following future developments in geodetic theory, observations and modelling.- Refinement of the IHRS/IHRF standards/conventions based on the unification of the standards/conventions used by the geometric and gravity IAG Services.- Development of strategies for collocation of IHRF stations with existing gravity and geometrical reference stations at different densification levels.- Identification of the geodetic products associated with the IHRF and description of the elements to be considered in the corresponding metadata.- Servicing the vertical datum needs of other geosciences such as, e.g., hydrography and oceanography.Implementation of a registry containing the existing local/regional height systems and their connections to the global IHRS/IHRF., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

With the definition of the International Height Reference System (IHRS) and the development of a roadmap for its implementation through the International Height Reference Frame (IHRF), an analytical evaluation of the various approaches for the practical determination of potential values at IHRF is necessary. In this work we focus on two main approaches to estimate geopotential values at IHRF stations. The first approach resides on the use of either local gravity anomalies and gravity disturbances around each site and the geopotential determination based on Stokes’ and Molodensky’s boundary value problems, respectively. In this scheme, the influence of the classical residual terrain model (RTM) reduction as well as that of RTM effects based on spherical harmonics expansion of the topographic potential are investigated. Furthermore, the introduction of possible biases within the various pre- and post-processing steps are thoroughly investigated, as e.g., during the estimation of station geometric heights, along with the influence of the quasi-geoid to geoid separation estimation. In the second approach, we investigate the determination of geopotential values based on either national and regional geoid models, i.e., resembling the case that access to local gravity data is not available, and the determination has to be based on some available geoid model. In the present work we analyze the theoretical and methodological steps that need to be followed in each approach, identifying the possible sources of biases. Finally, some early results are presented aiming at providing a roadmap and an error assessment for the practical realization of the IHRF.

One of the main applications of the gravity field and steady-state Ocean Circulation Explorer (GOCE) satellite data is their combination with local gravity anomalies for geoid and gravity field modeling purposes. The aim of the present paper was the determination of an improved geoid model for the wider Hellenic area, using original GOCE SGG data filtered to retain only useful signals inside the measurement bandwidth (MBW) of the satellite. The filtered SGGs, originally at the satellite altitude, were projected to a mean orbit (MO) and then downward continued to the Earth’s surface (ES) in order to be combined with local gravity anomalies. For the projection to an MO, grids of disturbing gravity gradients from a global geopotential model (GGM) were used, computed per 1 km from the maximum satellite altitude to that of the MO. The downward continuation process was then undertaken using an iterative Monte Carlo (MC) simulated annealing method with GGM gravity anomalies on the ES used as ground truth data. The final geoid model over the wider Hellenic area was estimated, employing the remove–compute–restore method and both Fast Fourier Transform (FFT) and Least Squares Collocation (LSC). Gravity-only, GOCE-only and combined models using local gravity and GOCE data were determined and evaluation of the results was carried out against available GNSS/levelling data in the study area. From the results achieved, it was concluded that even when FFT is used, so that a combined grid of local gravity and GOCE data is used, improvements to the differences regarding GNSS/levelling data by 14.53% to 27.78% can be achieved. The geoid determination with LSC was focused on three different areas over Greece, with different characteristics in the topography and gravity variability. From these results, improvements from 14.73%, for the well-surveyed local data of Thessaly, to 32.88%, over the mountainous area of Pindos, and 57.10% for the island of Crete for 57.10% were found.

Machine Learning-based models gain increasingly momentum regarding the problem of path loss prediction. The work at hand deploys four machine learning algorithms (k Nearest Neighbors - kNN, Support Vector Regression - SVR, Random Forest - RF and AdaBoost), in order to simulate the radio coverage provided from a flying base station in the greek city of Tripolis. Their comparison shows that tree-based ensemble models (RF and AdaBoost) can be used as fast and reliable alternatives to the Ray Tracing technique.

In this paper we investigate the ionospheric turbulence from TEC observations before and during the intense seismic activity of October of 2018 at Zakynthos area (λ = 20.51°E, φ = 37°34'N), (Main shock: ML 6.6, 60.6 km SW of Zakynthos). The Total Electron Content (TEC) data of 5 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analyzed using Discrete Fourier Analysis in order to investigate the TEC variations. The results of this investigation indicate that the High-Frequency limit fo, of the ionospheric turbulence band content, increases as the site and the moment of the earthquake occurrence are approaching, pointing to the earthquake locus. We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology.

In this paper we investigate the ionospheric turbulence from observations of TEC variations as well as from VLF/LF transmitter signal observations before and during the disastrous seismic activity of August and October 2016 in Central Italy. The Total Electron Content (TEC) data of 8 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations. The data acquired for VLF/LF signal observations are from the receiver of Thessaloniki (40.59N, 22,78E), Greece, which monitor the VLF/LF transmitters of the International Network for Frontier Research on Earthquake Precursors (INFREP). A method of normalization according to the distance between the receiver and the transmitter is applied on the above data and then they are processed by the Hilbert Huang Transform (HHT) to produce the corresponding spectra for visual analysis. The results of both methods indicate that the High- Frequency limit fo, of the ionospheric turbulence content, increases as the site and the moment of the earthquake occurrence is approaching, pointing to the earthquake locus.

In this paper we investigate the variation of the tidal triggering effect efficiency, by means of the tidal seismicity compliance parameter p, and the lower Ionosphere variations, by means of the variation of the High-Frequency limit, fo, of the ionospheric turbulence content with the time and space proximity to the site of the earthquake occurrence as well as by the intensity variations of VLF signals transmitted over the seismic area. It is shown that the “Earth tides-seismicity compliance parameter” p may be used as a medium time earthquake warning while the frequency content of the ionospheric turbulence over the earthquake epicenter, deduced directly from GPS network TEC observations or indirectly through the VLF transmission network, may be used for the short time earthquake forecasting, deserving the special attention of the authorities, the scientists and the society.

Summarization: This work presents the latest calibration results for the Copernicus Sentinel-3A and -3B and the Jason-3 radar altimeters as determined by the Permanent Facility for Altimetry Calibration (PFAC) in west Crete, Greece. Radar altimeters are used to provide operational measurements for sea surface height, significant wave height and wind speed over oceans. To maintain Fiducial Reference Measurement (FRM) status, the stability and quality of altimetry products need to be continuously monitored throughout the operational phase of each altimeter. External and independent calibration and validation facilities provide an objective assessment of the altimeter’s performance by comparing satellite observations with ground-truth and in-situ measurements and infrastructures. Three independent methods are employed in the PFAC: Range calibration using a transponder, sea-surface calibration relying upon sea-surface Cal/Val sites, and crossover analysis. Procedures to determine FRM uncertainties for Cal/Val results have been demonstrated for each calibration. Biases for Sentinel-3A Passes No. 14, 278 and 335, Sentinel-3B Passes No. 14, 71 and 335, as well as for Jason-3 Passes No. 18 and No. 109 are given. Diverse calibration results by various techniques, infrastructure and settings are presented. Finally, upgrades to the PFAC in support of the Copernicus Sentinel-6 ‘Michael Freilich’, due to launch in November 2020, are summarized. Presented on: Remote Sensing

In this paper, we investigate the ionospheric turbulence from TEC observations, before and during the intense seismic activity of September 2019 at Albania (main shock at l=19.445oE, j=41.372o N, Mw=5.6) and at Marmara sea (main shock at l=28.19 oE, j=40.872oN, Mw=5.7), as well as of November 2019 at Albania (main shock at l=19.470oE, j=41.381oN, Mw=6.4), and at Bosnia-Herzegovina (main shock at l=17.961oE, j=43.196oN, Mw=5.4).The Total Electron Content (TEC) data of 6 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations. The results of this investigation indicate that the High- Frequency limit fo, of the ionospheric turbulence content, increases by aproaching the site and the time of the earthquake occurrence, pointing to the earthquake location (epicenter). We conclude that the LAIC mechanism, through acoustic or gravity wave, could explain this phenomenology. In addition the proximity of the tectonic active areas to the GPS stations offer us an opportunity to discriminate the origin of the disturbances

In this paper we investigate the ionospheric turbulence from TEC observations before and during the tectonic activity of the last quarter of 2019 in the Hellenic Arc, Greece (main shock at l=23.26oE, j=35.69oN, Mw=6.1). The Total Electron Content (TEC) data of 6 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations. The results of this investigation indicate that the High- Frequency limit fo, of the ionospheric turbulence content, increases by aproaching the site and the time of the earthquake occurrence, pointing to the earthquake location (epicenter). We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology.

Lake morphometry is essential for managing water resources and limnetic ecosystems. For reservoirs that receive high sediment loads, frequent morphometric mapping is necessary to define both the effective life of the reservoir and its water storage capacity for irrigation, power generation, flood control and domestic water supply. The current study presents a methodology for updating the digital depth model (DDM) of lakes and reservoirs with wide intra and interannual fluctuations of water levels using satellite remote sensing. A time series of Terra MODIS satellite images was used to map shorelines formed during the annual water level change cycle, and were validated with concurrent Landsat ETM+ satellite images. The shorelines were connected with in-situ observation of water levels and were treated as elevation contours to produce the DDM using spatial interpolation. The accuracy of the digitized shorelines is within the mapping accuracy of the satellite images, while the resulting DDM is validated using in-situ elevation measurements. Two versions of the DDM were produced to assess the influence of seasonal water fluctuation. Finally, the methodology was applied to Lake Kerkini (Greece) to produce an updated DDM, which was compared with the last available bathymetric survey (1991) and revealed changes in sediment distribution within the lake.

The definition and realization of vertical datum is a key concept in support of not only geodetic works but also for surveying and hydraulic studies to name a few. In the GOCE era, this is customarily done by estimating height and/or geopotential offsets with respect to a conventional reference geopotential value or to available GNSS/Leveling observations on trigonometric BMs and a GOCE-based geoid. This work investigates the influence of GOCE errors in the determination of the Hellenic Local Vertical Datum. This is facilitated through a least-squares adjustment of collocated GNSS/Leveling and GOCE geoid heights over a network of 1,542 BMs. TIM-R5, GOCO05s and GOCO05c Global Geopotential Models (GGMs) are used for representing the contribution of GOCE and GRACE to the Earth’s gravity field. First, a weighted adjustment is carried out employing the GGMs commission error as indicative of the geoid height variance for all stations. Then, full variance-covariance matrices of the GGMs are employed for utilizing realistic GOCE error information and investigating their influence on the adjustment results. Using the available GNSS/Leveling formal errors, a Variance Component Estimation (VCE) is performed to evaluate height (h, H, N) error matrices and assess the stochastic model for the corresponding observational noise. VCE is used to address the impact of a simplified uniform variance assumption for all geoid height data on the final prediction variances in contrast to using the full covariance matrices. Finally, zero-level geopotential values are estimated for the Greek mainland following weighting schemes as the ones described above.

Summarization: The aim of this work has been to examine the relationship of steep bathymetry in the coastal areas around the permanent Cal/Val facility of Gavdos, and their influence on the produced calibration values for the Jason-2 satellite altimeter. The paper describes how changes in seafloor topography (from 200 to 3500 m depth over a distance of 10 km) are reflected on the determined altimeter parameters using different reference surfaces for satellite calibration. Finally, it describes the relation between these parameter trends and the region’s local characteristics. Using 3.5 years of Jason-2 calibration data and careful data analysis, certain features and variations, with permanent signatures at some locations south of Gavdos, have been found in the altimeter bias values. Causes of these variations have been interpreted. Some of these features, for example, at 12 km south of the Cal/Val site seem to be related to the general oceanographic circulation, but others of short wavelength (in the order of 1 km) are because of the insufficient geoid model resolution. Along Pass No. 109, the concealed effect of bathymetry on the geoid has produced a slope of 3.1 cm over 14–21 km from Gavdos. Along the other Pass No.18 and in the range 12–21 km, a slope in the geoid model, caused again by the sharp changes in the topography of seafloor (from 300 to 3200 m depth over a distance of 10 km), have been discovered. Those geoid model imperfections of short wavelength do not change the previously reported bias values. Finally, boat campaigns that took place for verifying the previous features, were also used to determine an altimeter bias of B = +19.6 ± 4.5 cm, for cycle = 80, at 15:14:58 UTC, on 7-Sept-2010, along Pass No. 109. Calibration took place at a distance of about 19 km south of Gavdos, in the open sea while Jason-2 was flying over. Presented on: Advances in Space Research

Summarization: Satellite altimetry provides exceptional means for absolute and undisputable monitoring of changes in sea level and inland waters (rivers and lakes), over regional to global scales, with accuracy and with respect to the center of mass of the Earth. Altimetry system's responses have to be continuously monitored for their quality, biases, errors, drifts, etc. with calibration. Absolute calibration of altimeters is achieved by external and independent to satellite facilities on the ground. This is the mainstay for a continuous, homogenous, and reliable monitoring of the earth and its oceans. This paper describes the development of the Permanent Facility for Altimetry Calibration in Gavdos/Crete, Greece, as of 2001 along with its infrastructure and instrumentation. Calibration results are presented for the reference missions of Jason-1, Jason-2, and Jason-3. Then, this work continues with the determination of relative calibrations with respect to reference missions for Sentinel-3A, HY-2A, and SARAL/AltiKa. Calibration results are also given for Jason-2 and Jason-3 altimeters using the transponder at the CDN1 Cal/Val site on the mountains of Crete, with simultaneous comparisons against sea-surface calibration and during their tandem mission. Finally, the paper presents procedures for estimating uncertainties for altimeter calibration to meet the Fiducial Reference Measurement standards. Presented on: Remote Sensing

Βασισμένοι στα αποτελέσματα των μελετών μας για την εκδήλωση παλιρροϊκών επιδράσεων στην σεισμικότητα του Ελληνικού Χώρου, θεωρούμε το Επίπεδο εμπιστοσύνης p της προσαρμογής της κατά φάση κατανομής του αριθμού των σεισμών και της περιοδικής μεταβολής των σεληνοηλιακών παλιρροιών ως ένα δείκτη της οριακού επιπέδου τεκτονικής τάσεως για την εκδήλωση σεισμού, και ελέγχομε αν η πρόσφατη μικροσεισμική δραστηριότητα στο Νησιώτικο Σύμπλεγμα της Σαντορίνης δείχνει ότι τα ρήγματα Καμένης και Κολούμπο (στα οποία η σεισμική δραστηριότητα εστιάζεται) είναι ώριμα για ένα ισχυρότερο σεισμό. Στην ανάλυση αυτή χρησιμοποιούμε όλους τους σεισμούς ανεξαρτήτως μεγέθους από τον κατάλογο του Γεωδυναμικού Ινστιτούτου του Εθνικού Αστεροσκοπείου Αθηνών για το χρονικό διάστημα 1968- 2012, και μελετούμε συγκριτικά το Επίπεδο εμπιστοσύνης p της προσαρμογής της κατανομής των σεισμών με τις περιόδους των παλιρροϊκών συνιστωσών, για τις χρονικές περιόδους 1968-2010 και 2011-2012 (την χρονική περίοδο της αυξημένης μικροσεισμικής δραστηριότητας). Το συμπέρασμα είναι ότι υπάρχει ένδειξης αυξημένης ευαισθησίας για εκδήλωση σεισμών την διετία 2011-2012. Πρέπει να σημειωθεί ότι Γεωδαιτικές παρατηρήσεις δείχνουν αυξημένη παραμόρφωση στην περιοχή της Σαντορίνης το 2010-2011 που πιθανόν να οφείλεται σε άνοδο μάγματος και πιθανόν συνδέεται με την αύξηση της μικροσεισμικής δραστηριότητας. Εδώ θα πρέπει να τονίσουμε ότι το μέγεθος ενός πιθανού σεισμού εξαρτάται από την τεκτονική δομή, δυναμική και ιστορία του τόπου, και δεν είναι δυνατόν να προσδιορισθεί από την απόφανση επί της ωριμότητας του ρήγματος για την εκδήλωση σεισμού., Based on the results of our studies for the tidal triggering effect on the seismicity of the Hellenic area, we consider the confidence level of earthquake occurrence - tidal period accordance as an index of tectonic stress criticality for earthquake occurrence and we check if the recent increase in the seismic activity at the Santorini island complex indicate that the faults Kameni and Columbo (to which the seismicity is clustered), (Chouliaras et al., 2013) are mature for a stronger earthquake. In this paper, we present the results of this test.

Summarization: This work presents the first calibration results for the SARAL/AltiKa altimetric mission using the Gavdos permanent calibration facilities. The results cover one year of altimetric observations from April 2013 to March 2014 and include 11 calibration values for the altimeter bias. The reference ascending orbit No. 571 of SARAL/AltiKa has been used for this altimeter assessment. This satellite pass is coming from south and nears Gavdos, where it finally passes through its west coastal tip, only 6 km off the main calibration location. The selected calibration regions in the south sea of Gavdos range from about 8 km to 20 km south off the point of closest approach. Several reference surfaces have been chosen for this altimeter evaluation based on gravimetric, but detailed regional geoid, as well as combination of it with other altimetric models. Based on these observations and the gravimetric geoid model, the altimeter bias for the SARAL/AltiKa is determined as mean value of −46mm ±10mm, and a median of −42 mm ±10 mm, using GDR-T data at 40 Hz rate. A preliminary cross-over analysis of the sea surface heights at a location south of Gavdos showed that SARAL/AltiKa measure less than Jason-2 by 4.6 cm. These bias values are consistent with those provided by Corsica, Harvest, and Karavatti Cal/Val sites. The wet troposphere and the ionosphere delay values of satellite altimetric measurements are also compared against in-situ observations (−5 mm difference in wet troposphere and almost the same for the ionosphere) determined by a local array of permanent GNSS receivers, and meteorological sensors. Presented on: Marine Geodesy

Summarization: In this work, absolute calibration of the Chinese HY-2 satellite altimetry mission is carried out, employing Pass No. 280 and the calibration facility, CRS1, located in the Southwest end of the island of Crete, Greece. Satellite Pass No. 280 is descending and follows a ground track almost parallel to the west coast of Crete. It comes close to the coast, at a distance of about 9 km from the CRS1 calibration site, and finally goes away south of Crete. The HY-2 sensor geophysical data records (S-GDR) have been incorporated into the calibration procedures and processing has taken place for cycles No. 54–62, at 20 Hz data rate. Some peculiarities in the HY-2 satellite altimeter data, as delivered and depicted in the I-GDR and S-GDR data, have also been noticed. All calibration results have been determined using a regional, precise and detailed geoid, along with a good knowledge of local ocean circulation and site characteristics and a well-defined sea-surface calibration methodology. The first preliminary results for the HY-2 altimeter calibration have shown that the initial cycles, up to No. 51, display an erratic behavior. After those cycles, the altimeter range bias values seem to be stable and reach a value of B = −45.6 cm ± 4.4 cm, when applying the net instrument corrections as provided in the GDR. If the relativistic effects of the satellite clocks are properly applied for the net instrument corrections, then the altimeter range bias goes down to B = −27 cm ± 3 cm. Also, preliminary cross-over analysis with the SARAL/AliKa and Jason-2 satellites show a bias of B = −23 cm, and B = −28.5 cm, respectively. The performance of the HY-2 on-board radiometer has also been examined in terms of the wet troposphere corrections and shows a mean difference of −1 cm ± 0.1 cm with respect to in-situ GNSS-derived corrections. Finally, the ionosphere path corrections of the HY-2 satellite show a difference of +1 cm ± 1.1 cm, when compared against the GNSS-derived ionosphere values. Presented on