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Within the GeoGravGOCE project, funded by the Hellenic Foundation for Research Innovation, one of the main goals is the investigation of downward continuation schemes for the GOCE Satellite Gravity Gradiometry (SGG) data. It is well known that once the original SGG observations have been filtered to the GOCE Measurement Band Width (MBW), in order to remove noise and long-wavelength correlated errors, a crucial point for gravity field and geoid determination refers to the combination of GOCE data with local gravity field information. One possible way to exploit GOCE data is to use them in a Spherical Harmonic Synthesis (SHS) to derive a GOCE-only and/or a combined Global Geopotential Model. Our aim is to overcome the inherent smoothing of SHS and use directly the SGG data in order to investigate their contribution to regional gravity field and geoid determination. For that, methods based on the input-output-system-theory (IOST) are used for the combination of heterogeneous data at the Earth’s surface and at the satellite altitude or a mean sphere. The GOCE Level 2 gradients are first processed, transformed and reduced to a mean orbit using the IOST methods and then are downward continued to the Earth’s surface with an iterative Monte Carlo method (simulated annealing - SA). In this work we present the theoretical background of the proposed methodology and key-concepts for its implementation.
Ilias N. Tziavos; Dimitrios A. Natsiopoulos; Georgios S. Vergos; Eleftherios A. Pitenis; Elisavet G. Mamagiannou. Theoretical frame for the application of IOST in the downward continuation of GOCE SGG data. 2021, 1 .
AMA StyleIlias N. Tziavos, Dimitrios A. Natsiopoulos, Georgios S. Vergos, Eleftherios A. Pitenis, Elisavet G. Mamagiannou. Theoretical frame for the application of IOST in the downward continuation of GOCE SGG data. . 2021; ():1.
Chicago/Turabian StyleIlias N. Tziavos; Dimitrios A. Natsiopoulos; Georgios S. Vergos; Eleftherios A. Pitenis; Elisavet G. Mamagiannou. 2021. "Theoretical frame for the application of IOST in the downward continuation of GOCE SGG data." , no. : 1.
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.
Stelios Mertikas; Achilleas Tripolitsiotis; Craig Donlon; Constantin Mavrocordatos; Pierre Féménias; Franck Borde; Xenophon Frantzis; Costas Kokolakis; Thierry Guinle; George Vergos; Ilias Tziavos; Robert Cullen. The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards. Remote Sensing 2020, 12, 2642 .
AMA StyleStelios Mertikas, Achilleas Tripolitsiotis, Craig Donlon, Constantin Mavrocordatos, Pierre Féménias, Franck Borde, Xenophon Frantzis, Costas Kokolakis, Thierry Guinle, George Vergos, Ilias Tziavos, Robert Cullen. The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards. Remote Sensing. 2020; 12 (16):2642.
Chicago/Turabian StyleStelios Mertikas; Achilleas Tripolitsiotis; Craig Donlon; Constantin Mavrocordatos; Pierre Féménias; Franck Borde; Xenophon Frantzis; Costas Kokolakis; Thierry Guinle; George Vergos; Ilias Tziavos; Robert Cullen. 2020. "The ESA Permanent Facility for Altimetry Calibration: Monitoring Performance of Radar Altimeters for Sentinel-3A, Sentinel-3B and Jason-3 Using Transponder and Sea-Surface Calibrations with FRM Standards." Remote Sensing 12, no. 16: 2642.
The peculiar features of the Mediterranean Sea in terms of both gravity variations and geodynamic complexities resulted in a wide range of studies and research projects during the last decades focusing on the modeling of static and dynamic processes of this almost closed sea basin. The scope of the present study is twofold. The first part shall provide a synopsis of the data and methods used and the results achieved for geoid determination in the first phase of the GEOMED project (GEOid in the MEDiterranean) in the early 90s. The second part focuses on the recent activities and improvements of the continuation of the previous research in the framework of the GEOMED-2 project which is still an ongoing work. The collection of all available surface and satellite data towards the creation of improved gravity databases and the computation of marine geoid models are discussed. The crucial role of global geopotential models and digital topography and bathymetry is analyzed, focusing on the efficient modeling of the low and high frequencies of the gravity signal. The numerical methodology is discussed in the context of the available data sets and computer facilities over the duration of GEOMED. Finally, gravity databases and geoid models for the whole test area are presented and evaluated for both phases of the project. Some recommendations are also drawn along with a discussion on new methodological tools for the implementation of new data sources, targeting to the future improvement of gravity field over the Mediterranean Sea.
Ilias N. Tziavos. Gravity and geoid in the Mediterranean Sea: the GEOMED project. Rendiconti Lincei. Scienze Fisiche e Naturali 2020, 31, 83 -97.
AMA StyleIlias N. Tziavos. Gravity and geoid in the Mediterranean Sea: the GEOMED project. Rendiconti Lincei. Scienze Fisiche e Naturali. 2020; 31 (S1):83-97.
Chicago/Turabian StyleIlias N. Tziavos. 2020. "Gravity and geoid in the Mediterranean Sea: the GEOMED project." Rendiconti Lincei. Scienze Fisiche e Naturali 31, no. S1: 83-97.
This work presents calibration results for the altimeter of Sentinel-3A Surface Topography Mission as determined at the Permanent Facility for Altimetry Calibration in west Crete, Greece. The facility has been providing calibration services for more than 15 years for all past (i.e., Envisat, Jason-1, Jason-2, SARAL/AltiKa, HY-2A) and current (i.e., Sentinel-3A, Sentinel-3B, Jason-3) satellite altimeters. The groundtrack of the Pass No.14 of Sentinel-3A ascends west of the Gavdos island and continues north to the transponder site on the mountains of west Crete. This pass has been calibrated using three independent techniques activated at various sites in the region: (1) the transponder approach for its range bias, (2) the sea-surface method for the estimation of altimeter bias for its sea-surface heights, and (c) the cross-over analysis for inspecting height observations with respect to Jason-3. The other Pass No.335 of Sentinel-3A descends from southwest of Crete to south and intersects the Gavdos calibration site. Additionally, calibration values for this descending pass are presented, applying sea-surface calibration and crossover analysis. An uncertainty analysis for the altimeter biases derived by the transponder and by sea-surface calibrations is also introduced following the new standard of Fiducial Reference Measurements.
Stelios Mertikas; Craig Donlon; Pierre Féménias; Constantin Mavrocordatos; Demitris Galanakis; Achilles Tripolitsiotis; Xenophon Frantzis; Costas Kokolakis; Ilias N. Tziavos; George Vergos; Thierry Guinle. Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece. Remote Sensing 2018, 10, 1808 .
AMA StyleStelios Mertikas, Craig Donlon, Pierre Féménias, Constantin Mavrocordatos, Demitris Galanakis, Achilles Tripolitsiotis, Xenophon Frantzis, Costas Kokolakis, Ilias N. Tziavos, George Vergos, Thierry Guinle. Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece. Remote Sensing. 2018; 10 (11):1808.
Chicago/Turabian StyleStelios Mertikas; Craig Donlon; Pierre Féménias; Constantin Mavrocordatos; Demitris Galanakis; Achilles Tripolitsiotis; Xenophon Frantzis; Costas Kokolakis; Ilias N. Tziavos; George Vergos; Thierry Guinle. 2018. "Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece." Remote Sensing 10, no. 11: 1808.
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.
Stelios P. Mertikas; Craig Donlon; Pierre Féménias; Constantin Mavrocordatos; Demitris Galanakis; Achilles Tripolitsiotis; Xenophon Frantzis; Ilias N. Tziavos; George Vergos; Thierry Guinle. Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece. Remote Sensing 2018, 10, 1557 .
AMA StyleStelios P. Mertikas, Craig Donlon, Pierre Féménias, Constantin Mavrocordatos, Demitris Galanakis, Achilles Tripolitsiotis, Xenophon Frantzis, Ilias N. Tziavos, George Vergos, Thierry Guinle. Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece. Remote Sensing. 2018; 10 (10):1557.
Chicago/Turabian StyleStelios P. Mertikas; Craig Donlon; Pierre Féménias; Constantin Mavrocordatos; Demitris Galanakis; Achilles Tripolitsiotis; Xenophon Frantzis; Ilias N. Tziavos; George Vergos; Thierry Guinle. 2018. "Fifteen Years of Cal/Val Service to Reference Altimetry Missions: Calibration of Satellite Altimetry at the Permanent Facilities in Gavdos and Crete, Greece." Remote Sensing 10, no. 10: 1557.
This paper presents a fusion method for combining outputs acquired by low-cost inertial measurement units and electronic magnetic compasses. Specifically, measurements of inertial accelerometer and gyroscope sensors are combined with no-inertial magnetometer sensor measurements to provide the optimal three-dimensional (3D) orientation of the sensors’ axis systems in real time. The method combines Euler–Cardan angles and rotation matrix for attitude and heading representation estimation and deals with the “gimbal lock” problem. The mathematical formulation of the method is based on Kalman filter and takes into account the computational cost required for operation on mobile devices as well as the characteristics of the low-cost microelectromechanical sensors. The method was implemented, debugged, and evaluated in a desktop software utility by using a low-cost sensor system, and it was tested in an augmented reality application on an Android mobile device, while its efficiency was evaluated experimentally.
Photis Patonis; Petros Patias; Ilias N. Tziavos; Dimitrios Rossikopoulos; Konstantinos G. Margaritis. A Fusion Method for Combining Low-Cost IMU/Magnetometer Outputs for Use in Applications on Mobile Devices. Sensors 2018, 18, 2616 .
AMA StylePhotis Patonis, Petros Patias, Ilias N. Tziavos, Dimitrios Rossikopoulos, Konstantinos G. Margaritis. A Fusion Method for Combining Low-Cost IMU/Magnetometer Outputs for Use in Applications on Mobile Devices. Sensors. 2018; 18 (8):2616.
Chicago/Turabian StylePhotis Patonis; Petros Patias; Ilias N. Tziavos; Dimitrios Rossikopoulos; Konstantinos G. Margaritis. 2018. "A Fusion Method for Combining Low-Cost IMU/Magnetometer Outputs for Use in Applications on Mobile Devices." Sensors 18, no. 8: 2616.
In this study, the consistency of the Greek Vertical Datum (GVD) is examined, focusing on an area in central Greece and following similar efforts made in previous researches for the establishment of an International Height Reference System (IHRS). High precision GNSS measurements are available at trigonometric benchmarks located along the Gulf of Corinth, with benchmarks residing on both coasts along. First, the zero-level geopotential value (\( {W}_o^{LVD} \)) for the two areas, north and south coast, is determined, based on the classical Helmert theory using GNSS/leveling data and surface geopotential values derived from GOCE-based global geopotential models (GO-DIR-R5, GO-TIM-R5, GOCO05s and GECO) and EGM08. Then, the relative offset between the two areas is estimated while subsets of the computed benchmark values are also examined. Significant inconsistencies are detected that depend on the choice of benchmarks used in the computations. Moreover, a per benchmark analysis showed that the inconsistencies present a random spatial distribution and are attributed mainly to the orthometric height values of the benchmarks. Furthermore, the local \( {W}_o^{LVD} \) estimates are compared with previous results related to the GVD and the VD of the Greek islands and the corresponding value adopted by the IHRS. Finally, some remarks are drawn on the feasibility of the unification of the GVD with a global one.
V. N. Grigoriadis; E. Lambrou; G. S. Vergos; I. N. Tziavos. Assessment of the Greek Vertical Datum: A Case Study in Central Greece. International Symposium on Earth and Environmental Sciences for Future Generations 2017, 185 -191.
AMA StyleV. N. Grigoriadis, E. Lambrou, G. S. Vergos, I. N. Tziavos. Assessment of the Greek Vertical Datum: A Case Study in Central Greece. International Symposium on Earth and Environmental Sciences for Future Generations. 2017; ():185-191.
Chicago/Turabian StyleV. N. Grigoriadis; E. Lambrou; G. S. Vergos; I. N. Tziavos. 2017. "Assessment of the Greek Vertical Datum: A Case Study in Central Greece." International Symposium on Earth and Environmental Sciences for Future Generations , no. : 185-191.
The use of terrain and elevation data is critical for a number of applications in science and engineering. Typically, the quality of digital elevation models (DEMs) is assessed using external and independent point data sources to arrive at an overall RMS value for the errors. The utility of such a single-valued overall assessment depends on the spatial extent of the area under consideration and the terrain variability (both over time and space), as well as the application requirements. This paper aimed to understand the suite of parameters that are important to consider in deriving a DEM error budget. Specifically, terrain slope, land-cover type, information loss, and data measurement schemes were investigated. A region in western Canada spanning the Rocky Mountains was used to numerically quantify errors using two Global Positioning System (GPS) datasets: precise point positioning (PPP) profiles and GPS on benchmarks. Three digital elevation models [Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Global Digital Elevation Model Version 2 (GDEM2), Shuttle Radar Topography Mission 1 Arc-Second Digital Elevation Model Version 3 (SRTM1v3), and Canadian Digital Elevation Model (CDEM)] were assessed. Results highlight the importance of selecting ground-control points based on the region’s characteristics (e.g., slope, tree cover). This leads to more representative RMS values that improve DEM uncertainty estimations. Finally, a mathematical method [projection onto convex sets (POCS)] for filling data gaps in the GPS data profiles was implemented, and results demonstrate the utility of this approach over conventional interpolation schemes.
D. Bolkas; G. Fotopoulos; A. Braun; I. N. Tziavos. Assessing Digital Elevation Model Uncertainty Using GPS Survey Data. Journal of Surveying Engineering 2016, 142, 04016001 .
AMA StyleD. Bolkas, G. Fotopoulos, A. Braun, I. N. Tziavos. Assessing Digital Elevation Model Uncertainty Using GPS Survey Data. Journal of Surveying Engineering. 2016; 142 (3):04016001.
Chicago/Turabian StyleD. Bolkas; G. Fotopoulos; A. Braun; I. N. Tziavos. 2016. "Assessing Digital Elevation Model Uncertainty Using GPS Survey Data." Journal of Surveying Engineering 142, no. 3: 04016001.
In this paper, the development of a Web-based GIS system for the monitoring and assessment of the Black Sea is presented. The integrated multilevel system is based on the combination of terrestrial and satellite Earth observation data through the technological assets provided by innovative information tools and facilities. The key component of the system is a unified, easy to update geodatabase including a wide range of appropriately selected environmental parameters. The collection procedure of current and historical data along with the methods employed for their processing in three test areas of the current study are extensively discussed, and special attention is given to the overall design and structure of the developed geodatabase. Furthermore, the information system includes a decision support component (DSC) which allows assessment and effective management of a wide range of heterogeneous data and environmental parameters within an appropriately designed and well-tested methodology. The DSC provides simplified and straightforward results based on a classification procedure, thus contributing to a monitoring system not only for experts but for auxiliary staff as well. The examples of the system's functionality that are presented highlight its usability as well as the assistance that is provided to the decision maker. The given examples emphasize on the Danube Delta area; however, the information layers of the integrated system can be expanded in the future to cover other regions, thus contributing to the development of an environmental monitoring system for the entire Black Sea.
Ilias N. Tziavos; Thomas K. Alexandridis; Borys Aleksandrov; Agamemnon Andrianopoulos; Ioannis D. Doukas; Ion Grigoras; Vassilios N. Grigoriadis; Ioanna D. Papadopoulou; Paraskevas Savvaidis; Argyrios Stergioudis; Liliana Teodorof; George Vergos; Lyudmila Vorobyova; Georgios C. Zalidis. Development of a Web-based GIS monitoring and environmental assessment system for the Black Sea: application in the Danube Delta area. Environmental Monitoring and Assessment 2016, 188, 492 .
AMA StyleIlias N. Tziavos, Thomas K. Alexandridis, Borys Aleksandrov, Agamemnon Andrianopoulos, Ioannis D. Doukas, Ion Grigoras, Vassilios N. Grigoriadis, Ioanna D. Papadopoulou, Paraskevas Savvaidis, Argyrios Stergioudis, Liliana Teodorof, George Vergos, Lyudmila Vorobyova, Georgios C. Zalidis. Development of a Web-based GIS monitoring and environmental assessment system for the Black Sea: application in the Danube Delta area. Environmental Monitoring and Assessment. 2016; 188 (8):492.
Chicago/Turabian StyleIlias N. Tziavos; Thomas K. Alexandridis; Borys Aleksandrov; Agamemnon Andrianopoulos; Ioannis D. Doukas; Ion Grigoras; Vassilios N. Grigoriadis; Ioanna D. Papadopoulou; Paraskevas Savvaidis; Argyrios Stergioudis; Liliana Teodorof; George Vergos; Lyudmila Vorobyova; Georgios C. Zalidis. 2016. "Development of a Web-based GIS monitoring and environmental assessment system for the Black Sea: application in the Danube Delta area." Environmental Monitoring and Assessment 188, no. 8: 492.
A research project called ELEVATION (Evaluation of the HelLEnic Vertical Network in the FrAme of the European SysTems and Control Networks InterconnectiON—Application in the Areas of Attica and Thessaloniki) for the validation of the Hellenic vertical network is currently in progress. Two investigation areas in central and northern Greece were chosen for the project. The areas include several benchmarks of the national trigonometric and leveling networks. In the current research, static global positioning system (GPS) observations as well as classical terrestrial leveling were performed to assess the internal accuracy of the two networks. This paper presents some of the numerical tests based on GPS and leveling measurements and outlines the goals of the project. The strategies followed in the processing of GPS data are presented, with emphasis on their future use within this project. GPS observations were processed using various commercial as well as scientific software packages to examine the influence of the processing algorithms on the final results. Significant differences between the results of the various software packages were revealed, particularly in the case of challenging observation conditions. Finally, comparisons of the estimated geoid heights at GPS benchmarks (BMs) to Earth Gravitational Model 2008 (EGM2008) geoid information are presented as a first step toward the evaluation of the Hellenic vertical network. These comparisons indicate that the two investigation areas are of different internal accuracy, namely, 8.3 and 15.8 cm in terms of the standard deviation of the differences at the Attica and Thessaloniki test areas.
V. D. Andritsanos; O. Arabatzi; M. Gianniou; V. Pagounis; I. N. Tziavos; George Vergos; E. Zacharis. Comparison of Various GPS Processing Solutions toward an Efficient Validation of the Hellenic Vertical Network: The ELEVATION Project. Journal of Surveying Engineering 2016, 142, 04015007 .
AMA StyleV. D. Andritsanos, O. Arabatzi, M. Gianniou, V. Pagounis, I. N. Tziavos, George Vergos, E. Zacharis. Comparison of Various GPS Processing Solutions toward an Efficient Validation of the Hellenic Vertical Network: The ELEVATION Project. Journal of Surveying Engineering. 2016; 142 (1):04015007.
Chicago/Turabian StyleV. D. Andritsanos; O. Arabatzi; M. Gianniou; V. Pagounis; I. N. Tziavos; George Vergos; E. Zacharis. 2016. "Comparison of Various GPS Processing Solutions toward an Efficient Validation of the Hellenic Vertical Network: The ELEVATION Project." Journal of Surveying Engineering 142, no. 1: 04015007.
Stelios P. Mertikas; Xinghua Zhou; Fangli Qiao; Antonis Daskalakis; Mingsen Lin; Hailong Peng; Ilias N. Tziavos; George Vergos; Achilleas Tripolitsiotis; Xenophon Frantzis. First preliminary results for the absolute calibration of the Chinese HY-2 altimetric mission using the CRS1 calibration facilities in West Crete, Greece. Advances in Space Research 2016, 57, 78 -95.
AMA StyleStelios P. Mertikas, Xinghua Zhou, Fangli Qiao, Antonis Daskalakis, Mingsen Lin, Hailong Peng, Ilias N. Tziavos, George Vergos, Achilleas Tripolitsiotis, Xenophon Frantzis. First preliminary results for the absolute calibration of the Chinese HY-2 altimetric mission using the CRS1 calibration facilities in West Crete, Greece. Advances in Space Research. 2016; 57 (1):78-95.
Chicago/Turabian StyleStelios P. Mertikas; Xinghua Zhou; Fangli Qiao; Antonis Daskalakis; Mingsen Lin; Hailong Peng; Ilias N. Tziavos; George Vergos; Achilleas Tripolitsiotis; Xenophon Frantzis. 2016. "First preliminary results for the absolute calibration of the Chinese HY-2 altimetric mission using the CRS1 calibration facilities in West Crete, Greece." Advances in Space Research 57, no. 1: 78-95.
In the present work we examine the crustal structure in the wider Adriatic Sea region using a newly presented Bouguer database derived from the integration of satellite, marine and inland gravity measurements. The observed large-wavelength Bouguer anomalies clearly correlate with the deeper crustal - upper mantle structural features, revealing new information on the Moho undulations, allowing the quantitative assessments of the Moho geometry in the study area. This task was performed using three different approaches, namely the multiple source Werner deconvolution (MSWD), the isostatic admittance method and the Parker-Oldenburg algorithm. The interpretation of the MSWD estimates was performed along nine selected Bouguer gravity profiles, leading to a pseudo-3D Moho map, as opposed to the other two methodologies from which 3D models are derived. All three maps exhibit similar large scale features but also local differences for the Moho structure in the Adriatic Sea area. Since this region exhibits complex geotectonic features, such as a double plate subduction, the main goal of the present work is to highlight the main areas of crustal thinning and thickening, using the aforementioned results. Moreover, the results provided by the different methods are evaluated against the current geological and geophysical data and models developed for the area.
G.A. Tassis; Costas Papazachos; G.N. Tsokas; I.N. Tziavos; Ivana Vasiljevic; A. Stampolidis. Moho Depth Determination of the Adriatic Sea Region Using a New Bouguer Anomaly Database. 8th Congress of the Balkan Geophysical Society 2015, 1 .
AMA StyleG.A. Tassis, Costas Papazachos, G.N. Tsokas, I.N. Tziavos, Ivana Vasiljevic, A. Stampolidis. Moho Depth Determination of the Adriatic Sea Region Using a New Bouguer Anomaly Database. 8th Congress of the Balkan Geophysical Society. 2015; ():1.
Chicago/Turabian StyleG.A. Tassis; Costas Papazachos; G.N. Tsokas; I.N. Tziavos; Ivana Vasiljevic; A. Stampolidis. 2015. "Moho Depth Determination of the Adriatic Sea Region Using a New Bouguer Anomaly Database." 8th Congress of the Balkan Geophysical Society , no. : 1.
The main objective of this study is the determination of the Moho discontinuity in the Hellenic area based on gravity data inversion. High-resolution (2 arcmin) and accuracy (±2 to ±3 mGal) gravity data were used in the computations following the Parker–Oldenburg iterative method, and a low-pass filter was applied to gravity information towards the convergence of the final solution. To determine the cut-off wavenumber of the filter, we tested different mean depths and density contrasts of the crust and the mantle (Mohorovičić discontinuity). The selected density contrast and mean depth of the Mohorovičić discontinuity are those which agree with the Moho depth interpreted from deep seismic imaging data. By comparing the Moho depth estimated, covering both land and marine areas and including the Ionian and Aegean Sea, with those interpreted from previous studies, it is shown that our solution is consistent with the available seismic data interpretation. Moreover, the resolution of the gravity data and the appropriate filtering used in this study contributed to the representation of local features of the Moho discontinuity. Finally, area-dependent discrepancies detected between the Moho depths of the current solution and those derived from previous studies are extensively discussed.
Vassilios N. Grigoriadis; Ilias N. Tziavos; Grigorios Tsokas; Alexandros Stampolidis. Gravity data inversion for Moho depth modeling in the Hellenic area. Pure and Applied Geophysics 2015, 173, 1223 -1241.
AMA StyleVassilios N. Grigoriadis, Ilias N. Tziavos, Grigorios Tsokas, Alexandros Stampolidis. Gravity data inversion for Moho depth modeling in the Hellenic area. Pure and Applied Geophysics. 2015; 173 (4):1223-1241.
Chicago/Turabian StyleVassilios N. Grigoriadis; Ilias N. Tziavos; Grigorios Tsokas; Alexandros Stampolidis. 2015. "Gravity data inversion for Moho depth modeling in the Hellenic area." Pure and Applied Geophysics 173, no. 4: 1223-1241.
The GOCESeaComb project, funded by ESA in the frame of the PRODEX program, aims to utilize GOCE data within combination schemes in order to achieve high-quality and accuracy predictions related to Earth’s gravity field, sea level and dynamic ocean topography. In this work the results from the detailed validation of the latest GOCE, GOCE/GRACE and combined global geopotential models are presented referring to the fourth release of the models and the various strategies (TIM, DIR, GOCO, EIGEN-S/c) employed for their determination. The validation is performed following two approaches. The first one refers to the evaluation of the GGMs signal and error in the form of the provided degree and error variances. The second refers to an external evaluation of the GGMs against local gravity, GPS/Leveling data and deflections of the vertical. In this validation step we follow a spectral enhancement approach of GOCE GGMs, where EGM08 is used to fill-in the medium and high-frequency content along with RTM effects for the high and ultra high part. From the evaluation with GPS/Levelling benchmarks, it is concluded that the GOCE/GRACE GGMs provide improved accuracies compared to EGM2008 by about 2 cm in the spectral range between d/o 120–230. Finally, GOCE/GRACE GGMs manage to provide the same, as EGM2008, level of reduction to the local gravity anomalies, with a standard deviation at the 6.1–6.2 mGal level and marginally better residuals, at the sub-arcsec level in the reduction of deflections of the vertical.
I. N. Tziavos; George Vergos; V. N. Grigoriadis; E. A. Tzanou; D. A. Natsiopoulos. Validation of GOCE/GRACE Satellite Only and Combined Global Geopotential Models Over Greece in the Frame of the GOCESeaComb Project. International Symposium on Earth and Environmental Sciences for Future Generations 2015, 297 -304.
AMA StyleI. N. Tziavos, George Vergos, V. N. Grigoriadis, E. A. Tzanou, D. A. Natsiopoulos. Validation of GOCE/GRACE Satellite Only and Combined Global Geopotential Models Over Greece in the Frame of the GOCESeaComb Project. International Symposium on Earth and Environmental Sciences for Future Generations. 2015; ():297-304.
Chicago/Turabian StyleI. N. Tziavos; George Vergos; V. N. Grigoriadis; E. A. Tzanou; D. A. Natsiopoulos. 2015. "Validation of GOCE/GRACE Satellite Only and Combined Global Geopotential Models Over Greece in the Frame of the GOCESeaComb Project." International Symposium on Earth and Environmental Sciences for Future Generations , no. : 297-304.
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.
Stelios P. Mertikas; Antonis Daskalakis; Ilias N. Tziavos; George Vergos; Xenofon Fratzis; Achilleas Tripolitsiotis. First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities. Marine Geodesy 2015, 38, 249 -259.
AMA StyleStelios P. Mertikas, Antonis Daskalakis, Ilias N. Tziavos, George Vergos, Xenofon Fratzis, Achilleas Tripolitsiotis. First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities. Marine Geodesy. 2015; 38 (sup1):249-259.
Chicago/Turabian StyleStelios P. Mertikas; Antonis Daskalakis; Ilias N. Tziavos; George Vergos; Xenofon Fratzis; Achilleas Tripolitsiotis. 2015. "First Calibration Results for the SARAL/AltiKa Altimetric Mission Using the Gavdos Permanent Facilities." Marine Geodesy 38, no. sup1: 249-259.
Within the frame of the Elevation project, recently acquired collocated GPS/Leveling observations over trigonometric benchmarks (BMs) have been used for the evaluation of the recent GOCE/GRACE Global Geopotential Models (GGMs) and the unification of the Greek Local Vertical Datum (LVD). To this extent all available satellite-only and combined GOCE/GRACE GGMs were evaluated to conclude on the possible improvement brought by GOCE in the determination of the geoid over Greece. At a second stage, the present work focuses on the determination of the zero-level geopotential value W 0LVD for the Greek LVD. The estimation of W 0LVD was carried out using a least squares adjustment of Helmert orthometric heights, surface gravity disturbances and geopotential values computed from EGM2008 and GOCE/GRACE GGMs over the available GPS/Levelling BMs. Moreover, given that the BMs used belong to two distinct areas, i.e., one over Attica and another in Thessaloniki, the W 0LVD determination was carried out for each region separately, to conclude on the possible biases of the Hellenic LVD itself. From the evaluation of the GOCE/GRACE models it was concluded that the latest releases provide a significant, compared to EGM2008, improvement in the comparisons with the GPS/Levelling data, by as much as 3 cm, in terms of the standard deviation. Furthermore, the W 0LVD determined for the Greek LVD indicates a bias of about −4.95 m2/s2 compared to the conventional value of 62,636,856.0 m2/s2.
G. S. Vergos; V. D. Andritsanos; V. N. Grigoriadis; V. Pagounis; I. N. Tziavos. Evaluation of GOCE/GRACE GGMs Over Attica and Thessaloniki, Greece, and Wo Determination for Height System Unification. International Symposium on Earth and Environmental Sciences for Future Generations 2015, 101 -109.
AMA StyleG. S. Vergos, V. D. Andritsanos, V. N. Grigoriadis, V. Pagounis, I. N. Tziavos. Evaluation of GOCE/GRACE GGMs Over Attica and Thessaloniki, Greece, and Wo Determination for Height System Unification. International Symposium on Earth and Environmental Sciences for Future Generations. 2015; ():101-109.
Chicago/Turabian StyleG. S. Vergos; V. D. Andritsanos; V. N. Grigoriadis; V. Pagounis; I. N. Tziavos. 2015. "Evaluation of GOCE/GRACE GGMs Over Attica and Thessaloniki, Greece, and Wo Determination for Height System Unification." International Symposium on Earth and Environmental Sciences for Future Generations , no. : 101-109.
Topographic effects. The effects on gravity field functionals due to the masses residing outside the reference equipotential surface of the geoid and below the topographical surface of the Earth. The topographic, bathymetric, and compensated masses create a gravity signal that strongly dominates the short-wavelength band of the gravity spectrum. The effect of the gravitational attraction of the masses on the various observables of the gravity field (e.g., gravity anomalies and disturbances, geoid and quasi-geoid heights, deflections of the vertical, gravity gradients) is taken into account through appropriate reductions and has a significant contribution to gravity field modeling. The high-frequency contribution of topographic and bathymetric effects to gravity-field-related quantities is mainly due to the strong correlation of the short-wavelength gravity features with topography and bathymetry (Schwarz et al., 1990; Tziavos and Sideris, 20 ...
Ilias N. Tziavos; George Vergos. Topographic Effects. Encyclopedia of Solid Earth Geophysics 2014, 1 -8.
AMA StyleIlias N. Tziavos, George Vergos. Topographic Effects. Encyclopedia of Solid Earth Geophysics. 2014; ():1-8.
Chicago/Turabian StyleIlias N. Tziavos; George Vergos. 2014. "Topographic Effects." Encyclopedia of Solid Earth Geophysics , no. : 1-8.
Estimation of the zero-height geopotential level represented by W oLVD in a local vertical datum (LVD) is a problem of main importance for a wide range of geodetic applications related to different height frames and plays a fundamental role in the connection of traditional height reference systems into a global height system or even a modern geoid-based vertical datum. This paper aims primarily at the estimation of W oLVD for the continental part of Greece, with the use of surface gravity data and geopotential values computed from EGM08 in conjunction with GPS and orthometric heights over an extensive network which covers sufficiently the test area. The method used focuses on the estimation of W oLVD from a least squares adjustment scheme that is applied on the Helmert model for orthometric heights, using surface geopotential and gravity values (as obtained from EGM08 and the known 3D geocentric coordinates of each benchmark) along with the local Helmert heights over all network stations. Moreover, an attempt is made towards the modeling and removal of any height correlated errors in the available data according to this adjustment procedure. Different weighting schemes are tested, and, finally, some conclusions are drawn considering the accuracy of the obtained results.
V. N. Grigoriadis; C. Kotsakis; I. N. Tziavos; George Vergos. Estimation of the Reference Geopotential Value for the Local Vertical Datum of Continental Greece Using EGM08 and GPS/Leveling Data. International Symposium on Earth and Environmental Sciences for Future Generations 2014, 249 -255.
AMA StyleV. N. Grigoriadis, C. Kotsakis, I. N. Tziavos, George Vergos. Estimation of the Reference Geopotential Value for the Local Vertical Datum of Continental Greece Using EGM08 and GPS/Leveling Data. International Symposium on Earth and Environmental Sciences for Future Generations. 2014; ():249-255.
Chicago/Turabian StyleV. N. Grigoriadis; C. Kotsakis; I. N. Tziavos; George Vergos. 2014. "Estimation of the Reference Geopotential Value for the Local Vertical Datum of Continental Greece Using EGM08 and GPS/Leveling Data." International Symposium on Earth and Environmental Sciences for Future Generations , no. : 249-255.
The advent of the GOCE and GRACE missions during the last decade have brought new insights and promising results both in the static and time-variable representation of the Earth’s gravity field. The focus of this work is directed to the evaluation of most available Global Geopotential Models (GGMs) from GOCE and GRACE, both satellite only as well as combined ones. The evaluation is carried out over an extensive network of collocated GPS/Levelling benchmarks (BMs) which covers the entire part of continental Greece and with respect to the reductions the GGMs provide in existing gravity data in order to assess their performance in a scenario that a remove-compute-restore procedure would be followed for geoid determination. From the evaluation with GPS/Levelling BMs, it was concluded that the GOCE/GRACE GGMs provide an absolute accuracy at the 12–15 cm level, up to degree and order (d/o) 250, when considering the geoid omission error. This is comparable and in some cases better than the performance of EGM2008 in Greece. Moreover, the latest (Release 3) versions of the GGMs provide considerably better results compared to the earlier version by 1–5 cm. In terms of relative errors, GOCE/GRACE GGMs reach the 1 cm level for baselines between 50 and 60 km, while for longer ones, 80–90 km, their performance is analogous to the local geoid model and the ultra-high degree combined GGMs. Finally, GOCE/GRACE GGMs manage to provide the same, as EGM2008, level of reduction to the local gravity anomalies, with a std at the 26.7–27.8 mGal level, when evaluated up to d/o 250.
G. S. Vergos; Vassilios Grigoriadis; I. N. Tziavos; C. Kotsakis. Evaluation of GOCE/GRACE Global Geopotential Models over Greece with Collocated GPS/Levelling Observations and Local Gravity Data. International Symposium on Earth and Environmental Sciences for Future Generations 2014, 85 -92.
AMA StyleG. S. Vergos, Vassilios Grigoriadis, I. N. Tziavos, C. Kotsakis. Evaluation of GOCE/GRACE Global Geopotential Models over Greece with Collocated GPS/Levelling Observations and Local Gravity Data. International Symposium on Earth and Environmental Sciences for Future Generations. 2014; ():85-92.
Chicago/Turabian StyleG. S. Vergos; Vassilios Grigoriadis; I. N. Tziavos; C. Kotsakis. 2014. "Evaluation of GOCE/GRACE Global Geopotential Models over Greece with Collocated GPS/Levelling Observations and Local Gravity Data." International Symposium on Earth and Environmental Sciences for Future Generations , no. : 85-92.
Stelios P. Mertikas; Antonis Daskalakis; Ilias N. Tziavos; Ole B. Andersen; George Vergos; Achilles Tripolitsiotis; Vassilis Zervakis; Xenophon Frantzis; Panagiotis Partsinevelos. Altimetry, bathymetry and geoid variations at the Gavdos permanent Cal/Val facility. Advances in Space Research 2013, 51, 1418 -1437.
AMA StyleStelios P. Mertikas, Antonis Daskalakis, Ilias N. Tziavos, Ole B. Andersen, George Vergos, Achilles Tripolitsiotis, Vassilis Zervakis, Xenophon Frantzis, Panagiotis Partsinevelos. Altimetry, bathymetry and geoid variations at the Gavdos permanent Cal/Val facility. Advances in Space Research. 2013; 51 (8):1418-1437.
Chicago/Turabian StyleStelios P. Mertikas; Antonis Daskalakis; Ilias N. Tziavos; Ole B. Andersen; George Vergos; Achilles Tripolitsiotis; Vassilis Zervakis; Xenophon Frantzis; Panagiotis Partsinevelos. 2013. "Altimetry, bathymetry and geoid variations at the Gavdos permanent Cal/Val facility." Advances in Space Research 51, no. 8: 1418-1437.