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407 results on '"Mertikas, Stelios P."'

3. Absolute Calibration of Sentinel-3A and Jason-3 Altimeters with Sea-Surface and Transponder Techniques in West Crete, Greece

Author

Mertikas, Stelios P., Donlon, Craig, Femenias, Pierre, Mavrocordatos, Constantin, Galanakis, Demitris, Guinle, Thierry, Boy, Francois, Tripolitsiotis, Achilles, Frantzis, Xenophon, Tziavos, Ilias N., Vergos, Georgios S., Freymueller, Jeffrey T., Series Editor, Sánchez, Laura, Assistant Editor, Mertikas, Stelios P., editor, and Pail, Roland, editor

Published
2020
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10. The ESA Permanent Facility for Altimetry Calibration in Crete: Advanced Services and the Latest Cal/Val Results

Author

Mertikas, Stelios P., primary, Donlon, Craig, additional, Kokolakis, Costas, additional, Piretzidis, Dimitrios, additional, Cullen, Robert, additional, Féménias, Pierre, additional, Fornari, Marco, additional, Frantzis, Xenophon, additional, Tripolitsiotis, Achilles, additional, Bouffard, Jérôme, additional, Di Bella, Alessandro, additional, Boy, François, additional, and Saunier, Jerome, additional

Published
2024
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15. Contributors

Author

Abdelkader, Mohamed, primary, Al-Obeidat, Feras, additional, Al Awadi, Saed, additional, AlBlooshi, Durra M., additional, Ali, Ayub, additional, Astitha, Marina, additional, Banerjee, Neil, additional, Bataineh, Mahmoud, additional, Bernabeo, R. Alberto, additional, Boura, Angeliki, additional, Dimitropoulos, Vasileios, additional, Elhakeem, Abubaker A., additional, Elshorbagy, Walid A., additional, Gallampois, Christine M.J., additional, Ghrefat, Habes, additional, Hamza, Waleed, additional, Howari, Fares M., additional, Imran, Hassan D., additional, Iqbal, Jibran, additional, Kamareddine, Lina A., additional, Karydis, Vlassis A., additional, Khan, Qasim, additional, Khan, Niaz, additional, Klingmüller, Klaus, additional, Lelieveld, Jos, additional, Maraqa, Munjed A., additional, Marir, Farhi, additional, Mavrocordatos, Constantine, additional, Maximenko, Nikolai A., additional, Mertikas, Stelios P., additional, Mickus, Kevin, additional, O. Mohamed, Abdel-Mohsen, additional, Paleologos, Evan K., additional, Paleologos, Katrina E., additional, Partsinevelos, Panagiotis, additional, Rao, Madduri V., additional, Rodrigues, Valéria G.S., additional, Samitas, Aristeidis, additional, Schymanski, Emma L., additional, Selim, Mohamed Y.E., additional, and Siriopoulos, Costas, additional

Published
2021
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20. Absolute Calibration of Sentinel-3A and Jason-3 Altimeters with Sea-Surface and Transponder Techniques in West Crete, Greece

Author

Mertikas, Stelios P., primary, Donlon, Craig, additional, Femenias, Pierre, additional, Mavrocordatos, Constantin, additional, Galanakis, Demitris, additional, Guinle, Thierry, additional, Boy, Francois, additional, Tripolitsiotis, Achilles, additional, Frantzis, Xenophon, additional, Tziavos, Ilias N., additional, and Vergos, Georgios S., additional

Published
2019
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22. Absolute Calibration of the Chinese HY-2B Altimetric Mission with Fiducial Reference Measurement Standards

Author

Mertikas, Stelios P., primary, Lin, Mingsen, additional, Piretzidis, Dimitrios, additional, Kokolakis, Costas, additional, Donlon, Craig, additional, Ma, Chaofei, additional, Zhang, Yufei, additional, Jia, Yongjun, additional, Mu, Bo, additional, Frantzis, Xenophon, additional, Tripolitsiotis, Achilles, additional, and Yang, Lei, additional

Published
2023
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25. A new isotropic covariance function on the sphere based on a self-convolving spherical cap

Author

Piretzidis, Dimitrios, Kotsakis, Christopher, Mertikas, Stelios P., and Sideris, Michael G.

Abstract

Modeling the stochastic behavior of global-scale geophysical signals, such as the Earth’s external gravity field, requires the use of valid (i.e., positive-definite) covariance functions on the sphere. The construction of such functions is still a challenging task, especially when a compactly-supported (i.e., finite) covariance function is needed. A standard approach for deriving a positive definite function is by convolving any function with itself. Accounting for the convolution-multiplication duality, the spectrum of the resulting function is always positive, since it is equal to the multiplication of the spectrum of the self-convolving function with itself. The validity of the resulting function can therefore be easily established. In this study, we utilize this property and develop a new compactly-supported covariance function on the sphere based on the self-convolution of a spherical cap. The shape of this new covariance function resembles other models developed using the self-convolution property. Two representative examples are the spherical covariance function, which is based on the self-convolution of a two-dimensional ball, and the more generalized “Euclid’s hat” function, which is described by the self-convolution of an n-dimensional ball. Although the spatial representation of our newly-developed covariance function is given by a somewhat involved mathematical expression, its spectral representation is rather simple and only requires the calculation of spherical harmonic coefficients of a spherical cap. The latter are also known as Pellinen smoothing coefficients and are commonly evaluated by a well-known recurrence relation for local gravity field modeling applications., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Published
2023

26. Isotropic windows and filters on the sphere

Author

Piretzidis, Dimitrios, Kotsakis, Christopher, Mertikas, Stelios P., and Sideris, Michael G.

Abstract

The study of geophysical data on global scale requires the modification of conventional spectral analysis and signal processing techniques from the real line to the sphere. These techniques often depend on the use of window functions (e.g., to improve the detection of periodic constituents and to suppress ripple effects in the impulse response of digital filters). Normalized window functions are also utilized as averaging filters. In this work, we present a comprehensive list of window functions for the analysis of signals defined on a spherical surface. We focus on classic tapering windows that are systematically used in signal processing applications, such as the Bartlett, Welch, generalized cosine, Bartlett-Hann and Tukey windows. The approach followed for their adaptation to the sphere results in isotropic (i.e., rotationally symmetric) window functions. We also examine their related filter kernels and provide expressions for their representation in the spatial and spectral domains. The spectral representation corresponds to a set of spherical harmonic coefficients, the calculation of which is entirely based on recurrence relations. We utilize these newly-developed relations to evaluate the spectrum of all filter kernels and compare their main characteristics, such as the main lobe width, first side lobe level and side lobe decay rate. Since the majority of these windows and filters have not been examined before, the present work extends significantly the current methods for localizing and filtering geophysical signals on the sphere., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Published
2023

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