Πλοήγηση ανά Συγγραφέα "Vergos, G. S."
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Τεκμήριο Estimation of the Geopotential Value W0 for~the~Local Vertical Datum of Argentina Using~EGM2008 and GPS/Levelling Data W 0 LVD(2015-04-09) Tocho, C.; Vergos, G. S.The main purpose of this paper is to estimate the zero-height geopotential value for the Argentinean Local Vertical Datum (LVD). The methodology is based on the computation of the mean geopotential offset between the value W 0 = 62,636,856.0 m2 s−2, selected as reference in this study, and the unknown geopotential value of the LVD (W 0 LVD ). This estimation is based on the combination of ellipsoidal heights, levelled heights (referring to the LVD), and some physical parameters derived from the EGM2008 model (namely, geopotential values, gravity values, and geoid undulations). This combination is performed following two approaches: The first one compares levelled heights and geopotential values derived from the EGM2008 model using the Least Squares method to increase the robustness of the adjustment, while the second one analyses the differences between GPS/Levelling and EGM2008 geoid undulations. Both approaches are evaluated at more than 540 benchmarks (BMs) belonging to the vertical network of Argentina. The numerical computations include in addition the assessment of possible correlations of the estimated zero-height geopotential value with the height of the included BMs. The results show that the best possible estimation at present is 62,636,853.9 m2 s−2; however, it is necessary to improve these computations by including proper physical heights (instead of the levelled ones) and global gravity models containing GOCE data.Τεκμήριο Estimation of the Reference Geopotential Value for the Local Vertical Datum of Continental Greece Using EGM08 and GPS/Leveling Data(2014) Grigoriadis, V. N.; Kotsakis, C.; Tziavos, I. N.; Vergos, G. S.Estimation of the zero-height geopotential level represented by W o LVD 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 o LVD 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 o LVD 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.Τεκμήριο Evaluation of GOCE/GRACE Derived Global Geopotential Models over Argentina with Collocated GPS/Levelling Observations(2014) Tocho, C.; Vergos, G. S.; Pacino, M. C.This paper presents the results of the evaluation of recent GOCE/GRACE Global Geopotential Models (GGMs) over Argentina. Since the Gravity and steady state Ocean Circulation Explorer (GOCE) dedicated satellite gravity field mission was launched in March 2009, several global geopotential models have been computed and released. GOCE’s mission was designed to provide models of the Earth’s gravity field on a global scale with high-accuracy in the medium wavelength spectral band (maximum degree/order 200–250). Comparisons of geoid heights derived from different GGMs with GPS/Levelling derived geoid heights over Argentina have been carried out in both absolute and relative sense, to assess and validate the accuracy of GGM models over the entire country. The analysis has been carried out with actual GOCE-only, GOCE/GRACE and combined global gravity field models. In all cases, EGM2008 has been used as the baseline model, since it provides the overall best results. From the results, it was concluded that the latest Release 3 GOCE-only, TIM and SPW, GGMs provide improved accuracies by 1–4 cm compared to the Release 1 models. As far as the combined GOCE/GRACE models, GOCO and DIR, are concerned, the overall best results come from the Release 1 of the DIR model, probably due to the a-priori information from EIGEN5C used in its development. The Release 3 version of the GOCO GGMs improves the Release 1 model by 4 cm, while the same level of improvement is found between the Release 3 and Release 2 of the DIR GGMs.Τεκμήριο Validation of GOCE/GRACE satellite only and combined global geopotential models over Greece in the frame of the GOCESeaComb project(2015-01) Tziavos, Ilias N.; Vergos, G. S.; Grigoriadis, V. N.; Tzanou, E. A.; Natsiopoulos, D. A.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.