ESTUDIO PRELIMINAR DE GEOIDE MARINO EN EL OCÉANO ATLÁNTICO ARGENTINO

The objective of this paper is to present the studies conducted for the determinationof marine geoid model in the Atlantic coastal region of Argentina using satellitealtimetry and shipborne gravity data. The altimetry data of the satellite altimetrymission ERS1 and shipborne gravity data were used to compute a pure altimetricgeoid and a pure gravimetric geoid, respectively using the well known removerestoretechnique. The effect of the Quasi-Stationary Sea Surface Topography(QSST) and the effect of the Sea Surface Variability (SSV)were taken into accountduring the processing of the different solutions. From the results achieved it wasconcluded that the pure altimetric geoid has an accuracy of near 8 cm in someareas, while the pure gravimetric solution gives poorer results. The accuracy of thefinal geoid models was assessed through comparisons with stackedTOPEX/POSEIDON (T/P) SSHs, known for their high precision. In order toimprove the accuracy of the gravimetric geoid, a combined solution using theMultiple input –Multiple output system theory , known as MIMOST were used.MIMOST allow to combine heterogeneous data in the spectral domain. Thecombination of satellite and shipborne data with the proposed algorithm improvesthe accuracy of the gravimetric geoid model by about 2 cm.En este trabajo se presentan estudios realizados sobre la determinación de geoidesmarinos preliminares en el océano Atlántico próximo a Argentina, utilizando datosde altimetría satelital y datos de gravedad marina. Los datos correspondientes a lamisión espacial de altimetría satelital ERS1 y los datos de gravedad medidos enbarco permitieron determinar un geoide altimétrico puro y un geoide gravimétricopuro en el océano Atlántico argentino utilizando la técnica remover-restaurar. Elefecto de la componente casi estacionaria de la topografía del mar (QSST) y elefecto de la variabilidad de la superficie del mar (SSV) se tuvieron en cuentadurante el procesamiento de las diferentes soluciones. De los resultados obtenidos,se concluye que el geoide altimétrico puro tiene una exactitud de 8 centímetros enalgunas áreas mientras que los resultados de la solución gravimétrica pura son máspobres. La exactitud de los modelos de geoide finales se evalúo a través decomparaciones con datos de altura de la superficie del mar (SSHs) medidos por lamisión TOPEX/POSEIDÓN (T/P), conocidas por su gran precisión. Con el fin demejorar la exactitud del geoide gravimétrico se obtuvo una solución combinadautilizando la Teoría de Múltiple Entrada-Múltiple Salida, conocida como MIMOSTque permite la óptima combinación de datos heterogéneos en el dominio espectral.La combinación de la solución satelital pura con la solución obtenida con los datosde gravedad marina utilizando el algoritmo propuesto, mejoraron la exactitud,comparada con el geoide gravimétrico puro, en aproximadamente 2 centímetros.En este trabajo se presentan estudios realizados sobre la determinación de geoidesmarinos preliminares en el océano Atlántico próximo a Argentina, utilizando datosde altimetría satelital y datos de gravedad marina. Los datos correspondientes a lamisión espacial de altimetría satelital ERS1 y los datos de gravedad medidos enbarco permitieron determinar un geoide altimétrico puro y un geoide gravimétricopuro en el océano Atlántico argentino utilizando la técnica remover-restaurar. Elefecto de la componente casi estacionaria de la topografía del mar (QSST) y elefecto de la variabilidad de la superficie del mar (SSV) se tuvieron en cuentadurante el procesamiento de las diferentes soluciones. De los resultados obtenidos,se concluye que el geoide altimétrico puro tiene una exactitud de 8 centímetros enalgunas áreas mientras que los resultados de la solución gravimétrica pura son máspobres. La exactitud de los modelos de geoide finales se evalúo a través decomparaciones con datos de altura de la superficie del mar (SSHs) medidos por lamisión TOPEX/POSEIDÓN (T/P), conocidas por su gran precisión. Con el fin demejorar la exactitud del geoide gravimétrico se obtuvo una solución combinadautilizando la Teoría de Múltiple Entrada-Múltiple Salida, conocida como MIMOSTque permite la óptima combinación de datos heterogéneos en el dominio espectral.La combinación de la solución satelital pura con la solución obtenida con los datosde gravedad marina utilizando el algoritmo propuesto, mejoraron la exactitud,comparada con el geoide gravimétrico puro, en aproximadamente 2 centímetros

Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece

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

GOCE GGM analysis through wavelet decomposition and reconstruction and validation with GPS/Leveling data

Over the last decade, wavelets (WL) have been exploited widely in all fields of geosciences while they have provided significant outcomes in analyzing gravity field related data in the frequency domain. In this work, we focus on the spectral analysis of GOCE, GOCE/GRACE and combined Global Geopotential Models (GGMs) through wavelet decomposition, filtering and reconstruction in order to improve their performance as to their spectral content in the higher bands of the spectrum. The GGMs evaluated refer to the latest DIR-R4, TIM-R4 and GOCO03s models, which are compared with local GPS/Leveling geoid heights and gravity anomalies, while EGM2008 is used as a reference. Within a wavelet multi-resolution analysis, both gravity anomalies and geoid heights are analyzed to derive their approximation and detail coefficients for various levels of decomposition, which correspond to different spatial scales. The spectral content at each level is analyzed in order to conclude on the gravity field signal power that GOCE/GRACE GGMs represent compared to EGM2008, especially in the targeted waveband up to 110-150 km. Moreover, various types of low-pass and thresholding denoising filters are investigated to remove high-frequency information from the low resolution GOCE models and adjust the WL reconstruction, respectively. The model synthesis that follows, through coefficient reconstruction, aims at the generation of new synthesized GGMs, where both GOCE, GRACE and EGM2008 information is used, in order to investigate possible improvements in the representation of the Earth’s gravity field. Validation of the synthesized combined GGMs with available GPS/Leveling geoid heights and terrestrial gravity anomalies is performed, to further assess the improvement brought by the WL analysis. 

GOCE/GRACE GGM evaluation over Attica and Thessaloniki, Greece and local geoid modelling in support of height unification

Within the frame of the “Elevation” project, supported by the action “Archimedes III – Funding of research groups in T.E.I.”, co-financed by the E.U. and Greek national funds, an extensive evaluation of the latest GOCE, GOCE/GRACE and combined GGMs has been carried out. The evaluation was performed using a set of collocated GPS and levelling BMs covering the regions of Attica and Thessaloniki. To this extent the latest satellite-only and combined GOCE/GRACE GGMs were evaluated to conclude on the possible improvement brought by GOCE, given the two main methodologies used for the GGM development (DIR and TIM) and the latest releases of GOCE data (Release 5). For these GGMs, local height transformation parameters have been determined, employing low and higher order parametric models, to accommodate surveying and engineering applications. Moreover, local geoid models have been determined for the two areas under study through the well-known Multiple-Input Multiple-Output System Theory (MIMOST) method, employing GOCE GGMs and the local GPS/levelling data. The so-determined geoid models are validated against the set of available GPS/levelling BMs and conclusions are drawn w.r.t. the improvement brought by GOCE in resolving the lower and medium band of the gravity field spectrum with higher accuracy.

Variation of some planetary seismic hazard indices on the occasion of Lefkada, Greece, earthquake of 17 November, 2015

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

Ionospheric turbulence from TEC variations and VLF/LF transmitter signal observations before and during the destructive seismic activity of August and October 2016 in Central Italy

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

Use of Modis Satellite Images for Detailed Lake Morphometry: Application to Basins with Large Water Level Fluctuations

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

Absolute Calibration of the European Sentinel-3A Surface Topography Mission over the Permanent Facility for Altimetry Calibration in west Crete, Greece

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

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

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