Partenaires et organisations internationales
(Anglais)
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University College, London (UK); National Technical University of Athens (EL); University of Oxford (UK); Institute of Physics of the Earth, Paris (F); Delft University of Technology (NL); National Observatory of Athens (EL)
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Résumé des résultats (Abstract)
(Anglais)
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The SING project ran from 1 December 1997 to 30 November 2000. The main aim of the project was to study the strain field of Greece through seismic and satellite geodetic investigations, and to build up a picture of hazard assessment. The ETH group was mainly involved in the working packeges a) 'Geodetic Strain' and b) 'Mitigation of refracion effects due to water vapor'.
a) The main bulk of GPS observations consisted in a number of repeated observed field stations and a permanently operated site at Dionysos (near Athens). The latter serves as important reference station for the individual GPS campaigns. By combining the data of Dionysos with the data of several IGS stations daily solutions for the coordinates of Dionysos were calculated. These time series were rotated to a Eurasia fixed reference frame and filtered using a common-mode approach. The nearly linear motion of the station allowed to deduce a three-dimensional velocity vector [Peter 2000].
In the years 1998, 1999, and 2000, a total of about 60 points were reoccupied by the ETH group. These sites had already been measured in earlier campaigns in the years 1993 - 1996. These older data were reprocessed and combined with the newly acquired data in order to estimate most recent velocities. The results obtained showed a consistent velocity field with relative rates ranging from a few mm/yr in the north-west to more than 30 mm/yr on the island of Crete [Cocard et al., 1999].
The velocity field was then used to calculate the strain rate field using the method of collocation. The area considered extended over the entire Eastern Mediterrean region and the Near East [Kahle et al., 2000]. The dominant features observed are the large dilatation in the Gulf of Corinthos, the low strain rates in the Central Aegean area, and the compression along the West Hellenic Arc. The Kephalonia Fault Zone and the North Aegean Trough are dominated by large shear strain [Kahle et al., 1999].
b) Changes in the water vapor content of the atmosphere during a space geodetic measurement cause path delay fluctuations which result in significant errors of the coordinate solution. Remote sensing techniques have been developed for the determination of integrated precepitable water vapor (IPWV). Water Vapor Radiometers (WVR) measure the radiation intensity of the atmosphere in a frequency band usually ranging from 20 to 32 GHz. Their deployment is especially useful in geodetic applications where the accuracy of the vertical component is crucial. A comparison of different types of WVR's were carried out. A new technique, called GPS-Meteorology, has been developed which reverses the refraction problem of GPS measurements by using continuously operating receivers whose positions are accurately known [Kruse et al, 1999]. A new application in the field of GPS is applying the Tomographic Method to resolve spatial structures of tropospheric water vapor. The atmosphere above the GPS-receivers network is divided into a box model and the wet microwave refractivities within these boxes are calculated using a tomographic software package called AWATOS (Atmospheric Water Vapor Tomography Software) [Kruse, 1999]. The Solar Atmospheric MOnitoring Spectrometer (SAMOS) is another approach for determination of atmospheric water vapor content, which is especially designed for high-resolution measurements of water vapor absorption lines using solar radiation. A corresponding software package called COMEDIE (Collocation of Meteological Data for Interpretation and Estimation of Tropospheric Path Delays) has been developed at ETH Zurich to calculate the 4-dimensional (spatial and temporal) distribution of refractivity from surface measurements, ballon soundings and airborne measurements. All different remote sensing techniques were deployed, compared and validated during test campains.
An optimal estimation of path delay for GPS networks such as in Greece requires an advanced deployment of ground-based systems measuring the water vapor independently from each other. The number of remote sensing devices depends on the density of GPS network and distance between the receivers as well as availability of continuously operating GPS stations.
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