**Computation of the marine gravity gradient tensor from satellite altimetry data**
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Amin, Hadi; Safari, Abdolreza; sharifi, mohammad ali; foroughi, ismaeil
university of tehran, IRAN, ISLAMIC REPUBLIC OF
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With the appearance of satellite altimetry and after the launch of the first altimetry satellite SKYLAB in 1973, a new window was opened in oceanic, marine and Earth sciences. Development and maturity of the sensor technology and new satellite altimetry missions in the recent years led to an incredible evolution in geodesy and modeling of the Earth's gravity field. Satellite altimetry provides a huge source of information for geoid determination with high accuracy and spatial resolution. The information from this method is a suitable alternative for marine gravity data in the high frequency modeling of the Earth's gravity field in sea regions. Marine gravity observations contain high noise levels due to environmental effects. Further, it is not possible to model the high frequencies of the Earth's gravity field in a global coverage. Modeling the Earth's gravity field in sea for geoid determination and high precision explorations has been always of great significance in geodetic and geophysical sciences. In particular, the gravity gradient tensor gives the most information among other measurements of the gravity field like the gravity anomaly. In this paper, a new approach is introduced for the determination of the gravity gradient tensor in the sea surface based on satellite altimetry and using radial basis functions. The particular property of these functions is their high ability for local modeling of the Earth's gravity field. The algorithm used in this method is as follows: (1) Determination of the geoid heights from satellite altimetry data; (2) Calculation of the potential anomaly from geoid undulations using the inverse ellipsoidal Bruns formula; (3) Determination of the true potential at reference ellipsoid; (4) Elimination of the effects of the reference ellipsoid harmonics up to the degree and order of 240 and the centrifugal field, and determination of the residual potential at reference ellipsoid; (5) Local Modeling of the Earth’s gravity field based on radial basis functions using the residual potential data; (6) Restoring the gravitational field effect of the ellipsoidal harmonic expansion; (7) Computation of the gravity gradient tensor using the gravity gradient operator in the gravity potential determined in the previous step. Advantages of the approach used in this paper include the use of satellite altimetry data in geoid determination, and automatic determination of the unknown parameters of the radial basis functions by the least-squares method using only 0.7% of the observations as the primary centers of the radial basis functions. In order to investigate the proposed method, the gravity gradient tensor is determined for the Persian Gulf region based on satellite altimetry data and radial basis functions. This tensor is then compared with the gravity gradient tensor resulted from the Earth’s gravity field modeling in the sea by harmonic splines, and the results are presented.