1,720,964 research outputs found
Optimal multi-step collocation: application to the space-wise approach for GOCE data analysis
No full textCollocation is widely used in physical geodesy. Its application requires to solve systems with a dimension equal to the number of observations, causing numerical problems when many observations are available. To overcome this drawback, tailored step-wise techniques are usually applied. An example of these step-wise techniques is the space-wise approach to the GOCE mission data processing. The original idea of this approach was to implement a two-step procedure, which consists of first predicting gridded values at satellite altitude by collocation and then deriving the geo-potential spherical harmonic coefficients by numerical integration. The idea was generalized to a multi-step iterative procedure by introducing a time-wise Wiener filter to reduce the highly correlated observation noise. Recent studies have shown how to optimize the original two-step procedure, while the theoretical optimization of the full multi-step procedure is investigated in this work. An iterative operator is derived so that the final estimated spherical harmonic coefficients are optimal with respect to the Wiener–Kolmogorov principle, as if they were estimated by a direct collocation. The logical scheme used to derive this optimal operator can be applied not only in the case of the space-wise approach but, in general, for any case of step-wise collocation. Several numerical tests based on simulated realistic GOCE data are performed. The results show that adding a pre-processing time-wise filter to the two-step procedure of data gridding and spherical harmonic analysis is useful, in the sense that the accuracy of the estimated geo-potential coefficients is improved. This happens because, in its practical implementation, the gridding is made by collocation over local patches of data, while the observation noise has a time-correlation so long that it cannot be treated inside the patch size. Therefore, the multi-step operator, which is in theory equivalent to the two-step operator and to the direct collocation, is in practice superior thanks to the time-wise filter that reduces the noise correlation before the gridding. The criteria for the choice of this filter are investigated numerically
A simulated space-wise solution using GOCE kinematic orbits
No full textOne of the purposes of the GOCE data analysis is to derive a GOCE-only gravity field model. To this aim satellite gradiometry has to be complemented with satellite tracking data in order to estimate the low degrees of the geopotential spherical harmonic expansion.
In this paper a full space-wise solution based on simulated GOCE data is presented. In particular, kinematic orbits are used for the determination of the low degree coefficients of the geopotential. This implies a preprocessing of these orbits for data gap interpolation, outlier detection and velocity estimation from satellite positions.
The accuracy of the final spherical harmonic coefficients confirms that the proposed method can be used to produce a GOCE-only solution based on kinematic orbits. This solution is less accurate than the solution based on reduced dynamic orbits, but it is less dependent on a-priori information
Utilizzo dei dati della missione GOCE per la stima del geoide locale sul territorio della Regione Piemonte
No full text
GOCE: a full-gradient simulated solution in the space-wise approach
No full textGOCE will be the first satellite gradiometric mission with the purpose of estimating the stationary gravitational field to a high accuracy and spatial resolution. The on-board gradiometer will provide measurements of the second derivatives of the gravitational potential along the instrumental axes, which are used together with the satellite tracking data to retrieve the spherical harmonic coefficients of the geopotential model.
To this aim, a possible strategy is the so-called space-wise approach, basically exploiting the spatial correlation between data. This requires a pre-processing procedure consisting in filtering the observations in time and then producing grids of second derivatives and potential on a boundary sphere at mean satellite altitude.
In the past, this approach was applied only to simulated second radial derivatives. A time-series of the anomalous potential along the orbit was also used in order to improve the estimate at low frequencies. In this work we aim at integrating the information coming from all the three diagonal components, as measured in the gradiometer reference frame. In particular the performance of the space-wise approach has been tested on the basis of realistic end-to-end simulated data, showing that the proposed method is able to estimate the spherical harmonic coefficients up to degree and order 200. The results have to be evaluated also taking into account the time-length of the available data, namely one month, which is critical for any space-wise approach
An error model for the GOCE space-wise solution by Monte Carlo method
No full textThe aim of the data analysis of the GOCE satellite mission is to estimate spherical harmonic coefficients of the gravitational potential and the corresponding error covariance matrix. However this error estimation is generally very complicated and computationally demanding because of the large number of observations. In particular, this is true for the space-wise approach, which is a collocation solution implemented by several steps.
Up to now the error covariance matrix for this approach has been computed under simplifying hypotheses. In order to get a more realistic assessment of the true estimation error a Monte Carlo method can be applied. This requires the generation of several stochastic simulations and the computation of the corresponding solutions. This procedure is made numerically feasible by storing and not recomputing every time the operators that form the space-wise approach. Tests on realistic scenarios are performed and positive results are achieved
Combination of ground gravimetry and GOCE data for local geoid determination: a simulation study
No full text
Exploitation of GOCE data for a local estimate of gravity field and geoid in the Piemonte area (northern Italy)
No full text
- …
