M. Fermi

ESTIMATION OF LOW DEGREE GEOPOTENTIAL COEFFICIENTS USING SLR DATA

Abstract Geodetic satellites have been providing the low frequency part of the geopotential models used for precise orbit determination purposes (e.g. JGM3, EGM96, …). Nevertheless they can be used to estimate the temporal variation of selected coefficients, helping to clarify the complex interrelations in the earth-ocean-atmosphere system. In this paper we present the two years long analysis of SLR data from the seven available geodetic satellites (Lageos I–II, Stella, Starlette, Ajisai, Etalon I–II) to recover monthly estimates of low degree geopotential coefficients; the results are obtained analysing the satellites separately and in proper combination. An accurate modelling of the satellite orbits is required in order to separate the geopotential coefficients: we assume as a priori geopotential the JGM3 model together with its associated tides and we take care of non-gravitational effects on the satellites by means of proper empirical estimated accelerations. The time series of the estimated coefficients (J2, J3, J4, J5) are inspected to detect the sub-annual perturbations related to seasonal variation of mass distribution. Huge residual seasonal signals in the orbit of Stella indicate a strong model deficiency related to the Suns influence on the environment. The remaining six satellites are homogeneously modelled and build up a three cycles per year oscillation on J2 and a seasonal oscillation (1 year and six month periods) revealed on the J4. The origin and possible causes of these signals are further discussed in the text. We also present a preliminary J 2 estimate, using twelve years of Lageos-I and Lageos-II observations, that is compared with previous obtained values.

A contribution in the estimation of tectonic motion in crucial areas: the CGS96 SLR solution

Abstract The CGS96 is a global SLR solution based on the reduction of the whole available data set for the Lageos I and II satellites, starting from 1986. The accurate modelling adopted and the huge quantity of data of continuously growing quality, lead to reliable estimates of geodetic and orbital parameters. The solution collects a wide variety of parameters, daily earth orientation parameters, coordinates and site velocities of almost 60 sites, plus coordinates for sites with short tracking history, orbital parameters and non-modelled acceleration for both satellites. The core of the paper is the review of the tectonic results and their comparison with the models: the pointers of the goodness of the solution are those results regarding sites centrally located on the major plates, henceforth comparable to model predictions, being a prerequisite for the reliability of the estimates for the crucial sites. The tectonic results are particularly interesting in those areas not suitably described by the global models, such as the Mediterranean, periodically occupied by mobile systems.

The Italian GPS Fiducial Network: Services and products

Abstract Starting from 1995 new permanent GPS stations have been established in Italy to constitute the Italian GPS Fiducial Network (IGFN), together with the fundamental station in Matera. The present operational stations are: Cagliari, Matera, Medicina, Noto and Venezia, while a new station in Genova is under set-up. The whole data set acquired is collected at Matera Space Geodesy Center (CGS of the Italian Space Agency). Since August 1996, the Matera ASI/CGS has become EUREF Local Analysis Center, taking part in the IGS Pilot Project for densification of the ITRF through regional GPS analysis. The present ASI/CGS contribution consists of weekly solutions of 10 GPS sites providing the coordinates of the 5 stations of the IGFN (mentioned above) plus 5 foreign stations: Ebre, Madrid, San Fernando, Villafranco and Wettzell. In this paper a full description of all the services associated to the IGFN will be given and the results based on 10 months of continuous data analysis will be discussed. Furthermore future developments of the IGFN are outlined and possible applications in studies of regional deformations are shown.

Investigation on the combination of space geodetic techniques

Abstract The Space Geodesy Center (CGS) of the Italian Space Agency (ASI), located near Matera (Italy), hosts permanent Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR) and Global Positioning System (GPS) systems and manages the other receivers of the Italian GPS Fiducial Network. The presence of these collocated space geodetic systems in the area and the data analysis activities usually carried out at the CGS for each of the three techniques makes the construction of fully integrated solutions a natural objective. This paper describes the preliminary work carried out to assess the consistency of the various geodetic solutions and to explore their compatibility in order to combine the information content from the different techniques. The consistency of the CGS geodetic global solutions providing site positions and velocities is assessed, with particular attention to the Mediterranean and European areas. In addition a direct comparison between baselines and baseline rates for the collocated european GPS and VLBI sites is made.

Earth Orientation Parameters Measured by Space Geodesy Techniques

Nowadays the space geodetic techniques are providing the geophysicists with a lot of highly accurate products, as the EOP series. Even if at an international level a huge effort to standardize some guidelines to be followed in the analyses has been done (e.g. IERS standards), the peculiar features of each technique permeate the results and make their comparison and/or integration a topic to be handled with care.

Height Time Series of the Italian GPS Fiducial Network compared with VLBI and SLR Estimations

In this work, we present the results of the determinations of vertical motions obtained from the analysis of data coming from different space geodetic techniques, Global Positioning System (GPS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI). All the geodetic solutions have been computed at the Centre of Space Geodesy (CGS) at Matera.

Nearly Diurnal Structure of Polar Motion Analysing Lageos SLR Data

Since more accurate and precise measurements are available in space geodetic techniques, there is a growing interest for the scientific community in studying the short periodic fluctuation of the Earth’s rotation axis. In particular subtle nearly diurnal variations of the orientation of the rotation axis, as viewed from the Earth, reflect dynamical properties of the deep interior of the Earth. Recently Very Long Baseline Interferometry (VLBI) data analysis sets a stringent upper limit to the nearly diurnal wobble whose amplitude is well below the milliarc second (mas) level [Herring et al., 1991].

Tectonic Motion from LAGEOS SLR Data Using a Network Adjustment Technique

In this paper we present the most recent results from the analysis of the LAGEOS SLR data concerning the present plate tectonic motion. The horizontal site velocities are estimated via a network adjustment by a weighted least squares technique; the geodesic rates between pairs of stations, estimated by a linear fitting of the semiannual values are modeled by an observation equation, linear in latitude and longitude time derivatives, assumed as solve-for-parameters. This observation equation is based on the mapping of the reference ellipsoid onto the so called ‘image sphere’, properly transforming geodesics on ellipsoid to great circle arcs on the sphere. The method is applied to the results of the TPZ90 solution in the case of a worldwide network.

European Baselines Determined with LAGEOS: The TPZ-88.1 Solution

The possibility of monitoring a large scale geodetic network in southern Europe and East Mediterranean Area represents one of the most far reaching perspectives in modern experimental research in geodynamics. In the WEGENER/MEDLAS project the approach to such a monitoring is based on the LAGEOS satellite being tracked by a number of fixed and mobile stations which locally densify the worldwide laser tracking network. The analysis includes, but is not limited to, the repeated estimation of the baselines. Our main objective is to reliably determine the deformation of the WEGENER geodetic network (see fig.l) and in this case the time series of the baselines estimates are analysed in conjunction with other results, such as polar motion, LAGEOS kinematical and dynamical model, geological survey and local field measurements including the monitoring of the performance of LASER stations partecipating to the WEGENER project.