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CCD-Astrometrie von Objekten des geostationären Ringes / M. Ploner (1996)

Public Titre : CCD-Astrometrie von Objekten des geostationären Ringes Titre original : [Astrométrie CCD des objets de l'anneau géostationnaire] Type de document : Rapport Auteurs : M. Ploner, Auteur Editeur : Vienne [Autriche] : Technische Universität Wien Année de publication : 1996 Collection : Geowissenschaftliche Mitteilungen, ISSN 1811-8380 num. 46 Importance : 133 p. Format : 21 x 30 cm Note générale : Bibliographie Langues : Allemand (ger) Descripteur : [Vedettes matières IGN] Géodésie physique [Termes IGN] astrométrie [Termes IGN] campagne d'observations [Termes IGN] chambre DTC [Termes IGN] champ de pesanteur terrestre [Termes IGN] orbitographie [Termes IGN] Pascal [Termes IGN] satellite géostationnaire [Termes IGN] satellite météorologique [Termes IGN] transformation de coordonnées Index. décimale : 30.40 Géodésie physique Résumé : (Auteur) The motions of artificial satellites are influenced by gravitational and non-gravitational perturbations. Of special interest among the gravitational perturbations is the anisotropy of the earth's gravitational field. Orbits of geostationary satellites are extremely sensitive to perturbations caused by the earth's equatorial ellipticity, which is described by the coefficients C22 and S22 in the harmonic series expansion of the geopotential. These coefficients cause resonant perturbations, which lead to a librating motion of geostationary objects around two defined points of the earth's equator. These points are located at longitude east 75' and longitude west 105' of Greenwich. The period of the librating motion varies between 1000 and 2000 days, changes in the semi-major axis can reach up to 35 km. On the other hand, these large variations allow for a very exact calculation of the above mentioned terms by means of observing geostationary satellites over a period of only a few weeks. Meteosat satellites are extremely useful for this type of study, because in contrast to communication satellites, orbit manoeuvres are carried out only every 2 or 3 months. In the course of this project, the Zeiss BNIK 75 ballistic camera of the Graz-Lustbühel satellite observation centre was adapted for CCD observations. Instead of the automatic plate-changing device, the CCD camera SITe 1024B (1024*1024 pixel, pixel size 24µm, backside illuminated, 1 pixel = 6.6") was mounted. Thanks to a special shuttering technique, the epoch registration of the observations can be carried out with an accuracy of a few microseconds. After installing the hardware, a two-month observation campaign of the satellites Meteosat 5 and Meteosat 6 as well as the Russian satellites Gorizont 11 and Statsionar-raduga 8 was carried out. Whereas, due to orbital manoeuvres, the orbits of the Meteosat satellites are very close to the theoretical geostationary orbit, the latter mentioned satellites have not been controlled for a quite long time. Inevitably this causes an increasing motion leading away from their geostationary orbit. They also are marked by heavy tumbling movements. The astrometric evaluation of the observations was carried out with the help of the program CCD. Its development was - besides the hardware installation - one of the main tasks of this project. This Windows application, which is written in Borland Pascal 7.0, permits the use of different polynomes for the transformation of the measured coordinates into tangent coordinates and makes use of various fit algorithms (Gaussian, Laplace and Cauchy distributions) for the calculation of the object coordinates. Usually the star tracker is switched off when observations of geostationary satellites are carried out. Therefore special attention has to be paid to the determination of the starting and final point of star trails. The reference stars can be selected from the PPM and the GSC catalogue. The calculated coordinates of the measured objects either refer to the true date and equinox or to the mean date and equinox (J2000.0). The orbit integration program ORBDET of the Astronomical Institute of the University of Berne was used for the calculation of the osculating orbital elements and the direct solar radiation pressure of all satellites. The residuals of the two Meteosat satellites did not show any remaining signals. The average deviation of a single observation was 0.5" for these extremely faint objects, which corresponds to less than 1/10 of the pixel size. This high accuracy was a consequence of calculating the satellite coordinates by fitting a two-dimensional Gaussian distribution. The situation concerning Gorizont 11 and Statsionar-raduga 8 is different. Significant signals were found in the residuals, which can be explained by extreme difficulties in establishing an appropriate model for the direct solar radiation pressure. While Meteosat satellites have a simple cylindrical shape, the Russian satellites are marked by large solar panels. The establishment of a model for the direct solar radiation pressure is further complicated because of the heavy tumbling movements. Finally, the geopotential coefficients C22 and S22 were calculated from observations of the satellites Meteosat 5 and 6. The results show an extremely low deviation from the corresponding numbers of the well-known gravity model JGM-3). By combining this data with observations made by the satellite observation centre Zimmerwald (Switzerland), the accuracy of the coefficients C22 and S22 could be further increased. The calculations result in the following values for the two normalized coefficients : C22 = 2.43923E-06 ± 3.9OE-10 S22 = -1.4003IE-06 ± 9.77E-11 The results show that CCD observations of geostationary satellites can contribute considerably to the determination of the above mentioned parameters. A further increase in accuracy is to be expected by the use of the Hipparcos star catalogue, which is going to be available in summer 1997. Numéro de notice : 55044 Affiliation des auteurs : non IGN Thématique : POSITIONNEMENT Nature : Rapport d'étude technique Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=59754

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