ASTROMETRIC POSITIONING OF GEOSTATIONARY SATELLITES (PASAGE)
T. L´pez Moratalla,1 F. J. Montojo,1 C. Abad,2 J. Pizarro,3 J. L. Mui˜os,1 J. Palacio,1 and P. Galindo3 o n RESUMEN El principal objetivo del proyecto PASAGE es utilizar observaciones astrom´tricas desde tierra para obtener e efem´rides precisas de sat´lites geoestacionarios ydeterminar sus orbitas. Este uso de la astrometr´ es bastante e e ´ ıa distinto del habitual y requiere el desarrollo de t´cnicas y algoritmos especiales para reducir las observaciones. e ABSTRACT The PASAGE project major goal is to use earth-based astrometric observations to calculate precise ephemeris of geostationary satellites and for orbital determination of these satellites. This special use ofastrometry is quite diﬀerent from classical one and has required the development of the necessary techniques and algorithms for processing the observations.
Key Words: astrometry — ephemerides — space vehicles
© 2009: Instituto de Astronomía, UNAM - 12th IAU Regional Latin American Meeting of Astronomy Ed. G. Magris, G. Bruzual, & L. Carigi
1. INTRODUCTION Precise ephemerides ofgeosynchronous satellites available at any time is a key point for satellite’s station keeping routines, both for planning maneuvers and for checking out the results of these maneuvers. The major goal of PASAGE project is to use earth-based astrometric observations both for obtaining precise ephemeris of geosynchronous satellites and for orbit determination of these satellites. It requires the developmentof the necessary techniques and algorithms to process the observations. Nevertheless, optical observations depends on atmospheric conditions and, therefore, this technique must operate together with other alternatives. Optical observation of objects in geostationary ring, including space debris, is also used by other groups with several purposes; see for instance Alby et al. (2004), Beutler et al.(2005), Sabol & Culp (2001), and Schildknecht et al. (2004). Topocentric equatorial coordinates of the satellite can be obtained using one single telescope and, when having a suﬃcient number of observations, these can be used for orbit determination purposes. The Gautier astrographic telescope of the Real Instituto y Observatorio de la Armada (ROA) is an adequate instrument for doing the task. Theimprovement of the telescope’s performances by a CCD device has supposed the recovery of this telescope.
1 Real Instituto y Observatorio de la Armada, San Fernando (C´diz), Spain (email@example.com). a 2 Centro de Investigaciones de Astronom´ ıa, M´rida, e Venezuela. 3 Grupo de Sistemas Inteligentes de Computaci´n, Univero sidad de C´diz, Spain. a
An Apogee Alta U9 CCD camera has beenbought and ﬁtted to the telescope for this purpose. The project also involves the telescopes of the Observatorio Astron´mico Nacional de Venezuela o (OAN). By means of astrometric observations taken from San Fernando and Venezuela, the parallax can be derived and a better determination of the satellite position can be achieved. Some previous results of the project are presented in Abad et al. (2007).A better satellite position can be computed when astrometric observations are combined with the “Two Way Satellite Time and Frequency Transfer” system (TWSTFT). This technique is used at the San Fernando Time Laboratory for synchronizing with other laboratories. Range measures can be obtained with this procedure and, in fact, both techniques could be mutually validated. This method can only beapplied to geostationary satellites which support TWSTFT; in our case INTELSAT 707. 2. GEOSTATIONARY APPARENT MOTION An ideal geostationary satellite should remain at rest when observed from the Earth. So, its local hour angle will be nearly ﬁxed, whereas stars are deriving with sidereal time. This obvious fact is determinant when designing the observational technique. Satellites reﬂect the light...