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1 Introduction

Observations of mutual eclipses and occultations of planetary satellites are a source of valuable data on the dynamics of the satellites. Present-day ground-based facilities make it possible to observe mutual occultations and eclipses of the Galilean satellites of Jupiter as well as major Saturnian and Uranian satellites.

For the sake of brevity, mutual occultations and eclipses in the systems of planetary satellites are referred to as mutual phenomena. These rare events occur during about six-month long seasons separated by half the orbital period of the corresponding planet. About 300 mutual phenomena occur during each such season. Most of the mutual phenomena last from 2 to 10 minutes. What makes observations of mutual phenomena valuable is the high accuracy of astrometric data derived from the satellite photometry obtained. The data inferred from such observations are several tens to hundreds of times more accurate than the results of usual direct measurements of satellite positions.

Many observatories throughout the world observe on a regular manner mutual phenomena of planetary satellites, and both the data amount and the accuracy of satellite photometry is steadily increasing. Beginning in 1973, regular international campaigns were organized with the aim to observe mutual phenomena of the Galilean satellites in accordance with observing seasons. The description of specific features of mutual phenomena of planetary satellites and brief reviews of campaigns conducted can be found in Aksnes et al. [(1984)], Arlot [(1984)], Arlot et al. [(1992)], Nasonova [(1996)]. Only a few researchers from the former Soviet Union had experience in performing such observations. In 1985 two light curves of the Galilean satellites were obtained at the observatory of the Fesenkov Astrophysical Institute of the Academy of Sciences of the Republic of Kazakhstan (FAI AS RK) in Almaty (Grigorjeva et al. [1986a]; Grigorjeva et al. [1986b]). Two observatories of CIS countries - the Crimean laboratory of the Sternberg Astronomical Institute (CL SAI) and the observatory of FAI AS RK - took part in the last campaign of observations of Saturnian satellites. A total of four light curves of the mutual phenomena of Saturnian satellites have been obtained. The techniques used and the results of astrometric reduction of satellite photometry have been published by Emel'yanov et al. ([1997]). This was our first experience in reducing such observations.

Seven observatories in Russia, Ukraine, and Kazakhstan took part in the next observing campaign of mutual phenomena of the Galilean satellites in 1997. Such observations have also been performed at many observatories throughout the world during this season. A large amount of satellite photometry has been obtained. These observations proved to be more accurate than those of Saturnian satellites with the Galilean satellites observed at a smaller geocentric distance and being brighter than the Saturnian ones All this made it necessary to refine the photometric model of mutual phenomena of planetary satellites and develop more sophisticated methods for extracting positional information from such observations. Theoretical issues and the development of the tools used to construct the models of mutual phenomena of natural planetary satellites have been addressed by Emel'yanov ([1999]) and Emel'yanov ([2000]).

During the 1997 observing season of mutual phenomena of the Galilean satellites more than 40 sets of successful photometric observations have been obtained at seven observatories in Russia, Ukraine, and Kazakhstan, yielding the same number of satellite light curves.

New series of photometric satellite observations are required to refine the theory of satellite motion. This process involves the use of a huge amount of satellite photometry interpreted in terms of a model of mutual occultations and eclipses as observed from ground-based observatories, theory of planetocentric motion of satellites, and the theory of motion of planets. In this paper we suggest and apply a special technique for reduction of photometric observations of mutual phenomena of natural planetary satellites. To simplify the process of refinement of satellite orbits, we subdivide it into two stages. The input data at the first stage are the results of photometric measurements and parameters describing the observer-satellites-planet-Sun configuration. The output, after reduction of each light curve, has the form of differences of planetocentric rectangular satellite coordinates referred to a single instant of time. At the second stage the elements of satellite orbits are refined using a series of satellite coordinate differences inferred by reducing the observations all mutual phenomena carried out at different observatories.

The advantage of this approach for a user of the observational data consists of the fact that one will have a deal with a planetocentric satellite motion only to fit the satellite orbit to observations. On the other hand these data keep the whole observational reality including the observational errors.

We describe the observations made and the results of their reduction, i.e., the mutual planetocentric positions of the Galilean satellites as inferred from photometry of their mutual phenomena. Reduction of photometric observations yields to satellite position data that can be used, combined with the results of other observations, to refine the elements of satellite orbits.

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