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This paper summarizes Galileo's orbit determination activities leading up to its encounter with asteroid 243-Ida on August 28, 1993. In addition to the nominal 2-way S-band range and Doppler radio metric data obtained from the Deep Space Network (DSN), several navigational aids were brought together to make this encounter successful.

As the time of the Ida encounter by Galileo approached, some unfortunate circumstances occurred, causing a very worrisome but exciting encounter, especially with regards to navigation. This paper reports on Galileo's orbit determination stratagy during the period after the last Earth encounter through the Ida flyby. Details in the modeling of Galileo's orbit, and in the use of various navigation tools, will be explained and the results of several key orbit solutions will be given.

The tracking and Data Relay Satellite System (TDRS) was successfully used to track the Galielo spacecraft on December 8, 1992, during the Galielo Earth-2 flyby. This flyby enabled Galielo to obtaina gravity assited energy increase as part of the Venus-Earth-Earth trajectory en route to the planet Jupiter.

The galileo spacecraft was launched on October 18, 1989 towards Jupiter using the Venus-Earth-Earth Gravity Assist (VEEGA) trajectory.

The Tracking and Data Relay Satellite System (TDRSS) was successfully used to track the Galileo spacecraft on December 8, 1992, during the Galileo Earth-2 Flyby. This flyby enabled Galileo to obtain a gravity-assisted energy increase as part of the Venus-Earth-Earth trajectory en route to the planet Jupiter.

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This paper discusses orbit determination results for the Galileo satellite tour. Lacking a high gain antenna, the mission will use a low gain antenna for communication and tracking. This change implies far less navigation data will be available than previously expected. A baseline orbit analysis was completed assuming this decreased data schedule. Variations on this baseline were studied to determine sensitivity to data loss. Results indicate that the probability of completing the tour is less than 90 percent, although future improvements in orbit determination promise to raise the probability of completion above 90 percent.

The Galileo mission is an ambitious attempt to explore the Jovian system by spacecraft. This voyage of exploration is a logical successor to the reconnaissance voyages of Pioneers 10 and 11, Voyagers 1 and 2, and Ulysses. These spacecraft merely flew past Jupiter, spending relatively little time in its system. Galileo differs from these earlier spacecraft in that it will remain within the Jovian system, studying the planet and its four major satellites for a period of two years. Insertion into orbit around Jupiter will occur on December 7, 1995. The portion of the mission encompassing observations of Jupiter and the major and minor satellites, and magnetospheric mapping, has become known as the Jovian tour. During this period Galileo will encounter each of Europa, Ganymede, and Callisto at least three times on trajectories that will bring it to altitudes from 200 to 3100 kilometers.

This paper presents an overview of the orbit determination for Galileo's epochal encounter with the asteroid Gaspra on October 29, 1991. Topics discussed are a ground-based observation campaign to improve Gaspra's ephemeris before encounter, and the use of optical navigation together with Doppler and range data. The paper concludes by noting the steady improvement in the B-plane dispersions during the two months prior to encounter.

This paper presents the orbit determination strategy and results in navigating the Galileo spacecraft from launch through its Venus and first earth flybys. Many nongravitational effects were estimated, including solar radiation pressure, small velocity impulses from attitude changes and eight trajectory correction maneuvers. Tracking data consisted of S-Band Doppler and range. The fitting of Doppler was difficult since one of the cpacecraft's two antennas was offset from the spin axis, thus producing the sinusoidal velocity fluctuation seen in the data. Finally, Delta Differential One-way Range data was used during the last three months of the earth approach to help deliver the spacecraft to within desired accuracy.