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       It used to take scores of spacecraft coordinates, long-hand arithmetic and a good old-fashioned slide ruler to calculate a spacecraft's location in space. Now computer-savvy flight operators are using a new, three-dimensional software tool to pinpoint spacecraft positions instantly.

       Software engineers at NASA's Jet Propulsion Laboratory have developed the new software tool -- called "TrajTool" for trajectory tool -- especially for the Galileo navigation team as their spacecraft begins a two-year tour of Jupiter and its moons. The trajectory visualization program, though, has broad applicability for use on other spaceflight missions.

       With the ease of a simple point-and-click menu, TrajTool will present a three-dimensional picture of a spacecraft's trajectory and provide vital orbital information. In Galileo's case, TrajTool gives navigators instant measurements on the spacecraft's distance from Jupiter, any of its moons or Earth.

       "The program reads in two ephemeris files, which are tables of the computed positions of celestial bodies," said Alan Quan, technical group leader and developer of the software application along with Dr. Patti Koenig, for JPL's Flight Projects Office Information Systems Testbed. "One of these files is for the spacecraft and one is for the planets. The software then calculates the Cartesian coordinates of the objects and their trajectories."

       The information is next plotted and displayed on a graphical user interface. Using the mouse the user can manipulate a slider on the graphical interface to move the spacecraft and planets forward or backward in time to their relative positions at any point within the time span of the ephemeris file.

       TrajTool can also overlay on the display certain parameters that are of interest to navigators, including the distances between the spacecraft and planets and the angles between the Sun, Earth and spacecraft, added Dr. Ursula Schwuttke, manager of the Flight Projects Office Information Systems Testbed.

       "The tool gives us a three-dimensional view of space," she said, "in which the user can translate and rotate the viewing position along any axis."

       Perhaps the most beneficial feature of the new software tool is its ability to let users see trajectory information, rather than having to gather data that were previously only available in text form from a trajectory characteristics document, Schwuttke said.

       "The navigator can observe when critical events occur, when occultations or eclipses begin, or when the Sun- Earth-probe angle will reach its maximum or minimum," she said. "It gives us a global, intuitive understanding of the trajectory and trajectory events at a glance."

       Navigators also can go immediately to a point of interest in a trajectory, whereas in the past it took about 15 minutes of manual searching through paper text, tables and charts to accomplish the task.

       First developed for the Galileo magnetometer team, who wanted to display their data in three dimensions, TrajTool is being refined for other spaceflight missions, Quan said. The software is currently being developed for NASA's New Millennium missions, which will feature miniaturized spacecraft and a host of revolutionary new technologies designed to study phenomena in Earth's atmosphere, the solar system and celestial events unfolding in galaxies beyond the Milky Way.

       "We've demonstrated the software to the New Millennium project and are working on incorporating modifications that will allow us to tailor the software to their specific needs," Quan said. "TrajTool will also be used after launch, during normal operations, to verify trajectory data that will be sent from spacecraft to the Deep Space Network."

       Development of TrajTool was carried out with funding from the Jet Propulsion Laboratory Multimission Ground Systems Office for NASA's Office of Space Science, Washington, D.C.

1/19/96 DEA