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Autonomous Operations Communications with FUSE will be made through a single ground station antenna located at the University of Puerto Rico in Mayaguez. The satellite's circular 775 kilometer orbit, which takes just over 100 minutes for a single orbit, brings it over the ground station for less than 10 minutes at a time (on average) for about 6 orbits in a row, followed by about 8 orbits without contact. Hence, the satellite must operate autonomously (or without direct contact from the ground) most of the time, moving from target to target, identifying star fields, centering each target in the spectrograph apertures, and performing the observations. Controlling FUSE All of the instructions the satellite needs to perform these tasks must be pre-planned and upinked. This activity occurs in the Satellite Control Center (SCC) located in the Bloomberg Center at Johns Hopkins University. Potential observations are planned based on predicted viewing intervals and many other constraints in a process known as mission planning. These mission plans, or timelines of activities, are then turned into detailed instrument commands by a team of engineers and uplinked to the satellite for execution. Acquiring Targets FUSE knows where it is pointed using a combination of software and gyroscopes known as the Attitude Control System, or ACS. For each potential FUSE pointing, a list of field stars within a degree region around the target are uplinked to the telescope's computer. When FUSE arrives at the approximate position of the target, the telescope compares the stars it "sees" in the Fine Error Sensor camera with the expected star patterns and locates exactly where it is. Usually a small movement is then required to place the object of interest into the "aperture" or opening of the spectrograph. When this is accomplished, the "acquisition" of the target is complete and the computer commands a period of data collection (an "exposure") with the spectrograph. A typical exposure with FUSE will be a half an hour to 40 minutes, depending on the orbital visibility of the target. (Remember, for most targets the earth gets in the way for roughly half of the time!) If more than a single exposure is required for a faint target, the satellite will simply wait for the earth to get out of the way, re-acquire the target, and expose again as needed. FUSE Data Handling Data are stored in computer memory on the satellite until contact with the ground station is established, and then the precious scientific cargo is transmitted down to earth. The data are stored temporarily at the ground station, and then transferred back to the Satellite Control Center at JHU, where they are checked to verify they are complete and then sent on for processing into a form that the scientists can measure and use for their investigations.
Last modified: 7/98
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