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OFJ Field Journal from Lou D'Amario - 11/29/95

This morning, the Navigation Team presented the options for the OTM-1 strategy and the latest orbit determination results to the Project. The material on OTM-1 generated a great deal of discussion. That's understandable -- this material is particularly detailed and complicated, but I believe that the Project now understands much better the issues surrounding OTM-1 and the implications of changing the Ganymede 1 encounter date to reduce the OTM-1 velocity change. We should be getting the requested OTM-1 design ground rules by the end of the week.

Today's orbit determination solution for the Io flyby show the trajectory to be at an altitude of 956 kilometers, 44 kilometers below the desired altitude. The uncertainty is now about the same size as this error. The trajectory is South of the aimpoint by about 300 kilometers. This "too far South" error in the trajectory is not a problem, because the Project some time ago decided that there would be no remote-sensing observations (for example, pictures) at the Io flyby, so as not to risk breaking the tape recorder, on which all of the atmospheric Probe data will be stored. The "too far South" error does not affect the gravity assist effect (slowing the speed of the Orbiter) we are getting from Io.

At the end of the day, word came down from the Project that they were leaning heavily towards the OTM-1 strategy that would make maximum use of changing the Ganymede 1 flyby date in order to minimize the velocity change. This strategy would help us to conserve as much propellant as possible, with the major downside of having to make changes to all of the computer sequences that would be running the spacecraft during the Ganymede 1 encounter. But these sequences have to be redone anyway, because of the tape recorder problem. So accommodating a change in the Ganymede 1 flyby date does not have as big an impact as we had originally thought.



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