


OFJ Field Journal from Lou D'Amario  12/1/95
Today I got to work at 6:00 AM, but I wasn't the first one in. Our orbit
determination specialists were already hard at work generating a new trajectory
that includes the latest tracking data. Our maneuver design and trajectory
analysis experts were eagerly awaiting the new trajectory so that they could
get to work on the maneuver design. We had five hours in which to do all
the work required to calculate the desired velocity change for TCM28A before
passing on the information to the Orbiter Engineering Team. No problem 
by noon, we were presenting arguments for and against doing TCM28A at the
Maneuver Design Status meeting.
But first, some background. The new strategy for designing OTM1 allows
us to move the Ganymede 1 flyby date earlier (in one week increments).
This allows us to minimize the OTM1 velocity change (and propellant consumption)
in some situations  namely, when the time it takes for Galileo to orbit
around Jupiter after JOI is longer, or higher, than expected. So, if the
spacecraft has a higher "orbit period" than was initially planned, we
have a straightforward way in which to save considerable propellant.
Does this work the other way? If the orbit period is shorter than expected,
or "low," can we move the Ganymede 1 flyby date later to conserve propellant?
Unfortunately, no, since that would require flying closer to Ganymede
than is safe. So we want to avoid errors that will result in a low period,
but we can deal effectively with high period errors. Now back to the TCM28A
meeting.
The situation was as follows: If we don't do TCM28A, that leaves the
trajectory with a slightly high period (by about 3.5 days), a result of
the predicted Io flyby altitude now being about 60 kilometers low. If
we do TCM28A, then we will get rid of this period error, and move the
trajectory toward the "low period" cases  precisely the situation we
want to avoid. The Navigation Team recommended that TCM28A be canceled
so that the trajectory would have a slight "bias" toward a low period.
Of course, the orbit determination results can (and will) change as we
get closer to Io, and the JOI burn may underperform (moving the period
lower) or overperform (moving the period higher). After considerable discussion,
the Project agreed with our recommendation, and TCM28A was canceled.
(The new OTM1 strategy was made official today; the Project Manager issued
a memo declaring that the Ganymede 1 date should be moved to minimize
the propellant required for OTM1.)
With the cancellation of TCM28A, that makes three maneuvers in a row
that have been canceled. In many jobs, having an upcoming work activity
get canceled would be regarded as a failure. For us, it means that we're
doing our jobs right. It has been possible to cancel these maneuvers because
of a combination of factors: excellent navigation (smart, hard working
navigators :), the new strategy for OTM1 (more navigation ingenuity
 what can I say, I work with a group of smart people), the removal of
any science requirements on the Io flyby (since we don't have to worry
about camera pointing angles, this allows us to leave the ever present
"too far South" error uncorrected), and, finally, a little bit of luck.
The next big event is the JOI "Tweak" which starts next Monday afternoon
(three days before the Io flyby). Even though TCM28A was canceled (it
would have been done on Saturday), the Navigation Team will be working
this weekend to update the trajectory predictions as more tracking data
comes in and to practice the design of OTM1.

