MISSION PLAN - PDS Small Bodies Node

** vectors evaluated at the time of closest approach
This attitude description is also used in the formulation of a predict attitude C-kernel file.
The DSM attitude option is used primarily for this purpose. It is not used during small
forces calculations as turns related to DSMs are assumed to be part of the DSM design.
The inclusion of slew events in the modeling also expands the number of deadband
options available to the analyst. In addition to those listed in Table 10.1-2, deadband
options are made available for OPNAVs and MGA communications. The near encounter
option in Table 10.1-2 is the same as is used for HGA communications. Table 10.2-4
lists the complete set of deadband options available in the slew ∆V model.
Table 10.2-4 Spacecraft Deadband Options - Slew ∆V Model
Option Deadband Comments
1 x, y, z = 15° Cruise option 1
2 x, y, z = 10° Cruise option 2
3 x, y = 2°, z = 10° HGA comm / Near encounter
4 x, y, z = 6° MGA comm
5 x, y, z = 0.25° Image
The spacecraft’s attitude and slew history contribute to the calculation of the slew ∆V’s
by providing the initial and final attitudes that allow computations of turn angles. In a
semi-iterative process, the turn angles are used by the analyst to schedule the appropriate
slew group for each slew event. This process places the burden of selecting the
appropriate slew group on the analyst, but this approach is selected in favor of the
extensive coding that would be required to make the slew group selection completely
autonomous.
Three events are considered in the construction of the slew history: communications,
OPNAVs and background attitude mode transitions. Slews for communications comprise
the bulk of slewing activity on STARDUST with 630 events. Current plans call for 44
OPNAV events and 30 background attitude mode transitions. There are 40 different
attitude modes, but 9 transitions occur naturally and do not require a slew.
Construction of the communications schedule is strongly guided by the desired to
maintain the +x-axis of the spacecraft pointing away from the sun to eliminate shadowing
across the solar panels from the whipple shields. This results in the pitch-yaw-pitch slew
sequence described in the slew group options (Table 10.2-2). Some mitigation to using
this sequence is possible at low Sun-Probe-Earth angles, where the off-sun angles
required for communications and corresponding array shadowing is minimal. Selection
of this strategy typically involves scheduling of the HGA instead of the MGA to
eliminate the 7 deg off-z-axis boresight angle of the MGA and invoking a ‘spacecraft
flip’ input flag that overrides the typical HGA attitude and the +x-axis to be pointed
toward the sun.
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