Environmental Engineering Reference
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upon again in a more detailed fashion when current and possible future flight
system autonomous capabilities are discussed.
Flight Autonomy Enablers of Ecient Science Execution
For science that is predictable and can be scheduled in advance (e.g., science
that is characteristic of a LEO celestial-pointer spacecraft), the objective is to
pack as much science into the schedule as possible with minimal time overhead
or wastage. Traditionally, it is the responsibility of the ground system to solve
the “traveling salesman” problem by generating an error-free schedule that
optimizes data collection per unit time elapsed. Although schedule generation
is the most complex part of the problem, the ground cannot cause its schedule
to be executed effectively without the cooperation and support of several
autonomous flight capabilities.
Command Execution
First, the flight system must execute the activities specified by the ground at
the required times. Traditionally, the ground-generated schedule has defined
both the activities and their execution times in a highly detailed manner. An
activity can be viewed as a collection of directives that specify the steps that
must be performed for the activity to be completed successfully. The directives
themselves are decomposed into actual flight hardware or software commands
that cause the directives to be executed. Depending on the sophistication of
the flight system, the decomposition of the directives into commands may
be done by the ground system and uplinked in detail to the spacecraft, or
may be specified at a much higher level by the ground system, leaving the
decomposition job to the flight system. In practice, most missions share the
decomposition responsibility between ground and flight systems, trading on-
board storage resources vs. onboard processing complexity.
However the decomposition issues are decided, the collection of directives
and commands are uplinked and stored onboard until executed by the flight
system in one of the following three ways: absolute-timed, relative-timed, or
conditional. Note that this discussion is limited to stored commanding, as
opposed to other approaches for commanding the spacecraft in realtime. The
way the FSW orchestrates the potentially competing demands of stored com-
manding with realtime commanding - the commanding originating externally
from the ground as well as internally from within the flight system - will be
touched upon in later sections dealing with FSW infrastructure.
Absolute-timed commands include an attached time specified by the
ground defining precisely when that command is to be executed. This ap-
proach is most appropriate for a ground scheduling program that accurately
models both planned spacecraft activities as well as ground track and external
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