Environmental Engineering Reference
In-Depth Information
events or phenomena. It allows an extremely ecient packing of activities, pro-
vided no serious impacts occur due to unforeseen events. Major anomalous
events would break the plan and potentially invalidate all downstream events.
In such cases, science observations (especially for LEO celestial-pointers)
might have to be postponed until the ground scheduling system can re-plan
and intercept the timeline. Alternatively, for events only affecting the current
observation, the spacecraft could simply skip the impacted observation and
recommence science activities (supported by any required engineering activi-
ties, such as antenna slewing) at the start time of the next observation. This
latter alternative can be particularly appropriate for earth-pointers, where
the spacecraft's orbit will automatically carry it over the next target. For
such cases, resolving a problem in the scheduled timeline can simply involve
“writing off” the problem target and reconfiguring the spacecraft and SI so
that they are in the correct state when the orbit ground track carries them
over the next target. Minor anomalous events can be handled by padding the
schedule with worst-case time estimates, thereby reducing the operational ef-
ficiency, or uplinking potentially extensive “what if” contingency scenarios,
increasing demands on onboard storage.
Use of relative-timed commands reduces somewhat the accuracy demands
on ground-system modeling. The ground system still specifies an accurate
delta-time (with respect to a spacecraft or orbital event) for executing the
activity, but the flight system determines when that key event has occurred.
Although the treatment of timing issues differs in the two cases (absolute- vs.
relative-timed), the treatment of the activity definition (i.e., how the activity
is decomposed into directives and commands) would remain the same.
By contrast, conditional commanding requires the flight system to make
realtime decisions regarding which directives or commands will be executed,
as well as
when
they will be executed. When conditional commanding is em-
ployed, the ground specifies a logic tree for execution of a series of directives
and/or commands associated with the activity, but the flight system deter-
mines when the conditions have been met for their execution, or chooses be-
tween possible branches based on observed realtime conditions. For current
GSFC spacecraft, conditional commanding typically is used for detailed-level
commanding within a larger commanding entity (e.g., an activity), with time-
padding used to ensure that no time conflicts will occur, regardless of what
decisions are made by the flight system within the conditional block. These
various commanding methods provide an infrastructure enabling accurate and
effective execution of ground-specified activities. Traditional applications uti-
lizing this commanding infrastructure include pointing control and SI con-
figuration. Conditional commanding can also enable more flexible onboard
planning and scheduling functions than would be achieved through absolute-
timed commanding, permitting selective target scheduling and autonomous
target rescheduling (e.g., High Energy Astronomical Observatory-2's (HEAO-
2's) very flexible onboard scheduling scheme driven by its target list).
