Environmental Engineering Reference
In-Depth Information
Pointing Control
The second major enabler of science execution is autonomous pointing control.
Without autonomous spacecraft pointing control, ecient spacecraft opera-
tions would be impossible. In fact, the survival of the spacecraft itself would
be highly unlikely for other than passively stabilized spacecraft. For space-
craft requiring active attitude control, it is the exclusive responsibility of the
flight system to maintain the spacecraft at the desired fixed pointing within
accuracy requirements, to reorient the attitude to a new pointing (as specified
by the ground), or (in the case of survey spacecraft) to cause the attitude to
follow a desired trajectory. Put somewhat more simply, it is the flight system's
job to point the spacecraft in the direction of a science target and maintain
that pointing (or pointing trajectory) throughout the course of the science
data collection.
Once the required spacecraft orientation for science activities has been
achieved, the SI(s) must be configured to support target identification, ac-
quisition, and observation. Although target identification and acquisition can
be performed with the ground system “in the loop” for missions where real-
time communications are readily available and stable, typically optimization
of operations requires that these functions be conducted autonomously by the
flight system. For most missions, routinely supplying realtime science data to
the ground is precluded by orbit, geometry considerations, timing restrictions,
and/or cost.
As a general rule, the flight system will autonomously identify the science
target (sometimes facilitated by small attitude adjustments) and acquire the
target in the desired location within the field of view (FOV) of the SI (also, at
times, supported by small spacecraft re-orientations). Once these goals have
been achieved, the flight system will configure the SI(s) to perform the de-
sired observation in accordance with the activity definition uplinked by the
ground. Note that the flight system may even be assigned some of the respon-
sibility for defining the details for how the science observation activity should
be performed. This autonomy is enabled by relative-timed and conditional
commanding structures. For example, for a LEO spacecraft whose SIs are ad-
versely impacted by energetic particles within the South Atlantic Anomaly
(SAA), the flight system could determine start and stop times for data tak-
ing relative to exit from and entrance into SAA contours. Or, based on its
realtime measurement of target intensity, the flight system could determine
via conditional commanding how long the SI needs to observe the target to
collect the required number of photons.
Data Storage
Once a target has been successfully acquired and science data start flowing out
of a SI, the flight system must store the data onboard in a manner such that
unnecessary burdens are not forced on the ground system's archive and science
data processing functions. To that end, the FSW may evaluate science data
