Environmental Engineering Reference
In-Depth Information
schedule can be a lengthy process during which precious observing time will
be lost, potentially irretrievably depending on the mission lifetime and the
nature of the science. So, a lesser response to anomalies than safemode entry
is highly desirable.
An inertial fixed pointing mode serves this purpose. For celestial pointing
spacecraft, being inertially fixed is effectively being in observing mode with-
out a science target and without making use of fine error sensor or SI data.
The pointing accuracy and stability will, therefore, not meet mission require-
ments, but will be adequate to facilitate re-initiation of the science program.
And virtually all spacecraft, independent of mission type, require an inertial
fixed pointing mode to support sensor calibrations, such as gyro scale fac-
tor and alignment. For these reasons, and since ground controllers may need
to command the spacecraft to an inertial mode either during the immediate
postlaunch checkout phase or in response to unusual spacecraft performance
during mission mode, the FSW backbone will require the capability to transi-
tion to and maintain the spacecraft at an inertially fixed pointing. Note that
typically the following associated functions are part of and subsumed under
an inertial hold mode:
1.
Gyro data processing and drift bias calibration
2.
Attitude control law(s) for fixed pointing
3.
Reaction wheel commanding and momentum distribution
4.
Actuator commanding for momentum management
6.1.3 Ground Commanded Slewing
Just as ground controllers need access to an inertial fixed pointing mode in
response to an anomalous condition on-orbit, the need may arise to slew the
vehicle attitude to a different orientation. Control of nominal spacecraft slew-
ing activities in support of science execution or sensor/instrument calibration
could be allocated to an appropriate Remote Agent. But some basic slewing
capability must also reside within the FSW backbone to ensure that in the
event of the onset of an anomaly originating from within (or at least affecting)
the agent itself, the backbone retains the capability for slewing the spacecraft
to a needed orientation, such as sun pointing.
6.1.4 Ground Commanded Thruster Firing
Similarly to slewing, control of nominal thruster firing could be allocated to an
appropriate Remote Agent. Nominal use of the propulsion subsystem might
well be highly automated, coupled with autonomous onboard planning and
execution of orbit stationkeeping maneuvers. However, the need will still exist
to provide the ground controller a “back door” into the propulsion subsystem
to enable ground commanding of emergency angular momentum dumps or or-
bit changes. For that matter, the FSW backbone itself may need to command
