Environmental Engineering Reference
In-Depth Information
Also in the area of diagnostic functionality, Landsat-7 (launched in 1999)
experimented with a high level FDC capability, in addition to flying a
more traditional one. The FDC functions for most GSFC spacecraft have
a one-to-one quality, i.e., a trigger is received and an associated response
is executed. The potential problem with this approach is that a higher level
problem (for example, a failure in the ACE) can corrupt output data from sev-
eral ACS sensors that could be misinterpreted as the individual failures of all
those ACS sensors, potentially resulting in unnecessary autonomous switches
to their redundant components. To protect against a problem of this kind,
Landsat-7 implemented a Boolean logic function that would examine all error
flags generated at a component level and, by comparing the error flag pattern
to a set of patterns maintained onboard defining the signature of higher level
problems, deduce the true cause of the current anomaly and respond accord-
ingly. By setting the counters associated with the triggers for the higher level
failures to lower values than for the counters associated with the component
level failures, the Landsat-7 FSW would be able to switch out the higher level
hardware element before the cascade to redundant components commenced.
At a somewhat more detailed level, SAMPEX incorporated a new au-
tonomous calibration function, which also has been flown on other spacecraft
in the small explorer (SMEX) series. SAMPEX possesses the capability to cal-
ibrate its magnetometer coupling constants (relative to the magnetic torquer
bars) inflight, relieving the FOT of the burden of collecting the necessary engi-
neering data, processing it, and uplinking the modified calibration parameters
to the spacecraft.
Lastly, some very interesting new ideas have been implemented at JPL.
Because of the long cruise periods until their spacecraft achieve their mission
orbits or swing-bys, JPL has the luxury of experimenting with their FSW
after launch and even making wholesale changes (or simply completing the
original coding effort) after launch. Their deep space missions also, by their
very nature, may require more autonomy than is typical of GSFC missions.
Because of the long communications-delay times inherent in a deep space mis-
sion and because of the time critical aspects associated with celestial flybys,
JPL has been experimenting with autonomous target identification and acqui-
sition functions that are more elaborate than those flown at GSFC. At a more
fundamental structural level, the New Millennium Program's Deep Space One
(DS1) FSW was initially designed with Remote Agents having responsibility
for multitask management, planning and scheduling, and model-based FDC
[
99
,
108
]. In practice (due to schedule conflicts), the mission was flown using a
more conventional FSW implementation, but the Remote Agent-based version
was activated briefly for test purposes.
3.2.4 Current Spacecraft
A number of interesting new autonomy capabilities have been flown on GSFC
spacecraft launched in the 2000s. First, the recent development of quaternion
