Environmental Engineering Reference
In-Depth Information
miles, so communications-bandwidth needs do not demand excessively large
antennas or transmitter power.
From a planning and scheduling perspective, the space environment is
simple enough (contrary to the conditions at LEO) to enable considerable
onboard scheduling autonomy. The absence of complex time-dependent con-
straints makes event-driven processing a preferred choice (over absolute time-
driven, totally preprogrammed processing) for higher operational e
ciency,
and in fact, has been considered for use by the JWST mission. However, the
value of onboard short-term scheduling is highly mission dependent.
Although communications with the ground can be regular, frequent, and
of long duration, there still can be almost equally long periods when the
spacecraft is out of contact. At these times, a smart fault detection, diagnosis,
isolation, and correction capability, in conjunction with onboard data mon-
itoring and trending, would (depending on designed redundancy capacity)
allow the spacecraft to “fly through” a failure and continue its mission while
out of contact with the ground. However, an equally viable approach would
be simply to rely on conventional fault detection with a transition to a ro-
bust safemode to guarantee platform and payload H&S until contact with the
ground is regained. Currently, the answer to the question of which approach
is best is highly mission dependent and can only be determined following
a rigorous cost-benefit trade. In the future, the answer will depend on how
much progress has been made in standardizing onboard smart fault process-
ing and in reducing FSW development costs relative to comparable ground
software costs.
Similar arguments can be made for many of the other functions that might
potentially be assigned to Remote Agents. Onboard SI data processing could
be very helpful to opportunistic identification, scheduling, and acquisition of
science targets (as well as TOO response). It could also enable a reduction in
downlink bandwidth by telemetering only processed science products, not raw
data, to the ground. But for some missions, the science targets are quite pre-
dictable and greater overall scheduling eciencies may be obtained via a con-
ventional ground scheduling system. And for most missions, the astronomer
customer will insist on receiving the raw data, rather than the downstream
processed product. So again, an evaluation of the desirability requires rigorous
cost-benefit analysis.
Summarizing, implementing the following additional onboard functions
as Remote Agents would be consistent with the lights-out control center
philosophy:
1.
Fine attitude determination (needed for target acquisition)
2.
Orbit determination (not needed)
3.
Attitude sensor/actuator and SI calibration (need is a function of H/W
design)
4.
Attitude control (execution of slew requests; fine pointing)
5.
Orbit maneuvering (infrequent; planned on ground)
