Environmental Engineering Reference
In-Depth Information
the important role of making sure the actions commanded by the sequence
planning group do not jeopardize the spacecraft or damage its instruments.
When such a condition occurs, the spacecraft automatically takes control and
places itself into a safe mode. This type of automation is critical to spacecraft
that operate very far from earth: the delay in long-range communications, even
at the speed of light, means receiving status information, sending commands,
and receiving confirmations will take a long time and the spacecraft can be
lost before ground control has a chance to react to an unexpected condition.
Furthermore, the delay means that the changed conditions at the spacecraft
will invalidate the commands based on the earlier conditions, rendering con-
trol by the ground ineffective and potentially counterproductive. In short,
ground control of a very remote space asset (e.g., a rover or a spacecraft)
under dynamic risk conditions is, in general, not an option.
7.4.2 Analysis of Spacecraft Mission Model
The first thing to note in Fig.
7.8
is the limited use of automation technologies.
There are large amounts of human cooperation, communication, and negoti-
ation, but the only fully automated processing occurs at the lowest level of
the hierarchy. This is where commands are sent to the spacecraft and au-
tomatically verified. This lack of automation dictates a large staff of expert
personnel in each of the mission domains. While this helps to ensure the safety
and success of the mission, it also means substantial operational costs.
The large degree of human communication and negotiation also severely
limits the speed at which the organization can perform decision making. The
time required for human decision making has three major impacts:
Planning time:
The entire planning hierarchy has developed to accommodate
the slowness of human deliberations. Decisions requiring long deliberations
are accomplished at the top of the hierarchy and are infrequent. The
middle tier of the hierarchy is focused upon near-term decision making and
the lowest level on those of the immediate future. If human deliberations
can be reduced or eliminated at any of the levels, improvements can be
made in the time required for the planning process.
Reaction time:
The cornerstone of the planning hierarchy is predictive sched-
uling. This requires all possible activities to be preplanned, with humans
involved in all decision making. In the case of a spacecraft anomaly,
the mission planning group must be called in to examine the anomaly
and re-plan the short-term activities. While this re-planning does not
have a major impact on single platform operations, it can easily lead
to nonproductive time when one instrument of a platform fails. An au-
tomated mission planner could easily redirect the properly functioning
instruments to another activity while the anomalous instrument is being
examined.
