TECHNOLOGY DIGEST - Draper Laboratory

details many of the considerations for planning the attitude
profile of a satellite during rendezvous.
Once the activities have been identified for the nominal
mission, failure and off-nominal conditions are considered
to complete the problem decomposition. We
already have the capability to handle many failure types
since the mission activity agent can request the highthrust
system to take over should low-thrust failures
occur, and the low-thrust agent can replan the lowthrust
profile should burn errors or navigation state
updates require it. However, when low-thrust failures
occur very early in the rendezvous, or if high-thrust failures
prevent completion of the rendezvous, then it may
not be possible to achieve the desired offset point. When
this occurs, the mission agent may invoke an “abort”
agent to place the spacecraft in a safe orbit co-elliptic
with the RSO, or if this is not possible, to safe the spacecraft
subsystems. Note that the abort agent utilizes the
same burn co-elliptic agent used by the high-thrust
agent to accomplish its safe orbit objective. Only the
agents required to accomplish the current mission
objectives are actually invoked by the mission agent and
instantiated by the ADEPT framework, so the abort
agent does not appear in the successful cases shown in
the results section below.
Once the desired offset point has been achieved, the
PAMM spacecraft may conduct proximity operations,
docking operations, or other RSO-relative maneuvers.
The proximity operations and docking agents are not
implemented here, but are displayed in the results section
as placeholders for future development. Reference
[3] describes some considerations and techniques for
autonomous proximity operations and docking.
PAMM Functional Decomposition
Once the hierarchy of agents has been determined
through the temporal decomposition process, we can
begin the process of defining the required functionality
of each agent by defining its monitoring, diagnosis,
planning, and execution functions. For the sake of
brevity, this section will describe only the more interesting
functions associated with the shaded elements in
Figure 7.
Mission Agent Planning
Beginning with the mission activity agent, we start by
discussing planning, since the planning process actually
creates and reconfigures the agent hierarchy. The mission
agent planning function looks at the current relative navigation
state of the chaser with respect to the RSO, along
with subsystem state and status information. If subsystems
are nominal and the altitude differential between
the vehicles is large, then the mission agent will spawn a
low-thrust child and assign it the objective of reaching a
nominal target state below and behind the RSO (refer
48
Autonomous Mission Management for Spacecraft Rendezvous Using an Agent Hierarchy
again to Figure 6). The low-thrust child agent in turn
invokes the optimal guidance algorithm to plan a trajectory
to the assigned relative state. If low-thrust planning
is successful, the mission agent creates a high-thrust
child, assigns it the low-thrust termination state as an
initial condition and the desired offset point as an objective.
The high-thrust agent in turn calculates a series of
burns to achieve the desired state by invoking the corresponding
burn agents. If high-thrust planning is
successful, then the mission agent terminates planning
and begins the monitoring cycle.
If low-thrust planning is unsuccessful, the mission agent
attempts to accomplish the remaining rendezvous by
assigning the high-thrust agent the current state as initial
condition, rather than the predicted low-thrust
termination state. As part of the high-thrust problem
specification, the mission agent may also relax the
arrival time requirement. If the high-thrust agent is successful,
the mission is continued with high thrust only;
if not, an abort is commanded by invoking the abort
agent.
Note that no assumptions were made about the initial
vehicle conditions for the planning process. This means
that the replanning process is exactly the same as the initial
planning process, although the resulting plan – and
corresponding agent hierarchy – may be different.
Mission Agent Monitoring
Once the rendezvous plan is complete, the mission agent
begins to monitor subsystem status, as well as the status
from all its executing children. The job of the monitoring
function is to determine whether the mission objectives
may be accomplished given the current plan and the current
vehicle state and status. Two types of conditions may
cause the mission agent to trigger a replan by executing its
diagnosis function. First, a child agent (e.g., low thrust)
may report that it can no longer achieve the objective
assigned by the mission agent, and second, a subsystem
failure may be detected that directly triggers the mission
diagnosis function. The distinction between these two
types of failure conditions becomes important in the diagnosis
process.
Mission Agent Diagnosis
Diagnosis is executed whenever monitoring detects a
condition that prevents achieving the mission objective
with the current plan (set of activities). The job of diagnosis
is to determine the reason for the failure of the
plan in order to select a replanning strategy. If, for example,
diagnosis is triggered because the low-thrust agent
has determined (through its own monitoring and diagnosis
process) that it can no longer reach its commanded
state, then the mission agent may attempt to assign a
new goal to the low-thrust child agent. This could occur
for example if low-thrust propellant usage was larger