Environmental Engineering Reference
In-Depth Information
These functions may be thought of generically as the spacecraft's auto-
nomic system, in much the way that one views a human's respiratory and
circulatory system. Since they are so common from spacecraft to spacecraft,
and so essential to spacecraft operations, they ironically are often neglected in
discussions of spacecraft autonomy. In the following sections, the various ele-
ments of this spacecraft operational infrastructure will be described in more
detail. Some of this discussion will be a bit redundant with material presented
in Sect. 3.1.1 , which dealt with how the FSW enables satisfaction of space-
craft mission objectives. Such material is repeated here more in the context of
what the flight system must do in order to keep the spacecraft available and
responsive to the ground's needs, as opposed to what it does to accomplish
what the ground wants done.
Flight Autonomy Enablers of Command Execution: Validation
Earlier, the various stored command types (absolute-timed, relative-timed,
and conditional) were described to illustrate how the FSW is able to execute
ground requests faithfully, while still exploiting realtime information that was
unavailable to the ground system at the time their requests were generated
and uplinked - so as to provide a value-added response to the ground's needs.
However, to make safe and reliable use of these command structures, the
FSW independently validates commands on receipt, validates them again on
execution, and monitors their passage through the C&DH subsystem as they
make their way to their local destination for execution.
The first step in the process is to verify that a command (or a set of com-
mands) has not been garbled. For this purpose, when the C&DH subsystem
first receives a command packet, the C&DH checks the bit pattern in the
header and verifies it matches the expected pattern. At the same time, the
C&DH examines the command packet at a high level to make sure it recog-
nizes the packet as something an onboard application could execute. Second,
when it is time for the command to be executed, the C&DH determines to
which application the command should be sent and ships the command out to
be loaded into that application's command buffer. The C&DH then looks for
a message verifying that the command was successfully loaded into the buffer
(i.e., that there was room in the buffer for the command). Third, as the ap-
plication works its way through the buffer contents, it examines the contents
of the individual commands to verify they are valid. It also will check the
command itself to verify there are no inherent conflicts to executing that kind
of command given the current spacecraft state. Finally, once the application
determines that the command may be executed, it carries out its prescribed
function or launches it toward its final destination for execution and verifies
that it then executes successfully. This multiple-tiered validation process en-
sures that only valid commands are executed, that they reach their proper
destination, and that they are executed properly once they get there.
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