Robin Mueller
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43 lines
2.1 KiB
Markdown
# Working with Actions
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Space systems generally need to be commanded regularly. This can include commands periodically
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required to ensure a healthy system, or commands to reach the mission goals.
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These commands can be modelled using the concept of Actions. the ECSS PUS standard also provides
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the PUS service 8 for actions, but provides few concrete subservices and specification on how
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action commanding could look like.
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`sat-rs` proposes two recommended ways to perform action commanding:
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1. Target ID and Action ID based. The target ID is a 32-bit unsigned ID for an OBSW object entity
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which can also accept Actions. The action ID is a 32-bit unsigned ID for each action that a
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target is able to perform.
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2. Target ID and Action String based. The target ID is the same as in the first proposal, but
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the unique action is identified by a string.
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The library provides an `ActionRequest` abstraction to model both of these cases.
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## Commanding with ECSS PUS 8
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`sat-rs` provides a generic ECSS PUS 8 action command handler. This handler can convert PUS 8
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telecommands which use the commanding scheme 1 explained above to an `ActionRequest` which is
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then forwarded to the target specified by the Target ID.
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There are 3 requirements for the PUS 8 telecommand:
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1. The subservice 128 must be used
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2. Bytes 0 to 4 of application data must contain the target ID in `u32` big endian format.
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3. Bytes 4 to 8 of application data must contain the action ID in `u32` big endian format.
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4. The rest of the application data are assumed to be command specific additional parameters. They
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will be added to an IPC store and the corresponding store address will be sent as part of the
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`ActionRequest`.
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## Sending back telemetry
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There are some cases where the regular verification provided by PUS in response to PUS action
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commands is not sufficient and some additional telemetry needs to be sent to ground. In that
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case, it is recommended to chose some custom subservice for action TM data and then send the
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telemetry using the same scheme as shown above, where the first 8 bytes of the application
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data is reserved for the target ID and action ID.
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