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Hwurzburg merged 1 commit into from
Aug 25, 2026
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dev: document dflogger streaming and MAVFTP for companion computers #7992

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19 changes: 19 additions & 0 deletions dev/source/docs/mavlink-log-download.rst
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Expand Up @@ -56,3 +56,22 @@ exchange above automatically:
- ``log download <lognumber> <filename>`` — download a single log to a file
- ``log download all`` — download all logs
- ``log erase`` — erase all logs

Streaming the Current Log to a Companion Computer
--------------------------------------------------

The methods above download a *completed* log after the fact. A
companion computer can instead have ArduPilot stream the *current*
log to it live, as it is being written, by setting bit 1 ("MAVLink")
of :ref:`LOG_BACKEND_TYPE <copter:LOG_BACKEND_TYPE>`. The size
of the in-memory ring buffer used for this is set by
:ref:`LOG_MAV_BUFSIZE <copter:LOG_MAV_BUFSIZE>` (KB).

With that bit set, ArduPilot sends `REMOTE_LOG_DATA_BLOCK <https://mavlink.io/en/messages/ardupilotmega.html#REMOTE_LOG_DATA_BLOCK>`__ messages (200 bytes of log data per message, identified by a ``seqno``) once a receiver signals it is ready by sending a `REMOTE_LOG_BLOCK_STATUS <https://mavlink.io/en/messages/ardupilotmega.html#REMOTE_LOG_BLOCK_STATUS>`__ with ``seqno`` = ``MAV_REMOTE_LOG_DATA_BLOCK_START`` (2147483646). The receiver acks/nacks each block it receives (or notices missing) with another ``REMOTE_LOG_BLOCK_STATUS``, ``seqno`` set to the block number and ``status`` = ``MAV_REMOTE_LOG_DATA_BLOCK_ACK`` (1) or ``_NACK`` (0); ArduPilot re-sends nacked blocks. Sending ``REMOTE_LOG_BLOCK_STATUS`` with ``seqno`` = ``MAV_REMOTE_LOG_DATA_BLOCK_STOP`` (2147483645) stops the stream.

`dflogger <https://github.com/peterbarker/dronekit-la/blob/master/dataflash_logger.cpp>`__ is one existing implementation of this protocol suitable for running on a companion computer.

MAVFTP
------

:ref:`MAVFTP <mavlink-mavftp>` can also fetch a log file directly from the autopilot's filesystem, but this is not the recommended method -- use the ``LOG_*`` messages above (for a completed log) or MAVLink log streaming (for the current log) instead.
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