This describes the new logger, SYS_LOGGER set to 1.

The logger is able to log any ORB topic with all included fields. Everything necessary is generated from the .msg file, so that only the topic name needs to be specified. An optional interval parameter specifies the maximum logging rate of a certain topic. All existing instances of a topic are logged.

The output log format is ULog.


By default, logging is automatically started when arming, and stopped when disarming. A new log file is created for each arming session on the SD card. To display the current state, use logger status on the console. If you want to start logging immediately, use logger on. This overrides the arming state, as if the system was armed. logger off undoes this.


logger help

for a list of all supported logger commands and parameters.


The list of logged topics can be customized with a file on the SD card. Create a file etc/logging/logger_topics.txt on the card with a list of topics:

<topic_name>, <interval>

The <interval> is optional, and if specified, defines the minimum interval in ms between two logged messages of this topic. If not specified, the topic is logged at full rate.

The topics in this file replace all of the default logged topics.


There are several scripts to analyze and convert logging files in the pyulog repository.


Logging dropouts are undesired and there are a few factors that influence the amount of dropouts:

  • Most SD cards we tested exhibit multiple pauses per minute. This shows itself as a several 100 ms delay during a write command. It causes a dropout if the write buffer fills up during this time. This effect depends on the SD card (see below).
  • Formatting an SD card can help to prevent dropouts.
  • Increasing the log buffer helps.
  • Decrease the logging rate of selected topics or remove unneeded topics from being logged (info.py <file> is useful for this).

SD Cards

The following provides performance results for different SD cards. Tests were done on a Pixracer; the results are applicable to Pixhawk as well.

SD Card Mean Seq. Write Speed [KB/s] Max Write Time / Block (average) [ms]
SanDisk Extreme U3 32GB 461 15
Sandisk Ultra Class 10 8GB 348 40
Sandisk Class 4 8GB 212 60
SanDisk Class 10 32 GB (High Endurance Video Monitoring Card) 331 220
Lexar U1 (Class 10), 16GB High-Performance 209 150
Sandisk Ultra PLUS Class 10 16GB 196 500
Sandisk Pixtor Class 10 16GB 334 250
Sandisk Extreme PLUS Class 10 32GB 332 150

More important than the mean write speed is the maximum write time per block (of 4 KB). This defines the minimum buffer size: the larger this maximum, the larger the log buffer needs to be to avoid dropouts. Logging bandwidth with the default topics is around 50 KB/s, which all of the SD cards satisfy.

By far the best card we know so far is the SanDisk Extreme U3 32GB. This card is recommended, because it does not exhibit write time spikes (and thus virtually no dropouts). Different card sizes might work equally well, but the performance is usually different.

You can test your own SD card with sd_bench -r 50, and report the results to https://github.com/PX4/Firmware/issues/4634.

Log Streaming

The traditional and still fully supported way to do logging is using an SD card on the FMU. However there is an alternative, log streaming, which sends the same logging data via MAVLink. This method can be used for example in cases where the FMU does not have an SD card slot (e.g. Intel® Aero Ready to Fly Drone) or simply to avoid having to deal with SD cards. Both methods can be used independently and at the same time.

The requirement is that the link provides at least ~50KB/s, so for example a WiFi link. And only one client can request log streaming at the same time. The connection does not need to be reliable, the protocol is designed to handle drops.

There are different clients that support ulog streaming:

  • mavlink_ulog_streaming.py script in Firmware/Tools.
  • QGroundControl:


  • If log streaming does not start, make sure the logger is running (see above), and inspect the console output while starting.
  • If it still does not work, make sure that Mavlink 2 is used. Enforce it by setting MAV_PROTO_VER to 2.
  • Log streaming uses a maximum of 70% of the configured mavlink rate (-r parameter). If more is needed, messages are dropped. The currently used percentage can be inspected with mavlink status (1.8% is used in this example):
    instance #0:
          GCS heartbeat:  160955 us ago
          mavlink chan: #0
          type:           GENERIC LINK OR RADIO
          flow control:   OFF
          tx: 95.781 kB/s
          txerr: 0.000 kB/s
          rx: 0.021 kB/s
          rate mult: 1.000
          ULog rate: 1.8% of max 70.0%
          accepting commands: YES
          MAVLink version: 2
          transport protocol: UDP (14556)
    Also make sure txerr stays at 0. If this goes up, either the NuttX sending buffer is too small, the physical link is saturated or the hardware is too slow to handle the data.

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