journalctl JSON exports

The companion computer's systemd journal records what the autonomy stack, systemd and the kernel did during a run. Export it with journalctl -o json, which writes one JSON object per line, and import the file with the flight log. Foxborne reads it with parser journal 0.6.0.

If the companion collector is installed, it delivers the allowlisted journal units on its own. An export still matters for vehicles without a collector, and for lines from units outside the collector's allowlist.

Export the journal#

Run the export on the companion computer as root, or as a member of the systemd-journal group, so it includes the system and kernel entries. Name the file for the host, as in uas04-orin-journal.json.

To export the window of a run, give its start and end in UTC:

sudo journalctl -o json --since "2026-09-24 14:10:00 UTC" --until "2026-09-24 14:40:00 UTC" > uas04-orin-journal.json

To export a whole boot, find its boot ID, then pass it to -b:

sudo journalctl --list-boots
sudo journalctl -o json -b 9b2e4c1d0f8a4e57b3c2a1d9e8f7c6b5 > uas04-orin-journal.json

Export the whole journal for the window or the boot, not single units. The rules read the kernel's lines and systemd's lines about each unit, and a -u filter leaves the kernel's lines out.

Fields Foxborne uses#

Each field has a job in the investigation:

• Time. __REALTIME_TIMESTAMP places each entry on the companion's wall clock. __MONOTONIC_TIMESTAMP and _BOOT_ID place it on the monotonic clock of its boot, the clock that collector samples share.
• Level. PRIORITY sets how an entry shows: the timeline marks priority 3 or lower red and 4 amber. The event page labels 2 or lower Critical, 3 Error and 4 Warning.
• Source. _HOSTNAME, SYSLOG_IDENTIFIER, _PID, _TRANSPORT and UNIT say who logged the entry and what it is about. The event page's tags carry them as host, identifier, pid, transport and unit.
• Text. MESSAGE is quoted exactly wherever the entry appears as evidence, with the line it came from.

How Foxborne recognizes the export#

Foxborne looks for __REALTIME_TIMESTAMP in the file's first 4,096 bytes. The file's row then reads journalctl JSON export with Journal fields found. journalctl's default short output carries no field names, so Foxborne cannot recognize it.

Clocks and chrony#

Both timestamps mark when journald received the entry, not when the program wrote it. __REALTIME_TIMESTAMP is usually a little later than _SOURCE_REALTIME_TIMESTAMP, the earliest trusted time of the message.

Wall time is only as good as the clock under it, and chrony disciplines that clock. Foxborne anchors the journal to UTC while chrony reports a synchronized source. For run R-0931 the run page reads chrony, synchronised, offset −1.8 ms, and the example dataset carries ±6 ms on the journal.

When chrony has no source, chronyc tracking shows Leap status : Not synchronised. In INC-0139, chronyd logged No suitable source for synchronisation at boot, and the run page reads None: chrony never synchronised. The journal then stays on its own clock, and an engineer can align it by hand.

Record chronyc tracking before and after each run, and keep the output with the journal.

Example: the kill in INC-0142#

The kernel killed perception_node for running out of memory at 14:32:04.118 UTC. This is the entry in the example dataset's export, on several lines to read:

{
  "__REALTIME_TIMESTAMP": "1790260324118000",
  "__MONOTONIC_TIMESTAMP": "1152904000",
  "_BOOT_ID": "9b2e4c1d0f8a4e57b3c2a1d9e8f7c6b5",
  "_HOSTNAME": "uas04-orin",
  "PRIORITY": "3",
  "SYSLOG_IDENTIFIER": "kernel",
  "_TRANSPORT": "kernel",
  "MESSAGE": "Out of memory: Killed process 2213 (perception_node) total-vm:11873248kB, anon-rss:6823516kB, file-rss:10240kB, shmem-rss:0kB, UID:1001 pgtables:14720kB oom_score_adj:0"
}

systemd's lines about the units follow. Each carries SYSLOG_IDENTIFIER systemd, _PID 1 and the unit in UNIT.

The event page's Unit history lays out the same lines. The process_exit rule fired on the exit of perception.service and on the stop of mavlink-router.service, and service_restart fired on the restart.

Related#

• Clocks, anchors and error bounds explains the wall clock, the monotonic clock and chrony's bound.
• The companion collector delivers allowlisted journal units without an export.
• process_exit and service_restart describe the rules that read these lines.
• Import files in the console covers the upload itself.