Fault modes

Each mode is applied per channel by kraken_faults/fault_injector. A channel is a named subscription/publication pair declared in kraken_faults/config/channels.yaml; the shipped ones are gnss (sensor_msgs/NavSatFix), imu (sensor_msgs/Imu) and wheel (nav_msgs/Odometry).

Set one from a scenario file:

- label: kill_gnss
  action: set_fault
  channel: gnss
  mode: dropout

or at runtime:

ros2 service call /fault_injector/set_fault kraken_interfaces/srv/SetFault \
  "{fault: {channel: gnss, mode: 1}}"
ros2 service call /fault_injector/clear_faults std_srvs/srv/Trigger

dropout (mode 1)

channel behaviour
gnss keeps publishing, but with STATUS_NO_FIX and unknown covariance
imu stops publishing entirely
wheel stops publishing the topic, keeps broadcasting the last transform

GNSS reports NO_FIX rather than going silent because that is what a real receiver does when it loses lock, and it is the harder case: a consumer that only checks “did a message arrive” will sail straight past it.

The wheel case keeps its transform alive deliberately. odom -> base_link is part of the TF chain the EKF needs in order to publish map -> odom at all, so dropping it would break the whole tree rather than test the filter.

degrade (mode 2)

Adds white noise of noise_stddev and multiplies the sensor’s reported covariance by covariance_scale. Native units per channel: metres for gnss, rad/s for imu, m/s and rad/s for wheel. GNSS is additionally downgraded from STATUS_GBAS_FIX to STATUS_FIX.

This models an honest degradation — the sensor gets worse and says so. A well-tuned filter should widen its own uncertainty and lean on dead reckoning.

bias_ramp (mode 3)

Adds bias_rate units per second, accumulating from the moment the fault is set. GNSS walks off on a fixed 45° bearing; the IMU gets a growing yaw-rate bias.

Unlike degrade, the reported covariance stays tight. The sensor is lying confidently, which is the case filters handle worst.

Known gap: the stack currently has no innovation gating, so it follows a GNSS spoof more or less exactly (~8 m of error for a 0.2 m/s ramp over 40 s). scenarios/gnss_spoof.yaml asserts this failure so it stays visible. Adding a Mahalanobis gate, or a consistency check between GNSS and dead reckoning, is an open and very welcome contribution.

slip (mode 4)

Odometry only. Multiplies reported linear and angular velocity by slip_ratio:

healthy (mode 0)

Passes measurements straight through. Faults also revert to this automatically once their duration elapses; a duration of zero means “until cleared”.

Adding a mode

  1. Add the constant and any parameters to kraken_interfaces/msg/FaultSpec.msg.
  2. Add it to SUPPORTED in fault_injector.py for the channel types it applies to, and implement it in the matching _apply_* method.
  3. Add a scenario file that exercises it, with thresholds taken from actual measured runs rather than guessed.