Vision Navigation: camera-IMU time offset

Optical flow needs no camera calibration. The plugin pairs each frame with the closest gyro sample, and by default it assumes the camera and IMU clocks line up. If your camera has a known delay, you can give the plugin a measured camera-IMU time offset. This is optional. The only value the plugin uses is timeshift_cam_imu from a Kalibr-style camchain.yaml. It does not use the intrinsics or T_cam_imu.

1. Measure the offset with Kalibr

Run Kalibr’s camera-IMU calibration (kalibr_calibrate_imu_camera) offline for the camera and IMU on the drone. It writes a camchain.yaml. The extension does not run Kalibr for you.

2. Validate the file

The extension’s repository includes a small Python package that loads and checks a camchain.yaml before it goes onto a drone:

git clone https://github.com/altnautica/ADOSExtensions.git
cd ADOSExtensions/extensions/vision-nav/agent/calibration-helper
python -m pip install -e .
python -c "from altnautica_vision_nav_calib import load_intrinsics, load_extrinsics; \
print(load_intrinsics('camchain.yaml')); print(load_extrinsics('camchain.yaml'))"

The loaders raise an error on a malformed block, for example a T_cam_imu rotation that is not orthonormal, and the extrinsics object flags a suspicious timeshift (has_suspicious_timeshift). The file holds a cam0 block (a bare block without the cam0: wrapper also loads):

cam0:
  camera_model: pinhole
  intrinsics: [460.0, 460.0, 320.0, 240.0]
  distortion_model: radtan
  distortion_coeffs: [-0.28, 0.07, 0.0002, 0.00002]
  resolution: [640, 480]
  T_cam_imu:
    - [0.0, -1.0, 0.0, 0.02]
    - [1.0, 0.0, 0.0, 0.00]
    - [0.0, 0.0, 1.0, -0.01]
    - [0.0, 0.0, 0.0, 1.0]
  timeshift_cam_imu: 0.004

timeshift_cam_imu is in seconds, with Kalibr’s convention t_imu = t_cam + timeshift.

3. Copy it to the drone

Copy the file into the plugin’s per-drone data directory on the drone, named camchain.yaml:

ls /var/ados/plugin-data/com.altnautica.vision-nav/drones/
sudo cp camchain.yaml /var/ados/plugin-data/com.altnautica.vision-nav/drones/<agent-id>/camchain.yaml

<agent-id> is the directory the first command lists for this drone.

4. Restart the plugin

The plugin reads the file once, at start-up:

ados plugin disable com.altnautica.vision-nav
ados plugin enable com.altnautica.vision-nav

The camera row on the Vision Nav tab’s Sensors card then shows Calibrated instead of Not calibrated.

Sync offset on the Sensors card

The Sensors card shows the live camera-IMU sync offset: green at 10 ms or less, yellow up to 30 ms, red above 30 ms. The colour is informational. It does not change the estimator state or the pre-arm report. A sync offset that drifts usually means the camera mode changed (resolution, frame rate or exposure); measure again in the mode you fly.

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Extension guides

Vision Navigation: camera-IMU time offset

1. Measure the offset with Kalibr

2. Validate the file

3. Copy it to the drone

4. Restart the plugin

Sync offset on the Sensors card

Next steps

Troubleshooting

Modes and fallback

​1. Measure the offset with Kalibr

​2. Validate the file

​3. Copy it to the drone

​4. Restart the plugin

​Sync offset on the Sensors card

​Next steps

Troubleshooting

Modes and fallback

1. Measure the offset with Kalibr

2. Validate the file

3. Copy it to the drone

4. Restart the plugin

Sync offset on the Sensors card

Next steps