> ## Documentation Index > Fetch the complete documentation index at: https://docs.altnautica.com/llms.txt > Use this file to discover all available pages before exploring further. # FAQ > The questions that come up every time. Do I need a rangefinder, which board for VIO, what about night flight, how does it compare to GPS. # Frequently Asked Questions The questions operators ask before, during, and after the first GPS-denied flight. Short answers; deeper pointers at the end of each. ## Do I need a rangefinder? If you pick the `optical_flow` mode, yes. The rangefinder provides the scale that turns angular flow into metric velocity. If you pick `optical_flow_degraded`, no. The plugin pulls scale from the FC's barometer instead. The cost is reduced accuracy; the GCS labels the mode as degraded and the EKF auto-de-weights the emissions. If you pick `vio_openvins` or `vio_vins_fusion`, no. VIO computes metric pose directly from the camera and IMU. A rangefinder helps indirectly (you can mix in OF mode later for redundancy) but is not required. See [Fallback methods](/drone-agent/vision-nav-fallback-methods) for the rangefinder-free scale ladder. ## Which board do I need for VIO? OpenVINS runs on any board with at least one Cortex-A76 core or equivalent. In practice that means Radxa ROCK 5C Lite (RK3582), Radxa CM4 (RK3588S2), or Rockchip RK3576 boards. A Raspberry Pi 5 will run it with the NEON fallback at reduced rate. VINS-Fusion needs roughly twice the CPU OpenVINS uses. It only runs well on RK3582 and RK3588S2 boards. A Raspberry Pi 4B is fine for OF and OF-degraded modes but cannot run either VIO engine at the rates the EKF expects. See [Mode comparison](/drone-agent/vision-nav-mode-comparison) for the per-board mode matrix. ## What about night flight? Vision navigation needs light. The Lucas-Kanade tracker (used by all OF modes) needs at least 20 lux to extract corner features. VIO has a similar threshold; below 20 lux the tracker loses features per frame and the estimator goes degraded. Workarounds: * Add an IR illuminator. The tracker is colour-blind; it works fine with IR illumination. * Use a mono global-shutter camera. Mono sensors have higher quantum efficiency per pixel and tolerate lower light. * Drop the camera frame rate. Lower rate means longer exposure per frame; this trades motion-blur tolerance for light sensitivity. The plugin's fallback banner fires when flow quality drops below 50 so the operator sees the degradation before the EKF rejects too many samples. ## How does it compare to GPS? GPS gives the EKF an absolute position. Vision gives it a relative position (where the drone is now compared to where it was a moment ago). The two are not interchangeable. For most flights GPS is better. It works above clouds, at night, in featureless terrain, and over water. Vision is the GPS-denied fallback or the only option when GPS is unreliable. Vision shines in environments GPS cannot reach: indoor flight, under canopy, in canyons, between tall buildings, in tunnels, near infrastructure that reflects GPS multipath. The EKF source-set switcher on Mission Control's Navigation tab lets the operator flip between GPS and vision sources in flight. On ArduPilot the switch is safe at altitude. ## Can I fly outdoors with vision navigation? Yes. Outdoor flight is sometimes easier than indoor flight because the texture is richer (grass, asphalt with paint markings, gravel, sand textures). The two outdoor challenges are: 1. **Featureless ground:** snow, water, fresh asphalt without markings. The OF tracker has nothing to lock onto. VIO modes use forward-facing cameras and are less affected; OF-degraded mode can fall back to the GPS scale rung if the outdoor flag is enabled. 2. **Fast yaw:** the OF tracker handles yaw up to roughly 90 °/s using gyro derotation. Beyond that, features fall off the frame between samples. Slow the yaw or switch to a wider FOV lens. ## What happens if the camera unplugs mid-flight? The plugin watches for capture-source failure. When the camera goes silent for more than 2 seconds, the estimator state flips to `failed` and the fallback banner appears. The flight controller's EKF stops receiving samples from the source the plugin owns. Its own innovation gate notices and downgrades the local position estimate. On ArduPilot, the operator can use the EKF source-set switcher to flip back to GPS at altitude. On PX4, the EKF will eventually surface its own warning; the operator should land or RTL. The plugin does not auto-RTL. Mode is an operator decision and the plugin reports honestly so the operator makes the right call. ## Do I need to recalibrate after every flight? No. Camera intrinsics are a one-time per-camera calibration. They stay stable for the life of the camera. Camera-IMU extrinsics are a one-time per-pair calibration. They stay stable as long as the camera and IMU are rigidly mounted to the same frame. The static time offset between the camera clock and the IMU clock is stable for a given camera mode (resolution + frame rate + exposure profile). If you change camera modes, the offset shifts and the calibration should be refreshed. The rolling time-sync residual the GCS shows is a per-flight measurement against the calibrated offset; if it stays in the green band, no recalibration is needed. ## When can I switch the EKF source to VIO? The plugin ships the VIO engines, so the agent's heartbeat reports `vioSupported: true` whenever Vision Navigation is installed. The VIO source-set switch is gated on the estimator actually being ready: the button enables only when the estimator state is `converged`, so the switch never silently fails mid-flight. Until the VIO estimator converges (calibration loaded, a VIO mode selected, enough features tracked), the switch stays disabled and the tooltip explains which condition is unmet. ## How accurate is it? Honest answer: it depends on the mode, the scene, and the calibration. For `optical_flow` with a working rangefinder in a textured indoor scene at 1 to 3 m altitude: 20 to 50 cm drift radius over a 60-second hover is typical for a well-tuned setup. For `optical_flow_degraded` in the same conditions: 50 cm to 1 m drift radius. Roughly 2× worse than OF with a rangefinder. For `vio_openvins` in feature-rich indoor flight: 5 to 15 cm position accuracy after convergence, drift of roughly 0.1 to 0.5 percent of distance travelled. For `vio_vins_fusion`: slightly tighter than OpenVINS in feature-rich scenes (3 to 10 cm). Roughly the same in feature-poor scenes. These are typical numbers from upstream OpenVINS and VINS-Fusion benchmarks plus the OF performance the existing public datasets imply. Per-installation accuracy depends on calibration quality, scene texture, motion profile, and exposure. The plugin does not currently publish per-installation accuracy numbers from real flights. Numbers will be characterised in a published validation matrix before any "tested at X accuracy" claim ships. ## Can I run two of these plugins on the same drone? No. The plugin claims MAVLink components 197 and 198 exclusively. Two instances would clash on the component IDs and the second install would refuse to start. If you want redundant vision navigation, use the `hybrid_of_plus_vio` mode, which runs an OF estimator and a VIO estimator inside the same plugin process. ## Where do I file a bug? The plugin lives in the public [`altnautica/ADOSExtensions`](https://github.com/altnautica/ADOSExtensions) repository. File an issue there with: * The drone's mode at the time of the failure * The journalctl excerpt for the plugin's systemd unit (the agent's logs surface as `ados-plugin-com.altnautica.vision-nav*`) * The Mission Control screenshot of the Navigation tab at the moment of the failure * The MAVLink tlog from the flight controller covering the failure window The more context, the faster the fix. ## Where to go next * [Getting started](/drone-agent/vision-nav-getting-started) for the install + first-flight walkthrough. * [Modes](/drone-agent/vision-nav-modes) for the full per-mode reference. * [Mode comparison](/drone-agent/vision-nav-mode-comparison) for the side-by-side decision table. * [Troubleshooting](/drone-agent/vision-nav-troubleshooting) for what to do when something goes wrong.