I guess you want to fly indoors in LOITER mode?
If you fly with modes that don’t require GPS you don’t need anything.
If you want to use LOITER or other modes that require a GPS but indoors, you need optical flow and so you also need sonar.
The Bebop sonar is already supported in ardupilot though it may require some tricks to make it work.
For the optical flow. I have implemented a basic version but I never spent enough time, and also never had a really good knowledge of how the EKF handles the optical flow so I haven’t been able to debug it and make it usable in practice to fly indoors.
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