WebXR based drone HUD for AR (Hololens 2).

Intention is to run server on a RasPi with access to GPS + MAVLink streams.

The project is split into three parts:

• A web client and server
• A GPS position server
• A MAVLink server

The web client and server together serve an A-frame-based WebXR app for the Hololens, displaying a compass for alignment, drone positions and drone mission waypoints (flight plans).

The GPS position server provides position in local tangential plane coordinates (NED), an endpoint for setting the local plane origin, and an endpoint for GPS fix details.

The MAVLink server is the bridge to the drones. It is used to retrieve any drones' positions, their flight plans or missions and some limited status information (currently flight mode and battery).

The Hololens/WebXR API does not provide access to a globally stable coordinate system (e.g. WGS84). Instead, measurements of the GPS position and the Hololens position estimate are used to generate a transform from the global frame into the Hololens/WebXR frame. This process is especially important to generate a rotational alignment with grid north.

Users are asked to "Move around in the world" to allow the app to gather GPS and corresponding Hololens measurements. For north alignment, the movement needs to be significant enough for GPS position inaccuracies to be dealt with by the least squares alignment. Using an RTK-enabled GPS unit will make this a much less challenging requirement.

The Hololens coordinate system may drift over time, especially if users spend a lot of time looking upwards without nearby features for the system to track. Unfortunately, this is likely the exact sort of thing that will happen when trying to look at drones... The rotational system should remain relatively consistent, but may suffer from increased drift. Small drifts in horizontal position should not be too obvious at typical drone operation distances.

With a drone selected, the system will attempt to load the flight plan for it over MAVLink. The waypoints from this flight plan will be shown as a line in the projected view.