EKF2 with optical flow drifts without flow data

[mhkabir]

So I have been bench-testing a vehicle with a PX4Flow sensor + EKF2. EKF2_AID_MASK is set to fuse Flow and GPS. EKF2_OF_QMIN is set to 50.

When the vehicle is on the ground (PX4Flow lens blocked), the position estimate from EKF2 keeps drifting around, even the though the flow module returns 0 quality and 0 integrated_x and integrated_y. Here is a log from boot : https://logs.px4.io/plot_app?log=6628dc88-c567-4796-95a6-f0815ab1a535

Would be great if you could have a look @priseborough @ChristophTobler!

[priseborough]

This is by design. Flow is always fused, but if on ground and quality metric is too low, motion compensated flow data is set to zero. See reply in #9398.