Unstable final 1-2m of Autoland

Hi All!

We are just trying to chase down an odd but unnerving condition we have noticed.

When looking at the logs, look at the last stage in the landing. Using a HERE Flow meter also.

We are flying long range survey with large multicopters and currently using Dual RTK with bothe EK2 and EK3 set to GPS for alt measurement .

It does a great job however whenever we bring it back to land, in the last meter or two it starts moving away from its landing position. View calculated height vs baro or Rangefinder. Almost toilet bowling, so we reject the RTL, put it into loiter and climb up. It becomes rock solid after this, yet in Loiter again when we approach the ground it starts the same thing and seems unable to hold position. We then change to ALT HOLD and perform the landing by manually controlling the position.

Two things that i have noticed are, this only seems to happen when close to the ground we get into negative altitude on “Calculated height” - yet GPS and BARO are accurate, including rangefinder… As soon as it goes negative it starts this affect. I have also noticed in our logs that Optical Flow goes a bit nuts around the same time.

Anyone able to have a look at our two logs and see? The first log was an unstable RTL, the Second was a rejected RTL into ALT Hold for completion of the landing.

Does EKF switch to flow at negative altitude? My Flow hasnt been set up properly by the looks of it, when does it switch to this for position hold during RTL? Can i get rid of flow altogether and just use our RTK? Why does Calculated height not reflect the RTK GPS or Rangefinder close in?

Any help appreciated guys and girls!!!

https://drive.google.com/file/d/1JJpPgGaQCoiT0lG6iuuKINwUuvMNggAe/view?usp=sharing

https://drive.google.com/file/d/1JJb4cnNbKK_qC-hhpEQNXNZyyiEm6iGu/view?usp=sharing

Turned off flow. Rock solid landings.

It was the flow switching over and also feeding bad info as not calibrated!

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