Question about EKF Failsafe Action

Hi, everyone.
I’m newbie of Ardupilot.

Recently I’m playing with ArduCopter on SITL and I have a question about EKF failsafe.

In the document, EKF failsafe offers Land, AltHold, and Land even in stabilize.

When I trigger EKF failsafe using param set SIM_GPS_DISABLE 1,
copter always hit the ground more then 2 m/s speed.

APM: SIM Hit ground at 2.402549 m/s
APM: Disarming motors

Land option looks like more safe because it always under 1 m/s.
But AltHold option looks like falling of drone for me.

Could you please explain why it is normal and cannot hold the alt?

Thank you for reading this :slight_smile:

If a failsafe happens the drone no longer knows here it is. It needs to land as soon as possible. By using altholt you force it to be in the air. At some point it no longer can stabilize the copter and it falls of the sky. AFAIK That is why it hits the ground harder in AltHold than in land.

Thank you for answering my question.

I agree quick landing is best option when failsafe is occurred.
My question is why AltHold option is provided if drone cannot stabilize on the EKF failsafe.
If copter cannot stabilize without GPS, I think it is better to remove AltHold option in the EKF failsafe option.

Please let me know what I misunderstand, and any background history.
Thanks :slight_smile:

I found answer about this.

old article had same problem about this.
It was not EKF failsafe problem, but althold problem.

I can check the well failsafe action with EKF failsafe.
Thanks again @amilcarlucas !