Automatic flight without using GPS

Hi all! I’m interested in the possibility of flying in automatic mode WITHOUT USING GPS. When trying to arm in MissionPlaner messages, I see that EKF3 won’t start. The engine won’t start either. I understand that using only a magnetic compass, accelerometer, barometer, gyroscope, and airspeed sensor will be more error than it would be with GPS, however, I would like to be able to.

Can I achieve this in Arduplane 4.1 firmware? And if this cannot be achieved in the standard firmware, then where should I make changes in the code?
Thanks a lot for the answers!

This is for Copter but it generally applies. See the note:
Non-GPS Navigation