Operating APM2.6 without external GPS/Compass

Can anyone tell me if I can operate an APM2.6 without the GPS and compass if I’m only interested in Stabilization and FBW A modes?
I already have a full system installed in a Apprentice 15e and it works great. For my next plane, I have severe space constraints and do not want to install the GPS.

Its not recommend to fly without a GPS - the attitude estimation won’t be very good.
If you have an airspeed sensor that will help but we really wouldn’t recommend flying without a GPS.

Thanks, Grant