FW Position Controller: fix Altitude mode without valid z reference (…

…e.g. no GPS)

Signed-off-by: Silvan Fuhrer <[email protected]>

src/modules/fw_pos_control/FixedwingPositionControl.cpp

@@ -2304,7 +2304,7 @@ FixedwingPositionControl::Run()
 			_reference_altitude = 0.f;
 		}
 
-		_current_altitude = -_local_pos.z + _local_pos.ref_alt; // Altitude AMSL in meters
+		_current_altitude = -_local_pos.z + _reference_altitude; // Altitude AMSL in meters
 
 		// handle estimator reset events. we only adjust setpoins for manual modes
 		if (_control_mode.flag_control_manual_enabled) {