Copter: use rangefinder to prevent auto-disarm in the air

only consider the vehicle to be landed if either no rangefinder or
rangefinder shows an altitude below 2m

ArduCopter/config.h

@@ -370,6 +370,9 @@
 #ifndef LAND_CANCEL_TRIGGER_THR
  # define LAND_CANCEL_TRIGGER_THR   700     // land is cancelled by input throttle above 700
 #endif
+#ifndef LAND_RANGEFINDER_MIN_ALT_CM
+#define LAND_RANGEFINDER_MIN_ALT_CM 200
+#endif
 
 //////////////////////////////////////////////////////////////////////////////
 // Landing Detector

ArduCopter/land_detector.cpp

@@ -71,7 +71,10 @@ void Copter::update_land_detector()
         // check that vertical speed is within 1m/s of zero
         bool descent_rate_low = fabsf(inertial_nav.get_velocity_z()) < 100;
 
-        if (motor_at_lower_limit && accel_stationary && descent_rate_low) {
+        // if we have a healthy rangefinder only allow landing detection below 2 meters
+        bool rangefinder_check = (!rangefinder_alt_ok() || rangefinder_state.alt_cm_filt.get() < LAND_RANGEFINDER_MIN_ALT_CM);
+
+        if (motor_at_lower_limit && accel_stationary && descent_rate_low && rangefinder_check) {
             // landed criteria met - increment the counter and check if we've triggered
             if( land_detector_count < ((float)LAND_DETECTOR_TRIGGER_SEC)*scheduler.get_loop_rate_hz()) {
                 land_detector_count++;