EKF: Use parameter for stuck range test threshold

The default has been reduced to make it suitable for low altitude optical flow navigation.

EKF/common.h

@@ -283,6 +283,7 @@ struct parameters {
 	int32_t range_aid{0};			///< allow switching primary height source to range finder if certian conditions are met
 	float range_aid_innov_gate{1.0f}; 	///< gate size used for innovation consistency checks for range aid fusion
 	float range_cos_max_tilt{0.7071f};	///< cosine of the maximum tilt angle from the vertical that permits use of range finder and flow data
+	float range_stuck_threshold{0.1f};	///< minimum variation in range finder reading required to declare a range finder 'unstuck' when readings recommence after being out of range (m)
 
 	// vision position fusion
 	float ev_innov_gate{5.0f};		///< vision estimator fusion innovation consistency gate size (STD)

EKF/control.cpp

@@ -1184,7 +1184,7 @@ void Ekf::checkRangeDataValidity()
 	    _control_status.flags.in_air) {
 
 		// require a variance of rangefinder values to check for "stuck" measurements
-		if (_rng_stuck_max_val - _rng_stuck_min_val > 1.0f) {
+		if (_rng_stuck_max_val - _rng_stuck_min_val > _params.range_stuck_threshold) {
 			_time_last_rng_ready = _range_sample_delayed.time_us;
 			_rng_stuck_min_val = 0.0f;
 			_rng_stuck_max_val = 0.0f;