rtl: added loiter radius for descending

src/modules/navigator/rtl.cpp

@@ -257,6 +257,7 @@ void RTL::on_activation()
 
 	const vehicle_global_position_s &global_position = *_navigator->get_global_position();
 
+	_rtl_loiter_rad = _param_rtl_loiter_rad.get();
 	if (_navigator->get_vstatus()->vehicle_type == vehicle_status_s::VEHICLE_TYPE_ROTARY_WING) {
 		_rtl_alt = calculate_return_alt_from_cone_half_angle((float)_param_rtl_cone_half_angle_deg.get());
 
@@ -410,6 +411,10 @@ void RTL::set_rtl_item()
 				_mission_item.yaw = _destination.yaw;
 			}
 
+			if (_navigator->get_vstatus()->is_vtol) {
+				_mission_item.loiter_radius = _rtl_loiter_rad;
+			}
+
 			_mission_item.acceptance_radius = _navigator->get_acceptance_radius();
 			_mission_item.time_inside = 0.0f;
 			_mission_item.autocontinue = true;

src/modules/navigator/rtl.h

@@ -148,6 +148,7 @@ private:
 
 	float _rtl_alt{0.0f};	// AMSL altitude at which the vehicle should return to the home position
 	bool _rtl_alt_min{false};
+	float _rtl_loiter_rad{50.0f};		// radius at which a fixed wing would loiter while descending
 	bool _climb_and_return_done{false};	// this flag is set to true if RTL is active and we are past the climb state and return state
 	bool _deny_mission_landing{false};
 
@@ -159,7 +160,8 @@ private:
 		(ParamInt<px4::params::RTL_TYPE>) _param_rtl_type,
 		(ParamInt<px4::params::RTL_CONE_ANG>) _param_rtl_cone_half_angle_deg,
 		(ParamFloat<px4::params::RTL_FLT_TIME>) _param_rtl_flt_time,
-		(ParamInt<px4::params::RTL_PLD_MD>) _param_rtl_pld_md
+		(ParamInt<px4::params::RTL_PLD_MD>) _param_rtl_pld_md,
+		(ParamFloat<px4::params::RTL_LOITER_RAD>) _param_rtl_loiter_rad
 	)
 
 	// These need to point at different parameters depending on vehicle type.

src/modules/navigator/rtl_params.c

@@ -159,3 +159,17 @@ PARAM_DEFINE_FLOAT(RTL_FLT_TIME, 15);
  * @group Return To Land
  */
 PARAM_DEFINE_INT32(RTL_PLD_MD, 0);
+
+/**
+ * Loiter radius for rtl descend
+ *
+ * Set the radius for loitering to a safe altitude for VTOL transition.
+ *
+ * @unit m
+ * @min 25
+ * @max 1000
+ * @decimal 1
+ * @increment 0.5
+ * @group Return Mode
+ */
+PARAM_DEFINE_FLOAT(RTL_LOITER_RAD, 50.0f);