Plane: use WP height for height above ground in vtol landing

this allows for landings above or below the takeoff height without
requiring the use of terrain data. It allows both the use of vfwd
motor for holding against wind, and the correct height for land final
descent rate

ArduPlane/altitude.cpp

@@ -137,6 +137,16 @@ float Plane::relative_ground_altitude(bool use_rangefinder_if_available)
         return altitude;
     }
 #endif
+
+    if (quadplane.in_vtol_land_descent() &&
+        !(quadplane.options & QuadPlane::OPTION_MISSION_LAND_FW_APPROACH)) {
+        // when doing a VTOL landing we can use the waypoint height as
+        // ground height. We can't do this if using the
+        // LAND_FW_APPROACH as that uses the wp height as the approach
+        // height
+        return height_above_target();
+    }
+
     return relative_altitude;
 }
 

ArduPlane/quadplane.cpp

@@ -2521,7 +2521,15 @@ bool QuadPlane::verify_vtol_land(void)
         plane.auto_state.wp_distance < 2) {
         poscontrol.state = QPOS_LAND_DESCEND;
         gcs().send_text(MAV_SEVERITY_INFO,"Land descend started");
-        plane.set_next_WP(plane.next_WP_loc);
+        if (plane.control_mode == AUTO) {
+            // set height to mission height, so we can use the mission
+            // WP height for triggering land final if no rangefinder
+            // available
+            plane.set_next_WP(plane.mission.get_current_nav_cmd().content.location);
+        } else {
+            plane.set_next_WP(plane.next_WP_loc);
+            plane.next_WP_loc.alt = ahrs.get_home().alt;
+        }
     }
 
     // at land_final_alt begin final landing
@@ -2640,8 +2648,7 @@ int8_t QuadPlane::forward_throttle_pct(void)
     int8_t fwd_throttle_min = plane.have_reverse_thrust() ? 0 : plane.aparm.throttle_min;
     vel_forward.integrator = constrain_float(vel_forward.integrator, fwd_throttle_min, plane.aparm.throttle_max);
 
-    if (in_vtol_auto() && is_vtol_land(plane.mission.get_current_nav_cmd().id) &&
-        (poscontrol.state == QPOS_POSITION1 || poscontrol.state == QPOS_POSITION2)) {
+    if (in_vtol_land_approach()) {
         // when we are doing horizontal positioning in a VTOL land
         // we always allow the fwd motor to run. Otherwise a bad
         // lidar could cause the aircraft not to be able to
@@ -2933,3 +2940,27 @@ void QuadPlane::update_throttle_thr_mix(void)
         }
     }
 }
+
+/*
+  see if we are in the approach phase of a VTOL landing
+ */
+bool QuadPlane::in_vtol_land_approach(void) const
+{
+    if (in_vtol_auto() && is_vtol_land(plane.mission.get_current_nav_cmd().id) &&
+        (poscontrol.state == QPOS_POSITION1 || poscontrol.state == QPOS_POSITION2)) {
+        return true;
+    }
+    return false;
+}
+
+/*
+  see if we are in the descent phase of a VTOL landing
+ */
+bool QuadPlane::in_vtol_land_descent(void) const
+{
+    if (in_vtol_auto() && is_vtol_land(plane.mission.get_current_nav_cmd().id) &&
+        (poscontrol.state == QPOS_LAND_DESCEND || poscontrol.state == QPOS_LAND_FINAL)) {
+        return true;
+    }
+    return false;
+}

ArduPlane/quadplane.h

@@ -499,6 +499,16 @@ private:
     QAutoTune qautotune;
 #endif
 
+    /*
+      are we in the approach phase of a VTOL landing?
+     */
+    bool in_vtol_land_approach(void) const;
+
+    /*
+      are we in the descent phase of a VTOL landing?
+     */
+    bool in_vtol_land_descent(void) const;
+    
 public:
     void motor_test_output();
     MAV_RESULT mavlink_motor_test_start(mavlink_channel_t chan, uint8_t motor_seq, uint8_t throttle_type,