Plane: add param LAND_RNGFND_HLD to hold-off lidar during landing

ArduPlane/commands_logic.cpp

@@ -252,12 +252,22 @@ bool Plane::verify_command(const AP_Mission::Mission_Command& cmd)        // Ret
             return landing.verify_abort_landing(prev_WP_loc, next_WP_loc, current_loc, auto_state.takeoff_altitude_rel_cm, throttle_suppressed);
 
         } else {
+            float height = height_above_target();
+
             // use rangefinder to correct if possible
-            float height = height_above_target() - rangefinder_correction();
+            if (landing.get_rangefinder_holdoff_distance() <= 0 ||  // rangefinder holdoff is disabled (never hold off, always use rtanegfinder)
+                auto_state.wp_proportion >= 1 ||                    // or we have reached the land waypoint
+                landing.is_flaring() ||                             // or we are flaring
+                current_loc.get_distance(next_WP_loc) <= landing.get_rangefinder_holdoff_distance()) // or we've reached the holdoff distance
+            {
+                height -= rangefinder_correction();
+            }
+
             // for flare calculations we don't want to use the terrain
             // correction as otherwise we will flare early on rising
             // ground
             height -= auto_state.terrain_correction;
+
             return landing.verify_land(prev_WP_loc, next_WP_loc, current_loc,
                                        height, auto_state.sink_rate, auto_state.wp_proportion, auto_state.last_flying_ms, arming.is_armed(), is_flying(),
                                        g.rangefinder_landing && rangefinder_state.in_range);

libraries/AP_Landing/AP_Landing.cpp

@@ -163,6 +163,15 @@ const AP_Param::GroupInfo AP_Landing::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("WIND_COMP", 18, AP_Landing, wind_comp, 50),
 
+    // @Param: RNGFND_HLD
+    // @DisplayName: Rangefinder hold-off
+    // @Description: Rangefinder hold-off distance from land point. Hold off using the rangefinder's altitude correction until you're this close horizontally to the NAV_LAND point. Requires RNGFND_LANDING = 1. This is useful to avoid the rangefinder reacting to objects/trees, or pits, before the level-ish runway starts. Set to zero to disable the hold-off (don't inhibit the rangefinder). Using this feature will cause the rangefinder to correct the baro offset later in the landing which will lead to a larger slope correction. It is suggested to use LAND_ABORT_DEG with this feature so that unacheivable slopes will cause automatic go-arounds while retaining the rangefinder correction in the baro for a smoother landing on the next attempt.
+    // @Range: 0 1000
+    // @Units: m
+    // @Increment: 1
+    // @User: Advanced
+    AP_GROUPINFO("RNGFND_HLD", 19, AP_Landing, rangefinder_holdoff_before_land_point, 0),
+
     // @Param: TYPE
     // @DisplayName: Auto-landing type
     // @Description: Specifies the auto-landing type to use

libraries/AP_Landing/AP_Landing.h

@@ -105,6 +105,7 @@ class AP_Landing {
     int8_t get_abort_throttle_enable(void) const { return abort_throttle_enable; }
     int8_t get_flap_percent(void) const { return flap_percent; }
     int8_t get_throttle_slewrate(void) const { return throttle_slewrate; }
+    float get_rangefinder_holdoff_distance(void) const { return rangefinder_holdoff_before_land_point; }
     bool is_commanded_go_around(void) const { return flags.commanded_go_around; }
     bool is_complete(void) const;
     void set_initial_slope(void) { initial_slope = slope; }
@@ -166,6 +167,7 @@ class AP_Landing {
     AP_Int8 abort_throttle_enable;
     AP_Int8 flap_percent;
     AP_Int8 throttle_slewrate;
+    AP_Float rangefinder_holdoff_before_land_point;
     AP_Int8 type;
     AP_Int8 flare_effectivness_pct;
     AP_Float wind_comp;