Plane: add rngfnd. engagement distance parameter

Introduces the RNGFND_LND_DIST parameter, which defines the horizontal
distance to the landing point at which the rangefinder engages when
RNGFND_LANDING is enabled. This is useful for landing on platforms or
small plateaus, and to avoid interference from uneven terrain or
obstacles during the approach.

When set to the default value of 0, the rangefinder engages as soon as
it is within range, matching existing behavior.

Key changes:
- Adds RNGFND_LND_DIST parameter to control rangefinder engagement
  horizontal distance during landing.
- Updates rangefinder logic to engage only when within the specified
  distance.
- Requires rangefinder_state.in_use for rangefinder correction to be
  applied.
- Requires both in_use and in_range for rangefinder_active to be true.

Author: rubenp02

ArduPlane/Parameters.cpp

@@ -1279,6 +1279,17 @@ const AP_Param::GroupInfo ParametersG2::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("GUIDED_TIMEOUT", 40, ParametersG2, guided_timeout, 3.0f),
 
+#if AP_RANGEFINDER_ENABLED
+    // @Param: RNGFND_LND_DIST
+    // @DisplayName: Rangefinder landing engagement distance
+    // @Description: The horizontal distance to the landing point at which the rangefinder engages when RNGFND_LANDING is enabled. This is useful for landing on platforms or small plateaus, and to avoid interference from uneven terrain or obstacles on approach. A value of 0 engages the rangefinder as soon as it reports valid readings within its range limits. Very small values are not recommended unless required by the landing scenario, as they can force large slope corrections near the ground, increase auto-abort frequency if LAND_ABORT_DEG is set, and provide no slope-correction benefit if the rangefinder is engaged after flare.
+    // @Range: 0 500
+    // @Units: m
+    // @Increment: 1
+    // @User: Standard
+    AP_GROUPINFO("RNGFND_LND_DIST", 41, ParametersG2, rangefinder_land_engage_dist_m, 0),
+#endif
+
     AP_GROUPEND
 };
 

ArduPlane/Parameters.h

@@ -590,6 +590,9 @@ class ParametersG2 {
 #if AP_RANGEFINDER_ENABLED
     // orientation of rangefinder to use for landing
     AP_Int8 rangefinder_land_orient;
+
+    // distance to the landing point to engage the rangefinder for landing
+    AP_Int16 rangefinder_land_engage_dist_m;
 #endif
 
 #if AP_PLANE_SYSTEMID_ENABLED

ArduPlane/altitude.cpp

@@ -686,8 +686,10 @@ float Plane::rangefinder_correction(void)
         return 0;
     }
 
-    // for now we only support the rangefinder for landing 
-    bool using_rangefinder = (rangefinder_use(RangeFinderUse::TAKEOFF_LANDING) && flight_stage == AP_FixedWing::FlightStage::LAND);
+    // for now we only support the rangefinder for landing
+    bool using_rangefinder = (rangefinder_use(RangeFinderUse::TAKEOFF_LANDING) &&
+                              flight_stage == AP_FixedWing::FlightStage::LAND &&
+                              rangefinder_state.in_use);
     if (!using_rangefinder) {
         return 0;
     }
@@ -783,9 +785,18 @@ void Plane::rangefinder_height_update(void)
                 flightstage_good_for_rangefinder_landing = true;
             }
 #endif
+
+            // Check if the aircraft is within RNGFND_LND_DIST meters from the
+            // landing point to engage it
+            const int16_t land_engage_dist_m = g2.rangefinder_land_engage_dist_m;
+            const bool is_within_engagement_distance =
+                (land_engage_dist_m <= 0) ||
+                (auto_state.wp_distance <= land_engage_dist_m);
+
             if (!rangefinder_state.in_use &&
                 flightstage_good_for_rangefinder_landing &&
-                rangefinder_use(RangeFinderUse::TAKEOFF_LANDING)) {
+                rangefinder_use(RangeFinderUse::TAKEOFF_LANDING) &&
+                is_within_engagement_distance) {
                 rangefinder_state.in_use = true;
                 gcs().send_text(MAV_SEVERITY_INFO, "Rangefinder engaged at %.2fm", (double)rangefinder_state.height_estimate);
             }
@@ -951,7 +962,8 @@ float Plane::get_landing_height(bool &rangefinder_active)
 #if AP_RANGEFINDER_ENABLED
     // possibly correct with rangefinder
     height -= rangefinder_correction();
-    rangefinder_active = rangefinder_use(RangeFinderUse::TAKEOFF_LANDING) && rangefinder_state.in_range;
+    rangefinder_active = rangefinder_use(RangeFinderUse::TAKEOFF_LANDING) &&
+                         rangefinder_state.in_use && rangefinder_state.in_range;
 #endif
 
     return height;