AC_WPNav: added WP_RFND_USE parameter

allows disable of rangefinder use for terrain following

libraries/AC_WPNav/AC_WPNav.cpp

@@ -97,6 +97,13 @@ const AP_Param::GroupInfo AC_WPNav::var_info[] = {
     // @User: Advanced
     AP_GROUPINFO("LOIT_MINA",   9, AC_WPNav, _loiter_accel_min_cmss, WPNAV_LOITER_ACCEL_MIN),
 
+    // @Param: RFND_USE
+    // @DisplayName: Use rangefinder for terrain following
+    // @Description: This controls the use of a rangefinder for terrain following
+    // @Values: 0:Disable,1:Enable
+    // @User: Advanced
+    AP_GROUPINFO("RFND_USE",   10, AC_WPNav, _rangefinder_use, 1),
+    
     AP_GROUPEND
 };
 
@@ -1153,7 +1160,7 @@ bool AC_WPNav::get_terrain_offset(float& offset_cm)
 {
 #if AP_TERRAIN_AVAILABLE
     // use range finder if connected
-    if (_rangefinder_use) {
+    if (_rangefinder_available && _rangefinder_use) {
         if (_rangefinder_healthy) {
             offset_cm = _inav.get_altitude() - _rangefinder_alt_cm;
             return true;

libraries/AC_WPNav/AC_WPNav.h

@@ -64,7 +64,7 @@ public:
     void set_avoidance(AC_Avoid* avoid_ptr) { _avoid = avoid_ptr; }
 
     /// provide rangefinder altitude
-    void set_rangefinder_alt(bool use, bool healthy, float alt_cm) { _rangefinder_use = use; _rangefinder_healthy = healthy; _rangefinder_alt_cm = alt_cm; }
+    void set_rangefinder_alt(bool use, bool healthy, float alt_cm) { _rangefinder_available = use; _rangefinder_healthy = healthy; _rangefinder_alt_cm = alt_cm; }
 
     ///
     /// loiter controller
@@ -362,7 +362,8 @@ protected:
     // terrain following variables
     bool        _terrain_alt = false;   // true if origin and destination.z are alt-above-terrain, false if alt-above-ekf-origin
     bool        _ekf_origin_terrain_alt_set = false;
-    bool        _rangefinder_use = false;
+    bool        _rangefinder_available;
+    AP_Int8     _rangefinder_use;
     bool        _rangefinder_healthy = false;
     float       _rangefinder_alt_cm = 0.0f;
 };