AP_PrecLand: consume distance_to_target as alternative to rangefinder_alt

Resolves issue #5636

libraries/AC_PrecLand/AC_PrecLand.cpp

@@ -163,8 +163,13 @@ void AC_PrecLand::update(float rangefinder_alt_cm, bool rangefinder_alt_valid)
 
             bool target_vec_valid = target_vec_unit_ned.z > 0.0f;
 
-            if (target_vec_valid && rangefinder_alt_valid && rangefinder_alt_cm > 0.0f) {
-                float alt = MAX(rangefinder_alt_cm*0.01f, 0.0f);
+            if (target_vec_valid && ( (rangefinder_alt_valid && rangefinder_alt_cm > 0.0f) || ((enum PrecLandType)(_type.get()) == PRECLAND_TYPE_COMPANION && _backend->distance_to_target()) ) ) {
+                float alt;
+                if ((enum PrecLandType)(_type.get()) == PRECLAND_TYPE_COMPANION && _backend->distance_to_target()) {
+                    alt = _backend->distance_to_target();
+                } else {
+                    alt = MAX(rangefinder_alt_cm*0.01f, 0.0f);
+                }
                 float dist = alt/target_vec_unit_ned.z;
                 Vector3f targetPosRelMeasNED = Vector3f(target_vec_unit_ned.x*dist, target_vec_unit_ned.y*dist, alt);
 

libraries/AC_PrecLand/AC_PrecLand_Backend.h

@@ -33,6 +33,9 @@ public:
     // return true if there is a valid los measurement available
     virtual bool have_los_meas() = 0;
     
+    // return distance to target
+    virtual float distance_to_target() = 0;
+
     // parses a mavlink message from the companion computer
     virtual void handle_msg(mavlink_message_t* msg) {};
 

libraries/AC_PrecLand/AC_PrecLand_Companion.cpp

@@ -48,6 +48,12 @@ bool AC_PrecLand_Companion::have_los_meas()
     return _have_los_meas;
 }
 
+// return distance to target
+float AC_PrecLand_Companion::distance_to_target()
+{
+    return _distance_to_target;
+}
+    
 void AC_PrecLand_Companion::handle_msg(mavlink_message_t* msg)
 {
     // parse mavlink message

libraries/AC_PrecLand/AC_PrecLand_Companion.h

@@ -31,6 +31,9 @@ public:
     // return true if there is a valid los measurement available
     bool have_los_meas();
     
+    // return distance to target
+    float distance_to_target();
+    
     // parses a mavlink message from the companion computer
     void handle_msg(mavlink_message_t* msg);