AC_PrecLand: return target distance if enabled from SIM_Precland

For rover, we cant measure distance towards target using a rangefinder. Hence, we should must return target distance if enabled through SIM_Precland.

libraries/AC_PrecLand/AC_PrecLand_SITL.cpp

@@ -19,6 +19,7 @@ void AC_PrecLand_SITL::update()
         const Vector3d position = _sitl->precland_sim.get_target_position();
         const Matrix3d body_to_ned = AP::ahrs().get_rotation_body_to_ned().todouble();
         _los_meas_body =  body_to_ned.mul_transpose(-position).tofloat();
+        _distance_to_target = _sitl->precland_sim.option_enabled(SITL::SIM_Precland::Option::ENABLE_TARGET_DISTANCE) ? _los_meas_body.length() : 0.0f;
         _los_meas_body /= _los_meas_body.length();
         _have_los_meas = true;
         _los_meas_time_ms = _sitl->precland_sim.last_update_ms();
@@ -33,7 +34,6 @@ bool AC_PrecLand_SITL::have_los_meas() {
     return AP_HAL::millis() - _los_meas_time_ms < 1000;
 }
 
-
 // provides a unit vector towards the target in body frame
 //  returns same as have_los_meas()
 bool AC_PrecLand_SITL::get_los_body(Vector3f& ret) {

libraries/AC_PrecLand/AC_PrecLand_SITL.h

@@ -32,11 +32,15 @@ public:
     // return true if there is a valid los measurement available
     bool have_los_meas() override;
 
+    // returns distance to target in meters (0 means distance is not known)
+    float distance_to_target() override { return _distance_to_target; }
+
 private:
     SITL::SIM           *_sitl;                 // sitl instance pointer
     Vector3f            _los_meas_body;         // unit vector in body frame pointing towards target
     uint32_t            _los_meas_time_ms;      // system time in milliseconds when los was measured
     bool                _have_los_meas;         // true if there is a valid measurement from the camera
+    float               _distance_to_target;    // distance to target in meters
 };
 
 #endif