Plane: power-off rangefinder at high alt without terrain avail

- moved terrain alt rangefinder power-off trigger from trerrain thread to rangefinder thread
- allow rangefinder to power-off using baro if terrain data not available

ArduPlane/ArduPlane.cpp

@@ -357,13 +357,6 @@ void Plane::terrain_update(void)
 {
 #if AP_TERRAIN_AVAILABLE
     terrain.update();
-
-    // tell the rangefinder our height, so it can go into power saving
-    // mode if available
-    float height;
-    if (terrain.height_above_terrain(height, true)) {
-        rangefinder.set_estimated_terrain_height(height);
-    }
 #endif
 }
 

ArduPlane/sensors.cpp

@@ -24,6 +24,29 @@ void Plane::init_rangefinder(void)
 void Plane::read_rangefinder(void)
 {
 #if RANGEFINDER_ENABLED == ENABLED
+
+    // notify the rangefinder of our approximate altitude above ground to allow it to power on
+    // during low-altitude flight when configured to power down during higher-altitude flight
+    float height;
+#if AP_TERRAIN_AVAILABLE
+    if (terrain.status() == AP_Terrain::TerrainStatusOK && terrain.height_above_terrain(height, true)) {
+        rangefinder.set_estimated_terrain_height(height);
+    } else
+#endif
+    {
+        // use the best available alt estimate via baro above home
+        if (flight_stage == AP_SpdHgtControl::FLIGHT_LAND_APPROACH ||
+            flight_stage == AP_SpdHgtControl::FLIGHT_LAND_FINAL) {
+            // ensure the rangefinder is powered-on when land alt is higher than home altitude.
+            // This is done using the target alt which we know is below us and we are sinking to it
+            height = height_above_target();
+        } else {
+            // otherwise just use the best available baro estimate above home.
+            height = relative_altitude();
+        }
+        rangefinder.set_estimated_terrain_height(height);
+    }
+
     rangefinder.update();
 
     if (should_log(MASK_LOG_SONAR))