Plane: fixed takeoff direction with no yaw source

in TAKEOFF mode with either very poor yaw source or no yaw source we
need to use ground vector and wait for sufficient ground speed

ArduPlane/mode_takeoff.cpp

@@ -83,7 +83,10 @@ void ModeTakeoff::update()
     if (!takeoff_mode_setup) {
         plane.auto_state.takeoff_altitude_rel_cm = alt * 100;
         const uint16_t altitude = plane.relative_ground_altitude(false,true);
-        const float direction = degrees(ahrs.get_yaw());
+        const Vector2f &groundspeed2d = ahrs.groundspeed_vector();
+        const float direction = wrap_360(degrees(groundspeed2d.angle()));
+        const float groundspeed = groundspeed2d.length();
+
         // see if we will skip takeoff as already flying
         if (plane.is_flying() && (millis() - plane.started_flying_ms > 10000U) && ahrs.groundspeed() > 3) {
             if (altitude >= alt) {
@@ -115,7 +118,15 @@ void ModeTakeoff::update()
 
             plane.set_flight_stage(AP_FixedWing::FlightStage::TAKEOFF);
 
-            if (!plane.throttle_suppressed) {
+            /*
+              don't lock in the takeoff loiter location until we reach
+              a ground speed of AIRSPEED_MIN*0.3 to cope with aircraft
+              with no compass or poor compass. If flying in a very
+              strong headwind then the is_flying() check above will
+              eventually trigger
+             */
+            if (!plane.throttle_suppressed &&
+                groundspeed > plane.aparm.airspeed_min*0.3) {
                 gcs().send_text(MAV_SEVERITY_INFO, "Takeoff to %.0fm for %.1fm heading %.1f deg",
                                 alt, dist, direction);
                 plane.takeoff_state.start_time_ms = millis();