added comment on using GPS ground course

ArduPlane/navigation.pde

@@ -106,6 +106,15 @@ static void calc_gndspeed_undershoot()
 static void calc_bearing_error()
 {
 	if(takeoff_complete == true  || g.compass_enabled == true) {
+        /*
+          most of the time we use the yaw sensor for heading, even if
+          we don't have a compass. The yaw sensor is drift corrected
+          in the DCM library. We only use the gps ground course
+          directly if we haven't completed takeoff, as the yaw drift
+          correction won't have had a chance to kick in. Drift
+          correction using the GPS typically takes 10 seconds or so
+          for a 180 degree correction.
+         */
 		bearing_error = nav_bearing - dcm.yaw_sensor;
 	} else {