Rover: use Y accel computed from gyro and velocity

this should be less susceptible to noise and attitude errors,
hopefully leading to better learning

APMrover2/GCS_Mavlink.pde

@@ -240,7 +240,7 @@ static void NOINLINE send_nav_controller_output(mavlink_channel_t chan)
     mavlink_msg_nav_controller_output_send(
         chan,
         lateral_acceleration, // use nav_roll to hold demanded Y accel
-        ins.get_accel().y, // use nav_pitch to hold actual Y accel
+        g_gps->ground_speed_cm * 0.01f * ins.get_gyro().z, // use nav_pitch to hold actual Y accel
         nav_controller->nav_bearing_cd() * 0.01f,
         nav_controller->target_bearing_cd() * 0.01f,
         wp_distance,

APMrover2/Steering.pde

@@ -286,10 +286,16 @@ static void steering_learning(void)
         return;
     }
     /*
-      the idea is to slowly adjust the turning circle 
+      the idea is to slowly adjust the turning circle to bring the
+      actual and desired turn rates into line      
      */
     float demanded = lateral_acceleration;
-    float actual = ins.get_accel().y;
+    /*
+      for Y accel use the gyro times the velocity, as that is less
+      noise sensitive, and is a more direct measure of steering rate,
+      which is what we are really trying to control
+     */
+    float actual = ins.get_gyro().z * 0.01f * g_gps->ground_speed_cm;
     if (fabsf(actual) < 0.1f) {
         // too little acceleration to really measure accurately
         return;