AP_Airspeed: added EAS2TAS in airspeed driver

this keeps the true airspeed ratio in the airspeed driver, which seems
the most logical place

libraries/AP_Airspeed/AP_Airspeed.h

@@ -26,7 +26,6 @@ private:
     const float Q1; // process noise matrix bottom right element
     Vector3f state; // state vector
     const float DT; // time delta
-
 };
 
 class AP_Airspeed
@@ -34,7 +33,8 @@ class AP_Airspeed
 public:
     // constructor
     AP_Airspeed() : 
-        _ets_fd(-1)
+        _ets_fd(-1),
+        _EAS2TAS(1.0f)
     {
 		AP_Param::setup_object_defaults(this, var_info);
     };
@@ -99,8 +99,18 @@ public:
         return max(_last_pressure - _offset, 0);
     }
 
+    // set the apparent to true airspeed ratio
+    void set_EAS2TAS(float v) {
+        _EAS2TAS = v;
+    }
+
+    // get the apparent to true airspeed ratio
+    float get_EAS2TAS(void) const {
+        return _EAS2TAS;
+    }
+
     // update airspeed ratio calibration
-    void update_calibration(Vector3f vground, float EAS2TAS);
+    void update_calibration(Vector3f vground);
 
     static const struct AP_Param::GroupInfo var_info[];
 
@@ -116,6 +126,7 @@ private:
     float           _airspeed;
     int			    _ets_fd;
     float			_last_pressure;
+    float           _EAS2TAS;
 
     Airspeed_Calibration _calibration;
     float _last_saved_ratio;

libraries/AP_Airspeed/Airspeed_Calibration.cpp

@@ -2,7 +2,7 @@
 /*
  *   auto_calibration.cpp - airspeed auto calibration
 
- * Algorithm by Paul Riseborough
+ * Algorithm by Paul Riseborough 
  *
  */
 
@@ -102,15 +102,15 @@ float Airspeed_Calibration::update(float airspeed, const Vector3f &vg)
 /*
   called once a second to do calibration update
  */
-void AP_Airspeed::update_calibration(Vector3f vground, float EAS2TAS)
+void AP_Airspeed::update_calibration(Vector3f vground)
 {
     if (!_autocal) {
         // auto-calibration not enabled
         return;
     }
-    // calculate true airspeed, assuming a ratio of 1.0
-    float airspeed = sqrtf(get_differential_pressure()) * EAS2TAS;
-    float ratio = _calibration.update(airspeed, vground);
+    // calculate true airspeed, assuming a airspeed ratio of 1.0
+    float true_airspeed = sqrtf(get_differential_pressure()) * _EAS2TAS;
+    float ratio = _calibration.update(true_airspeed, vground);
     if (isnan(ratio) || isinf(ratio)) {
         return;
     }