AP_Baro: more precise altitude calculation on PX4

if not using an AVR CPU then use a more computationally expensive
altitude calculation, which is more precise at higher altitudes

libraries/AP_Baro/AP_Baro.cpp

@@ -106,12 +106,21 @@ float AP_Baro::get_altitude(void)
         return _altitude;
     }
 
-    // this has no filtering of the pressure values, use a separate
-    // filter if you want a smoothed value. The AHRS driver wants
-    // unsmoothed values
+
+#if defined(__AVR_ATmega1280__) || defined(__AVR_ATmega2560__)
+    // on AVR use a less exact, but faster, calculation
     scaling                                 = (float)_ground_pressure / (float)get_pressure();
     temp                                    = ((float)_ground_temperature) + 273.15f;
     _altitude = logf(scaling) * temp * 29.271267f;
+#else
+    // on faster CPUs use a more exact calculation
+    scaling                                 = (float)get_pressure() / (float)_ground_pressure;
+    temp                                    = ((float)_ground_temperature) + 273.15f;
+
+    // This is an exact calculation that is within +-2.5m of the standard atmosphere tables
+    // in the troposphere (up to 11,000 m amsl).
+	_altitude = 153.8462f * temp * (1.0f - expf(0.190259f * logf(scaling)));
+#endif
 
     _last_altitude_t = _last_update;