Plane: isFlying redesign

The is_flying() function is too simplistic to be used as a generic
status. Added a sticky heuristic method that depends on the arm
state. More thought should go into the boolean result of isFlying but
this is at least safer than the original method.

ArduPlane/ArduPlane.pde

@@ -585,6 +585,10 @@ static bool throttle_suppressed;
 
 AP_SpdHgtControl::FlightStage flight_stage = AP_SpdHgtControl::FLIGHT_NORMAL;
 
+// probability of aircraft is currently in flight. range from 0 to 1 where 1 is 100% sure we're in flight
+static float isFlyingProbability = 0;
+static void determine_is_flying(void);
+
 ////////////////////////////////////////////////////////////////////////////////
 // Loiter management
 ////////////////////////////////////////////////////////////////////////////////
@@ -1024,6 +1028,9 @@ static void one_second_loop()
 
     update_aux();
 
+    // determine if we are flying or not
+    determine_is_flying();
+
     // update notify flags
     AP_Notify::flags.pre_arm_check = arming.pre_arm_checks(false);
     AP_Notify::flags.pre_arm_gps_check = true;
@@ -1536,6 +1543,52 @@ static void update_flight_stage(void)
     airspeed.set_EAS2TAS(barometer.get_EAS2TAS());
 }
 
+
+
+/*
+  Do we think we are flying?
+  Probabilistic method where a bool is low-passed and considered a probability.
+*/
+static void determine_is_flying(void)
+{
+    float aspeed;
+    bool isFlyingBool;
+
+    bool airspeedMovement = ahrs.airspeed_estimate(&aspeed) && (aspeed >= 5);
+
+    // If we don't have a GPS lock then don't use GPS for this test
+    bool gpsMovement = (gps.status() < AP_GPS::GPS_OK_FIX_2D ||
+                        gps.ground_speed() >= 5);
+
+
+    if(ahrs.get_armed()) {
+        // when armed, we need overwhelming evidence that we ARE NOT flying
+        isFlyingBool = airspeedMovement || gpsMovement;
+
+    } else {
+        // when disarmed, we need overwhelming evidence that we ARE flying
+        isFlyingBool = airspeedMovement && gpsMovement;
+    }
+
+    // low-pass the result.
+    isFlyingProbability = (0.8 * isFlyingProbability) + (0.2*(float)isFlyingBool);
+}
+
+static bool is_flying(void)
+{
+    if(ahrs.get_armed()) {
+        // when armed, assume we're flying unless we probably aren't
+        return (isFlyingProbability >= 0.1);
+    }
+    else
+    {
+        // when disarmed, assume we're not flying unless we probably are
+        return (isFlyingProbability >= 0.9);
+    }
+
+}
+
+
 #if OPTFLOW == ENABLED
 // called at 50hz
 static void update_optical_flow(void)

ArduPlane/Attitude.pde

@@ -545,29 +545,7 @@ static void flap_slew_limit(int8_t &last_value, int8_t &new_value)
     last_value = new_value;
 }
 
-
-
-/**
-  Do we think we are flying?
-  This is a heuristic so it could be wrong in some cases.  In particular, if we don't have GPS lock we'll fall
-  back to only using altitude.  (This is probably more optimistic than what suppress_throttle wants...)
-*/
-static bool is_flying(void)
-{
-    // If we don't have a GPS lock then don't use GPS for this test
-    bool gpsMovement = (gps.status() < AP_GPS::GPS_OK_FIX_2D ||
-                        gps.ground_speed() >= 5);
-    
-    bool airspeedMovement = (!ahrs.airspeed_sensor_enabled()) || airspeed.get_airspeed() >= 5;
-    
-    // we're more than 5m from the home altitude
-    bool inAir = relative_altitude_abs_cm() > 500;
-
-    return inAir && gpsMovement && airspeedMovement;
-}
-
-
-/* We want to supress the throttle if we think we are on the ground and in an autopilot controlled throttle mode.
+/* We want to suppress the throttle if we think we are on the ground and in an autopilot controlled throttle mode.
 
    Disable throttle if following conditions are met:
    *       1 - We are in Circle mode (which we use for short term failsafe), or in FBW-B or higher