AP_NavEKF: Reset mag and heading states to try and pass pre-flight checks

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4376,6 +4376,9 @@ void NavEKF::readMagData()
         // read compass data and scale to improve numerical conditioning
         magData = _ahrs->get_compass()->get_field_milligauss() * 0.001f;
 
+        // check for consistent data between magnetometers
+        consistentMagData = _ahrs->get_compass()->consistent();
+
         // get states stored at time closest to measurement time after allowance for measurement delay
         RecallStates(statesAtMagMeasTime, (imuSampleTime_ms - msecMagDelay));
 
@@ -4686,6 +4689,7 @@ void NavEKF::InitialiseVariables()
     ekfStartTime_ms = imuSampleTime_ms;
     lastGpsVelFail_ms = 0;
     lastGpsAidBadTime_ms = 0;
+    magYawResetTimer_ms = imuSampleTime_ms;
 
     // initialise other variables
     gpsNoiseScaler = 1.0f;
@@ -5218,6 +5222,18 @@ bool NavEKF::calcGpsGoodToAlign(void)
                            "GPS horiz error %.1f", (double)hAcc);
     }
     
+    // If we have good magnetometer consistency and bad innovations for longer than 5 seconds then we reset heading and field states
+    // This enables us to handle large changes to the external magnetic field environment that occur before arming
+    if ((magTestRatio.x <= 1.0f && magTestRatio.y <= 1.0f) || !consistentMagData) {
+        magYawResetTimer_ms = imuSampleTime_ms;
+    }
+    if (imuSampleTime_ms - magYawResetTimer_ms > 5000) {
+        // reset heading and field states
+        Vector3f eulerAngles;
+        getEulerAngles(eulerAngles);
+        state.quat = calcQuatAndFieldStates(eulerAngles.x, eulerAngles.y);
+    }
+
     // fail if magnetometer innovations are outside limits indicating bad yaw
     // with bad yaw we are unable to use GPS
     bool yawFail;

libraries/AP_NavEKF/AP_NavEKF.h

@@ -685,6 +685,8 @@ private:
     bool useGpsVertVel;             // true if GPS vertical velocity should be used
     float yawResetAngle;            // Change in yaw angle due to last in-flight yaw reset in radians. A positive value means the yaw angle has increased.
     bool yawResetAngleWaiting;      // true when the yaw reset angle has been updated and has not been retrieved via the getLastYawResetAngle() function
+    uint32_t magYawResetTimer_ms;   // timer in msec used to track how long good magnetometer data is failing innovation consistency checks
+    bool consistentMagData;         // true when the magnetometers are passing consistency checks
 
     // Used by smoothing of state corrections
     Vector10 gpsIncrStateDelta;    // vector of corrections to attitude, velocity and position to be applied over the period between the current and next GPS measurement