AP_NavEKF3: FuseVelPosNED uses ext nav vel err in obs data check

libraries/AP_NavEKF3/AP_NavEKF3.h

@@ -503,6 +503,7 @@ private:
     const float gpsNEVelVarAccScale = 0.05f;       // Scale factor applied to NE velocity measurement variance due to manoeuvre acceleration
     const float gpsDVelVarAccScale = 0.07f;        // Scale factor applied to vertical velocity measurement variance due to manoeuvre acceleration
     const float gpsPosVarAccScale = 0.05f;         // Scale factor applied to horizontal position measurement variance due to manoeuvre acceleration
+    const float extNavVelVarAccScale = 0.05f;      // Scale factor applied to ext nav velocity measurement variance due to manoeuvre acceleration
     const uint16_t magDelay_ms = 60;               // Magnetometer measurement delay (msec)
     const uint16_t tasDelay_ms = 100;              // Airspeed measurement delay (msec)
     const uint16_t tiltDriftTimeMax_ms = 15000;    // Maximum number of ms allowed without any form of tilt aiding (GPS, flow, TAS, etc)

libraries/AP_NavEKF3/AP_NavEKF3_PosVelFusion.cpp

@@ -630,7 +630,13 @@ void NavEKF3_core::FuseVelPosNED()
             // For data integrity checks we use the same measurement variances as used to calculate the Kalman gains for all measurements except GPS horizontal velocity
             // For horizontal GPS velocity we don't want the acceptance radius to increase with reported GPS accuracy so we use a value based on best GPS performance
             // plus a margin for manoeuvres. It is better to reject GPS horizontal velocity errors early
-            for (uint8_t i=0; i<=2; i++) R_OBS_DATA_CHECKS[i] = sq(constrain_float(frontend->_gpsHorizVelNoise, 0.05f, 5.0f)) + sq(frontend->gpsNEVelVarAccScale * accNavMag);
+            float obs_data_chk;
+            if (extNavVelToFuse) {
+                obs_data_chk = sq(constrain_float(extNavVelDelayed.err, 0.05f, 5.0f)) + sq(frontend->extNavVelVarAccScale * accNavMag);
+            } else {
+                obs_data_chk = sq(constrain_float(frontend->_gpsHorizVelNoise, 0.05f, 5.0f)) + sq(frontend->gpsNEVelVarAccScale * accNavMag);
+            }
+            R_OBS_DATA_CHECKS[0] = R_OBS_DATA_CHECKS[1] = R_OBS_DATA_CHECKS[2] = obs_data_chk;
         }
         R_OBS[5] = posDownObsNoise;
         for (uint8_t i=3; i<=5; i++) R_OBS_DATA_CHECKS[i] = R_OBS[i];