AP_NavEKF3: Update documentation

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -408,7 +408,7 @@ void NavEKF3_core::setAidingMode()
 void NavEKF3_core::checkAttitudeAlignmentStatus()
 {
     // Check for tilt convergence - used during initial alignment
-    // Once the tilt variances have reduced to equivalent of 3deg uncertainty, re-set the yaw and magnetic field states
+    // Once the tilt variances have reduced, re-set the yaw and magnetic field states
     // and declare the tilt alignment complete
     if (!tiltAlignComplete) {
         if (tiltErrorVariance < sq(radians(5.0f))) {

libraries/AP_NavEKF3/AP_NavEKF3_core.cpp

@@ -1043,7 +1043,9 @@ void NavEKF3_core::CovariancePrediction(Vector3f *rotVarVecPtr)
         P[16][16] = sq(frontend->_magNoise);
         P[17][17] = P[16][16];
         P[18][18] = P[16][16];
-        FuseDeclination(0.34f);
+        // Fusing the declinaton angle as an observaton with a 20 deg uncertainty helps
+        // to stabilise the earth field.
+        FuseDeclination(radians(20.0f));
     }
 
     if (!inhibitMagStates) {