From 64a3d8fe4e48eabd716c7eb1d8e6d6e415124acb Mon Sep 17 00:00:00 2001
From: Paul Riseborough <priseborough@sypaq.com.au>
Date: Sat, 24 Oct 2020 20:02:52 +1100
Subject: [PATCH] AP_NavEKF3: Update documentation

---
 libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp | 2 +-
 libraries/AP_NavEKF3/AP_NavEKF3_core.cpp    | 4 +++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp b/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp
index 53bc980f20..33ac7b2d19 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp
+++ b/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))) {
diff --git a/libraries/AP_NavEKF3/AP_NavEKF3_core.cpp b/libraries/AP_NavEKF3/AP_NavEKF3_core.cpp
index 6b65546e20..f7f9353763 100644
--- a/libraries/AP_NavEKF3/AP_NavEKF3_core.cpp
+++ b/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) {