diff --git a/libraries/AP_Math/quaternion.cpp b/libraries/AP_Math/quaternion.cpp
index 403158c74b..946bc97dc5 100644
--- a/libraries/AP_Math/quaternion.cpp
+++ b/libraries/AP_Math/quaternion.cpp
@@ -464,7 +464,7 @@ void Quaternion::rotate(const Vector3f &v)
 
 // convert this quaternion to a rotation vector where the direction of the vector represents
 // the axis of rotation and the length of the vector represents the angle of rotation
-void Quaternion::to_axis_angle(Vector3f &v)
+void Quaternion::to_axis_angle(Vector3f &v) const
 {
     const float l = sqrtf(sq(q2)+sq(q3)+sq(q4));
     v = Vector3f(q2,q3,q4);
diff --git a/libraries/AP_Math/quaternion.h b/libraries/AP_Math/quaternion.h
index 25ca6cf398..0848f11eed 100644
--- a/libraries/AP_Math/quaternion.h
+++ b/libraries/AP_Math/quaternion.h
@@ -89,7 +89,7 @@ public:
 
     // convert this quaternion to a rotation vector where the direction of the vector represents
     // the axis of rotation and the length of the vector represents the angle of rotation
-    void        to_axis_angle(Vector3f &v);
+    void        to_axis_angle(Vector3f &v) const;
 
     // create a quaternion from a rotation vector where the direction of the vector represents
     // the axis of rotation and the length of the vector represents the angle of rotation