diff --git a/libraries/AP_Math/quaternion.cpp b/libraries/AP_Math/quaternion.cpp
index 2c3e71d678..cae8b4a781 100644
--- a/libraries/AP_Math/quaternion.cpp
+++ b/libraries/AP_Math/quaternion.cpp
@@ -366,3 +366,9 @@ Quaternion Quaternion::operator/(const Quaternion &v) const
     ret.q4 = (rquat0*quat3 - rquat1*quat2 + rquat2*quat1 - rquat3*quat0);
     return ret;
 }
+
+// angular difference in radians between quaternions
+Quaternion Quaternion::angular_difference(const Quaternion &v) const
+{
+    return v.inverse() * *this;
+}
diff --git a/libraries/AP_Math/quaternion.h b/libraries/AP_Math/quaternion.h
index 43eb3c4f6c..2393b67ea3 100644
--- a/libraries/AP_Math/quaternion.h
+++ b/libraries/AP_Math/quaternion.h
@@ -141,4 +141,7 @@ public:
     Quaternion operator*(const Quaternion &v) const;
     Quaternion &operator*=(const Quaternion &v);
     Quaternion operator/(const Quaternion &v) const;
+
+    // angular difference between quaternions
+    Quaternion angular_difference(const Quaternion &v) const;
 };