diff --git a/message_definitions/v1.0/ardupilotmega.xml b/message_definitions/v1.0/ardupilotmega.xml
index be538df89e..c58cc0f627 100644
--- a/message_definitions/v1.0/ardupilotmega.xml
+++ b/message_definitions/v1.0/ardupilotmega.xml
@@ -423,6 +423,12 @@
       <entry value="2" name="LAND_IMMEDIATELY">
         <description>Flag set when plane is to immediately descend to break altitude and land without GCS intervention. Flag not set when plane is to loiter at Rally point until commanded to land.</description>
       </entry>
+      <entry value="4" name="ALT_FRAME_VALID">
+        <description>True if the following altitude frame value is valid.</description>
+      </entry>
+      <entry value="24" name="ALT_FRAME">
+        <description>2 bit value representing altitude frame. 0: absolute, 1: relative home, 2: relative origin, 3: relative terrain</description>
+      </entry>
     </enum>
     <!-- Camera event types -->
     <enum name="CAMERA_STATUS_TYPES">