HOME_POSITION - quaternion can have an invalid value (#1843)

* HOME_POSITION - quaternion can have an invalid value

message_definitions/v1.0/common.xml

@@ -6545,7 +6545,11 @@
       <field type="float" name="x" units="m">Local X position of this position in the local coordinate frame (NED)</field>
       <field type="float" name="y" units="m">Local Y position of this position in the local coordinate frame (NED)</field>
       <field type="float" name="z" units="m">Local Z position of this position in the local coordinate frame (NED: positive "down")</field>
-      <field type="float[4]" name="q">World to surface normal and heading transformation of the takeoff position. Used to indicate the heading and slope of the ground</field>
+      <field type="float[4]" name="q" invalid="[NaN]">
+        Quaternion indicating world-to-surface-normal and heading transformation of the takeoff position.
+        Used to indicate the heading and slope of the ground.
+        All fields should be set to NaN if an accurate quaternion for both heading and surface slope cannot be supplied.
+      </field>
       <field type="float" name="approach_x" units="m">Local X position of the end of the approach vector. Multicopters should set this position based on their takeoff path. Grass-landing fixed wing aircraft should set it the same way as multicopters. Runway-landing fixed wing aircraft should set it to the opposite direction of the takeoff, assuming the takeoff happened from the threshold / touchdown zone.</field>
       <field type="float" name="approach_y" units="m">Local Y position of the end of the approach vector. Multicopters should set this position based on their takeoff path. Grass-landing fixed wing aircraft should set it the same way as multicopters. Runway-landing fixed wing aircraft should set it to the opposite direction of the takeoff, assuming the takeoff happened from the threshold / touchdown zone.</field>
       <field type="float" name="approach_z" units="m">Local Z position of the end of the approach vector. Multicopters should set this position based on their takeoff path. Grass-landing fixed wing aircraft should set it the same way as multicopters. Runway-landing fixed wing aircraft should set it to the opposite direction of the takeoff, assuming the takeoff happened from the threshold / touchdown zone.</field>