diff --git a/msg/ReferenceTarget.msg b/msg/ReferenceTarget.msg
index af618f7..2ffa2c0 100644
--- a/msg/ReferenceTarget.msg
+++ b/msg/ReferenceTarget.msg
@@ -1,3 +1,9 @@
+# Constant representing that the value in question couldn't be measured
+# This often applied speed or heading/yaw for example in single-antenna GPS
+# For orientation, use NO_MEASUREMENT as the `w` value of the quaternion
+# This value is arbitrary and should be used by name rather than by value, but it's chosen to be exactly-representable in a float32 so there aren't floating point comparison issues
+float32 NO_MEASUREMENT = -9999.5
+
 # PoseStamped data for reference target pose
 geographic_msgs/GeoPoseStamped pose