diff --git a/docs/c_its-messages.proto b/docs/c_its-messages.proto
index c2ca54e..9848910 100644
--- a/docs/c_its-messages.proto
+++ b/docs/c_its-messages.proto
@@ -70,6 +70,8 @@ message RawMsgTx {
   required GNTransport    gnTransport           =  3;
   required GNArea         gnArea                =  4;
   required uint32         hopLimit              =  5;
+
+  optional uint32         stationType           =  6; // for source position vector
 }
 
 // ------------------------------