diff --git a/tool/ubx_filter.rb b/tool/ubx_filter.rb
index 1be0038b2..0dc5def75 100644
--- a/tool/ubx_filter.rb
+++ b/tool/ubx_filter.rb
@@ -177,6 +177,7 @@ def run(&b)
       event,2100:2000,drop,GPS:03 
           # start dropping GPS 3 after GPS time 2100[week] 20000[s]
 
+    Note: For old ubx, just PRN number (without satellite system) like "1" (<-instead of "GPS:01") works well. 
 __STRING__
 
 options = {