diff --git a/CMakeLists.txt b/CMakeLists.txt
index ee04483..0570cac 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,8 +32,6 @@ set(common_msg
   "msg/HelperHeading.msg"
   "msg/HelperLatLon.msg"
   "msg/HelperSpeed.msg"
-  "msg/HelperCOG.msg"
-  "msg/HelperSOG.msg"
   "msg/HelperDimensions.msg"
   "msg/HelperROT.msg"
 )
diff --git a/msg/HelperAISShip.msg b/msg/HelperAISShip.msg
index 9761d8a..20f0645 100644
--- a/msg/HelperAISShip.msg
+++ b/msg/HelperAISShip.msg
@@ -1,7 +1,11 @@
 # AIS identifier
 int32 id
 HelperLatLon lat_lon
-HelperCOG   cog
-HelperSOG   sog
+# course over ground in degrees from CW from true north
+HelperHeading   cog
+# speed over ground in kmph
+HelperSpeed   sog
+# rate of turn in degrees per hour
 HelperROT   rot
+# dimensions of the boat in meters
 HelperDimensions dimensions
diff --git a/msg/HelperCOG.msg b/msg/HelperCOG.msg
deleted file mode 100644
index 165d971..0000000
--- a/msg/HelperCOG.msg
+++ /dev/null
@@ -1,2 +0,0 @@
-# Unit: decimal degrees
-float32 cog
diff --git a/msg/HelperROT.msg b/msg/HelperROT.msg
index fb22852..59f8ef4 100644
--- a/msg/HelperROT.msg
+++ b/msg/HelperROT.msg
@@ -1,3 +1,2 @@
-# Unit: -126 to 126 ROT AIS scale corresponds to -708 to 708 degrees per min
-# ROTAIS = 4.733 * sqrt (rot [degrees/min])
+# Unit: degrees/hour
 float32 rot
diff --git a/msg/HelperSOG.msg b/msg/HelperSOG.msg
deleted file mode 100644
index 04c73ab..0000000
--- a/msg/HelperSOG.msg
+++ /dev/null
@@ -1,2 +0,0 @@
-# Unit: km/h
-float32 sog