diff --git a/msg/HelperROT.msg b/msg/HelperROT.msg
index 7b6f6ca..63b108a 100644
--- a/msg/HelperROT.msg
+++ b/msg/HelperROT.msg
@@ -1,3 +1,8 @@
-# Rate of turn of the ship, negative for CCW turning and positive for CW turning.
-# Unit: degrees/hour
-float32 rot
+# Rate of turn of the ship, negative for CCW (min -126) turning and positive for CW (max +126) turning.
+# Special unit: https://documentation.spire.com/ais-fundamentals/understanding-ais-performance-in-high-traffic-zones/
+# = int(4.733 * sqrt(degrees/minute))
+# -126 and +126 indicate CCW/CW at a rate of up to 708 degrees/minute or higher
+# -127 and +127 are special values indicates CCW/CW turning of more than 10 degrees/minute (only used if the reporting
+# vessel does not have an external ROT indicator device available)
+# -128 means no rot info
+int8 rot