AIS broadcast data message – what is in it?

The information that is broadcasted by the AIS unit is sorted into three independent data reports and transmitted at set schedules according to the agreed standards.

Marine traffic is interested in the vessels name, its status according the Rules of the Road, its course and speed, rate of turn (RoT) and what the CPA and TCPA are. This information is packed in the so-called Static and Dynamic data packages, all other data is for the primary use of the maritime authorities.

Static data (update every 6 minutes)
(pre-programmed and does not change)

o Ship’s name and call sign
o IMO number
o Length & beam
o Location of antenna
o Ship’s type

Voyage related data (update every 6 minutes)
(to be inputted every new voyage):

o Draft
o Cargo information
o Destination and ETA
o Other relevant information

Dynamic data (update varies)
(automatically derived from ship’s interfaces):

o Time
o Ship’s position
o Course over ground
o Speed over ground
o Gyro heading and Rate of Turn
o Navigational status (according to Rules of the Road)

The Dynamic reports are updated depending on ship’s speed and status, as follows:

o At anchor 3 minutes
o 0 – 14 knots 10 seconds
o 0 – 14 knots & changing course 3.3 seconds
o 14 – 23 knots 6 seconds
o 14 – 23 knots & changing course 2 seconds
o 23+ knots 2 seconds
o 23+ knots & changing course 2 seconds

The trouble is that that AIS on some commercial vessels I have met at sea are showing “At Anchor” when they are clearly underway and going at a fair rate of knots… this means the so called dynamic data is being refreshed every 3 minutes – more than enough time for a collision event!


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