AIS Binary Message updates - The Hydrographic Society of America

US Hydro conference 2011 – Tampa 

Ted Read MRIN (Presenter) 

Managing Director - Ohmex Ltd 

9 Gordleton Business Park 

Lymington , SO41 8JD, UK 

ted@ohmex.com 

AIS Binary Message updates 

IMO SN.1/Circ.289 and IMO SN.1/Circ.290 review 

Abstract 

This short presentation is a review, from a commercial perspective, of the recent changes to the 

transmission of information by way of AIS binary messages. The changes introduced by IMO 

SN.1/Circ 289 and Circ.290 are reviewed and the implications for software used to decode and 

present the embedded message data to the AIS user. Ohmex is a manufacturer of AIS tide and 

weather equipment so particular attention is given to the ‘TideMet’ binary message used to 

transmit current Tide and Weather information. The topic is of particular interest to VTS 

authorities for the safe navigation relating to ports and harbours using ECDIS equipment. The 

presentation includes data from a recent survey of the use of these messages and considers the 

reason why a fundamental standard navigation message has not, as yet, been adhered to on a 

regional or global basis. 

Fig 1 - TIDEMET HARDWARE LAYOUT 

IMO SN.1/Circ.289 and Circ.290 review Page 1


Binary AIS Messages 

Binary messages contain digital data transmitted and received using standard AIS equipment. The 

intention of these messages is to reduce verbal communications, enhance reliable information exchange 

and reduce VTS operator workload. AIS Binary Messages may be either addressed (point to point) or 

broadcast to all listeners. Static Aid to Navigation stations (AtoNs) are designed to report navigation 

information, current conditions and warnings together with optional binary messages. 

The generalized structure of an AIS message consists of a header and data packet encoded as readable 

text characters then transmitted in recognizable NMEA format. The decoded header contains unique 

parameters of registration, current location and time together with message specific codes and data. The 

primary message identifier is always „8‟ with the size and structure of its embedded data defined by the 

Designated Area Code (DAC) and Function Identifier (FI) parameters within the header. Mathematically 

this provides for the combination of 1023 possible DAC values each with 63 possible FI codes, it would 

appear these should provide sufficient combinations for all possible requirements of binary messages. 

Event History 

Fig 2 - TYPICAL TIDEMET BINARY MESSAGE 

In May 2004 the IMO Maritime Safety Committee approved SN.1/Circ.236 as guidance for the 

application of AIS binary messages. The purpose of this document was to define a provisional set of 7 

digital messages, including the TideMet message, for a trial period of 4 years with no change. The criteria 

for selecting the trial messages were to be a demonstrated operational need by a cross-section of users 

including ships, VTS, Pilots, Coastguard and port authorities. 

These 7 trial messages were reviewed during 2009 and published as 15 new messages within IMO 

SN.1/Circ.289, these new messages were defined and approved for international from June 2010. Prior 

messages, defined by Circ.236, were revoked with a caveat their use is to cease as of 1 st January 2013. 

Also in June 2010 the IMO SN.1/Circ.290 was released with the title “Guidance for the presentation and 

display of AIS Application-specific message information”. The document was consultative and showed 

various suggestions of how some binary data might be presented to the user. 



Changes to TideMet Message 

Fig 3. PREVIOUS TIDEMET HEADER - Circ.236 (121bit) 

Function Identifier FI has changed from 11 to 31 

Latitude & Longitude have been swapped around 

Position quality bit has been added 

Header section changed from 121 to 122 bits 

Water level now as 0.01m increments. 

Additional spare bits added to message 

Overall message size increased from 352 to 360 bits 

Consequences of message Changes 

Fig 4. TIDEMET HEADER CHANGES - Circ.289 (122bit) 

Aids to Navigation (AtoN) reprogramming 

Decoding/Display software now obsolete 

Binary message restructure, needs two parses 

Variance from ITU-R M.1371 standard 



Recent Survey of Binary Message use 

Figure 5 was extracted from a recent survey of DAC/FI usage and abbreviated to include just TideMet 

type messages, it shows a wide range of interpretation of the basic concept. ITU-R M.1371-4 defines the 

use of the DAC (country) code within the Application ID, this implies that FI codes are country specific 

and that every nation could define their own parochial use of these values. The DAC country code is also 

duplicate information, already contained within the registration number (MMSI) portion of the message 

header. 

Fig 5. *EXTRACT FROM A SURVEY OF DAC/FI TIDEMET USAGE 

As can be seen from the table above there is a wide interpretation of the use of the DAC/FI code to define 

the specific application. The problem tends to be either historic (St Lawrence Waterway were pioneers of 

the use of AIS binary messages) or hardware specific where manufacturers have embedded their own 

encoding to solve a problem without adhering to the IMO standards. To address this problem the Radio 

Technical Commission for Maritime Services (RTCM) is preparing a standard for the use of these binary 

messages particularly for Inland waterway applications. The International Association of Lighthouse 

Authorities ( IALA) are also in the process of compiling a register of regional applications for AIS binary 

messages to prevent duplication of effort. 



IMO SN.1/Circ.290 - Data display formats 

Further to work by CCOM-JHC the illustrations in Circ.290 offer some suggestions for presenting 

TideMet data to the user, as with all presentations the style is one of preference and weighted to specific 

requirements. The output is normally required in a simple alphanumeric format for instantaneous 

readings, in a spatial chart context for navigational needs and preferably in a graphic time series 

presentation for a temporal view of the data. 

Fig 6. EXAMPLE PRESENTATION OUTPUT FOR CIRC.236/289 MESSAGES 



Hydrographic Applications 

The International Hydrographical Organisation (IHO) is currently investigating a universal standard for 

transmitting Tide data for live use by survey vessels. The transmission of this data by AIS has the 

advantage that it is available to anyone with a low cost AIS receiver within a 25Km range. The tide data 

supplied is also accurately located and synchronised to GPS time. Hypack software has the ability to 

decode AIS messages and display vessels in the survey vicinity, Ohmex have recently supplied trial data 

transmitted using the new Circ.289 message to Hypack who have written a DLL driver for their popular 

hydrographic survey package to use the tide data and display the new message parameters. 

Other Environmental messages 

Fig 7. HYPACK IMPLEMENTATION OF IMO SN.1/CIRC.289 

In addition to the TideMet message, Circ.289 also defined binary message 26 which has some 16 short 

sub-messages which can contain some of the TideMet parameters. The requirement for binary messages 

on inland waterways is slightly different to coastal areas, some waterways are tidal while others are rivers 

or canals controlled by locks and levies. Of particular navigation interest is forward knowledge of lock 

openings and any awaiting traffic. In addition to under keel clearance, provided by the current water 

elevation, inland waterway skippers are interested in „air gap‟ clearance information pertinent to 

approaching bridges and overhead structures. The Environmental sub-messages defined within Circ.289 

using DAC code 200 (Inland) are better suited to these inland waterway requirements rather than using 

the TideMet message. The inclusion of the Area message 22 within the revised list of broadcasts provides 

the possibility of placing a temporary AtoN on drifting obstacles such as Icebergs. This would mark their 

current position and also give an indication of their speed, heading and danger area by transmitting a zone 

of influence. The AtoN device could be a buoy device dropped on an Iceberg and recovered at a later date 

when the hazard to navigation has melted away. Some work would need to be done on differentiating a 

floating AtoN from one riding on an Iceberg, however, there are sufficient parameters in existing binary 

messages to accommodate these specific requirements. 



Satellite AIS Applications 

Satellite networks such as Orbcomm have onboard VHF receivers which can track AIS transmissions 

within their footprint, coupling these together on their control network produces a global view of AIS 

traffic. The following images show a snapshot of the AIS targets seen within the Indian Ocean over a 

short selected time period. 

Fig 8. ORBCOMM AIS TRACKING OF INDIAN OCEAN AREA 

In addition to vessel traffic the Orbcomm network can also collect binary messages from AtoN 

transmissions. There is a potential use for this wide area of AtoNs viewed by the network to be of use for 

tracking or even the prediction of impending events such as impacts of a Tsunami wave. The latency of 

messages in networks like Orbcomm is small enough to actually enable prediction and warning of 

landfall by a large wave by up to 4 hours before the impact. A remote AtoN would probably be shore 

based or in a shallow water mooring enabling its tide gauge to detect the relatively large amplitude, slow 

moving wave as opposed to false triggering by the very fast moving, small amplitude deep ocean waves 

monitored by floating wave sensors. 

Future requirements 

Security - Of great concern to port and coastal authorities is the potential hazard of a rogue AtoN which 

is transmitting false information and cannot be deactivated remotely. There is a requirement for not only 

an addressed control message but also some form of security password/verification to prevent 

unauthorised „hacker‟ access to the remote device. 

Website information - As the potential bandwidth of AIS is limited the possibility of transmitting more 

detailed information to a vessel of say the latest ECDIS chart information for a port would be severely 

restricted. A suggestion would be to embed a URL within an AtoN message, this could then be accessed 

via the Internet obtaining more detailed information relevant to the location. The alternative would be an 

addressed message requesting further information from a VTS base station including URLs pointing to 

further information such as a port‟s website. This mechanism would in effect make AIS part of a larger 

„Mesh‟ based computer network that would link AIS, BGAN, WiMax and cellular networks. 



Conclusions ... 

Incorporation within the ECDIS standard - By changing the underlying binary message 

layout the update has rendered as useless existing software written to decode and display the 

previous message. To add to the problem it is also difficult for software to discern that a device is 

transmitting the old or new message format, backward compatibility between messages has 

become an issue. If the new TideMet message was incorporated within the ECDIS standard for 

chart display then all new software would incorporate the format as standard. 

Dual message decode - Until the expiry date in 2013 any adopted software standard for 

displaying the TideMet message should attempt to decode the new message first before 

attempting the older message. 

Expand on graphic display format – As suggested in IMO Sn.1/Circ.290 there should be a set 

of display formats adopted for presenting the TideMet information in graphic form in addition to 

alphanumeric text. The TideMet message makes use of non-SI combination of units (Miles, 

Knots etc) so these displays should be in user selected unit preference (i.e. feet/meters etc). 

Similarly the use of language filters for the titles and descriptions used should form part of any 

standard for presentation. 

Range and accuracy Water Level – The new message has overcome the previous limit on 

Water Level resolution from 0.1m to 0.01m. Unfortunately it has parted from the AIS standard 

for 2‟s compliment numbers by having a constant offset of -10.0m. 

Harmonize Inland/Coastal usage – As yet there is no clear distinction between the specific 

requirements of maritime and inland waterways usage of DAC/FI codes, or even the overlap 

between their two areas of message use. Standardization of message requirements and the 

introduction of an application register are now an urgent requirement to enable further software 

and equipment development. 

Subjective or derived values - Many of the parameters defined in the IMO TideMet standard 

message are subjective so are not suitable for automated instrumentation (e.g. Sea State, 

Visibility, Precipitation). Other parameters such as Dew Point and Significant Wave Height are 

derived so could be calculated by a client program. A standard calculation of derived values 

should be formally stated. 



Acronyms used 

AIS - Automatic Identification System 

AtoN - Aid to Navigation 

BGAN – Broadband Global Area Network 

CCOM-JHC - Centre for Coastal & Ocean Mapping - Joint Hydrographic Centre 

GNSS - Global Navigation Satellite System 

IALA - International Association of Lighthouse Authorities 

IMO - International Maritime Organization 

IEC - International Electro technical Commission 

MMSI - Maritime Mobile Service ID 

NMEA - National Marine Electronics Association 

R&TTE - Telecommunications Terminal Equipment (R&TTE) Regulations 

Reference materials 

IMO SN.1/Circ.289 - June 2010 

IMO SN.1/Circ.290 - June 2010 

New Standards for Providing Meteorological and Hydrographic Information via AIS - L.Alexander, 

K. Schwehr – International Hydrographic review, May 2010 

Tidal & Meteorological data over AIS – E.F.Read, W.S.Heaps – Hydrographic Society, Hydro8 

proceedings, November 2008. 

IMO SN.1/Circ.236 - May 2004 

Acknowledgements 

*Neal Arundale - UK 

Pat Sanders, Mircea Neacsu - Hypack Inc - USA 

Nick Ward -Trinity Light House Service - UK 

Kurt Schwehr - UNH, CCOM - USA 

Steve Shipman - IHO - Monaco 

Mariuxi Chávez - Orbcomm Inc. - Europe 

PRESENTING AUTHOR BIOGRAPHY 

Ted Read is managing director of Ohmex Ltd, a 

UK based manufacturer of marine instruments 

and application software. Born in Bahrain and 

educated in London he has been personally 

involved with advanced software and 

electronic design projects for the past thirty 

years

AIS Binary Message updates - The Hydrographic Society of America

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