US Hydro conference 2011 – Tampa
Ted Read MRIN (Presenter)
Managing Director - Ohmex Ltd
9 Gordleton Business Park
Lymington , SO41 8JD, UK
[email protected]
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
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US Hydro conference 2011 – Tampa
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.
Fig 2 - TYPICAL TIDEMET BINARY MESSAGE
Event History
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 1st 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.
IMO SN.1/Circ.289 and Circ.290 review
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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
Fig 4. TIDEMET HEADER CHANGES - Circ.289 (122bit)
Consequences of message Changes
Aids to Navigation (AtoN) reprogramming
Decoding/Display software now obsolete
Binary message restructure, needs two parses
Variance from ITU-R M.1371 standard
IMO SN.1/Circ.289 and Circ.290 review
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US Hydro conference 2011 – Tampa
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.289 and Circ.290 review
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US Hydro conference 2011 – Tampa
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
IMO SN.1/Circ.289 and Circ.290 review
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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.
Fig 7. HYPACK IMPLEMENTATION OF IMO SN.1/CIRC.289
Other Environmental messages
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.
IMO SN.1/Circ.289 and Circ.290 review
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US Hydro conference 2011 – Tampa
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.
IMO SN.1/Circ.289 and Circ.290 review
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US Hydro conference 2011 – Tampa
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.
IMO SN.1/Circ.289 and Circ.290 review
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US Hydro conference 2011 – Tampa
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
IMO SN.1/Circ.289 and Circ.290 review
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