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8 December 2003, 03:01 pm
Review of Automatic Identification Systems (AIS)
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ISAF at the International Maritime Organisation

ISAF was represented at the recent Conference on Automatic Identification Systems organised by the Royal Institute of Navigations in London. These notes have been produced as a result of the conference.
As a result of the conference, ISAF have the opportunity to the put the views of the leisure boat user to the ITEC Working Group that is drafting the performance standards for Class B sets.

In order to collate as wide a selection of views as possible, ISAF would be grateful if all interested parties could complete the QUESTIONNAIRE and return to ISAF by the 8 January 2004.

All ships subject to SOLAS have to be fitted with AIS not later than 31 December 2004. In US waters other commercial ships longer than 65 feet are also subject to mandatory fit. There are no plans for compulsory fit to leisure craft at the moment.

What does AIS do, and how does it work?

AIS is a system which broadcasts the position, identification and other data of a vessel at sea.

AIS is centred around automatic data transmissions regularly (typically every 1-3 minutes)broadcast by vessels at sea using a dedicated marine VHF. The power and range is just the same as in voice VHF.

Using tiny "time slots" accurately measured from GPS time signals, each AIS set checks with all others within range then occupies a vacant "slot" so that a large number of vessels can be accommodated in the same area (present capacity about 2,000 reports per minute).

An AIS set broadcasts its ship's identity (MMSI) position, course, speed, ship type and rate of turn and other information and incorporates a facility to send short text (binary) messages.

The receiving part of an AIS set collects all the data within range and displays it either as a text output on a screen, which can be very small -like some navtex-sets, integrated in a radar display, or on an ECDIS chart.
The graphical displays usually feature a mouse-sensitive "tag" allowing the operator to reveal more data.

Already lighthouse authorities and ports are transmitting AIS data from fixed Aids to Navigation and intend to create "virtual" Aids to Navigation of which data will be transmitted by AIS, even though no Aid is physically present.

Like radar, AIS works perfectly at night and thick fog. Unlike radar, because AIS depends on VHF which is neither cut off by intervening cliffs nor confused by sea or rain clutter, AIS offers a huge advantage over radar in supplying other ships with a continuous, clear information stream.

Apart from ships and vessels at sea, who else will use AIS?

AIS enables port control centres and VTS (Vessel Traffic Services) to be better informed about the positions, identities, details and precise movements of ships in their area.

Since 9/11, counter-terrorism organisations including e.g. Homeland Security in the US have shown a very keen interest in AIS and are vigorously pursuing its carriage by vessels entering US waters.

What are the principal drawbacks in the system as a whole?

Known drawbacks include:
  • Dependence on GPS (from which both time-slot governance and position-fixing is derived). In the long term these data may be got from other systems including e.g. Galileo, GLONASS and Loran but all are vulnerable in some way.

  • Integrity of ships' own data: the information broadcast by a ship may be wrong due to an individual malfunction: receiving stations may have no means of assessing the data received.

  • A ship may switch off its AIS transmissions at any time.

  • The system is vulnerable to spoofing (transmission of inaccurate or false information) and jamming.

  • In areas of piratical activity an AIS broadcast will be very helpful to the pirates.

  • In a very large ship the AIS GPS antenna location becomes the pinpoint location of the ship. A protocol needs to be developed to cope with this. It could be e.g. that a data field shows the distance from the bows of the AIS GPS antenna (as the data message will also show LOA the positions of bow and stern can then be deduced).

  • The AIS GPS may be differentially corrected or not (this could produce a small position variable).

  • The main usage of AIS at least initially is in conjunction with radar. Radar does not work on absolute positioning, only on echo positions relative to own ship. Therefore the overlay of radar echoes with AIS data leaves (some, perhaps not much) room for confusion.

  • Antennae must be mounted as high as possible and well separated from other antennae.

What about small craft AIS sets?

Generally AIS sets not fitted compulsorily are known as Class B sets, the format of which is not at present controlled or specified. It is envisaged that there will be 3 types of class B Set:
  • Professional - Aimed at the larger non-Solas ship. Performing close to Class A with similar functionality, power and hardware.

  • Standard - Similar to Professional with some functions as optional extras.

  • Basic - Lower power fewer options.

Some AIS sets are on the market with small text displays. These could be used by small craft. The cost of a receive-only set is about £650.

There are many questions as yet unanswered about the possible development and use of Class B AIS sets, including:-
  • Just as a fishing vessel may be fishing or not fishing (her status affecting her rights of way under COLREGS) so a sailing craft may be sailing or motoring. Also a ship may be "NUC" (Not Under Command). There are presently no plans for a data field for "fishing" or "sailing" etc.


  • In a crowded area like the Solent or Chesapeake Bay on a busy summer's day the number of AIS-fitted craft could exceed the total the system can handle. What then? A central station (e.g. VTS Southampton for the Solent) will have the power to refuse service to all Class B sets in the area, by denying the right to transmit messages in the zone.


  • A user of a Class A set can select to display only the vessel type in which he is interested, e.g. all ships of more than 300 grt. or to exclude from the display all Class B Ships.


  • Could a computer chart display or radar screen be connected to the AIS-received data. Yes.


  • Could AIS be so connected to calculate collision risk? Yes, (rather like ARPA) though no manufacturer has yet offered such a device on the market.


  • Could an existing VHF set be used to receive and transmit AIS data andcan position data obtained from an existing GPS set be used for Class B? This is not presently possible. The AIS set has to be stand alone to preserve the integrity of the data.


  • Could an AIS device be transmit-only? No, because it must receive first in order to determine which "slots" are currently free.


  • A Class B set will not have the ability to seek additional information from a Class A set. In the event of conflict between Class A and class B transmissions Class A will receive priority.


  • Class B sets will not be able to send text (binary) messages.


  • Would a ship in busy traffic lanes pay much attention to the AIS data received from a sailing yacht? No guarantee.


  • Would a ship deep-sea keep her AIS switched on? No guarantee.

It is unlikely that all small craft would have the same preference for an ideal form of AIS equipment? Probably all would agree on:-
  • Must be lightweight (would probably be similar to a DSC VHF)

  • Must have low power consumption (comment as (a))

  • Must be low cost (eventually, comment as (a))

  • Must need minimum expertise (no doubt the sophisticates would buy a top-of-the-range set anyway)

Some sailors may want (variously):
  • Absolutely no more equipment up the mast.

  • Only an extra "warning off" device that tells ships about the yacht but does not have any other facility (sounds good but there will be no guarantee)

  • A limited text display in small size (will reduce cost but operation will not be user-friendly)

  • A text display in large size (ok but needs larger display unit)

  • A plan display integrated with ECDIS (should appeal to most ECDIS users)

  • A plan display integrated with Radar (should appeal to most radar users except that re-equipping is likely to be needed in the radar department!)

  • A stand-alone plan display

  • A plan display on a lap-top computer (most top racing boats will have a lap-top or a pc in the navigation set-up)

Most of the above options will have variables (i) splash proof for operation in a wheelhouse etc or (ii) totally waterproof and suitable for use on an unprotected deck.

Other systems

A common comparison is with the aviation "IFF" (identification - Friend or Foe). In civil aircraft control the controller may direct an aircraft to transmit its identity code on a stated frequency. A successful "match" enables the controller to "tag" a radar target with the correct identity. This writer does not know how military aviation IFFs work but presumably they are automatic, fast, and do not depend on a continuous broadcast as in AIS.

Conclusions - What do small craft want?

The opportunity has arisen for ISAF to put the views of the leisure boat user to the ITEC Working group that is drafting the performance standards for Class B sets. To help in this please complete the attached questionnaire and return it to Sebastian Edmonds (bas@isaf.co.uk) by no later than 8 January 2004.
The Working Group is meeting later in that month.
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