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The implication of AIS- Automatic Identification System (Transponders) on ships and shore
Captain/ Senior Pilot Benny Pettersson FNI Sweden,
The technology, that makes it possible to identify and track ships from other ships and from shore stations, already exists. We already know without a doubt, that some type of AIS "transponder" will be fitted on ships in the future and there is an intensive work going on in IMO on this matter. The best future AIS solution should naturally fulfil the demands from both the mariner on the ships, and shore based VTS-stations monitoring and supervising the coast and harbours. . The main purpose of a ship-to-ship AIS function is to assist in the avoidance of collision but it is also important that the future AIS increases the safety of shipping as a whole.
IMO NAV 43 July 1997 has prepared a draft performance standard for a universal shipborne AIS.
Four manufactures from South Africa, USA and Sweden have agreed on the Technical Characteristics for a Universal Shipborne Automatic Identification System using Time Division Multiple Access in the Maritime Band fulfilling the draft performance standard from NAV 43. These draft recommendation is now delivered to ITU.
IMO NAV 43 also discussed carriage requirements for AIS by all passenger ships and cargo ships of 300 gt and upwards in a working group on the revision on SOLAS chapter V. Carriage requirements will be brought up again in NAV 44.
This paper will show, why there is a requirement for an AIS, which is as versatile as possible, using a broadcast and interrogating self-organising technologies, to handle as many ships as possible, at the same time on a designated frequencies.
Who could be helped by AIS
The demand from politicians and maritime authorities, to have control over movements of ships in their territorial waters, in order to prevent accidents and marine disasters, is growing increasingly stronger. Coastal states are responsible for traffic separation schemes, routing areas and fish catch areas in their territorial waters. These states demand compulsory compliance by those, who use these waters and have a need to monitor "areas to be avoided".
Just the fact, that the user is monitored and identified from shore, will most probably result in a greater compliance with traffic separation schemes, routing measures and fishing regulations.
It has been argued, that accidents in coastal areas could be avoided, if the ship is identified. This maybe true in some cases, but a shore station, that identified a ship, is limited to advise actions only over VHF. Furthermore, this advice may be misunder-stood because of language problems.
The only one that can stop the engine or change course to prevent a grounding or collision is the Officer Of the Watch (OOW). The OOW also decides on, which actions to follow to comply with COLREG, the collision regulations. If a ship is equipped with an AIS, it enables the man in charge of the ship, the OOW, to be the first one to benefit from the AIS and to get an improved situation awareness capa-bility.
Ambiguous calls, with a stressed voice, in the night, " Ship on my port side, ship on my port side, this is M/S XXX, I am turning to port", have made many OOW wishing, that an AIS could have identified who is M/S XXX , when and how much is she turning and whom is she directing her call to? All the OOWs, within radio range, will be put under severe stress due to this inaccurate and incomplete information, until at least those ships, equipped with an ARPA radar, after two to three minutes, will detect that one of the targets is turning and might be M/S XXX.
With AIS, it has been argued, that if the OOW knows the name of other ships, he will make contact with these ships and sometimes agree on actions, which are in conflict with COLREG. This might or might not happen, but we should remember that this kind of contacts is made anyway today, but without knowing exactly to whom they are speaking. Prohibiting the OOW from using AIS, would be like prohibiting the use of radar. This would result in ships keeping a very slow speed in bad visibility, in compliance with COLREG, but there would also be a greater risk of collision.
Pilots, barge captains, ferries etc.
In very busy areas, like harbours, rivers and archipelagos, the need for a high update rate beacon mode AIS is evident. The limitations of the ARPA radar to track ships due to target swapping from a ship to land, beacons, bridges and other ships makes the ARPA capabilities very limited in narrow and congested waters.
Today there is a strong feeling among mariners navigating in harbours, rivers and archipelagos, that an broadcast AIS would improve the safety and solve the limitations of the radar because of the following capabilities. A broadcast AIS is able to:
look behind the bend in a channel or behind an island in an archipelago, to detect the presence of other ships and identify them.
predict the exact position of a meeting with other ships in a river or in the archipelago to avoid meeting in e.g. a narrow river bend.
know which port and which harbour a ship is bound for
know the size and the draft of ships in the vicinity.
detect a change in a ships heading almost in real time
identify a ferry leaving the shore bank in a river.
Radar is today the main sensor of the VTS to detect a ship. The VTS radar has the almost same errors in range and bearing resolution as all other radar's, although it has a known position and is North oriented without the errors of a compass involved. The limitations, of the ARPA radar to track ships due to target swap to land, beacons, bridges and other ships, makes the tracking facility in the ARPA rather limited.
To track ships in a bay area or an approach from the sea to an harbour, is what VTSs are limited to do today. There is a requirement for improving the VTS to be able to:
cover areas where radar coverage is almost impossible to achieve, like rivers and archipelagos.
identify radar blips on the VTS radar automatically.
interrogate ships for information regarding type of cargo
track with a high up date rate, the ferry going between two ports in the bay or a river continuously, without needing to reacquire it every time the radar tracking has swapped to another target, when the ferry has been moored to a pier or passed too close to a beacon or a passing ship.
know which port and which harbour a ship is bound for
know the size and the draft of ships in the vicinity
detect a change in a ships heading almost in real time
The only AIS solution, that could solve all the requirements of the VTS, is a high update rate, broadcast AIS, with interrogating capabilities.
The establishment of a global AIS in general, or a landbased AIS reception network in particular, may actually mitigate the need for VTS in many areas. The cost of installing and maintaining an AIS network is minute compared to a VTS radar net-work.
Marine Rescue and Co-ordination
Marine Rescue and Co-ordination Centres (MRCC) SAR (Search And Rescue) operations would be much more efficient if they had all the rescue craft fitted with AIS, to quickly determine which ship is closest to a distress situation. During a search, all the crafts could be tracked and plotted, enabling the MRCC to monitor the progress, to direct the available resources efficiently and to ascertain that search coverage is without gaps. Furthermore, if a ship in distress had an AIS, it could be seen on displays of all the surrounding ships and also at the MRCC.
Fishing boats and pleasure crafts
If a broadcast AIS would be a carriage requirement for all SOLAS ships over 300 ton, it would enable smaller ships and boats equipped with AIS to detect and be detected in bad weather with high seas or heavy rain. Most small ships will, however, not carry any AIS onboard which makes the radar the most important sensor to detect the existence of small crafts without AIS.
If mandatory AIS carriage could be applied to fishing vessels, it would enable national fishing agencies to enforce fishing restrictions more efficiently.
Shore based pilotage
The captain on a ship with no radar or a radar disturbed by sea/ rain clutter and with a very weak radar return from shore and navigational aids, may today be assisted with his navigation if he is able to get navigational assistance from shore.
Shore based pilotage with radar is normally limited to small and moderately sized ships which generally are more manoeuvrable than, for example, large tankers. The limitations of radars, makes shore based pilotage suitable only in areas where navigable water is sufficient to separate the traffic, and where the ship could be supervised only by giving bearings to, for example, waypoints.
If the DGPS position from a ship is transmitted to the VTS operator via radio, he will get an exact position of where the antenna of the ship is at the moment, he will also get the speed and course over ground for the same antenna, but the ships heading and where the rest of the ship is at the same moment is unknown.
This could lead to mistakes on what actually is happening. For example, when a tanker of 350 meters length, with the antenna positioned above the wheel house at the stern of a ship, starts to turn to port. The heading will change to port, but initially the stern and the antenna will swing slightly to starboard, due to the fact that the ship is turning around the pivot point. There is a need, to determine a virtual antenna position and the exact position of the antenna including the heading. The ARPA radar tracks the part of the ship, which gives the best radar return, normally on a loaded tanker the superstructure at the stern. A big tanker, turning with the superstructure at the stern tracked by radar, could have turned 40-60 degrees, before this is detected by the ARPA radar, on another ship or at the VTS, and now up to 4-5 minutes has passed since the turn started.
To be able to improve the tracking and the detection of a course change of a ship from other ships or from the VTS, ship's heading and DGPS antenna position also have to be transmitted at a high update rate. The best way of transmitting this kind of information would be, if vessels were equipped with a radio transponder AIS working in beacon mode and a high update rate.
It has been argued, that with a AIS display presenting AIS ships, the OOW will forget to fine tune his radar to detect small crafts. It is important to emphasise, that the radar is and will remain one of the most important instruments on the bridge, even in the future. It is important, that the OOW is taught the difference between the radar and the AIS, and the importance of tuning the radar to detect small crafts without AIS onboard.
The radar has been the primary mean for preventing collisions at sea in restricted visibility. The necessity to use a plotted ships course and speed through water, to be able to get its actual heading as accurate as possible, is obvious. Still the difference in course and speed through water, between a tanker and a carcarrier in a strong wind, is considerable.
With a broadcast AIS the identification, heading and the change in heading could be determined with a high update rate and solving some of the inherent limitations of radars. The AIS also shows the navigational status of another ship with AIS, which means that two ships equipped with AIS, that meet in restricted visibility, have all the information, that the navigation lights could show, including name and and port of destination.
The heading is taken from the compass and is not affected, neither when the AIS is using a high accuracy nor a low accuracy positioning sensor. The faster the nominal update rate is, the faster the mariner on another ship, or an observer in a VTS, will detect a course change. This means that a ship with high speed needs a higher nominal update rate than a ship with slow speed.
The large potential of transmitting the rate of turn of the ship, together with all the other movement parameters of the ship, is still not fully recognised. This enables both the VTS and approaching ships to make a rather accurate prediction of the path a ship is taking, some 30-90 seconds ahead. This gives more time and better information for all the other players in the traffic environment to plan their moves.
Tests are also in progress to make use of a GPS attitude sensor, or a "GPS-compass" if you like, to determine the attitude of the ship in six degrees of freedom.
Shipping routes within a VTS area are normally well defined, including alternative routes between two points, to the point that they could be indexed. On could anticipate a requirement to plan the route of a ship in advance, either by the OOW or supplied by the VTS. In either case the plan, consisting of the appropriate index numbers could be interchanged utilising AIS. This would enable interested parties to compare the planned route of the ship with the actual path taken.
The information from the AIS should be displayed for the OOW and the VTS operator. The most cost-effective way to display the AIS is to have a radar like display on a PC with only the AIS targets with their information presented where bearing and distances to the targets could be compared and identified on the radar. Software programs like the one referred to above are freely available and there is only the cost for the PC which is a normal inventory on a ship today.
Some ships already have an Electronic Chart System (ECS) or a full SOLAS compliance Electronic Chart Display and Information System (ECDIS) where the AIS could be presented. Manufactures, like ADVETO, STN ATLAS, NORCONTROL, TRANSAS MARINE, ANS, SPERRY and more, are already handling the proposed 4S AIS in their software. For manufacturers of other chart systems it is a matter of implementing the ability to present the AIS into their software. For ECS it is important that it is working with the geodetic datum WGS 84, as with the charts.
To present the AIS on the display of an old radar is not always possible, but when AIS is mandatory on new ships, radar manufacturers would be able to include this facility. AIS targets, superimposed on the radar display, will give the operator information of which targets have AIS and which have not.
IEC TC80/WG7 already has a proposal for AIS symbology on displays for AIS broadcast techniques.
To reduce clutter on the radar or ECDIS display due to many AISs appearing at the same time the AIS symbols could be "Active" or "Sleeping".
"Active" means that a green triangle showing the targets heading and (if high accurate positions used) COG/SOG vector.
"Sleeping" means that the mariner has chosen to suppress vectors and heading line and only a small green triangle pointing in the direction of the heading is shown A sleeping target could always be activated if the mariner selects it.
An additional proposal to the AIS symbology is, that all targets (or all active targets) should be updated using deadreckoning, once per second, using information from a database, containing last received position reports. If the position in the database is older than 1 second, then the new updated position should be calculated by using the latest received information on speed, course and rate of turn. One way of presenting this, is to have a small vector with the foot at the last received position report and let the symbol move along the vector using deadreckoning.
High update rate
Today with a modern ARPA radar 20 targets could be selected and tracked with an update rate of 3 seconds. The symbol on the PPI will, most of the time, follow the target, except in the case of a target swap. The limitations of the ARPA make a course alteration on another ship undetectable, until one or two minutes after the course change has started and for a large tanker it could take up to five minutes before a change is detected. Furthermore, the ARPA also needs one or two minutes before it can present a vector with course and speed through water of a target.
For AIS targets, superimposed on a radar display, the need for a high update rate becomes obvious. If the update rate is too slow there will be situations on smaller ranges where the AIS symbol will not catch up with the radar target. The optimal solution would be, that the update rate would be the same as the radar, i.e every 3 seconds.
The maximum expected number of ships in the VHF range and the need for a high update rate has to be considered when nominal reporting rates for the AIS are decided. The limit of what could be transmitted on the VHF is also one factor to be considered, when deciding on optimal nominal update rates.
IMO Navigation Committee (NAV) 43
NAV 43 prepared a draft performance standard for a universal shipborne AIS.
NAV43 annex 2 1.2.1 states "in a ship-to-ship mode for collision avoidance"
The above is the first listed requirement from NAV43 and it is also the prime requirement to fulfil. This requirement should not at any time be weakened or alleviated due to a competent authority's desire to use the same AIS radio channel to cover a larger AIS area or to use the controlled mode. A ship should at all times have an as good situation awareness as possible. This means that ships closer than 10 nautical miles of the own ship should fulfil the requirements in NAV43 annex 2 6.5 "Information update rates for autonomous mode" and the table reporting intervals.
Ship at anchor 3 min
Ship 0-14 knots 12 sec
Ship 0-14 knots and changing course 4 sec
Ship 14-23 knots 6 sec
Ship >23 knots 3 sec
Ship >23 knots and changing course 2 sec
The same paragraph also states that "the system should be able to handle a minimum of 2000 reports per minute to adequately provide for all operational scenarios envisioned" The 2000 reports should be handled within the range of the normal radio range of a ship (15 - 20 nautical miles).
NAV43 annex 2 2.1.1 says "an "autonomous and continuous" mode for operation in all areas. _. " 2.1.2 an "assigned" mode for operation in an area subject to a competent authority and 2.1.2 a polling or controlled mode.
It should be remembered that a ship mostly are in areas where there is no VTS or competent authority to poll or assign the TDMA link. Consequently the autonomous mode is to be considered to be the most important operational mode for the AIS. If the competent authority likes to have some particular or ships in a particular subarea assigned and to update at a faster rate to enable a close monitoring of a special situation or a shore based piloting, the use of the TDMA link for this purpose should not amount to more than 10 to 20 % of the available link capacity.
Carriage requirements in SOLAS chapter V will be brought up again in NAV 44 in July 98 and will most probably get a lot of resistance if the size of ship that should carry AIS is to low (300 gt). If e.g. 5000 gt could be agreed the advantage for smaller ships will be so obvious after a while that they will fit AIS anyhow.
World Radio Conference WRC- 97
WRC-97 decided that two frequencies (86 and 87) was to be used for AIS on a world-wide operation on high seas unless other frequencies are designated on a regional basis for this purpose. USA, New Zealand and Singapore where the only countries who have difficulties to designate the decided frequencies and need to designate other channels on a regional basis.
International Electronic Commission IEC
IEC have started a working group that will work with the standardisation for a universal AIS after ITU:s meeting in mars.
To fulfil the requirements of the future requirements the AIS system should:
operate in both ship-to-ship and ship-to-shore mode
be autonomous, automatic and continuous
not interfere with navigation
decrease the OOWs workload, not increase
have a high update rate (down to two sec)
high capacity ( more then 2000 reports/min)
operate in a broadcast mode
be able to assign and poll from a competent authority
be internationally standardised
be made a carriage requirement
be capable of accommodating future expansion and development
An AIS, using broadcast techniques and with a high update rate, will enhance the possibility of detecting the whereabouts of other ships, even if they are behind a bend in a canal or river or behind an island in an archipelago. The AIS will also solve the inherent problem with all radars, by detecting fishing boats and smaller ships, fitted with AIS, in sea-clutter and in heavy rain.
Innovative technologies like ECDIS and ship-to-ship and ship-to-shore AIS have a big potential in improving safety in the maritime field in the future. It will also make it possible to identify, track and supervise ships from shore with a much higher and more sustainable accuracy than with a shore based radar.
On the other hand, the need for navigational assistance from shore on a ship, with ECDIS and ship to ship and ship-to-shore AIS onboard, is surely less, than on a ship only fitted with a radar, when both are affected by sea/rain clutter.
Make the OOW the one, which benefits the most from an AIS system and it will increase the safety of shipping and navigation significantly.
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