VTS Committee

Version 1

(September 2001)

AISM-IALA : 20ter, rue Schnapper - 78100 Saint Germain en Laye - France

Telephone: +33 1 34 51 70 01,- Telefax: +33 1 34 51 82 05

E-mail: iala-aism@wanadoo.fr Internet: http://www.iala-aism.org

　

　IMO Assembly Resolution A.857(20), Guidelines for Vessel Traffic Services,establishes the following tasks that should be performed by a VTS :

　"A VTS should at all times be capable of generating a comprehensive overview of the traffic in its service area combined with all traffic influencing factors. The VTS should be able to compile the traffic image, which is the basis for the VTS capability to respond to traffic situations developing in the VTS area. The traffic image allows the VTS operator to evaluate situations and make decisions accordingly. Data should be collected to compile the traffic image. This includes :

　This IALA Guideline on AIS seeks to identify for the benefit of VTS authorities the ways in which AIS contributes to the achievement of the above tasks.

　The proper use of AIS enhances the safety of life at sea, the safety and efficiency of navigation, and the protection of the marine environment.

　According to SOLAS Chapter V, Regulation 19

　AIS shall-:

　Thus, AIS will become an important supplement to existing communication systems. In general, data received via AIS will enhance the quality of the information available. AIS is an important tool :

　In short, the purpose of AIS is:

　AIS is a broadcast transponder system, operating in the VHF maritime band, that is capable of sending ship information, such as identification, position, course, speed, ship dimensions, draught, ship type, and cargo information, to other ships and to shore.

　It is to be capable of handling over 2,000 time slots per minute and updates as often as every two seconds. AIS uses self-organizing time division multiple access (TDMA) technology to meet this high broadcast rate and to ensure reliable operation.

　In general, each AIS system consists of the following :

　Timing information is derived from an integral global navigation satellite system (e.g., GPS) receiver. Additional information may be broadcasted by the AIS if electronically obtained from shipboard equipment through standard marine data connections (i.e., roll, pitch, etc.).

　The effectiveness of AIS may be significantly increased by integrating it into other devices such as electronic chart system (ECS) / electronic chart display and information system (ECDIS) or a radar / automatic radar plotting aid (ARPA).

　The AIS transponder works in an autonomous and continuous mode, regardless of whether it is operating in the open seas or in coastal or inland areas. As prescribed by ITU-R World Radio Conference (WRC97), it has two designated frequencies that are in the maritime mobile band, i.e., VHF-FM channels 87B (161.975 MHz) and 88B (162.025 MHz). Transmissions are over a 25 kHZ or 12.5 kHZ VHF channel.

　The robustness of this band allows for the relatively high transmission rate (i,e., 2,000 reports / minute using 25 kHz channel (duplex) or 1,000 reports / minute using a 12-5 kHz channel (simplex)). Although only one channel is necessary, each station transmits and receives over two radio channels to avoid interference problems, and to allow channels to be shifted without communications loss from other vessels. The system provides for automatic contention resolution between itself and other stations, and communications integrity is maintained even in overload situations. That is, TDMA allows the system to be overloaded 400% to 500% and still provides nearly I 100% throughput for ships within 8 to 10 NM of each other.

　There are different message types including the ship's data required by the IMO performance standards (as well as data necessary for communication management).

　The data is autonomously sent at different update rates as follows :

　Report Rate of Dynamic Information

　A ship and its messages are identified and allocated by its MMSI number (Maritime Mobile Service Identity).

　Except where noted otherwise (i.e., listed as optional or at the discretion of a competent authority or master), under the international standard, the following data will be required to be transmitted by an AIS transponder :

Automatic Vessel Identification

　AIS brings to the mariner many benefits. Principal amongst these, as the name implies, is the automatic and immediate provision of vessel identity (MMSI, call sign etc), thereby facilitating rapid radio communication where necessary. This benefrt is of equal, if not even greater value to VTS authorities.

　Most VTS organisations require vessels to report to the VTS centre on approaching or entering the VTS area. Achieving vessel identity relies on such vessels reporting both identity and location to the VTS centre, and the VTS operator then correlating this information with an unassigned radar track.

　The process is time consuming and wholly reliant on the co-operation of participating vessels. It is not uncommon for some vessels to fail to comply with this requirement, thereby creating a potentially dangerous situation, and creating further distraction for the VTS operator. Even where VHF direction finding equipment is fitted, the VTS traffic image is still reliant on vessels reporting identity via VHF thereby permitting the correlation of identity with the radar track identified by DF. AIS will help overcome the safety weaknesses and time consuming procedures, inherent in the present arrangements.

　

Improved Vessel Tracking

a) Wider geographical coverage

　AIS data will be received by other AIS units, or by base or repeater stations. Thus where a VTS organisation is fitted with such equipment, it will be capable of receiving both identity and precise location of a vessel at the maximum reception range of the VHF radio communications frequency. As a consequence, it will often permit detection of vessel target wel1 outside conventional radar range. Even where this is not possible due to the need to screen base stations from adjacent VHF interference, extended VTS detection range may be achieved by the installation of additional base or repeater stations connected into a network at much lower cost than radar.

　

b) Greater positional accuracy.

　AIS aims to achieve positional accuracy better than 10 metres when associated with DGNSS correction signals. This compares favourably with radar which as a function of frequency, pulse repetition rate, and beam width will often only achieve positional accuracy in the range 30 to 50 metres.

　

c) Absence of "radar shadow" areas.

　In coastal and harbour waters radar tracking of vessels can be masked, or otherwise affected by the proximity of land and buildings. The resultant "shadow" areas can cause a radar based VTS to lose track, thereby denying the VTS centre the ability to monitor accurately vessel movement at what could be a critical time. The loss of tracking will invariably result in the need to reacquire and re-identify lost tracks, thereby increasing the work load within the VTS centre.

　Whilst AIS tracks will avoid the great majority of such effects, the very close proximity of buildings and bridges, sometimes known as the "urban canyon" effect, can cause difficulties for AIS transponders in heavily built-up areas. This is a consequence of inhibiting either the reception of the differential GNSS signal by the AIS transponder, or the transmission of the subsequent AIS message.

　

d) Traffic image accuracy

　Vessel tracking can similarly be interrupted when two vessels pass close to one another, with the result that the radar tracking of one contact is confused by the proximity of the other. Importantly, this can result in the identity of one track transferring or "swapping" to the other. Self-evidently, such a situation introduces a potentially dangerous inaccuracy in the vessel trafflc image, unless noticed and rectified quickly by VTS operators. Again the consequences of this phenomenon are yet further work for the VTS centre. The more precise tracking associated with AIS has been shown to prevent the incidence of"track swap".

　

e) Real time manoeuvring data.

　Radar based VTS systems will typically provide details of a vessel's course and speed over the ground.Of necessity, this information is historical in that it is calculated from the track made good by a vessel. In contrast, AIS will provide all recipients with certain elements of real time manoeuvring data such as Ships Heading and Rate of Turn. These are derived directly from the vessel navigation systems and are Included automatically in the Dynamic Message broadcast by the AIS.

　

f) Weather effects on tracking performance.

　Navigational radar performance is often adversely affected by precipitation as a function of the radio frequency on which it operates. In heavy rain or snow, effective radar tracking is sometimes unachievable, even with the use of modern suppression techniques. VHF radio transmissions on the other hand are not so attenuated. As a consequence a VTS centre is much more likely to maintain an accurate traffic image in adverse weather where that tracking is based on AIS data.

　VHF radio transmissions can be affected by atmospheric ducting. In these conditions, VHF reception ranges can be greatly extended. Where such an enhanced reception range brings with it the detection of greatly increased AIS messages, the system will automatically overcome the risk of overloading by ignoring signals originating from vessels at greatest range, and re-using the slots so gained.

　

g) Provision ofmore precise navigational advice.

　It follows that where a VTS centre is able to receive AIS information from vessels within or adjacent to its area, the quality, accuracy and reliability of vessel tracking will be improved markedly. As a consequence, that VTS centre will be able to provide more precise navigational advice, as and when required, or when deemed necessary.

　Moreover, the availability of certain real time manoeuvring data within the VTS centre will enable VTS operators to appreciate more rapidly, and in greater detail, actual vessel movement. It should be stressed, however, that this facility alone wil] not enable a VTS centre remotely to manoeuvre a vessel with safety.

　

Electronicl transfer of sailing plan information

　Where AIS is integrated into a VTS system, it becomes possible for vessels and the VTS centre to exchange passage information such as intended way points, provided the appropriate software is available.

　

Electronic tranfer of safety messages.

　The facility available within AIS for the transmission of short safety messages makes possible the electronic broadcasting from a VTS centre of local navigation warnings, and similar safety related messages.

　It is anticipated, that VTS centres may have the capability to broadcast via AIS local chart corrections to ECDIS fitted ships.

　

Automatic indicatin of Voyage Related Information (cargoes,dangerous goods etc)

　Vessels are normally required to report to the VTS authority that any dangerous goods are being carried. The AIS voyage related message permits the inclusion and automatic transmission of this information.

　

Impact on VHF communications

　As described earlier, a major benefit of AIS is the consequential reduction of VHF voice messages. This In turn reduces the reliance placed on vessels understanding such messages from a VTS centre and vice versa.

　

Archiving data

　The automatic availability within a VTS centre of AIS data for each vessel facilitates the rapid and comprehensive recording, replay and archiving of data.

　

System redundancy

　By equipping VTS centres with AIS, an alternative method of tracking and monitoring vessel navigation is introduced, thereby improving system redundancy significantly.

　

Potential for interaction within regional AIS network

　Increasing emphasis is being placed on networking VTS centres on a regional basis. Such an arrangement facilitates greater efficiency by making possible the rapid transfer of vessel details between different centres. Adoption of AIS within the relevant VTS centres may contribute toward this process.

　

Improved SAR management

　Many marine and VTS authorities are equipping SAR capable units, including aircraft and helicopters, with AIS. The AIS voyage related message permits a vessel to transmit the number of persons onboard. Whilst this is not mandatory for vessels at sea, it can be made a formal requirement in a VTS area. The provision of such details, and the ready identification and location of SAR units greatly facilitates the management and evaluation of any SAR response.

　Although AIS has the potential to greatly enhance VTS operations, the system does have several limitations or potential drawbacks. For example :

Number/location of base stations/repeaters

　In deciding the size, and thus cost, of integrating AIS into a VTS system, a careful study needs to be undertaken to establish practically the number and location of base and repeater stations required to achieve full and reliable coverage of the expected traffic load. Although VHF reception is greatly influenced by antenna location and height, operation in a heavy electronic environment may necessitate the installation of stations in order to reduce susceptibility to interference.

　

Interoprability with adjacent VTS authorities

　Where it proves necessary to use more than one centre, or where a VTS authority involve more than one VTS centre, the method of connecting the component elements into a local network needs to be given careful consideration. In particular, the existence of, or plans for a regional network may necessitate using a local networking solution, which is compatible with national and international networks.

　

Availability of VHF Communication channels.

　Two maritime VHF Channels have been allocated by the ITU for the international use of AIS in its primary ship-to-ship mode. What is not yet certain, is whether additional local channels will need to be allocated to support the operation of VTS within certain congested VTS port environments. The need for such additional channels will be at its most acute where large numbers of vessels navigate within a VTS area, and where the VTS centre has a particular interest in deriving vessel identity at maximum range. As has been described above in the section on the Limitations of AIS, the AIS in an overload situation will discount AIS signals received from the extremity of an area, before those emanating from vessels or craft close to the receiving station.

　

Availability of national/regional/local DGNSS corrections

　In order to monitor vessel navigation with the 10 metre accuracy potentially possible, a reliable DGNSS correction signals will need to be made available to all vessels throughout the VTS area. Such services are provided nationally or regionally in some areas. Where such a service does not exist, a VTS authority may consider providing these corrections itself. It is technically possible to transmit the relevant corrections using the AIS itself.

Integration AIS into existng rader based VTS systems

　Radar based VTS systems often differ in the way radar video is handled and processed, prior to presentation of the traffic image. System design and age are thus likely to influence the options for successfully integrating AIS. A full appreciation of those options, together with any consequences, will normally only be possible after consultation with the relevant manufacturers.

　In many VTS areas, vessel traffic is varied and includes both SOLAS and non-SOLAS vessels. In these circumstances, radar will remain the primary sensor for detecting vessels not fitted with AIS. Economies in infrastructure are therefore unlikely.

　AIS data is transmitted at variable rates depending upon vessel speed and manoeuvre. In contrast, radar data is generated at a constant rate as defined by antenna rotation speed. The integration of AIS into a radar based VTS system thus needs to be capable of achieving and maintaining the correlation of AIS and radar data originating from the same vessel, despite unpredictable variations in data rates. The potential benefits of AIS would be quickly reduced, should the process of integration result in the generation of numerous false tracks.

　

USE of electrobnic charts

　VTS systems have traditionally used a diagrammatic representation of the geographical and hydrographic features of the relevant area as the background to the traffrc image. The accuracy of such representations, however, is not suitable for precise navigation. With the advent of electronic charts, there are clear benefits to be gained by using such charts as the background to the traffic image. By so doing, vessel navigation may be monitored, and advised, in relation to precise charted features. In VTS systems not fitted with electronic charts, such advice can only be given in relation to radar detectable features, such as coastline or navigational buoys, or as depicted on existing VTS display diagrams.

　Where reliance is to be placed on electronic charts for this purpose, it is important that they are issued by an approved hydrographic office, accurate, and up to date. It is anticipated that VTS authorities may be able to broadcast local chart corrections to suitably equipped (ECDIS/ECS) vessels and to issue navigational warnings electronically using AIS.

　In confined waters, it is likely that VTS operators in monitoring vessel manoeuvres will occasionally have need to reduce the scale of their displays. In such circumstances, it will be important that the electronic chart acting as the background to the traffic image, is capable of showing increasing levels of survey detail, as operators reduce the scale on their displays.

　This will only be possible where the electronic chart is compiled from source survey data, rather than from an existing paper chart. In these circumstances, it will also be important that the charted location of radar sites is accurate to a maximum of 10 metres, if errors between radar and AIS generated tracks, which will be all the more obvious at reduced range scales, are to be avoided.

　IHO Standard S52 defines the standards for symbols and colours on official electronic charts. Four variations of the basic colour scheme are available. These colour schemes, whilst optimised for navigation in varying light conditions on the bridge of a vessel, may not be suitable for VTS purposes ashore, particularly where operators are required to study a display constantly for long periods.

　

Choice of VTS Symbols

　The following table shows the choice of AIS symbols allocated for use onboard vessels.

　These symbols may be found to be unsuitable for VTS purposes, for two reasons. First, those selected to represent AIS tracks may need to be accommodated logically within an existing framework of symbols. Second, VTS centres will often have need to represent visually on the traffic image, a much wider range of information than is necessary onboard a vessel. For example traffic management may necessitate the use of symbols which depict different types and sizes of vessels. Alternatively, it may be necessary to show which vessels have pilots embarked, and which do not.

　When it is required for a VTS Centre to transmit a psuedo-AIS target to an AIS/ECDIS fitted vessel, it will be necessary for the information to be transmitted in terms that will be recognised by the receiving vessel.

　A further application of AIS is as AtoN. When positioned at a significant geographic point or danger to navigation, an AlS transponder can provide information and data that would s
erve to:

　The remote control and monitoring of aids to navigation has been developed primarily to enable service providers to ensure that aids and supporting systems are functioning correctly and where required to organise maintenance.

　Until now, there has been no simple and universal method of communicating such information. The introduction of AIS presents an opportunity to provide such information to service providers and mariners, using internationally standardised and recognised equipment, message protocols and frequencies.

　The operation and performance of aids to navigation can be monitored or controlled using an AIS data link as the interface with the service provider. It is possible to have an AtoN transmit its identity, operational status and other information such as real time wave height, tidal stream and local weather to ships nearby or to the service provider. Buoys that can transmit an accurate position, perhaps based on DGNSS, can be monitored to ensure that they are on station. Performance monitoring, remotely changing operating parameters, and activating back-up equipment are also made possible by the use of AIS.

　Another application, which is expected to be widely used is the transmission of meteorological and/or hydrological data. Where such an application is intended for international use, the message format will be registered by IALA prior to being made available to system manufacturers. This will facilitate the correct presentation of the information on systems from different manufacturers.

　

Options for implementing this application include:

　The information to be broadcast will depend on the operational requirement and the availability of measuring and processing equipment. Examples include :

　Such data permits the presentation of real time information at receiving stations, includlng onboard ships within VHF range.

　For information on personnel and training, refer to model courses V103-1, V 103 -2, V 103 -3 and its associated task book and V103-4.

　AIS equipment is to be implemented as a mandatory carriage requirement under SOLAS Chapter V for newly constructed vessels from 1 July 2002 and progressively thereafter to other vessels by 1 July 2008.

　VTS authorities therefore need to consider as matter of priority whether they intend integrating AIS into their VTS system. As the previous paragraphs will have demonstrated, the inclusion of AIS into a VTS system signifrcantly enhances the precision and reliability with which AIS equipped vessels may be monitored, and thus enhances safety.

　AIS also has the potential to improve effrciency in vessel traffic and port management. The degree to which this potential may be realised will vary depending on the operational circumstances. It Is for each VTS authority to make that assessment.

　In order to avoid a situation whereby AIS fitted vessels incorrectly believe that a VTS authority is receiving data being transmitted via the AIS, all VTS authorities will need to publish by appropriate means their status in respect of AIS. Where applicable, the date on which they intend to incorporate AIS should also be promulgated well in advance.