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17.11.2013

AIS in VTS

Universal Automatic Identification System (AIS)
 
Application of technology in AIS vessel traffic services (VTS)
 
 
 
John McDonald
 
Australian Maritime Safety Department (AMSA)
 
 
 
In the late eighties of the XX century originated the concept of ' transponder ' as a result of the increased need for increased information exchange vessels to the shore in order to increase the efficiency of ship operation and safety of navigation. Although initially intended only communication with the shore vessels in support of vessel traffic management systems (VTS), this concept was subsequently expanded to include an additional requirement to secure data transmission between the courts for solving the collision avoidance .
 
At the same time, the modern revolution in navigation and information technology provided the opportunity to meet these new requirements. Combining satellite positioning systems, electronic mapping , communication and open architecture of information systems , marine electronics industry can now deliver working samples of what is called the universal automatic identification system (UAIS) or simply AIS.
 
This article describes the development of the concept of AIS, adopted by the international community working and functional requirements , specifications and standards system. The paper then discusses the current activities in the world on the use of technical means of AIS to the safety of navigation and traffic-management , in particular to the VTS.
 
 
 
INTRODUCTION
 
In 1988-89 yy VTS Committee on the International Association of Lighthouse Authorities (IALA), including experts from the IAPH, IAIN, IFSMA and IMPA, undertook a study of ' the possibility of using transponders for the identification and tracking of ships when approaching , entering and swimming in the service area VTS'. The article, which summarizes the results of the first study was presented at the IALA thirty-sixth session of the Subcommittee on the safety of navigation (NAV 36 ) of the International Maritime Organization (IMO) in September 1990 This document titled ' System Requirements identification , survey , maintenance and automatic notification to work in conjunction with VTS ' became the basis of technical specifications for AIS.
 
After consultation with the other Member States IMO, including Germany, the Netherlands , Norway and the United States, which , as far as was known , developed while VHF transponders , IALA presented at session 37 in NAV 1991 document with more stringent technical requirements. Project specifications was called ' Radiotranspondery for VTS and opportunities identification vessel from the shore ,' but apart from applications to problems VTS recognized the new requirements - the ability to ' data ... between certain categories of suitably equipped vessels and other vessels .'
 
Thus, although initially concerned the concept of transponder data exchange vessel to the shore for solving traffic organization , such as assistance Vessel Traffic Service (VTS), this concept has been extended at the initial stage and included the additional requirement of data transmission between the courts in order to prevent collisions ships.
 
 
 
TWO DIRECTIONS OF TRANSPONDERS
 
After a long (and often intense ) discussion forums IMO and IALA for several years gradually emerged two systems based on the VHF radio :
 
 
 
Transponder protocol based digital selective calling (DSC), using VHF channel 70 , and
 
 
 
 
' Broadcasting ' Universal AIS, using the methods of self-organizing multiple access time division multiplexed (STDMA).
 
 
Option with DSC
 
System formed in accordance with Recommendations ITU-R M.825, using digital selective calling techniques , mainly used as a transponder for transmitting data from ship to shore on VHF channels. These transponders provide identification , tracking and control of ships from the shore.
 
For this purpose the vessel is queried ( requested ) at least when it first enters the area served by the center VTS. However, for the initiation of such a consultation process usually requires the radar target detection , and often requires operator intervention. In systems AIS VHF channel provides up to 500 messages per hour.
 
Sharing channel 70 also restricts GMDSS refresh messages AIS, since the channel can not be used over 15 % of its theoretical maximum capacity . Baud rate - only 1200 bits per second , although there are successful development with the aim of increasing up to 9600 bits per second.
 
As a partial revision of Recommendation ITU-R M.825 were adopted additional procedures ship communication protocols using a kind of DSC, but works perfectly on channel 70 VHF channels. Transponders made according to this standard , also provide recognition and support vessel ship , but only to a limited extent .
 
DSC transponders not reached a final support IMO adopted technical specifications , but DSC technology proved attractive to several states. VHF (DSC) channel 70 is defined as the VHF part of the Global Maritime Distress and Safety System (GMDDS) and became a must-have for ships engaged in international voyages since February 1999 In addition, most countries in Europe , North America and Japan chose the sea areas under A1/A2 activities deployment GMDDS. This means that all ships operating in their territorial waters and coastal stations serving these vessels must be equipped with VHF DSC equipment to support GMDSS.
 
Therefore, the transponder , which uses a similar technique , and marine equipment already installed , have become an attractive choice , the choice that can be implemented relatively quickly and cheaply . Several systems using VHF (DSC) transponders, was established in the UK and in the United States . They mainly serve VTS centers or established port support vessels or vessels operating on regular passenger ferry lines . Control Information Service Strait Pas-de- Calais (CNIS) regular ferry lines crossing the Strait of Dover , and at the port VTS Valdez ( , Alaska ) - two examples of systems based on AIS DSC.
 
In addition , DSC provides a flexible means for administrations automatic selection VHF frequency channels that are running AIS, in regions where selected for AIS channels are unavailable . For this reason, some countries were interested to IEC standards developed by the organization to carry out tests ' broadcast ' AIS allow backward compatibility with DSC.
 
' Broadcast ' or with ILS
 
' Broadcasting ' transponders, originally named 4S - ' ship -to-ship ' and ' ship-to- shore' ( a term coined by the Swedish developers) , formed the basis for what became known as the ' universal shipborne automatic identification system (AIS)'. This system replaced with DSC option and adopted IMO and ITU-R as a standard AIS.
 
Simply put , AIS - broadcast transponder operating in the range of VHF maritime mobile service . He is able to send information about the vessel ( id, position, course , speed, etc. ) to other ships and on shore . It can handle a large number of messages that are updated with great frequency , and uses technical means of self-organizing multiple access time division multiplexed (STDMA), ensuring reliable operation of the vessel with the ship at high speed information exchange.
 
In connection with these IALA Association has developed an initial draft of the 'universal ' standard for IMO, formed a task force of representatives from industry and government agencies. Project in a revised form was considered at the session of NAV 43 ( July 1997 ) and formally adopted by MSC 69 Conference May 11, 1998 , entitled 'Annex 3 to Resolution IMO MSC.74 ( 69) - Guidelines for technical specifications for a universal shipborne automatic identification System (AIS) '. State of technical standards and standards for testing , produced by international organizations ITU-R and IEC, described below.
 
 
 
SPECIFICATIONS AIS
 
As already briefly mentioned, the definition of universal AIS IMO and ITU-R - this ship broadcast transponder system operating in the VHF range maritime communications . It can send information on the vessel ( id, position, course , speed, length , draft , class ship) and information about the cargo to other ships and shore. It can handle more than 2,000 reports per minute and update messages every two seconds. AIS hardware uses self-organizing multiple access with time-division multiplexing (STDMA), ensuring stable and reliable operation of the vessel with the vessel at a high speed messaging .
 
The system is backward-compatible with DSC systems , allowing shore GMDSS cheaply create channels for AIS operating frequency and to identify and track vessels equipped with AIS.
 
Each AIS system consists of one VHF transmitter , two VHF TDMA receivers , one VHF DSC receiver and a standard electronic link with the ship's display and information systems . Coordinates and data synchronization usually comes from internal or external receiver global satellite navigation system (GNSS) ( eg, GPS), including the MF receiver differential GNSS (DGNSS), used to determine the exact coordinates in coastal and inland waters. Information about the direction of motion is usually transmitted by all vessels equipped with AIS, while other information - course and speed over the bottom of the sea, turning speed , roll angle , pitch and roll , the port of destination and estimated time of arrival - may be required only on certain ships .
 
 
 
Data transmission technology STDMA
 
AIS transponder normally works in autonomous and continuous mode regardless of whether they equipped vessel on the high seas , in coastal waters or in inland areas . Since VHF reports are mainly transmitted at a relatively small distance , require significant rate and because they should not be subject to interference problems , it uses two frequencies in the band of the maritime mobile service. Modulation method used - FM / GMSK ( frequency modulation / manipulation of Gaussian minimum shift ) due to its reliability, efficient use of bandwidth and wide application in digital mobile communication .
 
So the 25- kHz VHF simplex channel can transmit approximately 2,000 reports per minute at a baud rate of 9600 bits per second , 12.5 kHz VHF simplex channel can transmit approximately 1,000 reports per minute at a transmission rate of 4800 bits per second.
 
Sending messages is carried out in separate time windows (slots ) that are synchronized according to the GNSS system with an accuracy of 10 microseconds. Each station determines its own transmission scheme (slot ), based on history information exchange through the communication channel and knowledge of future actions by other stations.
 
AIS station continuously synchronized with each other so as to avoid transmission overlap in the windows. AIS station selects the window (slot ) in a certain range at random, and the slot is assigned a timeout value of a random length between 0 and 8 frames . When a station changes its window , it pre- announces a new cell and a timeout for this cell . Thus, the court will always accept new stations , including stations that suddenly appear in the radio range close to other vessels.
 
The required update rate posts
 
Specifications of IMO and IMO notice for ITU-R define the type of data being exchanged , but do not define the required intensity of messaging . Committee on the IALA VTS Association studied this problem, and it would be interesting to consider this issue in terms of needs and services VTS ship reporting systems in the future. Consideration based on modern methods of radar wiring time intervals between successive locations Serif DGNSS system and, finally, ( as a worst case scenario ), the maximum intensity of the movement in Singapore and the Strait of Dover .
 
Mode of operation of modern radars . In radar update rate determined by the frequency of rotation of the antenna of from 20 to 60 revolutions per minute. This enables the update data (in this case, the range and bearing of the target) from 1 to 3 seconds.
 
Successive position fixes in the system DGNSS. In areas of difficulty for reliable navigation support, collision avoidance and pilotage position of the courts shall be determined with an accuracy of less than 15 meters. Navigating using DGNSS provides positioning accuracy of approximately 5 meters . Application of algorithms for predicting the parameters of motion introduces an additional error of the order of 10 meters .
 
For vessels that do not change its course , the update rate necessary to achieve a level of accuracy of the coordinates , determined by the speed of the ship and gives the following intervals between transmissions of messages :
 
Lots of traffic - Singapore and the Strait of Dover . These calculations are based on the results of the survey of the Port of Singapore and the Strait of Dover Coast Guard , answers the question , how much they expect the courts within 20 nautical miles (based on 1990 data ) . Answer from Singapore was that at any moment there are about 300 ships moored or anchored wall and 60-70 merchant ships on the move.
 
VHF transponder might be expected , bude have a range of about 40 nautical miles . Therefore, extrapolation of the data was performed , the result will be 300 ships at anchor and 210 on the go. This figure was further added 100 ships to accommodate ferries, pilot boats , tugs , support , patrol vessels and fishing vessels . Table 2 shows the resulting estimates of the number of reports on the position of ships required every minute to the values update rate messages listed in Table 1.
 
This scenario provides an estimate of 3060 reports per minute. A similar calculation for the Dover Strait gives about 2550 reports per minute ( 480 vessels). For practical reasons, the figure was chosen in 2000 reports per minute as a minimum requirement .
 
Requirements for indicator
 
Next, you need to consider the requirements of the indicator so that the output data were useful to the navigator . Specifications IMO left uncertain the question of requirements for display of information , although ideally assumed that AIS information will be displayed on the ship's radar display , LED electronic maps and information system with color display (ECDIS) or on a dedicated indicator . It would be necessary to take into account the danger of overloading the screen data , and probably requires a correlation between primary radar targets and objectives derived from the transponder AIS.
 
Currently radars and ECDIS systems are not capable of ( and do not have type approval ) to take AIS system generated data. Therefore, in many samples tested so far AIS indicators used special graphic displays based PCs.
 
However, the development of a standard for testing IEC 61993-2 Technical Committee 80 of the International Electrotechnical Commission (IEC) found it necessary to determine the ' minimum requirements for indicator for AIS', in order to evaluate the proposed functions. The standard requires a minimum output 16 alphanumeric characters , which is enough to get the ID and coordinates the polled vessel .
 
Emphasizes that it is - the minimum requirements for the indicator for AIS ( mainly for testing purposes ), although it can be used in the sea in low activity. To fully take advantage of the AIS, the system should be integrated into one of the existing graphical indicators on the bridge or have a dedicated graphical display . Higher functionality can be obtained from the graphical display with advanced features, but the choice of this type of indicator depends on the needs of the consumer and the manufacturer offers .
 
 
 
APPLICABLE STANDARDS
 
Following is a brief overview of the current state of development of standards for AIS. Although some special requirements still need to finish (especially technical) , several companies are well advanced in the development of very complex systems VTS, using new technology AIS.
 
Resolution IMO MSC.74 ( 69) Annex 3 . Recommendations on specifications for shipboard universal automatic identification system (AIS)
Recommendation ITU-R M.825- 2 . Features transponders using equipment DSC ( DSC ) and used for the identification of the vessel with the VTS and other vessels
Recommendation ITU-R M.1371. Technical characteristics for a universal shipborne automatic identification system using multiple access time division in the band maritime mobile service
(Draft) standard IEC 61993-2 for testing AIS. In July 1998, the Working Party 8A Technical Committee 80 of the International Electrotechnical Commission (IEC) has begun work on developing standard establishes requirements for the performance, technical data, performance characteristics and test procedures for transponder AIS ( Document TC80/WG8-U.AIS) .
 
This standard will be developed as soon as possible ( working group meetings are held about four times a year) . It is expected that the working group will complete its work by the end of 2000 - beginning of 2001 . The current standard for transponder AIS, using technology DSC, - a standard IEC 61993-1.
Devices made only standard MSC.74 ( 69), will not necessarily provide the opportunity to interact with the devices using equipment DSC. Although Recommendation ITU-R M.1371 and define the technology used for systems and devices made by this standard must be able to interact, there is no guarantee interoperability without further testing .

 


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