Automatic Identification System - Final Version (1)

RADIONAVIGATION SYSTEMS

AIS: AUTOMATIC IDENTIFICATION

SYSTEM FOR VESSELS Authors:

Antonio Villavicencio

Juan Pablo Lasso

Abstract: This document is devoted to describe the Automatic Identification System and its

operation mode

Keyword list: AIS, radar, SOTDMA, ITDMA, CSTDMA, VHF, vessel, DSC, collision avoidance,

ARPA, ESDIS, IMO, ITU-R, COG, ROT, ITDMA, FTDMA, RTDMA, GMSK -FM, BTS, GNSS, DGNSS,

SOLAS.

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Automatic Identification System Juan Pablo Lasso Antonio Villavicencio

TABLE OF CONTENTS

Executive Summary...................................................................................................................................... 3 1. Introduction................................................................................................................................................ 4 2. Operation of AIS........................................................................................................................................ 7 2.1. Transmission of Data............................................................................................................ 7 2.2. Frequency Channels and Modulation............................................................................ 9 2.3. Data Link Capacity................................................................................................................. 9 2.4. Channel Management......................................................................................................... 10 2.5. Update Rates.......................................................................................................................... 10 2.6. Class A and Class B Ship-Borne Mobile Equipment.............................................. 11 2.6.1. Class A Ship-Borne Mobile Equipment..................................................... 11 2.6.2. Class B Ship-Borne Mobile Equipment..................................................... 13 2.7. Data Types.............................................................................................................. ................ 14 2.7.1. Static Data............................................................................................................. 14 2.7.2. Dynamic Data...................................................................................................... 15 2.7.3. Voyage Related Data........................................................................................ 15 2.8. Display Requirements....................................................................................................... 15 3. Application and Modes........................................................................................................................ 17 3.1. Collision Avoidance............................................................................................................. 17 3.2. Marine Search and Rescue............................................................................................... 18 3.3. Heading................................................................................................................. ................... 18 3.4. Path Prediction..................................................................................................................... 19 3.5. Route Planning..................................................................................................................... 19 3.6. Short Message Communication..................................................................................... 19 3.7. Ship-to-Ship-Mode for Collision Avoidance............................................................. 20 3.8. Ship to Shore Mode............................................................................................................. 20 3.9. Ship to Shore Mode Integrated with VTS.................................................................. 21 4. Benefits and Drawbacks of AIS........................................................................................................ 22 4.1. Benefits................................................................................................................ .................... 22 4.2. Drawbacks.............................................................................................................................. 23 5. Bibliography......................................................................................................... ................................... 25 Annex 1................................................................................................................................. .......................... 26

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Executive Summary

This document presents a description of the Automatic Identification System (AIS).

The introduction defines the general idea about why this system is important for

marine navigation and specially for collision avoidance. Secondly, the operation and

protocols about how the system works are proposed. Then, the applications and

modes of the AIS mode are explained. Finally, a series of equipment are presented in

order to observe the devices and their features.

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1. Introduction

Controlling the maritime traffic is a key element to prevent accidents in the nautical environment. A significant part of maritime activities relates to the movement of vessels. Moreover, accidents caused by commercial vessels are extremely challenging to remedy. Namely, larger ships are easier to see but require more time to change speed or track. Besides, when many ships are in a small area, an ambiguous action taken to avoid a collision may endanger other ships. For instance, an impact on the sea leading to an oil spill incident, no matter how small, may affect the lives of hundreds of people. That is why, the dreadful shipwrecks of Exxon Valdez in 1989, the Aegean Sea in 1992 and the Braer in 1993 brought forward the discussion of the marine safety and how important it is to prevent any maritime disaster. Namely, it has been argued, that most of shipwrecks and disasters are linked to human errors and the difficulty to locate on a radar a particular vessel when several targets are being tracked, especially when the visibility conditions are reduced. The occurrence of these incidents yielded in the improvement of navigation technology. The radar has been the traditional means to prevent marine collisions under restricted visibility conditions. However, there is a strong necessity to use a plotted ship’s course and speed through water to be able to obtain an actual heading as accurate as possible in order to avoid any collision. Therefore, a growing demand for technically advanced devices aiming the goal to provide always better performance in terms of navigation prediction and speed in the information distribution is soaring. One of the main improvements in navigation technology is the Automatic Identification System (AIS), which is a technical system which allows to monitor vessels from other vessels, avoiding the problem of confusing ships. AIS was developed as a collision prevention instrument to enable commercial vessels to notice each other more clearly. AIS does this by constantly broadcasting: vessel identity, position, speed and course along with other appropriate information to all other AIS equipped vessels and base stations on the shore. Not only can the position of a ship be plotted in real time, the units also transmit data like Course Over Ground (COG) or Rate of Turn (ROT). One of the key features of AIS is the provision of detailed traffic information in real time due to its operation on VHF radio frequency, detecting other equipped targets in situations where the radar detection is limited such as around bends, behind hills, and in conditions of restricted visibility by fog, rain, etc. AIS exchanges data regarding navigational and voyage related information of ships and other related messages with other ships and shore stations. The exchange of information is received and then used to create a real-time graphical display of traffic in the area. A transponder can be attached to many types of chart plotters or a PC charting software to spread over the surface vessel positions on the chart. It can similarly be overlaid on many radar screens. In the simplest application, target information is displayed on a text screen.

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The IMO (International Maritime Organisation) is a specialized agency of the United Nations in charge of improving the safety and security of international shipping and to prevent marine pollution from ships. In 2000, IMO approved the requirement for all ships to implement AIS with the following goals:

Improving maritime safety and efficiency of navigation Protecting the maritime environment and life at sea

IMO approved a regulation which became effective by the beginning of 2005 requiring AIS to be fitted aboard on all ships of more than 300 gross tonnage covering international voyages, cargo ships of more than 500 gross tonnage not engaged on international voyages and all passenger ships irrespective of size. Ships fitted with AIS should maintain AIS in operation at all times except where international agreements, rules or standards provide for the protection of navigational information. AIS employs Self-Organizing Time Division Multiple Access (SOTDMA) technology to meet this high broadcast rate and ensure stable and reliable ship-to-ship and ship-to-shore operation. Besides, It can handle multiple reports at different update rates, which vary according to speed and status of the ships. The information can be received by anyone equipped with a relatively low cost receiver. The AIS information consists of different types of data classified as static, dynamic, voyage related information, and short safety messages. Static data are entered into the AIS on installation and need to be changed only if the ship type changes by a major conversion or if her name or call sign changes. AIS uses digital VHF signals to transmit its packets' information. The range of the system is similar to VHF radios. These VHF radio signals can be picked up in recondite corners, over islands and through heavy raining conditions, giving better coverage than RADAR in some conditions or enhancing a RADAR picture when used together. AIS systems are one of the most flexible instruments in the maritime area. Ships, shore stations, and even aids to navigation can transmit and receive a wide variety of information. Furthermore, these systems have ability to expand as new technologies come out. By using a binary data stream it is possible to encode almost any type of information. In addition, the data exchange is entirely automatic and transparent to the users. So, the result is a remarkable enhancement in situational awareness for the people who control and operate the instrument on board.

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One of the first uses of AIS binary data was the transmission of lock status along the Saint Lawrence Seaway in North America. The Panama Canal also uses AIS data to transmit wind information to ships as they make passage.

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2. Operation of AIS

The operation of AIS is defined by several standards, including the International Telecommunications Union (ITU) recommendation ITU-R M. 1371-1, named "Technical Characteristics for a Ship-Borne Automatic Identification System(AIS) Using Time Division Multiple Access in the Maritime Mobile Band", approved in November 1998. It specifies technical communications characteristics concerned in the operation of AIS, including: transceiver characteristics, modulation, the type of TDMA used, channel management, data format, messages, and packaging.

2.1. Transmission of Data

The main objective of AIS is to take information from the sensors installed on different vessels and exchange it automatically. The information can be transmitted between vessels or between a vessel and a land station in a shore. Exchange of information between ships is made in order to increase awareness of other ships during navigation, in order to avoid collisions. On the other hand, when information is sent to a shore station, traffic management can be improved, especially in situations of congestion (near a shore, for example). Also, there are certain reporting areas where the system is used. In order to process the information from each vessel, AIS stations use a technology called Self Organizing Time Division Multiple Access, operating in an autonomous and continuous manner. Other types of TDMA area also supported, including Random TDMA, Incremental TDMA, Carrier-Sense Time Division Multiple Access CSTDMA, and Fixed TDMA protocols. Their purposes are:

RTDMA is used to gain access to radio resources and allocate a slot

randomly. It may also be used when a faster update rate needs to be used for the transfer of some data, when starting a course change, for example.

ITDMA is used to allocate slots in advance within one minute and to prepare for SOTDMA slot mapping.

SOTDMA is used to allocate slots from three to seven frames ahead, which means that an AIS station will have from three to seven chances to get slot allocation. This is the type of TDMA that is applied most of the time in AIS.

FTDMA is only used by AIS shore stations. CSTDMA is used in Class B transponders. Here, the transmitter first

listens to the channel to make sure that it is not being occupied, and that there are free slots to use. This is employed in order for Class B

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transponders not to interfere with communications started by Class A ones, which should have more priority in the usage of the VHF channels.

The system must provide transmissions in a short range of distance, while having an increased data rate, and a certain robustness to prevent the messages from suffering interference. In this manner, two VHF frequency channels are utilized, which are inside the maritime mobile band. These channels may be used in parallel. The time multiplexing of these channels allows for 2250 slots to be allocated within a period of one minute. The combination of these slots will form one frame per minute. This means that each slot will have a duration of 26.6 milliseconds. A position report message from an AIS station fits in one of these slots. Every AIS station is responsible for determining its own transmission schedule, based on the knowledge of future actions of other stations and data link traffic history. The synchronization of this time structure is achieved using Global Navigation Satellite System (GNSS) time information as a first phase time mechanism. For this, AIS transponders include one or more GPS/DGPS receivers that provide both timing and positional information. Thus, GPS plays a key role in AIS. Nevertheless, the system counts with a secondary time mechanism that may provide an accuracy of at least 10 microseconds. Figure 1 below shows the slot allocation mechanism used in AIS.

Figure 1. Slot allocation using TDMA in AIS

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2.2. Frequency Channels and Modulation

Two frequency channels from the maritime mobile band were designated by ITU-R for their utilization in AIS, in 1997. These channels are now called AIS 1-87B and AIS 2-88B. The first one operates at 161.975 MHz, while the second one is allocated at 162.025 MHz. The use of two channels was made in order to increase the capacity of the system and to reduce interference due to possible high amounts of simultaneous transmissions from a big number of ships located close to each other. It is worth mentioning that for regions where channels 87-B and 88-B were already occupied, regional frequency channels for AIS may be utilized. Given the channel management techniques of AIS, all channel shifts are transparent to the user of the system. The modulation method employed in AIS is Frequency Modulation/Gaussian Minimum Shift Key (FM(GMSK). It was chosen because it is robust, its discrimination possibilities, its bandwidth efficiency, and the fact that it has been widely used in mobile communications.

2.3. Data Link Capacity

Simplex channels are used with bandwidths of 12.5 KHz or 25 KHz. At either bandwidth, the capacity reached is of 2250 slots per minute with a transmission rate of 9.6 kbps. Now, when both the AIS 1 and the AIS 2 channel are used, capacity will double to 4500 slots per minute. If a channel comes close to achieve capacity, due to a big load of traffic, the SOTDMA technology and the GMSK/FM modulation will reduce the size of the coverage area of the radio cell. In this manner, the AIS station installed on the ship will receive reports and data from less stations around it, and the traffic will be reduced by this suppression of transmissions from and to AIS stations that are farther away. However, priority is given to closer stations, which are more important to the navigational interests of the local AIS station. It is important to notice that the communication range is reduced when using 12.5 KHz channels. The size of a radio cell is reduced to one half the size of that a 25 KHz cell when there is an overloaded situation. Finally, as the systems operates in the VHF band, the communication will be optimal in the presence of line of sight between antennas in different stations.

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2.4. Channel Management

Since the two frequencies in the VHF band were assigned for AIS, and also an alternative 12.5 KHz narrowband was defined for use in regions where lack of channel availability exists, Digital Selective Calling (DSC) is employed in order to support automatic frequency channel switching and to make full use of the frequency band. Channel switching occurs when shore stations switch ship stations to VTS/AIS working frequencies or regional frequencies. The frequency switching mechanism may be done automatically by shore base stations, or manually by an AIS operator on the ship. The automatic switching from shore may be executed by a Vessel Traffic Services (VTS) base station using SOTDMA or by a GMDSS A1 area station using DSC.

2.5. Update Rates

The update rates of the system were defined based on worst-case scenarios where the peak traffic of information exchange between vessels and shore stations is high. It was estimated theoretically that within a 40NM area of VHF radio coverage, about 3000 reports per minute could be transmitted. In practice, around 2000 reports per minute were observed and chosen as a maximum rate. Given this requirement, update rates were established for both Class A and Class B ship-borne mobile equipment. Class A equipment is more complete than Class B, and must be in full compliance with AIS performance standards. The following two tables reflect the updating intervals for both cases. It can be seen that for course changes, the rate is increased to be three times faster than standard:

Ship Condition Reporting Interval

Ship at anchor or moored and moving slower than 3 knots 3 minutes Ship at anchor or moored and moving faster than 3 knots 10 seconds Ship moving at 0-14 knots 10 seconds Ship moving at 0-14 knots and changing course 3.33 seconds Ship moving at 14-23 knots 6 seconds Ship moving at 14-23 knots and changing course 2 seconds Ship moving at more than 23 knots 2 seconds Ship moving at more than 23 knots and changing course 2 seconds

Table 1. Update Intervals for Class A Ship-Borne Mobile Equipment

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Ship Condition Reporting Interval Ship moving slower than 2 knots 3 minutes Ship moving at 2-14 knots 30 seconds Ship moving at 14-23 knots 15 seconds Ship moving at more than 23 knots 5 seconds Search and Rescue Aircraft (airborne mobile equipment) 10 seconds Aids to Navigation 3 minutes AIS Base Station 10 seconds

Table 2. Update Intervals for Class B Ship-Borne Mobile Equipment

2.6. Class A and Class B Ship-Borne Mobile Equipment

According to the ITU-R standard for AIS, mobile equipment may be defined as Class A or Class B. As mentioned before, Class A equipment is fully compliant with IMO AIS carriage requirements, and Class B equipment provides features good enough to perform compatibly and satisfactorily in the system., even though it does not fully comply to IMO requirements.

2.6.1. Class A Ship-Borne Mobile Equipment

Class A transceivers are fully compliant to IMO standards and the Safety of Life at Sea (SOLAS) convention. They include a VHF transmitter of 12.5 Watts of power, and can reach a coverage range of 20 to 40 miles (32 to 64 kilometers), which depends on antenna height. Also, a dual channel VHF, SOTDMA receiver and either a built-in GPS or an external GPS port are included. This is a connection to GNSS/DGNSS equipment and other sensors that provide navigational and synchronization information for the ship. The interface between sensors and the AIS equipment is defined in the IEC 61162 series standards. In case of failure of the connection to GNSS, there is a back-up internal synchronization mechanism. In addition, they have an integrated display, in order to present a diagram showing the information of other ships, and its own, to authorized personnel aboard. Finally, they support Digital Selective Channel (DSC) for channel switching. A block diagram of a Class A ship-borne mobile transceiver is shown in Figure 2:

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Figure 2. Block Diagram of a Class A Ship-Borne Mobile Transceiver

A Class A transceiver deals with all AIS information, without omitting any parameters. Information broadcast includes:

MMSI number: An unique reference identification number for

every ship. Navigation Status: Which may be "at anchor", "under way

using engine" or "not under command". Rate of Turn: Ranges from 0 to 720 degrees/minute, right or

left. Speed Over Ground (SOG): Ranges from 0 to 102 knots, with a

resolution of 1/10 knots. Position Accuracy: An estimate of the error given in current

position. Latitude and Longitude: with a resolution of 1/10000

minutes. Course Over Ground COG): Relative to true north, with a

resolution of 1/10 degrees. True Heading: 0 to 359 degrees derived from gyrocompass

input. Time Stamp: The time (according to universal time, UTC) this

information is generated, in hours, minutes, seconds.

The information above is transmitted every 2 to 10 seconds while the ship is moving, and every 3 minutes at anchor, as defined in tables 1 and 2.

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Additionally, the following information is transmitted every 6 minutes:

MMSI Number: An unique reference identification number for

every ship. IMO Number: Unique reference identification, related to ship's

construction. Radio Call Sign: International call sign assigned to the ship,

used in voice radio. Name: Name of the ship, which may be up to 20 characters long. Type of Ship/Cargo: Data about the kind of load in the ship,

according to certain defined tables. Type of Position Fixing Device: GPS, DGPS, or others. Location on ship where reference point for position reports is

located. Draught of Ship: Ranges from 0.1 to 25.5 meters. It determines

the vertical length between the bottom of the ship and the waterline (how much the ship is sunk in the water).

Destination: Name of the destination where the ship is headed. It may be as long as 20 characters.

Estimated Time of Arrival at Destination: Month, day, hour, and minute, synchronized to UTC time.

2.6.2. Class B Ship-Borne Mobile Equipment

As seen before, this type of equipment provides functionalities that are not fully compliant with IMO AIS carriage requirements, and more specifically, with the guidelines established for ships in the Safety of Life as Sea (SOLAS) convention. Usually, this equipment is used in small commercial ships or pleasure ships (non-SOLAS vessels). The main differences with Class A ship-borne mobile equipment are:

Reporting rates are smaller than those of Class A, as seen in the

tables presented in the previous section of this chapter. IT does not transmit the vessel's IMO number, nor the Radio

Call Sign. It does not transmit the Estimated Time of Arrival at

Destination, nor the destination. It must only to receive, but not necessarily transmit safety text

messages. It is required to receive application identifiers (binary

messages), but not to transmit them. It does not transmit the Rate of Turn of the ship.

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It does not transmit the Draught of the Ship. Transmission power is restricted to 2 Watts, providing a

coverage of 5 to 10 miles (8 to 16 Km). In this case, it is forbidden to take external GPS data. Therefore,

it is a requirement for them to have a built-in GPS. They may use CSTDMA instead of SOTDMA. As it may be obvious, they are less costly.

2.7. Data Types

The data involved in AIS is used in different messages that will provide the necessary information for navigation to the vessel's personnel, and to those in other vessels or at shore. This data is classified in three different types: static, dynamic, and voyage related. Static and Voyage Related data are transmitted every 6 minutes, while Dynamic data is transmitted according to tables 1 and 2. In addition to these three types of data, short safety related messages can be transmitted. They are either addressed to an specific ship (using its MMSI), or broadcast to all ships in an area containing an AIS station. They can include up to 160 ASCII characters (6-bit long characters). Their content is relevant to the safety of navigation, like to inform the presence of an iceberg in an area.

2.7.1. Static Data

Static information is entered into the AIS station upon installation. It only changes in case the ship changes its name or goes through an important transformation that makes it change from one ship type to another. Static data includes:

IMO Number (where available). MMSI Radio Call Sign. Name of the Ship. Length and Beam. Type of Ship/Cargo. Location on ship where reference point for position reports is

located (location of position-fixing antenna on the ship).

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2.7.2. Dynamic Data

Dynamic information is automatically updated taking the output data provided by the ship' sensors connected to AIS. Dynamic Data includes:

Ship's position (latitude and longitude)with accuracy indication

and integrity status. It is determined from a GPS source. Time stamp (in UTC). It is determined from a GPS source. Course Over Ground (COG). It is determined from a GPS source. Speed Over Ground (SOG). It is determined from a GPS source. True Heading Navigation Status ("at anchor", "not under command", "under

way using engine"). Rate of Turn

2.7.3. Voyage Related Data

Voyage related information is manually entered by the user, through a password-protected routine, and its updated along the trip. This type data includes: Draught of Ship Hazardous Cargo/Type of Cargo Destination Estimated Time of Arrival at Destination Route plan (waypoints). Optional transmission, by request of a

competent authority. Number of persons aboard. Optional transmission, by request of

a competent authority.

2.8. Display Requirements

Once all AIS information is gathered, it must be made available for an officer of the watch or authorized personnel of a ship at sea to see it and use it at their best interest. Information is presented either in a graphic or a text format. In order not to overload an AIS equipment's screen, the following information has been made mandatory to be displayed, according to the AIS standards:

Position (Longitude, Latitude) Course Over Ground (COG) Speed Over Ground (SOG)

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True heading Rate of Turn (if turning)

The information displayed classifies other vessels as "targets", and according to their distance and direction from the ship. This classification sees targets as: sleeping target, activated target, selected target, dangerous target, and lost target. The targets of interest are selected and dangerous targets, because they represent those that will have a risk of collision with the ship. For these targets, a Closest Point of Approach (CPA), and a Time of Closest Point of Approach (TCPA) are calculated. CPA and TCPA limits are set uniformly for all the sensors in the ship. If a CPA or TCPA calculation surpasses these thresholds, a dangerous target symbol is displayed, or an alarm is triggered. Different standard symbols are defined in AIS standards in order to identify targets and other parameters relevant to navigation. The actual display of information may be done using one of the following options: A display integrated in the transceiver Electronic Chart and Display Information System (ECDIS) Automatic Radar Plotting Aid (ARPA) Electronic Chart Display (ECS) In a laptop.

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3. Application and Modes As explained before, AIS is a vessel data broadcast system that can be used with multiple applications and modes. Particularly, a vessel will repeatedly send out its ID, position, course, speed and other data already clarified to all other nearby ships and to coastal authorities on a common VHF radio channel. The main applications of AIS can be described as the following:

Collision avoidance Marine Search and Rescue (SAR) Heading Path prediction Route planning Short message communication

3.1 Collision Avoidance

A study of the German Marine Board states that most part of the radar assisted collisions are produced by the lack of information under limited visibility conditions, the study also asserts that these incidents could have been evaded if the sailors in command had received specific dynamic information (position, heading, speed and rate) of the other vessels involved. AIS is before anything else an anti-collision system. Using AIS, the data can be transmitted at a high update rate and some of the natural limitations of radars could be solved. These two conditions make of AIS a system to avoid collision because the navigational status of another ship can be done really swiftly. For instance, in the case of two ships equipped with AIS meeting each other with restricted visibility conditions, they will have all the information necessary to stay away from a crash, including name and port of destination. In addition, AIS is an important enhancement to existing navigational systems, including radar. It would be reasonable to anticipate that, in regard to potential collision situations, near real time target information transfer, including course and speed being made good, results in improved decision-making and a corresponding reduction in human error.

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3.2 Marine Search and Rescue (SAR)

Under the conditions of a vessel in trouble, marine search and rescue procedures are much more efficient if all the rescue units are equipped with AIS to quickly identify the ship closest in a distress situation. During a search operation all the crafts could be tracked and plotted enabling the Marine Rescue Coordination Centres (MRCC) to monitor the progress, to direct the available resources efficiently and to ensure a search coverage without gaps. The actual emergency status of the ship could also be determined automatically. A Man Over Board (MOB) alarm will most probably be one of the features allowing all other ships in vicinity to have the same MOB position displayed, when one of the ships has activated the MOB alarm.

3.3 Heading

Heading or shore based pilotage is a method that enables ships to be directed from the shore. So, the captain's ship with no radar or a radar disturbed by difficult weather conditions and with a very weak radar return from shore and navigational aids, may today be assisted with the navigation if the capitan is able to obtain navigational assistance from shore. Shore based pilotage with radar is normally limited to small and moderately sized ships which generally are more maneuverable than, for example, huge tankers. The limitations of radars, makes shore based pilotage suitable only in areas where the navigable water is sufficient to separate the traffic. If only the Differential Global Positioning System position from a ship, but no heading is transmitted via the AIS, the VTS operator will get an exact position of the ship’s antenna at the moment, as well as the speed and course over ground for the same antenna. The ship's heading and the position of other parts of the same ship, at the same moment is, however, unknown. This uncertainty of the real situation could lead to mistakes of importance. For instance, a 350 meter long tanker with the antenna situated on top of the wheelhouse at the stern, starts to turn to starboard. The ship’s course will then change to starboard, but initially the stern with its antenna will swing slightly to port, as the ship is turning around its so called pivot point.

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There is obviously, besides the need to determine and transmit the exact position of the antenna, a need to include the heading in the transmission. The ARPA radar tracks the part of the ship that gives the best radar return, which normally on a loaded tanker is the superstructure at the stern. A big tanker, turning with the superstructure at the stern tracked by radar, could have turned 30-50 degrees before this is detected by the ARPA radar on another ship or at the VTS, and now up to 4-5 minutes have passed since the turn started.

3.4 Path Prediction

The large potential of transmitting the rate of turn of the ship, together with all the other movement parameters, is still not fully recognized. This enables both the VTS and approaching ships to make a rather accurate prediction of the path a ship is taking, some 30 to 90 seconds ahead. This gives more time and better information for all the other players in the traffic environment to plan their moves.

3.5 Route Planning

Shipping routes within a VTS area are normally well defined, including alternative routes between two points, to the point that they could be indexed. One could anticipate a requirement to plan the route of a ship in advance, either by the Officers of the Watch (OOW) or supplied by the VTS. In either case, the plan consisting of the appropriate index numbers could be interchanged utilizing AIS. This would enable interested parties to compare the planned route of the ship with the actual path taken.

3.6 Short Message Communication

The limitation of any radio system with all users on the same channel is the capacity. This is true both for voice and data communication. The proposed AIS would make it possible to use a short message scheme, i.e. to produce an indexed list of predetermined traffic messages. There could be addressed (point-to-point) messages and broadcast messages. The actual transmitted message is constituted of the index number only, together with the eventual attribute. If a common list of messages could be agreed upon, it should be multilingual, thereby avoiding misunderstandings because of language difficulties.

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Furthermore, AIS can operate in one of the following modes: Ship-to-ship mode for collision avoidance. Ship-to-shore mode to obtain information about a ship and its cargo. Ship-to-shore mode as a traffic management tool when integrated with a

Vessel Traffic System (VTS) ship-to-ship data exchange.

3.7 Ship to Ship Mode for Collision Avoidance

The main goal of AIS is for the data to be broadcast from ship to ship in order to avoid any collision. So, the principal operating mode for AIS will be the independent ship-to-ship reporting. In this mode, each ship transmits its data to all other AIS-equipped ships within VHF range. The unique communications scheme permits these data transmissions to take place independently without the need for a master control station. Each ship transmits its data to all other AIS-equipped ships within VHF range. Position and other data are fed automatically from the ship’s sensors into the AIS system, where the data is set-up and transmitted in a short data burst on one of two global VHF channels. When received by other ships, the data is decoded and displayed to the OOW, who can view AIS reports from all other AIS-equipped ships within range. AIS messages are automatically transmitted every few seconds, to keep the information up to date.

3.8 Ship to Shore Mode

The second most important mode of AIS is ship to shore mode, which allows to have vessel information on the coast. In coastal waters, shore side authorities may establish automated AIS stations to monitor the movement of vessels through the area. These stations may simply monitor AIS transmissions from travelling ships, or may actively poll vessels via the AIS channels, requesting data such as identification, destination, type of cargo and other information. Coast stations can also use the AIS channels for shore-to-ship transmissions, to send information on tides, notices to mariners and local weather forecasts. Multiple AIS coast stations and repeaters may be tied together into Wide Area Networks (WAN) for extended coverage. In very busy areas, as harbors, rivers and archipelagos, the need for a high update rate 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.

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Coastal nations may use AIS to monitor the movement of hazardous cargoes and control commercial fishing operations in their territorial waters. AIS data can be logged automatically for playback in investigating an accident, oil spill or other event. AIS will also be a useful tool in search and rescue (SAR) operations, allowing SAR coordinators to monitor the movements of all surface ships, aircraft and helicopters involved in the rescue effort.

3.9 Ship-To-Shore Mode Integrated With VTS

When integrated with shore-based vessel traffic systems (VTS), AIS provides a powerful tool for monitoring and controlling the movement of vessels through restricted harbors and waterways. The AIS can augment traditional radar-based VTS installations, providing an AIS “overlay” on the radar picture. It can also provide a cost-effective alternative in areas where it is not feasible to establish radar-based systems. When integrated with radar, the AIS can ensure continuous coverage, even when the radar picture is degraded by heavy precipitation or other interference. The AIS channels can be used to transmit port data, pilotage, berth assignments, shipping agency information, tides and currents, notices to mariners and other information from shore to ship, as well as ship-to-ship and ship-to-shore AIS reports. It is also possible for the VTS to broadcast the complete harbor picture to all ships in the area, so the masters and pilots all share the same “big picture.” The VTS center can assume control over the assignment of timeslots for AIS messages to ensure optimum data exchange within the coverage area. Special dedicated channels may be designated for local-area AIS operations. The ship-board AIS equipment will have the ability to shift to different channels automatically when directed by the shore side VTS controller.

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4. Benefits and Drawbacks of AIS

AIS displays a graphic screen showing the vessels around and shows warnings if the closest point of approach of any of those targets is too close. Also, this system is very much alike to a radar with an ARPA (Automatic Radar Plotting Aid) function. AIS has several distinct pros and cons in comparison to RADAR with ARPA:

4.1. Benefits

By far the most important advantage of AIS is unambiguous identification of other vessels, AIS system shows the MMSI number and description (and the call sign for Class A AIS systems). This allows users to easily establish VHF voice contact (by name or call sign) or to initiate a DSC VHF communication, facilitating discussion of joint actions, rather than having to call “big ship near the Bay Bridge” and hoping the right vessel answers.

The ability to know the ship’s MMSI number is valuable because it allows you to send a DSC message directly to them that will ring an alarm in the deckhouse and possibly be noticed by the OOW even if the VHF volume is turned down.

The AIS system provides real time information about any changes in the acceleration , rate of turn or heading of other vessels

The AIS system detects better through rain squalls, over islands and around corners than recreational radar systems.

The AIS system is aware of vessels that might otherwise be hidden in another vessel’s radar shadow

The AIS system expends less power than a radar system. But the difference is not so big. A 2.2kW (small) radar will draw about 3.5 amps (at 12 V) and a 4kW unit, about 4 amps. On the other hand, a class B unit will draw about 2 amps. Most radars also have watch functions which scan only every 30 or 60 seconds, further reducing the energy gap.

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4.2. Drawbacks The most drastic drawback is that AIS only shows other targets that

have functional AIS systems. This means that sailors on board do not observe all the other traffic. For example, pleasure vessels, small fishing vessels, commercial vessels with failed AIS (really uncommon), foreign flagged vessels offshore not in compliance with the IMO requirements, large weather buoys, long line marker buoys, among others, do not use AIS. Thus AIS is in no way a replacement or a substitute for a visual deck watch and, in low visibility, a radar watch. AIS can only be viewed as a supplement to other watch keeping. This is easy to say, but poses a difficult practical problem because modern AIS and plotter displays can be mesmerizing. They look very ‘real’, much more real than a RADAR screen.

AIS was developed for commercial ship use. There is some concern that

if thousands of pleasure vessels start transmitting AIS signals it will clutter the system to the point that it will be less useful. Near very busy harbors there can be so many targets (mostly boats at anchor or on docks) that it can be difficult to see if one is coming out the entrance without zooming way in. Recreational vessel should definitely turn their AIS off when they are not navigating.

AIS is a relatively complex system. It is mechanically simpler than the

moving parts of a radar, but with AIS, users are depending on the other vessel having properly maintained, interfaced and functional equipment. For the system to function you must have at least one AIS transceiver and one receiver working on two different vessels. Each transceiver must have a functional multi-channel digital VHF and GPS. As the system has evolved, there have been both interfacing and compatibility problems. These will hopefully be minimized as the system matures.

The power requirements, while much lower than for full time RADAR,

will still be significant for a sailboat. Furuno’s specs call for 7.3 amps (at 12 V) for a Class A, 2 amps for a Class B, and 1.2 amps for a receive only. Running a Class B unit for 24 hours will require 50amp-hrs, which could easily be a 33% increase in power usage for a simple boat. And that power does not include the draw of the screen (plotter/PC or radar) necessary to display the AIS data, which could be from 1.1 amps (for a small 6” screen) to 3 amps (for a 15” screen).

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Data received is only as good as the data entered into the AIS. To ensure that correct AIS information is broadcast to other vessels and shore authorities, mariners are reminded to enter current voyage related data such as draught, type of hazardous cargo, destination and ETA properly at the beginning of each voyage and whenever changes occur.

The AIS unit may not, in all cases, be installed in accordance with the

IMO Guidelines. This can result in poor performance and erroneous transmissions.

The OOW should always be aware that AIS fitted on other ships as a mandatory carriage requirement may, under certain circumstances, be switched off, particularly where international agreements, rules or standards provide for the protection of navigational information.

AIS is subject to the vagaries and limitations of VHF-FM propagation.

Mariners should be aware that the accuracy of AIS positional information is the accuracy of the Electronic Position-Fixing Device (EPFD) connected. For example, LORAN C can be used, but it will typically have a far lower accuracy than GNSS.

Mariners are reminded to periodically check that correct information is being broadcast by their own vessel, particularly position, heading (provided by the ship's master gyro) and speed.

The mariner must always remember that AIS is just one of the several tools available to watch-keepers, to fulfill their obligations under the Collision Regulations.

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5. Bibliography

[1] S. J. Chang, "Use of the Automatic Identification System (AIS) on Autonomous Weather Buoys for Maritime Domain Awareness Applications", September, 2006.

[2] S. J. Chang, "Development and Analysis of AIS Applications as an Efficient Tool for Vessel Traffic Service" , November, 2004.

[3] Transportation Research Board of the National academies , "Shipboard

Automatic Identification System Displays: Meeting the Needs of Marine", September 2004.

[4] Leica Geosystems Inc, "The Complete Guide to Automatic Identification

Systems", 2001, www.siitech.com/files/ais/ais_guide.pdf.

[5] AISM, "IALA Guidelines on the Universal Automatic Identification System" Volume 1, Part I – Operational Issues. Edition 1.1 Systems, December 2006.

[6] Swedish Maritime Administration, "AIS for ships in the future", February 2004 2008, www.sjofartsverket.se/upload/1486/a171_2.pdf

[7] Marine Ship and Supplies, "Universal Shipborne Automatic Identification System (AIS), December 2003, http://www.mss-marine.com/contactus.html

http://www.sjofartsverket.se/upload/1486/a171_2.pdf

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ANNEX 1

AIS SHIP-BORNE MOBILE EQUIPMENT

PRODUCT DATASHEETS

Documents

Automatic Identification System - Final Version (1)