How AIS Works_04-A5

7/27/2019 How AIS Works_04-A5

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Contents

PREFACE ................................................................................................................... 4

1. HOW AIS WORKS: AN INTRODUCTION ................................................................. 6

2. AIS COMPONENTS .............................................................................................. 11

3. THE AIS MESSAGE SET......................................................................................... 16

ADDITIONAL AIS MESSAGES ...........................................................................................20 Aids to Navigation message ...............................................................................20 Class B message ..................................................................................................21 Search And Rescue aircraft message ..................................................................21

4. AIS AS PART OF A SYSTEM .................................................................................. 22

5. AIS NETWORK FUNCTIONALITY .......................................................................... 26

AIS BASE STATION / AIS SHORE STATION ..........................................................................28 AIS Physical Shore Station reliability...................................................................31

AN INTERCONNECTION OR TELECOMMUNICATIONS NETWORK ...............................................31 Cost .....................................................................................................................31 Reliability ............................................................................................................32 Topology .............................................................................................................32

AIS NETWORK APPLICATION ...........................................................................................34 THE HUMAN MACHINE INTERFACE ..................................................................................35 INTERFACE TO EXTERNAL APPLICATIONS ............................................................................39

6. AIS NETWORK LOAD CAPACITY ........................................................................... 41

VDL MANAGEMENT......................................................................................................41 SLOT REUSE.................................................................................................................42 SLOT RESERVATIONS .....................................................................................................42

7. AIS NETWORK APPLICATIONS ............................................................................. 43

PORT AUTHORITIES .......................................................................................................43 COASTAL AUTHORITIES ..................................................................................................44 STRATEGIC INSTALLATIONS .............................................................................................45 AIDS TO NAVIGATION AUTHORITIES .................................................................................46




8. LONG-RANGE AIS ................................................................................................ 48

9. AIS NETWORK SECURITY ..................................................................................... 50

10. LOGICAL SHORE STATIONS ................................................................................ 51

APPENDIX A: SELECTED AIS SPECIFICATIONS .......................................................... 53

APPENDIX B: SELECTED INTERNET LINKS ................................................................ 54

NOTES .................................................................................................................... 55




Prefacehe maritime Automatic Identification System or AIS is noted as being a

mandatory carriage item by all Safety Of Life At Sea [SOLAS] vessels as of 31st December 2004. Aids to Navigation, AIS units, AIS base stations and AIS

for leisure craft have been added to the available product range.

We at IMIS Global felt that AIS, the way it works and how to use it, needs to

be explained to Operators, Administrators and Support staff in an easy to

understand and simple manner without the technical jargon, unexplained

acronyms and potentially confusing detail.

This booklet is therefore aimed at those who want to know the basics and

want to understand the principles but don’t need to know all the technical

specification details.

For anyone that wants to become deeply involved in the AIS technology, the

list of specifications that can be used as a starting point is provided in

Appendix A.

For the rest of us, we hope that this booklet provides an easy to understand

introduction to AIS and in particular, AIS networks.

Enjoy!

Version 4.1 of 2010

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SOLAS Chapter V, Regulation 19, section 2.4.5 states:

"AIS shall:

• provide automatically to appropriate equipped shore stations, other ships

and aircraft information, including ship's identity, type, position, course,

speed, navigational status and other safety-related information;

• receive automatically such information from similarly fitted ships;

• monitor and track ships; and

• exchange data with shore-based facilities."

The International Maritime Organisation [IMO] Performance Standards for

AIS state:

"The AIS should improve the safety of navigation by assisting in the efficient

navigation of ships, protection of the environment, and operation of Vessel

Traffic Services (VTS), by satisfying the following functional requirements:

• in a ship-to-ship mode for collision avoidance;• as a means for littoral States to obtain information about a ship and its

cargo; and

• as a VTS tool, i.e. ship-to-shore (traffic management)

• The AIS should be capable of providing to ships and to competent

authorities, information from the ship, automatically and with the required

accuracy and frequency, to facilitate accurate tracking.

• Transmission of the data should be with the minimum involvement of ship's

personnel and with a high level of availability."

Quoted from the IALA web site




1. How AIS works: An introductionhe Automatic Identification System [AIS] transponder is a specialised data

radio modem installed on all Safety Of Life At Sea [SOLAS] vessels from

July 2002 onwards with the last SOLAS vessel being installed on the 31stDecember 2004.

This data radio modem (the AIS terminal / transponder) transmits a number

of predefined messages (25 messages in total at this time) that contain data

collected from ships’ sensors such as gyrocompass, GPS and heading

indicator.

This data is compressed and is broadcast in very small data packets to anyand all vessels within the coverage range of the vessel. The coverage range is

20 to 40 Nautical Miles [NM] depending on the height of the AIS antennas

(both the receiving and transmitting antennas) above sea level.

An active AIS transponder broadcasts information (data) in small time slots

lasting 26.67 thousands of a second (26.67 milliseconds). In each of these

time slots, the AIS transponder inserts 256 bits (ones and zeros) of data to be

broadcast at a data rate of 9,600 bits per second. All the data is transmittedin one-minute frames. This allows AIS transponders to have access to 2,250

slots of information within one minute on one AIS channel.

In order to increase the amount of data that can be carried by an AIS system

in any one area, two Radio Frequency [RF] channels in the maritime Very

High Frequency [VHF] band have been assigned on a worldwide basis for AIS

data (AIS1 and AIS2).

This means that when 2,250 AIS1 slots and the 2,250 AIS2 slots are added

together, there are 4,500 slots available for data.

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The relationship between slots, frames, data and channels is shown below:

This process of dividing up a single Radio Frequency [RF] channel into small

slots is called Time Division Multiple Access [TDMA].

The data is inserted into each of the slots according to the method describedin an AIS specification ITU-R.M1371-3. This specification is issued by the

International Telecommunications Union [ITU].

Each AIS transponder will only use slots that are free to be used and thus an

AIS transponder will listen to the AIS1 and AIS2 frequencies to find a free slot

before transmitting in that slot. This makes sure that all vessels can receive

each other without any two AIS transponders sending data at the same time.

The actual capacity (number of vessels fitted with AIS transponders and use a

single AIS slot map) of a single AIS slot map is discussed later in this booklet.




AIS transponders share both AIS 1 and AIS 2 RF channels by hopping between

the two channels. This is illustrated below:

All AIS transponders installed on vessels and Aids to Navigation [AtoN] or that

are part of shore based AIS networks share the same slot map or set of 4,500slots. This means that there are only 4,500 slots available for data in any one

area and thus these slots have to be conserved and managed and used only

for the primary purpose of increasing the safety and security of navigation

and vessels.

AIS transponders automatically select slots that they are to use for

transmitting information from the local slot map and inform all of the other

AIS transponders within Radio Frequency [RF] range (20 to 40 Nautical Miles)that they have done this.

Each AIS transponder then transmits its data on a regular basis at periods

ranging from once every 2 seconds up to once every 180 seconds depending

on the navigational status of the vessel on which the AIS transponder has

been installed. This regular transmission period is known as the AIS update

rate.






The specification for Class A AIS transponders (used on SOLAS vessels)

mandates the following basic reporting requirements:

TYPE ACTION INTERVAL

Static None 180 seconds

Ship At anchor 10 seconds

Ship 0 -14 knots 10 seconds

Ship 0 -14 knots and changing course 3.33 seconds

Ship 14 - 23 knots 6 seconds

Ship 14 - 23 knots and changing course 2 seconds

Ship > 23 knots 2 seconds

Ship > 23 knots and changing course 2 seconds

The specification for Class B AIS transponders (used on leisure craft and

similar vessels) mandates the following basic reporting requirements:

TYPE ACTION INTERVAL

Class B Moving at < 2 knots 180 secondsClass B Moving at 2-14 knots 30 seconds

Class B Moving at 14-23 knots 15 seconds

Class B Moving at > 23 knots 5 seconds

Class B (CS) Not moving faster than 2 knots 180 seconds

Class B (CS) Moving faster than 2 knots 30 seconds

Note 1: CS indicates a Class B transponder using Carrier Sense technology.

Not all the information available from an AIS is transmitted all the time. Static

information such as vessel name, International Maritime Organisation [IMO]

number and Estimated Time of Arrival [ETA], is transmitted less frequently

and thus to build a complete information set from one vessel, the AIS RF

channels (AIS1 and AIS2) would have to be monitored for a few minutes.




2. AIS componentsn order to be able to receive and transmit AIS defined data, the AIS

transponder has been designed with the following receivers andtransmitters:

• 2 X AIS protocol receivers.

• 1 X Digital Selective Calling [DSC] protocol receiver.

• 1 X General protocol transmitter.

Each AIS transponder collects information [data] from various sensors

onboard the vessel, processes this data and then transmits this data in a

specific slot. In this way all other AIS transponders within range of thetransmitting AIS transponder are able to receive data.

The DSC receiver can be used by the AIS transponder to allow the network

operator to assign alternative Radio Frequency [RF] channels for use by the

AIS transponder.

Where the globally assigned Radio Frequency [RF] channels (AIS1 and AIS2)

are not available, the AIS transponder can be made to operate on other RadioFrequency channels within the same maritime Very High Frequency [VHF]

band.

The AIS transponders from any one manufacturer are able to communicate

with AIS transponders from any other manufacturer as all transponders are

built to and tested to the same specification and are designed to be fully

interoperable with each other.

There are various types of AIS transponders. These are known as Class A - for

the SOLAS vessel, Class B - for the leisure craft and AIS base stations for the

AIS shore requirement.

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The block diagram for a typical AIS transponder is shown below:

The three receivers operate all of the time allowing the AIS to receiver data

on AIS1, AIS2 and Digital Selective Calling [DSC] simultaneously. The standard

AIS transponder on vessels cannot receive and transmit at the same time.

The AIS transmitter is very sophisticated and can transmit signals in either the

DSC or AIS formats.

The internal GPS extracts a very accurate timing signal from the GPS satellites

and uses this to find out where the AIS frames and Time Domain Multiple

Access [TDMA] slots begin and end.

Typically the Power Supply Unit [PSU] allows most AIS transponders to

operate from +/- 20 Volts Direct Current [VDC] to +/- 30 VDC.

The Class A AIS transponder collects data from a number of sensors onboard

the vessel.




This data that is collected from the various sensors is used to compose the

various messages that are broadcast by an AIS transponder. These sensors

include:

• Gyrocompass

• Speed log

• Navigational GPS

For any AIS transponders to work effectively, it has to be connected to Very

High Frequency [VHF] and GPS antennas.

The VHF antenna should be mounted as high as is possible and the GPS

antenna should have a clear view of the sky from horizon to horizon.

Not only are AIS transponders installed on vessels. Some AIS transponders

are installed on the shore (known as AIS base stations and when combined

with power supplies and AIS Base Station Controllers [BSC] , they are known

as AIS Physical Shore Stations [PSS]) giving rise to the so called ‘4S’ (Ship to

Ship and Ship to Shore) communications capability.




The primary classes of AIS transponders are:

• AIS base station – used on the shore as part of a network

• Class A AIS – used on SOLAS vessels and other larger vessels.

• Class B AIS – used by leisure and similar type non-SOLAS vessels

• Aids to Navigation – used on Aids to Navigation (fixed and floating)

• AIS-SART – (AIS - Search And Rescue Transmitter [SART])

Class B AIS transponders have fewer features and do not need to interface to

gyrocompasses and the like since all the Speed Over Ground [SOG] and

Course Over Ground [COG] and similar information is obtained from the built

in GPS unit.

AIS units are manufactured in different sizes with various displays although

they all conform to the same minimum requirement standard.

The following graphic shows a selection of Class A AIS transponders from

Furuno and Nauticast.

Each of these AIS transponders have an integrated graphic display and

keyboard. Each AIS transponder has a Presentation Interface [PI]. This is a

data port that outputs all the data that is correctly received by the AIStransponder.




There are a number of Electronic Chart Systems [ECS] and Electronic Chart

Display and Information Systems [ECDIS] that can use the data from an AIS

transponder to graphically display remote AIS targets on calibrated ECS or

ECDIS equipment.




3. The AIS message sethe AIS system allows for many varied messages to be sent between any

two AIS transponders. Most messages are broadcast allowing any otherAIS transponder within range to receive and decode them. There are,

however, a number of addressed messages for which the AIS transponder

uses the Maritime Mobile Service Identifier [MMSI] as the identifier or

‘telephone’ type number. Each AIS has a unique MMSI allocated to it by the

flag country (country in which the vessel is registered).

The total message set can be easily divided up into various types.

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The dynamic information broadcast by the AIS transponder includes the

following:

• Actual position (latitude and longitude).

• Rate Of Turn [ROT].

• Heading.

• Speed Over Ground [SOG].

• Course Over Ground [COG].

This information is augmented by the ability of the vessel to provide selected

vessel static information that includes the following:

• Estimated Time of Arrival [ETA] at the next port or destination port.

• The destination port.

• The size and draught of the vessel.

• The HAZMAT code of the cargo carried.

• Navigational status.

When an AIS transponder is installed on a vessel, a few parameters are

programmed by the installer and cannot be easily altered without access to a

password. These parameters include:

• Vessel name

• MMSI number

• IMO number

Fortunately most of the above parameters are either entered once at the

start of a voyage or entered once when the AIS transponders is installed.

Most of the other data is automatically obtained from sensors and thus does

not require manual input. The GIGO [Garbage In, Garbage Out] principle

applies. For the AIS to be effective, significantly enhance safety, security,provide the economic benefits and allow protection of the environment, the

data entered into the AIS transponders has to be accurate to be of value.




As noted at the beginning of this section, the AIS system allows for a number

of special message sets. The two most interesting of these are the ability to

send addressed free format text and data messages either in a broadcast

mode or in an addressed mode.

There are limitations on the length of messages but as demonstrated by the

value of the Short Message Service or ‘Text’ messaging [SMS] on cellular

networks, the ability to send short messages can have significant value to the

AIS user.

When sending an addressed message, the address of the destination AIS

transponder has to be known and entered. Some AIS transponders allow this

address to be chosen from a list of AIS transponders that are within the

coverage area (i.e. can be received).

Once the address has been chosen and the message entered (only capital

alpha-numeric and a few formatting characters are allowed), the message

can be sent.

An addressed text or data message requires an acknowledgement from the

remote or addressed AIS transponder for the message to be successful. If no

acknowledgement is received from the remote AIS transponder, the message

is indicated as having failed.




In contrast to an addressed AIS text or binary message, a broadcast message

does not require an acknowledgement to be declared successful by the

sending AIS transponder.

Importantly addressed messages are only received by the AIS to which the

AIS message is addressed and broadcast messages are received by all AIS

transponders that are able to receive the message that was broadcast by the

transmitting AIS transponder.

It is important to note that a message should not be addressed to an AIS

receiver since no acknowledgement is able to be returned since the receive-

only AIS transponder does not have a suitable transmitter to respond to the

AIS transponder that sent the original message. This though does not mean

that the AIS receive-only transponder did not receive the message, all it

means is that the acknowledgement was not sent back or was the message

was not received.




Additional AIS messages

There are a number of AIS messages that have been included in the AIS

standard or have been added to provide additional functionality. The most

used special messages are:

• Aids to Navigation message

• Class B messages

• Search And Rescue Aircraft message

Aids to Navigation message

The international body for Aids-to-Navigation, IALA, defines an Aid-to-

Navigation as:

"a device or system external to vessels designed and operated to

enhance safe and efficient navigation of vessels and / or vessel

traffic."

(IALA Navguide, Edition 1997, Chapter 7).

The Aids to navigation [AtoN] message contains the following information:

• Type of AtoN

• Name of AtoN

• Position accuracy, Latitude and Longitude

• Dimension and reference point

• Type of position fixing device

• Off position indicator




Class B message

The Class B AIS message that is regularly broadcast contains the following

detailed information:

• MMSI

• Speed Over Ground


• Course Over Ground [COG]

• True Heading

Search And Rescue aircraft message

The Search And Rescue aircraft message contains the following detailed

information:• MMSI

• Altitude



The Search And Rescue [SAR] aircraft AIS message is sent at a default update

rate of once every 10 seconds.




4. AIS as part of a systemhe AIS technology was developed to be part of a total system that

embraced the whole of the maritime environment and included a shorebased network and a long range capability using any suitable long-range

communication capability.

Any AIS transponders that is fitted to a vessel, aircraft, AtoN or as part of a

shore based network is part of the Global AIS environment where each unit

has to work cooperatively with others in the same environment.

The IALA AIS system concept is shown below:

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Since any AIS transponder can affect the performance of others within the

local area AIS network, a lot of attention has been paid to making sure that

all AIS transponders are fully interoperable or compatible with each other

thus ensuring that the safety benefits of the AIS system are preserved.

AIS transponders, like any other electronic telecommunications system, can

fail. When an AIS transponder fails, it should not be allowed to affect others

within the Local Area AIS Network [LAAN].

To ensure that the LAAN is protected the installation of AIS transponder

equipment on vessels and along shore lines should be completed to the best

standards possible. It is clear that the more AIS transponders that are active

in any one area, the greater the chance of failure and thus disruption of the

LAAN.

The Global fleet of SOLAS vessels is estimated at 46,000 vessels. This gives a

very low average density (number of vessels per square kilometre) except

near ports or critical routes through which a large number of vessels pass

such as the Dover Straits, The Panama Canal, the Suez Canal, The Malacca

Straits and similar areas.

Significant caution has to be exercised when large numbers of leisure craft

are fitted with AIS transponders (Class B) and operate on the same two AIS

frequencies as the SOLAS fleet. This additional load may negatively influence

the LAAN unless Vhf Datalink Layer [VDL] management techniques are used

(see section on AIS network load).




To fully understand how the whole AIS system fits together, consider the

message flow between four assets (vessels, aircraft and shore based AIS

network) below.

Assume that vessel A broadcasts an AIS message updating all within its Radio

Frequency [RF] coverage area of its current position (20 to 40 Nautical Miles

[NM]). This message contains a number of data fields and is sent in 26.67

milliseconds every 3.33 seconds. This broadcast is received by all other AIS

transponders including the AIS shore stations.

On all of the other vessels and aircraft, the position of vessel A is updated

and displayed in its new position on the local Electronic Chart Display and

Information System [ECDIS]. On the shore side, the data is collected at a

central location, processed and then displayed on one or more operator’s

console that could include an Electronic Chart System [ECS] or other

Geographic Information System [GIS].




Since all vessels within the Local Area AIS Network share the same radio

channels, AIS1 and AIS2, they all have to get a turn (a time slot) to send or

broadcast their specific set of information.

If there are 4,500 slots available per minute, this means that in any one area

there are only 4,500 (number of slots) / 60 (seconds in a minute) slots per

second or 75 slots per second. Given that AIS transponders on moving vessels

report at an average of every 3.33 seconds (vessel moving at 0 to 14 knots

and changing course), this means that there could be 3.33 X 75 vessels or

~250 active vessels in any one Local Area AIS Network.

Near large ports or in congested waterways, there are potentially more than

250 AIS transponders operating within a typical coverage area of any vessel.

The AIS technology would not be considered safe if some AIS transponders

did not get an opportunity to inform vessels in close proximity of its

navigational status and thus disappear off the AIS target display.

To ensure that each AIS transponder is able to report its information, the AIS

protocol has a number of features that allow vessels to share the same time

slot between two vessels that are the furthest distance from each other andthus pose no immediate danger to each other. This happens without the user

being aware of this happening. The AIS VHF Datalink Layer [VDL] can also be

managed thus allowing more vessels to report safely.

The AIS technology has a number of message sets that allow interfacing with

existing and proposed communication systems. These include:

• Vessel Traffic Services [VTS]

• Long-range communication systems

These are covered in more detail later.




5. AIS network functionalityhe AIS technology was originally conceived as a safety system that

allowed vessels to interact with each other and with shore-based facilities

in a manner that would dramatically improve safety.

The European Union [EU] VTMIS-NET project identified a number of stake

holders and how they would benefit from the deployment of AIS systems as

well as the integration of AIS within the shore environment. The model below

is drawn from the results of this project.

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Since the VTMIS-NET model was created, the AIS technology, and specifically,

the shore side AIS network has matured into a more complex technology that

has safety, security, economic and environmental implications and continues

to support the VTMIS-NET model.

There are two participants in any AIS system, the ship side and the shore

side.

Importantly, both participant groups share the same radio channel and this

radio channel is a finite and critical resource that needs to be carefully

managed and protected. AIS

networks have a basic set of

functions that include collecting data,

storing the data so that there is an

audit trail, processing the data toenable it to be exported to existing

systems and also be easily

assimilated by network operators.

The data is then disseminated to AIS

network operators and systems and

vessels that are within the AIS

network.




AIS networks consist of five basic components:

• AIS base station / AIS shore station

• An Wide Area communications Network

• An AIS network application

• A Human Machine Interface [HMI]

• An interface to external applications and other AIS networks

AIS base station / AIS shore station

An AIS base station is an AIS transponder that is able to communicate with

vessels fitted with tested and approved Class A and Class B AIS transponders.

An AIS Base Station is subject to an approval procedure to ensure that it

meets the minimal specification for AIS Base Stations [BS].

To enable an AIS base station to communicate with a large AIS network and

also perform some basic AIS shore station functions, a computer is generally

provided which does the following:

• Connects up to four AIS base stations

• Eliminates all duplicate messages received at the AIS shore station due to

having multiple AIS base stations in the same place

• Interfaces the AIS base stations to the communications network transferring

the AIS data back to a central environment

• Providing a limited amount of storage so that should the Wide Area

Network [WAN] fail for a few hours, no data is lost but stored until the WAN

is restored.

AIS base stations and a local AIS base station interface / controller (known as

a Base Station Controller [BSC]) are integrated with power supplies and

antenna systems to form a complete AIS Physical Shore Station [PSS] that has

all the features required by the AIS network operator.

Some manufacturers include proprietary features in the AIS PSS that are

outside the scope of the IEC specifications that define an AIS PSS and the

Presentation Interface [PI] to the Information Technology [IT] network that

the AIS PSS connects to.




The standard integrated AIS Physical Shore Station [PSS] is shown below.

This style of AIS Physical Shore Station [PSS] allows for the connection of theAIS Physical Shore Station to a central office environment that processes the

data ready for distribution back to the AIS network environment as well as to

the shore based AIS network user. To increase the network security, a

firewall that protects the AIS physical AIS shore station from the VHF Datalink

Layer [VDL], the WAN network and operator / administrator access sides may

have to be included in the Base Station Controller [BSC] at each AIS Physical

Shore Station.

This AIS Physical Shore Station can be significantly improved by adding a

number of features that improve the AIS Physical Shore Station functionality

especially those features provide additional value to the local maritime

community, the security authorities and the maritime authorities.




The IMIS Global MariWeb™ AIS network is an example of the 4th

generation

AIS network that is now available offering the maritime community and

authorities significantly improved functionality and increased feature set at

lower costs. The MariWeb™ AIS Physical Shore Station is and can be used as a

standalone AIS network by design or in case of an emergency.

The MariWeb™ system design allows many shore stations to be connected

together to form large, more complex network using the same standard,

tested building blocks. This provides a network that is feature rich and very

resilient.

The transportable MariWeb™ AIS Physical Shore Station (3rd

Generation) is

shown below:

This type of AIS Physical Shore Station design fully integrates all the AIS

networks requirements, including security and Human Machine Interface, at

every level of an AIS network, even at the AIS shore station. This type design

presents advantages in configuration and reliability.

This level of integration enables a full AIS network to be available at each AIS

shore station and then the shore station becomes a very flexible building

block. This allows the AIS network to be deployed to meet the cost,

functional and business needs of the stake holders.

The MariWeb™ off the shelf, small port / off shore structure AIS network,

product is shown.




AIS Physical Shore Station reliability

The AIS PSS reliability is affected by two parameters:

• Mean Time Between Failure [MTBF]

• Mean time To Repair [MTTR]

The Mean Time Between Failure [MTBF] is a figure that is normally initially

provided by the supplier of the equipment. The higher the figure (in hours)

the more reliable the equipment is and the less attention has to be given to

dealing with failures. Most AIS PSS offer an MTBF of greater than 100,000

hours excluding local peripheral equipment such as batteries and Wide Area

Network [WAN] modems.

The Mean Time To Repair [MTTR] indicates how long it would take to repair a

failed AIS Physical Shore Station once getting to the AIS Physical Shore

Station location. The shorter this period of time, the more time that the

system is available for use by the operator.

An interconnection or telecommunications network

The interconnection or telecommunication Wide Area Network [WAN] has

three facets that need to be considered:• Cost

• Reliability

• Topology

Cost

The operational cost of the WAN can be very high especially where primary

and secondary (redundant) links are required and a large number of long

links are required to very remote areas.

The cost of the WAN is also normally proportional to the bandwidth required.

The bandwidth required for each AIS shore station link should be sufficient to

carry the real time data and also recover the data that may have been stored

at the AIS shore station after a WAN failure. This is covered in a later section.




Reliability

In a WAN, the actual infrastructure is spread out between the remote AIS

shore stations and the central office. The longer this link is and the more

sections are added, the more unreliable the WAN link will become.

Topology

There are four basic topologies used to connect AIS shore stations together:

• Mesh

• Star (hub and spoke)

• Daisy chain

• Hierarchical

Of these four topologies, the star (hub and spoke) and the hierarchical are

the most common topologies. These are however not the most ideal

topology for all AIS networks and circumstances.

For explanatory purposes, the four basic topologies are illustrated below:

Some of these topologies can be combined to provide the best WAN solution

dependent on the telecommunications available and the level of redundancy

/ duplication required.

In the above example only the mesh and the daisy chain provide redundant

telecommunication paths (more than one connection or path between one

node and another). In a star or hierarchical system, redundancy can be

provided by using duplicated WAN systems.




As an example, it is required to design and install an AIS network along a long

waterway or port approach. To do this, all the AIS

shore stations are placed at the ideal places

taking into account the facilities that are

available to house the AIS shore stations and

associated masts, batteries and other equipment.

Once the sites are chosen, the AIS shore stations

are installed and linked to each other by

installing a telecommunications system that

connects each AIS shore station to the other in a

chain. To complete the daisy chain, the last

station, E, is connected back to station A via a

dedicated but totally separate

telecommunications link.

This means that, should the daisy chained

telecommunications link fail at any point, a path

still exists for the AIS shore stations to share AIS data between any one AIS

shore station and another by going through AIS shore stations that areavailable and using then as gateways or data routers. Should two links fail at

any point, the network would become segmented and not all the AIS shore

stations would be able to share data.

If this was constructed using the 4th

generation IMIS Global MariWeb™ AIS

network then any user could connect to any one of the nodes (A to E) and

have access to all of the AIS data that has been collected by all of the various

AIS shore stations.




AIS network application

The AIS network application is expected to implement a number of basic

services / functionalities as required / recommended by the International

Association of Marine Aids to Navigation and Light House Authorities [IALA]

in the IALA A-124 Recommendation.

In order to make the design and deployment of an AIS network solution a

flexible and economically viable process, IALA published the IALA A-123

Recommendation for consideration by the many and varied potential users of

AIS network solutions.

The AIS application has the following basic functions:

• Collects AIS data from AIS shore stations

• Stores the AIS data

• Processes the AIS data

• Disseminates the AIS data to external applications, display environments

and also to the AIS transponders in the coverage area

This four stage process needs to ensure the following:

• The AIS database remains uncorrupted

• The AIS data is not able to be corrupted via external attacks (virus and / or

hacker)

• No AIS data is lost or changed

• No AIS data duplicates exist in any single database

• That all AIS data is accurately referenced to the GPS date and time

(temporally accurate)




The Human Machine Interface

The Human machine Interface [HMI] or Human System Interface [HSI] is what

the operator uses to interface to the AIS network and make use of the data

that is captured within the AIS network.

The software client or application that allows an operator to connect to the

AIS network applications requires a user name and password. These are

allocated by an AIS network administrator thus ensuring that only authorised

users have access to the AIS network and it features and services.

Each user / client is allocated a number of permissions. These permissions are

activities that any user with a selected user name and password is allowed to

perform using the AIS network.

Most HMIs have a Graphical User Interface [GUI] that is either an ECS / ECDIS

type chart or other geo-referenced image over which the AIS targets are

displayed.

Since shore side AIS

displays are not often usedfor navigation, besides the

standard Raster and S57

standard maritime charts,

geo-referenced images are

often used to display AIS

targets over.

For pilotage organizationsusing AIS data, this could

be a geo-referenced image

that shows the approach channel and obstructions in photographic mode as

well as the standard maritime chart mode.




For port security and port operations, the geo-referenced image could be a

satellite image (either a photograph or line image) showing shore side

infrastructure, access roads and physical attributes which are more

recognisable from a photograph than on a chart of the area.

The MariWeb™ AIS

network offers both

options to the operator

when using the

integrated web based

ECS.

When a number of

operators share the

control of the same area,

they should access the

same geo-referenced image. This requires that the image be distributed with

the same name and attributes or served off a single Geographic Information

System [GIS] server.

The HMI has to take into account the needs of the operating environment.

With the reduction in staff, many staff have increased functional

responsibility and are empowered by information such as AIS data. AIS data,

to be of benefit to the user, has to be easily available when and where

required. In modern IT systems, this normally means that the client / user

software application is a standard Internet browser.

Access to various features of the HMI is constrained by the permissionsallocated to the user / client name / password combination.




A typical set of user permissions will control access to the following set of AIS

data:

• Network configuration and upgrade

• Sending of AIS messages

• Reception of AIS messages

• View of selected area/s

• View of selected fleets / types of vessels

• Connection to Logical Shore Stations [LSS]

A typical users / client permission based configuration application is shown.

This is often linked to a

Logical Shore Station[LSS] feature set. The

LSS will often be

associated with a area

of interest (focus area),

a list of vessel or group

of vessels, a special set

of functionality (virtual

and synthesized Aids to

Navigation [AtoN] or

other special functions)

and overriding set of

permissions. The

functions of an LSS are

explained later.

The permissions based system, although providing some security, still

requires that firewalls be properly configured and installed at each AIS

network node thus protecting the users, the AIS network and all applications

and system to which the AIS network provides data.






Interface to external applications

AIS networks are one of the communication systems that are used within a

national, port or strategic maritime environment.

Others systems include:

• Radar / VTS / VTMIS

• Closed Circuit Television [CCTV]

• Vessel scheduling

• Facilities management

• Geographic Information Systems [GIS]

• Electronic Chart Systems [ECS]

• Digital Selective Calling [DSC]

• Analogue maritime voice radio (VHF)• Financial management systems

• Pilotage management systems

• Ship movement planning and logging systems

• Land transport scheduling systems

• Customs and excise systems

• Passport control and immigration

An AIS network has data that adds value to all of these peripheral systems byautomatically providing accurate vessel, load and positional data. The process

of integration is made a lot easier by using industry standard and open

interfaces.

These interfaces include:

• Structured Query Language [SQL]

• Discoverable web services in eXtensible Mark-up Language [XML]

• IEC61162 (TCP/IP encapsulated IEC61162 or serial port)

Older port information or operational systems may not have these interfaces

and some integration work may be required to make sure that the required

information is easily transferred and integrated.




The data that is generally required from an AIS network can be grouped as

follows:

• Near real-time vessel tracking data for a particular area

• Latest data regarding a specific vessel

• Latest data concerning all vessels or a group of vessels in a particular area

• Data that is for a particular are between two separate date and times

An example would require a response to a query that requests a set of data

that could defined as follows:

• Start Date: 14/12/2004

• End Date: 15/12/2004

• Start Time: 12h00

• End Time: 12h01• Area: Boston Inner

• Vessel group: Tankers

• Speed Over Ground: >0

• Course Over Ground: Any

• Heading: Any

• Navigational Status: Underway

With the adoption of the Internet and the use of Internet standards expand,XML has become the open standard interface of choice for connecting

external systems to AIS networks. This includes discoverable Web Services

that allows user ftriendly nterfacing to other enterprise systems such as

Oracle Enterprise Service Bus [ESB]. The interconnection of AIS networks

together either on a regional, national or International basis, is expected to

only use open standards and the eNavigation standards currently being

developed by IALA follow this trend.

Due to the large volumes of data carried in large AIS networks, the linking of

external systems often employ some form of filtering (often set up as a

Logical Shore Station data feed) that eliminates all data that is not needed by

the receiving party. This filtering also allows the use of lower cost

telecommunications links where systems are physically distant from each

other and processing and storage infrastructure on the part of the recipient.




6. AIS network load capacityThe load of an AIS network is not only related to the 4,500 slots that are

available in any one slot map but also is affected by:

• Vessel activity and therefore reporting rates (already explained in detail

above)

• Number of slots reserved by the AIS network operators for specific purposes

(i.e. Aids to Navigation broadcasts)

• VHF Datalink Layer [VDL] management

• The Self Organising Time Division Multiple Access [SOTDMA] slot reuse

feature

Using only the 4,500 slots in a single slot map and a reporting rate of one

report every 6 seconds (vessels travelling 14 to 23 knots in a straight line) will

allow 6 (seconds per single vessel report) X 75 (slots per second) = 450

vessels to report (this is a theoretical maximum figure: this will be less due to

the methods employed to achieve this and other protocols overheads).

In reality vessels within range of a typical AIS PSS are in various navigational

states and also consist of various types of vessels. A typical scenario could

have 300 vessels at anchor, approaching port and moored in berths. This

would result in a very low reporting rate and therefore a far higher number

of vessels per AIS PSS. If the active vessels were largely Class B CS vessels

(fishing craft), then the report rate from each vessel would be reduced to one

report per 30 seconds (approximately 1/10th

of an average SOLAS vessel

traveling at 20Knts).

The VDL management capability and the SOTDMA slot reuse feature require

further explanation.

VDL management

The VDL management allows the network operator to control the reporting

rates of vessels within range of any AIS Physical Shore Station connected to

the AIS network. The network operator could, for example, issue the

following AIS ‘instructions’:

All vessels within range of AIS Physical Shore Station ‘Demo’ to only reportonce every 180 seconds.




This will allow 180 (seconds per single report per vessel) X 75 (slots per

second) = 13,500 vessels to report (this is a theoretical figure: this will be less

due to the methods employed to achieve this and other protocols

overheads).

Slot reuse

When an AIS transponder detects that a slot map is full and there are no

vacant slots for it to transmit its position data in, the AIS transponder will

select a candidate slot (a slot that the AIS wishes to transmit in) that is being

used by the vessel that is largest distance away from itself.

This means that all vessels that are closest to the vessel transmitting the

position data will be able to receive it even though two AIS transponders are

using the exactly the same slot. This ensures that vessels that are the closest

to each other remain aware of each other and thus safe.

Slot reservations

The AIS Physical Shore Stations can reserve slots for itself, other AIS physical

Shore Stations, Aids to Navigation and other AIS transponders that need tohave fixed and reliable access to the VHF Datalink Layer [VDL]. The AIS

Physical Shore Station does this by transmitting a ‘slot reservation’ message

all AIS transponders that receive these reservation messages know not to

transmit in these slots unless they are part of the group for which the slots

were reserved for.

The designers of AIS networks have to collect a large amount of detail with

regards to actual AIS traffic, coverage of the AIS PSS, adjacent AIS PSS(including international AIS PSS within a 120 nautical mile range), all Aids to

Navigation [AtoN] and the expected traffic profile (for example how many

text messages are to be sent or received per minute by the AIS PSS being

considered) to be able to construct the slot reservation map for a particular

AIS PSS.




7. AIS network applicationshe large amount of near real-time data that is available within AIS

networks, has allowed a number of new applications to be developed and

/ or has allowed other applications that have existed within the maritimeoperational environment to become significantly more efficient.

We have included a number of hypothetical examples to show what is

possible.

AIS networks are being used mainly by the following groups:

• Port authorities / Maritime Exchanges

• Coastline authorities

• Strategic installations

• Aids to Navigation authorities

Port authorities

Port authorities have the safety, security, economic efficiency and the

environmental protection of the port as their responsibility on a day-to-day

basis.

AIS networks play a significant role in the following ways:

• Providing detailed information of a vessel approaching port up to 70

Nautical Mile [NM] away from a well located AIS base station or set of AIS

base stations. This allows the safety and security systems to prepare to deal

with the approaching vessel. If the AIS network and Closed Circuit Television

[CCTV] network have been interfaced with each other, the CCTV will follow

the vessel into port.

• Automatically updating port arrival documentation and automatically

tracking internal ship movements.

• Providing accurate time of arrival and departure information.

• Providing accurate details of vessels that can include photos, CCTV images

as well as other documentation captured within the port regarding vessels.

• Automatically informing vessels of exclusion or inclusion zones. The

exclusion zones could include maritime reserves, strategic installations that

are within ports (fuel depots, etc.). The inclusion zones could be the

waypoints to be used when approaching a port.

T






This is important where the AIS shore stations cover critical, strategic or high

security environments and the WAN availability cannot be guaranteed under

most circumstances.

AIS networks, such as MariWeb™, have a capability to include a complete AIS

network at each and every AIS shore station. Most other AIS network systems

require that complex client software be carried to the AIS shore station to

achieve the required functionality.

Strategic installations

Strategic installations include any maritime installation of local, regional or

national importance or installations that pose a risk to the maritime natural

environment.

These strategic installations could include:

• Offshore structures (oil, gas, wind farms, wave power generation

installations and exploration installations)

• Coastal power generation facilities

• Fuel storage depots

• Naval ports and installations

• Airports near to the shore line

• Marine nature reserves

• Maritime archaeological sites

The strategic installations can be in any part of the maritime environment for

which the national authority is responsible. AIS allows for gathering of

detailed information on a local basis and the transference of this data to any

control room that has authority for the area of interest.

When combined with low rate video or still images of the same area, the use

of AIS derived data can significantly add to the effectiveness of ensuring the

safety, security, the economic advantage and the environmental protection

of these areas of interest.

AIS networks should be combined with a suitable HMI system that further

increase the effectiveness of the coastal authorities. This may includeoverlaying AIS targets on satellite or other real-time geo-referenced images.




Aids to Navigation authorities

The competent international body for Aids-to-Navigation, IALA, defines an

Aid-to-Navigation [AtoN] as:

"a device or system external to vessels designed and operated to enhance

safe and efficient navigation of vessels and / or vessel traffic."

This definition includes buoys, navigation markers and lighthouses. The AIS

technology and various AIS applications could also fall into this definition.

AIS can be integrated into the current Aids to Navigation [AtoN] environment

in a number of ways. These include:

• Fitting low powered AIS transponders on existing physical AtoNs (a complex

and often expensive process especially when considering life cycle costing

on small and remote floating AtoNs).

• Integrating existing or new telemetry systems to broadcast AtoN

information from shore based networks using data gathered from the

equipment already installed on physical floating AtoNs.

• The creating of virtual or synthesised AtoNs by defining AtoN in software

and broadcasting this information to the maritime industry.

The ease of creating a virtual AtoN allows the marking of wrecks, dangerous

areas and other critical points very quickly.

Using AIS provides the facility to send warning messages to vessels that are

close to or are sailing towards dangerous areas on an ongoing way.

If the virtual AtoN message generation capability resides at the AIS shorestation, even when the WAN fails, the messages will still be broadcast and

the audit trail archived. This also applies to synthesized AtoN where the

position of the floating Aids to Navigation is monitored by some other means

(i.e. low powered Radio Frequency [RF] data link) and this data is then used

by the Base Station Controller [BSC] in the AIS Physical Shore Station to

create the AtoN AIS message that is broadcast for and on behalf of the AtoN.




The collection of real-time meteorological and hydrological data and

transmission of this information using AIS networks can make the local area

significantly safer.

MariWeb™ incorporates a number of AtoN focused applications that enable

the definition of focus areas, critical areas and critical points. MariWeb™

allows the transmission of automated AtoN messages as well as text, binary

and Safety Related Messages [SRM]. These can either be broadcast or

addressed to selected vessels. All of these are stored in a database for audit

purposes.

The United States Coast Guard [USCG] has defined a set of AIS messages that

allow the transmission of weather information as well as allowing the

transmission of other data to vessels entering and leaving USA ports and the

St Lawrence Seaway.

The collection of AIS data over long periods of time (two or more years)

allows the analysis of the routes used by vessels fitted with AIS transponders

and how this relates to localized conditions (meteorological and

hydrological). This data is then inserted into the risk model used by the Aidsto Navigation [AtoN] authority and conclusions can be drawn as to the

efficiency of the AtoN expenditure and how to make the system more

efficient.

The online availability of a number of years of AIS data enables analysis of

trends by AtoN authorities, maritime consultants as well as by academia.




8. Long-range AISach AIS transponder has a long-range interface that allows the connection

of a long-range communication system. The IALA AIS system diagram also

indicates the use of a long range system although the actual technology is notspecified. A long-range system is required since the range of a standard AIS

shore station is limited to +/- 70NM under ideal conditions.

There are a number of long range technologies available.

Each long-range technology has its own advantages and disadvantages. The

United States Coast Guard [USCG] in 2004 issued a ‘sole source’ contract to

evaluate the use of AIS base station on Orbcomm satellites as an alternative

long range system.

The AIS specification (ITU-R.M1371-1) proposed using an active interface

between the AIS transponder and any long range technology selected by the

vessel owner / operator. The active interface allows the AIS long-range

protocol to be interfaced to the existing long-range communication system

already installed or to be installed by the vessel owner / operator.

The active interface allows the connection of other ship side peripheralsbesides the AIS transponder. This could include the Voyage Data Recorder

[VDR].

Long-range AIS data can also be obtained from other AIS networks installed

in remote locations. Some AIS networks, such as the MariWeb network,

already have a suitable interface as part of the standard product.

The European Union [EU] regional project called SafeSeaNet [SSN] and the EUVessel Traffic Monitoring Directive [VTMD] go some way to providing the

basis of a long-range system using standard AIS networks installed along the

various EU national coastlines and ports.

E




Various satellite service providers provide long-range capability. These

include:

• Iridium

• Orbcomm

• Thuraya

• Inmarsat

Long-range communication alternatives also exist in the form of High

Frequency [HF] radio and Meteor Scatter systems.

Each has advantages, limitations and range considerations.

Long range AIS has largely been replaced by the International Maritime

Organisation [IMO] Long Range Information and Tracking [LRIT] system that is

currently being implemented on a global basis for all SOLAS vessels.




9. AIS network securityhe AIS data is broadcast in an unencrypted format using a published and

easily available specification. Low cost AIS receivers are easily available

that allow the receiving of AIS data. Simple, low cost (free in some cases)applications exist that allow for the decoding of AIS data and the displaying of

vessel positions on a simple chart or Google Map. The AIS data is stored in a

local database and can be viewed directly.

The above situation means that persons with a basic technical understanding

have access to data and can, with access to an AIS base station or a WAN that

connects an AIS network together, interfere with or create false or spoofed

AIS data.

Well designed AIS networks have subsystems (firewalls and access detection

systems) that detect false or spoofed AIS messages and either mark or

eliminate them. Access to the WAN infrastructure is protected as are the

operator Human Machine Interfaces using a number of security mechanisms

that include:

• User names and passwords

• Protected or encrypted data links• Connection and access audit trails

• Suitable administration tools allow user and password management

• Database password protection

• Limiting / eliminating client side applications so that only a standard

Internet browser is used for access and the Human Machine Interface

Systems do exist that also allow the use of encrypted messaging between AIS

transponders and between AIS transponders and AIS shore stations using AESencryption methods and the AIS binary messaging. This system allows for the

transfer of position and text messages in an encrypted format to be shared

and is normally used by port police and coastguard vessels under previously

agreed circumstances.

T




10. Logical Shore StationsLogical Shore Station [LSS] presents a set of features to the user that

simulates an AIS Physical Shore Station [PSS] with a defined feature set.

This is normally implemented by a bi-directional data filter.

In the receive direction, a LSS will allow only vessels that meet a particular set

of criteria to be viewed. This can be done for operational (for example to

allow a particular operator to view only tankers in a particular area for safety

reasons) or for security reasons (for example excluding all vessels in a nearby

naval port).

The basic principle of LSS is indicated below:

The operators communicate with the Logical Shore Station [LSS] that has

been configured according to the AIS network design requirements.

A




A LSS also allows the configuration of transmitted data. This could, for

example, allow an operator to only transmit broadcast safety Related

Messages [SRM] messages to vessels in a particular and predefined area.

Logical Shore Station filters often include the following filter parameters:

• Type of vessel

• Speed Over Ground [SOG]

• Rate Of Turn [ROT]


• Area

• Transmit Addressed Binary Messages [ABM]

• Transmit Broadcast Binary Message [BBM]

Logical Shore Stations [LSS] have a standard interface that allows an

Electronic Chart System [ECS] or Electronic Chart and Display Information

System [ECDIS] to be connected directly to the LSS using the internationally

agreed protocol defined in the IEC61162 specification.

The LSS should not change any of the received data to ensure that no detail is

lost when a forensic analysis of the data is required. The LSS should not

significantly delay any of the received data so that the near real-time

performance of the shore side AIS network as a whole can be maintained.

MariWeb™ allows any number of Logical Shore Stations to be created as well

as any number of Clients to be created allowing each client to have their own

LSS, if required.




Appendix A: Selected AIS specifications

Specificationnumber

Specification description

ITU-R.M13171-3

The technical specification that lays out how the AIS

protocols is to work and what is contained in each AIS

message

IEC61162

The technical specification that defines how AIS

transponders are to communicate with other peripherals.

This is supported by a number of other IEC specifications.

IEC62320-1 AIS Base Stations – Minimum operational and performancerequirements, methods of testing and required test results

IALA A-124An IALA recommendation that provides a guide to AIS

system design and deployment

IALA A-123

An IALA recommendation that provides a decision matrix

for those looking at designing and / or purchasing an AIS

network.

This is just a selection. There are many other technical specifications that

have some influence on the AIS system as a whole. Should you wish to get

more deeply involved in the technical details of AIS systems, the

specifications listed above are a good start. These specifications reference

other.

It is also important to note that some national telecommunication authorities

have issued their own series of specifications for AIS transponders when used

as base stations.




Appendix B: Selected Internet links

The following Internet links are provided to allow more detailed information

to be obtained.

International Maritime Information Systems [IMIS Global]

www.IMISGlobal.com

International Association of Navigation Aids and Lighthouse

Authorities [IALA]

www.iala-aism.org

International Telecommunications Union [ITU]

www.itu.int

International Maritime Organisation [IMO]

www.imo.org

International Electrotechnical Commission [IEC]

www.iec.ch

United States Coast Guard [USCG]

www.navcen.uscg.gov

European Maritime Safety Agency [EMSA]

www.emsa.eu.int



Notes

Documents

How AIS Works_04-A5