reference AIS-compressed - Diego Grimaldo Ramos

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Contents 
1. Introduction 
1.1 What is AIS? 
2. How does AIS work? 
2.1 AIS communications 
2.1.1 AIS modes 
2.1.2 Long-range communications 
2.2 AIS data 
2.3 Transmission intervals 
2.4 AIS in use 
3. AIS shipborne equipment 
4. Use of AIS at sea 
4.1 Routine checking procedures 
4.1.1 Static data checks 
4.1.2 Voyage related data checks 
4.1.3 Dynamic data checks 
4.2 Entering of ‘non-static’ data 
4.2.1 Voyage related data 
4.2.2 Dynamic data 
4.3 Safety related messages 
4.4 Channel management 
4.5 Use of AIS to aid situation awareness and the making of collision 
avoidance decisions 
4.5.1 MKD based systems 
4.5.2 ARPA and Electronic Chart Display of AIS targets 
4.6 AIS alarms 
4.7 Security and confidentiality 
4.8 Oil terminals 
5. Check-lists 
Appendix Common problems of AIS
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1. Introduction 
1.1 What is AIS? 
AIS, which stands for Automatic Identification System, has been mandated by 
IMO to enhance the safety of life at sea, the safety of navigation and the 
protection of the marine environment. Its correct title is Universal Automatic 
Identification System (UAIS) but the ‘U’ is rarely used. The purpose of AIS is 
to help identify vessels, assist in target tracking, assist in situational 
awareness and to simplify safety related information exchange between 
vessels and between vessels and the shore. 
 
AIS has an important role in improving the capabilities of Vessel Traffic Services 
(VTS) and can also be used to enhance environmental protection monitoring and 
coastal state security. In fact, a major oil spillage arising from the grounding of 
the tanker Braer on the Shetland Islands in the UK was an important instigator in 
driving the IMO legislation. 
 
This training package is designed to help mariners understand how AIS works 
and how its facilities should be used at sea. AIS is a complex communications 
and navigation system but it is straightforward to use once its basic principles 
are understood. It is an additional tool that can help mariners to navigate their 
vessels safely in increasingly congested waters but it must be used with a full 
knowledge of its capabilities and limitations. It does not replace other methods 
of navigation but adds significantly to the information available to the mariner, 
enabling better navigation decisions. 
 
The next section explains how AIS works. Section 3 looks at the equipment on 
the vessel in more detail. Section 4 explains the use of AIS at sea. 
 
2 How does AIS work? 
2.1 AIS Communications 
At the basic level, AIS is a Very High Frequency (VHF) radio system that 
continuously exchanges navigational information between ‘stations’. Stations 
include ships (and, in the future, small craft), shore stations and (again, in the 
future) aids-to-navigation (AToNs), such as buoys and other navigational 
marks. Figure 1 illustrates this concept. 
 
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Fig 1: The AIS Concept 
 
The navigational information exchanged includes ‘static’ parameters such as 
ship’s name, type and length, as well as dynamic data including position, 
heading, and speed over ground (SOG). It also includes voyage-related data 
such as destination, estimated time of arrival {ETA) and draught (draft). 
 
In order to increase the number of ships that can use AIS at any one time, the 
system operates on two separate channels in the VHF band. To enable this to 
happen, ships’ AIS equipment embodies two independent VHF receivers and a 
single transmitter, which alternates its transmissions back and forth between the 
two frequencies. With these two frequencies the ship station can cope with up to 
4,500 reports per minute from other stations. 
 
The AIS operating frequencies, referred to as AIS1 and AIS2, are assigned by 
shore stations. Many countries have adopted Channels 87B and 88B for AIS 
(operating at 161.975 and 162.025 MHz) but this is not universal. In the overlap 
between regions that use different frequencies, shore station commands will 
instruct the ships’ equipment to change frequency. This is an automatic function 
that needs no operator intervention . 
 
Frequency change can also be instructed through the use of Digital Selective 
Calling (DSC) on Channel 70. This means that a further receive channel is 
included in ships’ AIS equipment in order to receive DSC messages, making a 
total of three receivers, (AIS1, AIS2 and DSC). 
 
DSC messages from a shore station can also request a vessel’s AIS to provide 
data over Channel 70. This means the AIS VHF transmitter also has to respond 
on the DSC frequency. Figure 2 contains an illustrative block diagram of the 
transmit and receive systems required in a shipborne AIS. 
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Figure 2: Block diagram of ship’s AIS 
 
In any one area all stations are using the same AIS1 and AIS2 dedicated 
channels, therefore there has to be a sophisticated system for ensuring that 
only one station is transmitting on a particular frequency at any one time. This 
is arranged by dividing time into precisely defined slots. There are 2,250 of 
these time slots in each minute, with each minute being aligned exactly to 
Coordinated Universal Time (UTC). All AIS units have a built-in position-fixing 
receiver, such as GPS (Global Positioning System), which is primarily used to 
determine UTC accurately. This ensures that the transmissions from all units 
are time-aligned correctly. 
 
When making its first transmission, the AIS receiver ‘listens’ to establish what 
time slots are free. It then uses predefined algorithms to decide when it 
subsequently transmits. The number of times that it transmits in a minute is 
determined according to Table 1 in Section 2.3. Transmissions are equally 
spaced and alternate between the two AIS frequency channels, AIS1 and AIS2. 
 
Using a radio transmission system that is based on using defined time slots is 
known as Time Division Multiple Access (TOMA). The self-organising (SO) 
AIS use of TOMA is therefore referred to as SOTDMA. The principle of 
SOTDMA is illustrated in Figure 3. 
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Figure 3: Principles of SOTDMA 
 
 
 
The SOTDMA system used for AIS has the built-in ability to ‘degrade gracefully’, 
when it is becoming overloaded with vessels. In any one locality strong signals 
from closer vessels will override weaker signals from those further away. In 
effect, the range of contact around individual vessels expands or contracts 
around own-vessel, according to the local traffic density. 
 
There are two types of AIS fitted to vessels. Class A equipment is being fitted 
mandatorily to all ships of 500GT and upwards. In addition ships of 300GT and 
upwards that are engaged on international voyages have to be fitted with Class 
A equipment. 
 
Class B equipment is to a lesser requirement and is intended to be fitted to 
smaller vessels, including leisure craft. It does not have internationally defined 
carriage requirements but national laws governing carriage may apply for 
certain classes of vessels. This manual does not cover the use of Class B 
equipment. 
 
2.1.1 AIS Modes 
 
The ‘normal’ mode of operation is known as Autonomous and Continuous. 
This is the mode that has been described in the section above. There is also a 
mode of operation known as Assigned in which the ‘competent authority’ (in the 
form of coastal stations) can individually assign ships their data reporting 
transmission intervals and time slots. 
 
This mode automatically times-out after a period (4-8 minutes), when the ship 
reverts to Autonomous and Continuous mode. There is no user action required. 
 
A further mode is known as Polled. This allows coastal stations to request 
specific data from ship stations or obtain data updates from the vessel in a 
shorter time interval than that occurring in Autonomous mode transmissions. 
Again, no user action is required for the AIS to respond to this mode. 
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Because AIS equipment can automaticallytransmit information in response to 
requests from other stations they are often called Transponders. 
 
2.1.2 Long-range communications 
 
There is an additional connection port (socket) for AIS transponders intended 
for long-range communications. This port can be typically connected to a 
satellite communications system. It is a two-way port allowing both reception 
and transmission of messages. The user can set whether the system 
automatically or manually responds to interrogations over the long-range 
system. 
 
The facility was provided to allow possible use for long range security 
reporting. It is now unlikely that IMO will mandate AIS for this application. 
 
2.2 AIS Data 
The information reported by the ship station is divided into four groups: static, 
dynamic, voyage related and short safety-related messages. 
 
Static information 
 
o IMO number (where available) 
 
o Name 
 
o Call sign 
 
o Maritime Mobile Services Identity number (MMSI) 
 
o Type of vessel 
 
o Length and beam 
 
o Height over keel 
 
o Location of the position-fixing antenna (referred to bow and 
centreline) 
 
The location of the position-fixing antennas (eg GPS) is needed to ensure 
accuracy in close-quarters situations, when the system is perhaps being used 
to aid collision avoidance. The position of the internal ‘position-fixing’ antenna 
(which is normally just used for the generation of UTC time) also allows the 
internal system to be used to obtain position, in the event of a malfunction in 
the ship’s primary position-fix system.
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Dynamic information 
 
o Ship’s position, (with accuracy and integrity, if available) 
 
o Time in UTC 
 
o Course over ground (COG) 
 
o Speed over ground (SOG) 
 
o Heading 
 
o Rate of turn (where available) 
 
o Navigational status, eg at anchor, not under command, etc 
 
It is important that the AIS has been configured by the installer to transmit the 
navigational data that is identical to that being used to navigate the vessel. 
 
Voyage related information 
 
o Ship’s draught 
 
o (Hazardous) cargo type (See Table 2, Section 4.1.2) 
 
o Destination and ETA 
 
o Number of passengers 
 
o Route plan (optional) 
 
Short Safety related messages 
 
Limited to about 160 characters (See Section 4.3) 
 
2.3 Transmission Intervals 
The AIS unit automatically determines the interval between successive 
transmissions of the above data, according to the following rules: 
 
Static and voyage related information: 
 
Every 6 minutes, when data has been amended, and on request 
 
Safety related information: 
 
As required 
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Dynamic information, according to Table 1: 
 
Table 1 Type of Ship Reporting 
Interval 
(secs) 
Ship at anchor or moored and not moving faster than 3 
knots 
180 
Ship at anchor or moored and moving faster than 3 
knots 
10 
Ship with a speed of between 0 - 14 knots 10 
Ship with a speed of between 0 - 14 knots and changing 
course 
3.33 
Ship with a speed of between 14 - 23 knots 6 
Ship with a speed of between 14 - 23 knots and 
changing course 
2 
Ship with a speed of greater than 23 knots 2 
Ship with a speed of greater than 23 knots and 
changing course 
2 
 
 
Table 1 slightly differs from IMO’s original requirements. In particular it 
recognises that many users of AIS are negligent in entering the navigational 
status of a vessel, particularly with regards to stating that a vessel is at 
anchor or moored when actually underway. This is why it includes "Ship at 
anchor or moored and moving faster than 3 knots"! 
 
2.4 AIS in use 
At VTS stations the AIS information from vessels can be superimposed on 
the VTS radar screens. In principle AIS ‘target’ information can be combined 
(correlated) with radar tracked targets to get greater reliability of the position 
and movement of targets. The vessel’s name can be displayed alongside the 
target image; by clicking on that target all the AIS received information on 
that target can be instantly accessed. 
 
On board each vessel there is normally a Minimum Keyboard and Display 
(MKD), which the user can input voyage related data and view the data 
collected from surrounding AIS equipped vessels. Use of the MKD is 
discussed in Section 4. Optionally, the vessel may be fitted with an interface 
to allow AIS targets to be viewed on a radar, ECDIS or other navigation 
display . Similar features that are available to the VTS operator are then 
available to the ship’s navigator. 
 
 
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Because AIS is essentially a VHF radio system, its range is similar to regular 
VHF radios. Some differences occur because of the digital nature of the AIS 
signal and the fact that the two transmit power options are usually 2 watts 
and 12 watts, as opposed to the 1 and 25 watts that most VHF radios utilise. 
 
Typically, maximum ranges of 20-30 miles will be experienced. Under some 
conditions the maximum range can be both longer and shorter than these 
figures. 
 
In normal coastal use the transmit power of AIS ship stations is set by the shore 
station. Some units have a special 1 watt setting for use by tankers when in oil 
terminals, to meet safety requirements. (See Section 4.8) 
 
VHF signals have quite good ability to ‘bend’ around obstacles (actually by 
diffraction and reflection) and therefore, unlike radar, are not confined to ‘line- 
of-sight’ operation. This can be particularly helpful in coastal and river 
situations when headlands and tall buildings can obscure targets to marine 
radar but will often remain visible on AIS. It should be noted that obscuration 
of AIS signals can still occur in some circumstances, even over relatively short 
distances. 
 
3. AIS Shipborne Equipment 
A typical installation is illustrated in Figure 4. It usually consists of an MKD, a 
transponder unit and associated VHF and GPS antennas. In addition there are 
digital connections to: 
 
• The ship’s navigation equipment 
 
• A pilot port 
 
• Optionally, an ARPA and/or ECDIS 
 
• A long range communications system 
 
 
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Figure 4 : Typical AIS Shipborne Installation 
 
 
The pilot plug (actually a socket) enables the interconnection of a Personal 
Pilot Unit (PPU) to the AIS, allowing pilots to have special operation of the 
equipment. It is essential that this socket is accessible from the pilot’s normal 
position. Further information on siting is available from the International 
Marine Pilots Association (IMPA). 
 
It is important that the equipment is fitted by a competent and qualified 
installation engineer, specifically trained to install the particular manufacturer’s 
AIS equipment. IMO issues guidelines on AIS installation (SN.Circ 227) and it 
is essential that these are followed. 
 
In particular, poorly sited antennas can give significant problems to the correct 
operation of the equipment and also can give interference to other 
communications and navigational equipment. The VHF antenna needs to 
have a good field of view and be in an elevated position. There should be a 
minimum of 2m horizontal distance between it and any other metallic 
(conducting) object. 
 
To prevent interference with the ship’s normal (primary) 
VHF radio system, the AIS VHF antenna and the 
primary VHF antenna should not be on the same level. 
Ideally these antennas should be mounted vertically 
above each other with a minimum of 2 metres vertical 
separation. Otherwise there should be at least a 10 
metre horizontal gap, particularly if they are at the same 
level. Additionally the AIS antenna should be out of the 
beam of radar antennas and at least 3 metres away. 
 
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If users experience regular clicking on the primary VHF when making a call, it 
indicates the possibility of a faulty AIS installation. It is then quite possible that 
when calls are being made on the primary VHF the AIS receive system can be 
totally disabled. 
 
IMO states that ‘the functionality ofthe MKD should be available to the mariner 
at the position from which the ship is normally operated’. In some ships (mainly 
smaller vessels) this can be a particularly difficult requirement to achieve 
because of limited space. However, it is an important requirement to achieve for 
several reasons: 
 
• If used to aid situational awareness the target information must be 
visible at the operating position 
 
• Safety related messages need to be viewed and responded to as they 
are received 
 
• AIS is continually transmitting own ship parameters. If the equipment 
develops a fault it will be more quickly be spotted if it is normally in 
view 
 
 
It should be noted that having AIS targets displayed on an ARPA does not 
normally meet the IMO ‘visibility’ requirement since the full ‘functionality of the 
MKD’ is not generally available on ARPA. 
 
4. Use of AIS at Sea 
It is imperative that the user of an AIS system at sea should be familiarised 
with the operation of the actual equipment that is installed on the vessel 
before use at sea. The operators’ manual should be 
read or an approved equipment specific training guide 
followed. This is necessary to comply with IMO’s 
Standards, Training, Certification and Watchkeeping 
Code (STCW) and the relevant parts of the 
International Safety Management Code (ISM). 
 
AIS transponders on vessels are advised to be left 
switched-on in most ports and should always be left on when at anchorage. 
This should be realised during familiarisation training to avoid the transmission of 
erroneous or training specific data. 
Use of AIS should be incorporated into the ship’s procedures. These should 
cover the checking procedures applied to own-ship transmitted data and the 
recommendations on the use of AIS to improve situational awareness. 
 
The following sections discuss the use of AIS at sea. They are generally 
applicable to all AIS installations but access to the discussed functionality will 
differ from system to system. 
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4.1 Routine Checking Procedures 
The usefulness of AIS suffers greatly when ships are transmitting incorrect data. 
More importantly, ships transmitting incorrect information create a risk to other 
ships, as well as to themselves. Many instances of incorrect data transmission 
are occurring because of wrong settings in static and voyage related data. 
 
It is strongly recommended therefore that the officer in charge of a 
navigational watch checks the following at the 
commencement of the watch: 
 
• That the static data held in the AIS is correct 
 
 
• That the voyage related data is correct 
 
 
 
 
(IMO states that, as a minimum, static data should be checked once per voyage 
or once per month, whichever is shorter). 
 
4.1.1 Static data checks 
 
The static data consists of the following: 
 
o IMO number (where available) 
 
o Ship’s name 
 
o Call sign 
 
o Maritime Mobile Services Identity (MMSI) number 
 
o Type of vessel 
 
o Length and beam 
 
o Location of the position-fixing antennas (external and internal) 
 
 
This data is available for view on the MKD by menu selection. The data should 
be compared with a master printed list kept close to the MKD. If a discrepancy is 
found the vessel’s Master must be immediately informed, who may authorise 
correction to this data. This data must only be changed on specific authorisation 
by the Master and for this reason is password protected. 
 
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The AIS holds the location of both the internal and external position fixing 
antennas. The input parameters for these antennas are defined in Figure 5. 
 
 
 
 
Figure 5: Position fix (GPS) antenna location. 
 
 
As an example, for a ship of 200 metres in length by 30 metres in beam, with 
an antenna mounted 30 metres from the stern, offset from the centre-line by 
5 metres to port, the parameters would be: 
 
A= 170, B = 30, C = 10, D = 20 
 
Special care should be taken when any of the static parameters change, for 
example the location of a position-fixing antenna, to make sure that both the 
check-list and the contents of the AIS memory are correctly altered. 
 
Checking of this data on such a regular basis is recommended as certain AIS 
malfunctions, such as those associated with electrical interference, including 
the effects of electrical storms, may corrupt this data. Also, abuse of the 
equipment during an earlier watch may have resulted in changes to the static 
data. 
 
4.1.2 Voyage related data checks 
 
The voyage related data consists of: 
 
o Ship’s draught 
 
o Hazardous cargo type, if required by competent authority 
 
o Destination and ETA (at Master’s discretion) 
 Distance in metres 
A 0 − 511 
511 = 511 or greater 
B − 511 
511 = 511 or greater 
C 0 − 63; 
63 = 63 m or greater 
D 0 − 63; 
63 = 63 m or greater 
 
Reference point of reported position not available, but dimensions of ship are available: 
A = C = 0 and B ≠ 0 and D ≠ 0. 
 
Neither reference point of reported position nor dimensions of ship available: 
A = B = C = D = 0 (=default ) 
 
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o Total number of persons on-board (obligatory for some port 
administrations) 
 
This data is available from the menu structure of the AIS. 
 
It is essential that the Master gives clear rules concerning entering both the 
hazardous cargo type and the requirement for destination and ETA and that these 
are followed. Disobeying these rules could compromise the security of the vessel 
and contravene port and flag state regulations. 
 
Table 2 is extracted from the IMO regulations concerning ship type and 
hazardous cargo and should be used in entering these parameters. 
Hazardous cargo groups include DG (dangerous goods), HS (harmful 
substances) and MP (marine pollutants). 
AIS MKDs typically present the options within a ‘user friendly’ menu. 
 Table 2 Identifiers used by ships to report their type 
Identifier No. Special craft 
50 Pilot vessel 
51 Search and rescue vessels 
52 Tugs 
53 Port tenders 
54 Vessels with anti-pollution facilities or equipment 
55 Law enforcement vessels 
56 Spare - for assignments to local vessels 
57 Spare - for assignments to local vessels 
58 Medical transports (as defined in the 1949 Geneva 
Conventions and Additional Protocols) 
59 Ships according to Resolution No 18 (Mob-83) 
Other Ships 
First digit (*) Second digit (*) First digit 
(*) 
Second digit (*) 
1 - reserved for 
future use 
0 - All ships of this type - 0- Fishing 
2 - WIG 1 -Carrying DG, HS, or MP 
IMO hazard or pollutant 
category A 
- 1 -Towing 
3 - see right 
column 
2 -Carrying DG, HS, or MP 
IMO hazard or pollutant 
category B 
3 - Vessel 2 - Towing and 
length of the tow 
exceeds 200 m 
or breadth 
exceeds 25 m 
4 - HSC 3 -Carrying DG, HS, or MP 
IMO hazard or pollutant 
category C 
- 3 - Engaged in 
dredging or 
underwater 
operations 
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5 - see above 4 -Carrying DG, HS, or MP 
IMO hazard or pollutant 
category D 
 
5 -reserved for future use 
- 
 
 
 
- 
4 - Engaged in 
diving 
operations 
 
5 - Engaged in 
military 
operations 
6 - Passenger 
ships 
6 -reserved for future use - 6 - Sailing 
7 - Cargo ships 7 -reserved for future use - 7 - Pleasure 
Craft 
8 - Tanker(s) 8 -reserved for future use - 8 - reserved for 
future use 
9 - Other types 
of ship 
9 -No additional information - 9 - reserved for 
future use 
DG: DangerousGoods. 
HS: Harmful Substances. 
MP: Marine Pollutants. 
(*) NOTE - The identifier should be constructed by selecting the appropriate 
first and second digits. 
 
4.1.3 Dynamic data checks 
 
From time-to-time during the watch the OOW should briefly select the screen that 
shows the data that is actually being transmitted. By inspection, the OOW should 
ensure that the dynamic data is replicating the readings given on the ship’s 
navigation sensor displays. The frequency of checks should be increased in 
coastal waters. 
 
Heading offsetsare a particularly common problem and are often caused by the 
offset being incorrectly set at the heading sensor. This can happen on older 
equipment when a digital converter has had to be retro-fitted in order to provide 
data to the AIS. On these systems, since the user normally uses only the 
‘analogue’ display, the digital offset is not always immediately obvious. 
 
Unfortunately, on some AIS equipment, viewing of 
own ship’s transmitted data is under password 
protection in an ‘Engineering Mode’. The password 
for entering this area also allows static data to be 
altered. On ships with such equipment it is often 
recommended that the Master, or the Master’s 
appointee, checks this data on a once-daily basis. 
 
 
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4.2 Entering of ‘non-static’ data 
4.2.1 Voyage related data 
 
At the start of each voyage ‘voyage related data’ should be entered into the 
AIS by selecting the correct page in the MKD menu structure. Since this can 
be awkward on some MKDs, a computer may be interconnected to assist the 
input of data. 
 
In all instances draught should be entered (in metres). This should be the 
maximum draught for the voyage and amended as required, for example as the 
result of de-ballasting prior to port entry. Normally the destination port, ETA (date 
and time) and hazardous cargo rating should be entered. On the Master’s 
instructions, either or both of these could be omitted. There may be national, 
commercial, security or other reasons that will determine the Master’s decision, 
although flag and port state instructions will normally need to be observed. 
 
Many port states will also require the total number of persons onboard to be 
entered. 
 
The hazardous cargo rating is probably selected by menu dropdowns, or a 
similar tool. Table 2 shows the IMO defined possibilities. 
 
As it becomes apparent that the previously entered data needs updating this 
should be performed, according to the ship’s procedures. Obviously, draught may 
change according to the water type and loading and therefore should be kept 
updated. 
 
4.2.2 Dynamic Data 
 
The only dynamic data that needs to be manually input is the Navigation Status 
of the vessel. This is normally accessed by a dropdown list in the AIS menu 
structure and will have terms such as: 
 
o Underway by engines 
 
o At anchor 
 
o Not under command (NUC) 
 
o Restricted in ability to manoeuvre (RIATM) 
 
o Moored 
 
o Constrained by draught 
 
o Aground 
 
o Engaged in fishing 
 
o Underway by sail, etc 
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Changes in navigational status should take place concurrently with the 
necessary changes in lights and shapes. 
 
4.3 Safety related messages 
There is a ‘text’ message service on AIS which should be used only for safety 
related messages (SRM). Misuse of this facility has led to action on 
perpetrators by maritime administrations. Its use is not confined to emergency 
situations but should be relevant to the safety of navigation. In emergency 
situations full use of GMDSS should be made - SRMs then provide an 
additional means of communication, if appropriate. 
 
Messages can be sent to individual vessels (and coastal stations), or broadcast 
to all stations. When sending to individual vessels the maximum message length 
is 156 characters for an addressed message and 161 for a broadcast message. 
 
When an SRM is received, the AIS will alert the user. An appropriate response 
should be carefully considered. Facilities are normally provided to reply, forward, 
store or delete the message. 
 
Sending a message is straightforward using the menu pull downs of the MKD. 
To send an addressed message it is normally only necessary to ‘highlight’ the 
intended recipient vessels, avoiding having to re-enter MMSI information, etc. 
 
Great care should be taken in assessing whether AIS is the best tool for 
communicating specific data. For ship-to-ship communication VHF voice is often 
preferable, as it elicits an instant response as to whether the message ha been 
received and understood. AIS can be useful to communicate safety information 
widely but it should not be assumed that all ships will receive or notice the 
message. 
 
Messages to a single vessel should be acknowledged by the recipient to confirm 
receipt. This can be by AIS or VHF radio. 
 
4.4 Channel Management 
This is normally an automatic function and, as stated in Section 2.1 can 
involve the use of the inbuilt DSC function. In ocean regions, when there is no 
shore station control, ships’ AIS units will normally operate on the 
internationally defined channels, 87B and 88B. 
 
The shore stations send special commands to ships’ AIS equipment that define 
operating areas, with frequencies and other data to enable AIS to operate 
correctly. In some circumstances it may be necessary for the OOW to modify 
these settings. The ship’s AIS manual will have instructions to perform these 
settings. 
 
These settings should only be changed manually for a specific reason, which 
would normally be communicated to vessels by a competent authority 
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4.5 Use of AIS to aid situation awareness and the making of collision 
avoidance decisions 
It should be stressed that best principles in the use of AIS to aid collision 
avoidance decisions are still being developed in the light of experience. Great 
caution should therefore be applied in using AIS data to aid collision 
avoidance. However, AIS in some configurations does give the mariner a 
great deal of useful information to aid situational awareness. This should be 
its main navigational use. The use of AIS, particularly when confined to 
displaying data on an MKD, should be considered when determining bridge 
resource management. 
 
 
4.5.1 MKD based systems 
 
If a truly minimum system, as defined by IMO is in use then the display of targets 
will be limited to a simple list. Options to order this list by range or bearing may 
be provided. 
 
From this list it may be possible to pair (correlate) visual and or radar targets with 
corresponding AIS target information. This will need careful radar and/or chart 
work. It can yield some useful information about the likely intentions of the vessel 
that may help in enhancing situational awareness, but: 
 
• Manual target ‘correlation ‘ may be in error 
 
• The AIS information on that target may be in error, including 
o Dynamic information errors, eg heading errors 
o Navigational status errors, eg at anchor status but actually 
underway 
 
o Static data errors, eg length 
 
o Voyage related errors, eg draught 
 
• Using the vessel call sign to make radio contact to pre-agree a collision 
avoidance manoeuvre would remain highly dangerous in many situations 
because of this uncertainty 
 
Many so-called MKDs have an enhanced ‘graphical’ display system that can 
show AIS targets as a radar-type image with own-ship at the centre of the 
display. (This feature is not part of IMO’S definition of a Minimum Keyboard and 
Display). Graphical displays have the advantage of the operator being able to get 
a reasonable awareness of the displayed situation since: 
 
• All received AIS targets are graphically displayed visible, rather than just 
looking at one target at a time on a list 
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• The polar display helps orientate the navigator to correlate targets visually 
by ‘looking out of the window’ or using a radar display. However, bear in 
mind that their displays are usually ‘head-up’; the radar may be in use in 
‘North-up’ mode. 
 
On its own, the graphical display of the enhanced MKD may give some 
unwarranted confidence about an apparent situation. The displayed 
information can be incomplete or misleading - and sometimes can be in gross 
error. The incompleteness comes from vessels which are not fitted with AIS or 
have a completely non-functioning system and will not be shown on the 
enhanced MKD display. It is also quite common for vessels to be 
broadcasting erroneousAIS data. 
 
 
4.5.2 ARPA and Electronic Chart Display of AIS targets 
 
AIS targets are perhaps best displayed on an ARPA or ECDIS. Closest point of 
approach (CPA) and time to closest point of approach (TCPA) are 
automatically calculated for selected targets. Symbols for the display of AIS 
targets on such displays have been internationally agreed and are illustrated 
here in Figure 6. 
© Videotel 2020
 
 
 
Figure 6: AIS Symbols for ARPA and ECDIS 
 
The ‘geographic’ display of AIS targets combined with radar and/or electronic 
chart data gives a very powerful tool for improving situational awareness. 
However, even when displayed on such screens, AIS data should never be used 
as the sole source of information in making collision avoidance decisions. This is 
because there is a significant possibility that the data being transmitted by the 
vessel is erroneous or has been corrupted in its transmission or reception paths, 
and that there may be non-AIS fitted vessels in the vicinity. Looking ‘out of the 
window’ is also critically important. The only exception would be in a rare 
emergency where AIS data may be the only information available to help 
alleviate a dangerous situation. 
 
The use of AIS does not negate the responsibility of the OOW to comply at all 
times with the Collision Regulations. 
© Videotel 2020
 
However, in many situations AIS data can give enhanced performance compared 
to ARPA. For instance, it is not affected by clutter, target swop or from target 
loss, following a rapid manoeuvre. Also, turn information can be more quickly 
apparent, as it does not have to be inferred by the ARPA from radar returns. 
Additionally, AIS provides vessel identity, navigational status and other 
parameters that are not determinable by radar. 
 
When AIS targets are displayed on an ARPA the ‘raw’ radar data has to remain 
clearly visible. ARPA tools, (eg CPA calculations and alarms) will also be 
effective on AIS targets. Selecting an AIS target will display at least the target 
motional information, which will be supplemented by the vessel’s MMSI and 
probably other parameters such as ship’s name. The user interface for these 
tools will replicate those used on the ARPA. 
 
The motional information depends on whether the display is ground or sea 
stabilised, according to the following table: 
 
 
 
Ground Stabilised Sea Stabilised 
Position Position 
Course over ground Course through the water 
Speed over ground Speed through the water 
Heading Heading 
Rate of turn or direction of 
turn* 
Rate of turn or direction of 
turn* 
* As available 
 
Effectively an ARPA is given two additional modes: 
 
• Display of AIS targets 
 
• Simultaneous display of ARPA and AIS targets 
 
In the ‘simultaneous’ mode manufacturers are required to present AIS targets 
in preference to ARPA. It should be remembered that this may obscure vital 
ARPA information. 
 
The equipment may provide an automatic function to ‘correlate’ AIS and 
ARPA information in the ‘simultaneous’ mode and show a single vector. In this 
case the operator will be able to make some adjustments to the criteria for 
identifying when an AIS and an ARPA target may be considered to be the 
same vessel. For instance, this may include setting parameters on the 
magnitude of the difference in their positions, courses and speeds, which 
must not be exceeded as the criteria for targets to be combined. 
© Videotel 2020
 
Great care should be taken with the use of such a display mode and with 
inputting suitable correlation criteria in accordance with the Master’s instructions. 
 
 
4.6 AIS Alarms 
The on-board has a number of alarm functions concerning the integrity of many 
of its functions. As a minimum the following alarms are given: 
 
Alarm’s description text Reaction of the system 
AIS: Tx malfunction Stop transmission 
AIS: Rx VSWR exceeds limits Continue operation 
AIS: Rx channel 1 malfunction Stop transmission on affected channel 
AIS: Rx channel 2 malfunction Stop transmission on affected channel 
AIS: Rx channel 70 malfunction Stop transmission on affected channel 
AIS: General failure Stop transmission 
AIS: MKD connection lost Continue operation with "DTE" set to 
1 
AIS: External EPFS lost Continue operation, apply sensor fallback 
AIS: No sensor position in use Continue operation, set pos. accuracy 
to 0 
AIS: No valid SOG information Continue operation using default data 
AIS: No valid COG information Continue operation using default data 
AIS: Heading lost/invalid Continue operation using default data 
AIS: No valid ROT information Continue operation using default data 
 
These alarms will be repeated every 30 seconds, until acknowledged. 
 
On an ARPA or ECDIS alarms for AIS target conditions will also be generated, 
following those already used for ARPA targets. 
 
 
© Videotel 2020
 
4.7 Security and confidentiality 
All AIS units have a security mechanism that detects the disabling of the AIS. 
This consists of a memory function that records all periods when the AIS 
installation is non-functioning or is switched off. The memory is not accessible 
by the user and is not affected by power-outs. As a minimum it records the 
last 10 times when the equipment was non-functioning for more than 15 
minutes. This information includes UTC and the duration of the ‘off’ periods. 
 
In some circumstances, when it is believed that that the continued operation of 
AIS might compromise the safety or security of a vessel, it is quite legitimate 
for the master to order the equipment to be switched off. For example: 
 
• When there is a danger of pirates utilising AIS information to aid locating 
and boarding a particular vessel 
 
• In an oil terminal when the AIS does not have a 1 watt mode or there is a 
danger that a coastal station will remotely set the high power mode 
 
• When it is interfering with other critical systems on board the vessel 
 
In such circumstances the event should be recorded in the ship’s log book. 
EXCEPT ON THE MASTER’S INSTRUCTIONS THE UNIT MUST NOT BE 
SWITCHED OFF. 
 
When entering any data manually, consideration should be given to the 
confidentiality of the information, especially when international agreements, rules 
or standards provide for the protection of navigational information 
 
 
4.8 Oil terminals 
Oil transfer regulations from the International Safety Guide for Oil Tankers and 
Terminals (ISGOTT), state that radio transmission from a vessel within an oil 
terminal must be limited to 1 watt maximum. It should be realised that low power 
operation of an AIS is normally set at 2 watts, although some systems have a 1 
watt special setting for use within oil terminals. 
 
If this setting is not provided the Master should order the AIS to be switched off 
within such terminals. 
© Videotel 2020
 
5. Check-lists 
New users of AIS may find it useful to refer to check-lists based on the following 
examples: 
 
Before every voyage At change of navigation status 
 
• Check that the equipment is switched on • Input new navigational status, 
eg: 
 
o Underway by engines 
o Restricted in ability to 
manoeuvre 
o Constrained by draught 
o Moored 
o At anchor 
• Check that there are no error messages 
displayed. If there is a displayed error 
check for obvious causes; inform Master 
• Check correct navigational status is 
entered, eg; 
 
o Moored 
o At anchor 
• Check static data; inform Master if errors 
are apparent: 
 
o IMO number 
o Ship’s name 
o Call sign 
o MMSI number 
o Type of vessel 
o Length and beam 
o Location of position- 
fixing antennas 
At change of watch 
• Check for error messages 
• Check navigational status 
• Check validity of voyage related 
data and adjust if appropriate, 
following Master’s instructions 
• If time and resources permit check 
static data (as above) 
During watch 
• Utilise AIS data appropriately to 
improve situational awareness, after 
assessing validityof received data 
for vessels of interest. 
• DO NOT USE AIS DATA ALONE IN 
MAKING COLLISION AVOIDANCE 
DECISIONS 
• Occasionally check for validity of 
transmitted (own ship’s) dynamic 
data: 
 
• Ascertain, enter and check voyage 
related data, (as directed by Master): 
 
o Ship’s draught 
o Hazardous cargo type 
o Destination and ETA 
o Number of persons onboard 
o Position 
o SOG 
o COG 
o Heading 
o ROT (if available) 
 
© Videotel 2020
 
• Be aware of and respond to error 
messages 
• Respond appropriately to received 
messages 
• When appropriate, use AIS to send safely 
related messages, after assessing all 
options 
• Follow Master’s instructions on use of 
AIS in situations which may be a security 
risk, such as potential piracy 
 
Appendix 
Common problems of AIS 
 
When I use my VHF at sea there are clicks and buzzes every 6 (or) 10 
seconds. 
 
(i) The VHF and AIS antennas have been placed incorrectly with respect to 
each other. Recall the installation engineer, or: 
 
(ii) Inappropriate, or low-grade cable has been used to connect the AIS 
antenna. Recall the installation engineer 
 
 
My heading is being broadcast incorrectly (on ships retro-fitted with a 
digital interface to an existing heading indicator) 
 
The offset of the digital converter has not been set correctly. Refer to heading 
indicator manual. 
 
 
Most vessels are only displaying their MMSI (often where ship’s name is 
expected). 
 
This indicates that there is a poor reception of AIS data from other vessels causing 
many transmissions from other vessels to be missed. Call service engineer. 
 
 
On some ships there is poor tie-up with their position on radar. 
 
Many vessels have incorrectly installed or operated AIS. Be wary about all AIS 
data until properly assessed against visual or radar data. Unless all vessels 
display a rotational or a linear shift compared to radar it is unlikely to be an own-
ship problem.