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Automatic Identification System (AIS)

AIS is a shipboard transponder system in which ships continuously transmit their "Static" and "Dynamic" data to other nearby ships and shore authorities on a common VHF channel.

Principle:- In AIS the principle used to transmit data on a single VHF channel is obtained by SOTDMA i.e. Self Organized Time Division Multiple Access. Under SOTDMA each minute of time is divided into 2250 time slots and used by multiple vessels in a single time period.

Purpose:- Security of the flag or the coastal state. The only difference is flag state has got the right to have position information as well as track the movement of any ship which is within its near coastal water.

coastal water means its range is restricted to 35-40 NM, So AIS equipment works on a VHF frequency. VHF frequency is also used for oral ship to ship or ship to shore or shore to ship connections. To avoid interference, AIS works on two dedicated channels 87B (161.975 MHz) and 88B (162.025 MHz), no other communication can take place on these dedicated channels or frequency.

🖛 AIS equipment suppose to do - Transmit four basic information. They are 

• Dynamic Data - Data that is continuously changing
• Static Data - Data that is permanent with that ship
• Voyage Data - Information of the voyage, that particular voyage of the ship
• Safety Data - Any information related to the safety of life, property, and environmental protection or navigational safety.

All this information when it is put into AIS equipment it is computerized or changed into computerize language or digitalized language and then it is compressed and at any one-time maximum amount of information that can be transmitted by any AIS equipment is restricted to 254 bits of information.

Dynamic Data - Mandatory information as per IMO

• Identity of the ship in the form of MMSI No.

• position of the  ship in Latitude & Longitude

• Heading, course, and speed of the vessel


• Navigation Status of the vessel

• ROT if the Vessel is restricted with the ROT indicator.

• UTC date and time of a position that was transmitted 

All this information is not to be fed in, the only information to be fed in by the OOW is the navigation status of the vessel. Rest all information comes in directly from the various navigation aids which are connected to the AIS equipment and that is -

• Position from GPS 

• UTC date and time of position from GPS

• SMG & CMG from GPS

• Heading from gyro Compass

• Speed from Log

• ROT of Vessel, from ROT indicator

Static Data - Information to be transmitted is mandatory 

• Identity of the ship in four forms - Name of the vessel, Radio Call Sign, MMSI No., IMO No.

• Port of Registry (Flag of Vessel)

• Type of Vessel

• Length & Breadth of Vessel

• Location of the AIS antenna onboard the vessel

These are mandatory static data that is to be transmitted. Static Data is fixed and cannot be changed. It is fed in by the manufacturer when the equipment was installed, which is permanently stored in the AIS equipment.

Voyage Data - Not mandatory to be transmitted. If the master of the vessel agree then only information will be fed in -

• Next port of call

• ETA of next port of call

• Nature of cargo onboard

• Draft of the vessel forward and aft

• No. of passengers onboard the ship

• Once fed in, they will be transmitted automatically.

Safety Data - Any information regarding the safety of navigation, the safety of life, the safety of property, and environment protection. If there is such information, OOW can be fed it or not mandatory so that it does not occupy the space.

🖛 Static and voyage-related data is transmitted every 6 minutes. Safety data whenever needed. Dynamic data is transmitted as follows: 
• At Anchor - 3 minute
• 0 - 14 kts - 12 Sec, Changing Course - 4 Sec.
• 14 - 23 kts - 6 Sec, Changing Course - 2 Sec.
• 23+ kts - 3 Sec, Changing Course - 2 Sec.

• Every ship is not expected to transmit all four information data. some ships might have all 4 data, some ships only 3, and some ships only 2.

• The amount of data that is going to be transmitted by every ship AIS is not going to be the same.

• The maximum amount of information permissible under AIS protocol is always restricted to 254 bits of info.

• This 254 bits of information is transmitted at a fixed rate of 9600 bits/sec (9.6 k bits/sec)

• So to transmit 254 bits of information maximum time taken is 26.67 milliseconds and that time is called one slot. So in one minute of time, there are 2250 slots.

• AIS works on two different channels at the same time that means a total number of communication at least that can take place in one minute on two working channels can be4500.

• Every ship does not have 254 bits of information, so the time taken to transmit every ship data can be lesser than one slot time, the balance of slots is not wasted. That slots can be utilized or balanced by any other ship's AIS to transmit its own data.

• So in one particular slot, up to 4 different transmissions can take place depending upon what is the quantity of bits of information that is transmitted by every AIS of the ship.

• So up to 4 different transmissions in one AIS in one slot. So a maximum number of communication that can take place in one minute of time on two working channels. It can be as high as 18000 communication/minute of time.

• All this AIS equipment works on their own, organized among themselves, So the principle of working is Self Organized Time Division Multiple Access.

Related Post:-

🖛 Use of AIS in Collision Avoidance and SAR Operations

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