AIS
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Contents
Overview
The Automatic Identification System (AIS) is an automated, autonomous tracking system used for the exchange of navigational information between AIS-equipped vessels. In AIS, static and dynamic vessel information can be electronically exchanged between AIS-receiving stations (onboard, ashore or satellite).
Performance standard, testing standard & carriage requirement
Performance standard & ITU Resolution
- IMO Resolution MSC.74(69) contains the recommendation on performance standards for a universal shipborne AIS
- ITU Resolution ITU-R M.1371 - Technical characteristics for an automatic identification system using time division multiple access in the VHF maritime mobile frequency band
Testing standard
- IEC 61993-2 - Maritime navigation and radiocommunication equipment and systems - Automatic identification systems (AIS) - Part 2: Class A shipborne equipment of the automatic identification system (AIS) - Operational and performance requirements, methods of test and required test results
Carriage requirement
- The compulsory carriage requirements for AIS are covered in SOLAS V Regulation 19.2.4, under which compulsory carriage of AIS is limited to: "all ships of 300 gross tonnage and upwards engaged on international voyages and cargo ships of 500 gross tonnage and upwards not engaged on international voyages and passenger ships irrespective of size".
- There is no requirement for small commercial vessels or recereational vessels to carry AIS, however should they chose to do so they should be fitted with an AIS B transponder or receiver.
Annual testing
- SOLAS Chapter V Regulation 18.9 requires an annual test of shipborne AIS (as set out in MSC.1/Circ.1252: Guidelines on annual testing of AIS).
- Guidance on the United Kingdom's implementation of the requirements of MSC.1/Circ.1252 is set out in MGN 465 (M+F)
What it does
AIS ship station transponders include a GPS receiver (and may also be interfaced to other sensors) which collect the subject vessel's position and movement details. Such dynamic details along with other static information provided by the vessel's crew are automatically broadcasted at regular intervals via a built-in VHF transmitter using 2 specific VHF channels known as AIS 1 and AIS 2 (161.975 Mhz and 162.025 Mhz - 87 & 88 old VHF channels).
The periodic AIS-data information can be received by other vessel or base stations, provided they are within range. Then, with the use of special software, it can be processed and depicted on chart plotters or on computers (for example, on the MarineTraffic Live Map). AIS-data can also be received by satellites - in this case, the term Sat-AIS is used (Satellite AIS or S-AIS).
How it is used
The availability of the AIS information to the public domain led to a drastic change regarding the initial perception of the use of AIS. AIS was originally developed by IMO (International Maritime Organisation) as a standard which would help vessels to avoid collisions while assisting port authorities to control marine traffic with more efficiency. However, the fact that its capabilities could become applicable to a wider spectrum of maritime-related business fields soon became clear .
Nowadays, AIS information is used to serve various purposes and facilitates the work of people in various occupations, such as (among others):
- Port Authorities and Harbor Masters
- Ship Owners, Managers and Builders
- Ship Agents, Brokers and Charterers
- Researchers and Data Analysts
- Tug Operators and Pilots
- Search and Rescue teams
- Flag administrators and Classification Societies
- Vessels' crews and their families' members
- Coast Guard and Border Patrol
- Hotels and Tour operators
- Passengers or recreational sailors
- Environmental Protection agents
- Maritime Enthusiasts and Radio-amateurs
How it works
SOTDMA
Class A AIS transponders (usually onboard SOLAS-compliant vessels), make use of the SOTDMA protocol - the name stands for Self Organised Time Division Multiple Access:
- All AIS transmissions are based on GPS derived UTC time. This enables for accurate determination and global synchronisation of the start time of each transmission;
- Each vessel pre-occupies certain time-slots so that no transmission from other vessels sailing on the same group is made on the exact same time (ie using the same slot);
- Each UTC second can be divided to up to 2250 such slots. This means that one AIS data transmission can be made every 26.67 milliseconds;
- Each vessel occupies certain slots depending on her moving status, speed and course - for example, a vessel sailing faster than 14 knots while changing course would be transmitting AIS signals every 2 seconds. Upcoming transmissions will make use of the time-slot used before (pre-occupied);
- Vessels within a self-organised group automatically avoid making use of slots that are already marked for use by another vessel;
- When a vessel is moving from an area to another, it is possible to came across another vessel that uses the same slot. In such a case, the station will modify its own slot allocation looking for a free slot;
The result is a completely autonomous system able to self organise based on temporal and spatial criteria.
Even in very busy areas where all available slots are occupied, SODTMA is able to manage the situation using a slot re-using approach by prioritising the allocation of slots to stations that are closer to each other:
- The transponder scans for available slots;
- Occupies and reserves those that it will be using;
- Starts transmitting according to its selection. The slot used is also transmitted (UTC seconds) so that other nearby vessels can do the same (scan, occupy, transmit);
CSTDMA
Class B AIS transponders (used by non-SOLAS vessels) make use of the CSTDMA (Carrier Sense Time Division Multiple Access) protocol which interweaves with Class A transmissions in a way that does not cause mix-ups by giving priority to SOTDMA transmissions. The overall logic remains the same:
- The transponder scans for an available slot;
- If it is found, it makes use of it;
- If no empty slot is found, the transmission is delayed awaiting for a free slot;
- Once the transmission is made, the same process repeats.
Note that AIS transmissions may get scrambled and/or lost when received by satellites (Sat AIS). This can happen because satellites have a wide reception footprint which includes multiple groups of AIS-equipped vessels that transmit different data packets using the same slots - this situation is know as "packet collision".
Detailed requirements
(Performance Standard)
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