The world of AIS (or Automatic Identification System) can often be confusing, with many questions arising, such as, “What is AIS?” “Why do I need it?” and “What type of AIS does my ship need or have?” according to Marine Insight. Lets delve deeper into the topic.
About AIS
An automatic Identification System (AIS) is an automated tracking system that displays other vessels in the vicinity. The broadcast transponder system operates in the VHF mobile maritime band.
Your ship also shows on the screens of other nearby vessels, provided your vessel is fitted with AIS. If AIS is not fitted or not switched on, there is no exchange of information on ships via AIS.
The AIS onboard must be switched on at all times unless the Master deems it to be turned off for security reasons. The working mode of AIS is continuous and autonomous.
Why is AIS Provided?
It is fitted on ships for the identification of ships and navigational marks. However, it is only an aid to navigation and should not be used for collision avoidance. Vessel Traffic Services (VTS) ashore use AIS to identify, locate, and monitor vessels. The Panama Canal uses the AIS to provide information about rain along the canal and wind in the locks.
AIS Types
- Class A: Mandated for all vessels 300 GT and above engaged on international voyages as well as all passenger ships
- Class B: Provides limited functionality and is intended for non-SOLAS vessels. Primarily used for vessels such as pleasure crafts
AIS operates principally on two dedicated frequencies or VHF channels:
- AIS 1: Works on 161.975 MHz- Channel 87B (Simplex, for the ship to ship)
- AIS 2: 162.025 MHz- Channel 88B (Duplex for the ship to shore)
It uses Self Organizing Time Division Multiple Access (STDMA) technology to meet the high broadcast rate. This frequency limits the line of sight, which is about 40 miles.
Working Of AIS
How does AIS work exactly? How do we obtain all this data? Originally, AIS was used terrestrially, meaning the signal was sent from the boat to land and had a range of roughly 20 miles (also considering the earth’s curvature). As ships began sailing further away from land, they started sending the signal to low-orbit satellites, relaying information back to land. This meant ships could sail as far as they liked, and we’d always have peace of mind knowing exactly where they were and how they were doing.
The AIS system consists of one VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link to shipboard display and sensor systems. Position and timing information is normally derived from an integral or external GPS receiver. Other information broadcast by the AIS is electronically obtained from shipboard equipment through standard marine data connections.
Although only one channel is necessary, each station transmits and receives over two radio channels to avoid interference and communication loss from ships. A position report from one AIS station fits into one of the 2250 time slots established every 60 seconds. AIS stations continuously synchronize themselves to avoid overlap of slot transmissions.
It’s pretty easy to install as well, as AIS is generally integrated with ship bridge systems or multifunctional displays, but installing a standalone system is as straightforward as plugging in a couple of cables and switching on the plug.
AIS As a Surveillance Tool
In coastal waters, shoreside authorities may establish automated AIS stations to monitor the movement of vessels through the area.
Coast stations can also use the AIS channels for shore to ship transmissions and to send information on tides, NTMs and located weather conditions.
Coastal stations may use the AIS to monitor the movement of hazardous cargoes and control commercial fishing operations in their waters. AIS may also be used for SAR operations enabling SAR authorities to use AIS information to assess the availability of other vessels in the vicinity of the incident.
AIS As an Aid to Collision Avoidance
AIS contributes significantly to the safety of navigation. All the information transmitted and received enhances the effectiveness of navigation and can greatly improve situational awareness and the decision-making process.
As an assistant to the OOW, the tracking and monitoring targets by the AIS and determining information on the CPA and TCPA add great value to the safety of navigation overall.
However, the user should not rely solely on the AIS information for collision avoidance. AIS is only an additional source of information for the OOW and only supports the process of navigating the vessel. AIS can never replace human expertise on bridges!
Limitations of AIS
As with all navigational and/or electronic equipment, the AIS has limitations:
- The accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted
- The position received on the AIS display might not be referenced to the WGS 84 datum
- Over-reliance on the AIS can cause complacency on the part of the OOW
- Users must be aware that the AIS might transmit erroneous information from another ship
- Not all ships are fitted with AIS
- The OOW must be aware that AIS, if fitted, might be switched off by a certain vessel, thereby negating any information that might have been received from such a ship.
- It would not be prudent for the OOW to assume that the information received from other ships might not be fully accurate and of precision that might be available on its vessel.
To sum it up, the AIS only improves the safety of navigation by assisting the OOW/VTS or whatever entity. It’s pretty easy to install as well, as AIS is generally integrated with ship bridge systems or multifunctional displays, but installing a standalone system is as straightforward as plugging in a couple of cables and switching on the plug.
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Source: MarineInsight
