Automatic Identification System (AIS) an Overview

Overview of Automatic Identification System

The world of Automatic Identification System (AIS) can often be a confusing one to delve into.  Many questions arise such as “what is AIS?”, “why do I need it?”, and “what type of AIS does my ship actually need or have?”

Automatic Identification System (AIS) is an automated tracking system that displays other vessels in the vicinity. It is a broadcast transponder system which operates in the VHF mobile maritime band.  Your vessel will show on the screens of other vessels in the vicinity when it is fitted with AIS.  There is no AIS exchange of information on ships if AIS is not fitted or not switched on.  AIS on-board must be switched on at all times.  Only the Captain may decide it must be turned off for security or other reasons. The Automatic Identification System is continuous and autonomous in working mode.

Why is AIS provided?

Automatic Identification System identifies other ships and navigational marks when installed.  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.

virtual aid on radar display
AIS – Automatic Identification System – VirtualAid on Radar Display

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 intended for non SOLAS vessels. Primarily used for vessels such as pleasure crafts

Automatic Identification System operates principally on two dedicated frequencies or VHF channels:

(AIS 1): Works on 161.975 MHz- Channel 87B (Simplex, for ship to ship)
(AIS 2): 162.025 MHz- Channel 88B (Duplex for ship to shore)
It uses Self Organizing Time Division Multiple Access (STDMA) technology to meet the high broadcast rate.  This frequency has a limitation of line of sight which is about 40 miles or so.

Working: How does AIS work exactly? How do we obtain all this data?

Signals sent from boat to land are terrestrial.  AIS began with terrestrial usage.  It had a range of roughly 20 miles (also taking into account the curvature of the earth).  As ships began sailing further and further away from land, they began sending the signal to low orbit satellites.  These satellites then relayed information back to land.  This meant ships could sail as far as they like. Simultaneously, we’d always have peace of mind knowing exactly where they are and how they’re doing.

The AIS system consists of the following components.  One VHF transmitter, two VHF TDMA receivers, one VHF DSC receiver, and a standard marine electronic communications link.  This link connects to shipboard display and sensor systems.  An integral or external GPS receiver normally derives position and timing information.  Other AIS broadcast information is electronically obtained.  This information comes from shipboard equipment through standard marine data connections.

Although only one channel is necessary, each station transmits and receives over two radio channels.  This avoids interference and communication loss from ships.  A position report from one Automatic Identification System station fits into one of 2250 time slots established every 60 seconds. AIS stations continuously synchronize themselves to each other, to avoid overlap of slot transmissions.

It’s pretty easy to install as well.  AIS generally integrated with ship bridge systems or multifunctional display.  Installing a standalone system is as straightforward as plugging in a couple of cables and switching on the plug.

ship virtual AIS display
Ship Virtual AIS Display

AIS Data Transmitted

 1. Static Information (Every 6 minutes and on request):

MMSI number
IMO number
Name and Call Sign
Length and Beam
Type of ship
Location of position fixing antenna

2. Dynamic Information (Depends on speed and course alteration)

Ship’s position with accuracy indication
Position time stamp (in UTC)
Course Over Ground (COG)

3. Voyage Related Information (Every 6 minutes when data is amended or on request)

Ship’s draft
Type of cargo
Destination and ETA
Route plan (Waypoints)

4. Short safety related messages

Free format text message addressed to one or many destinations or to all stations in the area.  This content could be warnings such as a buoy missing, iceberg sighting, etc.

AIS as a Surveillance Tool

In coastal waters, shore side 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, 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.  SAR operations may also use Automatic Identification System (AIS).  This enables SAR authorities to use AIS information to assess availability of other vessels in the vicinity of the incident.

AIS as an Aid to Collision Avoidance

Automatic Identification System contributes significantly to the safety of navigation.  All the information transmitted and received enhances the effectiveness of navigation.  This can greatly improve situational awareness and the decision making process.  AIS tracking and monitoring of targets assists the OOW in safe navigation.  CPA and TCPA information combine with AIS to improve navigation. However, the user should not solely rely on information from AIS for collision avoidance.  AIS is only an additional source of information for the OOW.  It only supports the vessel navigating process.  AIS can never replace human expertise on the bridge!

Automatic Identification System Display
Automatic Identification System Display

Limitations of AIS

As with all navigational and/or electronic equipment, the AIS has limitations:

    1. The accuracy of AIS information received is only as good as the accuracy of the AIS information transmitted
    2. Position received on the AIS display might not be referenced to the WGS 84 datum
    3. Over reliance on the AIS can cause complacency on the part of the OOW
    4. Another ship may transmit erroneous information from it’s AIS.
    5. Not all ships are fitted with AIS
    6. The OOW must be aware AIS, if fitted, might be switched off by a certain vessel thereby negating any information that might have been received from such ship
    7. It would not be prudent for the OOW to assume information received from other ships might not be fully accurate and of precision that might be available on own 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.  hip bridge systems or multifunctional display integrate AIS.  Installing a standalone system is as straightforward as plugging in a couple of cables and switching on the plug.


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