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How the automatic identification system works
There are two dedicated frequencies used for AIS:
- AIS 1 (channel 87B)
- AIS 2 (channel 88B).
Each frequency is divided into 2250 time slots that are repeated every 60 seconds. The AIS units send packets of information which are transmitted in these time slots.
Learn more about how the AIS works by viewing our interactive AIS tutorial.
How displays can vary
Shipborne AIS units autonomously broadcast different AIS messages including:
- 'dynamic data' which includes latitude, longitude, position accuracy, time, course, speed, navigation status
- 'static data' which includes name, dimensions, type, draft, destination and estimated time of arrival.
Position reports are broadcasted frequently (between 2–10 seconds depending on the vessel’s speed, or every 3 minutes if at anchor), while static and voyage related reports are sent every 6 minutes.
It is common for an AIS user to receive numerous position reports from a vessel before the vessel’s name and type.
Displays on public websites
Some publicly available AIS websites are a crowd-based approach to AIS information. The International Maritime Organization (IMO) does not support the display of AIS on public websites. The AIS receivers used for this purpose are not always certified AIS base stations and may not provide accurate or valid data.
Under SOLAS and the relevant IMO guidelines, the master of any vessel has the discretion to turn off the AIS unit if its continual operation might compromise the vessel's safety or security.
Long range identification and tracking
Long range identification and tracking (LRIT) is not the same as AIS and does not replace AIS. AIS cannot be used for LRIT.
LRIT is an international system used to monitor the location of vessels travelling within 1000 nautical miles off the Australian coast. Vessels must willingly provide their location to the LRIT system and its use of satellites mean it can be used anywhere in the world. LRIT was adopted by the International Maritime Organization as an amendment to Chapter V of SOLAS and came into force on 1 January 2008.
Find out if your vessel should be using LRIT systems.