Automatic Identification System — principles, carriage requirements, data transmitted, modes, limitations
11 lessonsabout 12 min reading1 visuals and tools10 scenario questions3 oral questions
Lesson 1 of 11 · 1 min
AIS (Automatic Identification System) is a VHF transponder system that automatically broadcasts a vessel's identity, position, course, speed, and other data to other vessels and shore stations. It operates on two dedicated VHF channels (161.975 MHz and 162.025 MHz) using TDMA (Time Division Multiple Access) technology.
Say it out loud. Talk “AIS principles and message types” through with the live Tutor.
Start a live lesson →Lesson 2 of 11 · 1 min
| Vessel type | AIS required |
|---|---|
| All vessels 300 GT and over on international voyages | Class A AIS mandatory |
| All cargo vessels 500 GT and over | Class A AIS mandatory |
| All passenger vessels regardless of size | Class A AIS mandatory |
| Vessels below mandatory threshold | Class B AIS recommended — may be fitted voluntarily |
Say it out loud. Talk “AIS principles and message types” through with the live Tutor.
Start a live lesson →Lesson 3 of 11 · 1 min
Static Data (entered once)
Dynamic Data (updated automatically)
Voyage-Related Data (updated by crew)
Say it out loud. Talk “AIS data — static, dynamic and voyage” through with the live Tutor.
Start a live lesson →Lesson 4 of 11 · 1 min
| Class A | Class B | |
|---|---|---|
| Required on | SOLAS vessels (300 GT+) | Non-SOLAS / smaller vessels (voluntary) |
| Transmit power | 12.5 watts | 2 watts |
| Update rate (underway) | Every 2–10 seconds depending on speed | Every 30 seconds |
| Data transmitted | Full static, dynamic, and voyage data | Reduced — no voyage data, no ROT |
| DSC capability | Integrated DSC | No DSC |
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Start a live lesson →Lesson 5 of 11 · 1 min
| Limitation | Implication |
|---|---|
| Not all vessels are fitted | Fishing vessels under 300 GT, small craft, and vessels not required to carry AIS will not appear. The absence of a target on AIS does not mean the sea is clear |
| Data can be incorrect | Crew may input wrong draught, destination, navigational status, or cargo information. Static data errors are common — wrong vessel name or MMSI |
| Position accuracy depends on GPS | If the other vessel's GPS is faulty or spoofed, their AIS position will be wrong |
| AIS can be switched off | Vessels engaged in illegal activities, piracy avoidance, or security operations may switch off AIS intentionally. A missing vessel is not necessarily safe |
| Class B update rate is slow | 30-second updates at high closing speeds can give a dangerously outdated picture |
| Security concerns | In piracy areas, some guidance recommends switching off AIS — verify flag state and company policy |
Say it out loud. Talk “AIS limitations in collision avoidance” through with the live Tutor.
Start a live lesson →Lesson 6 of 11 · 1 min
| Code | Status |
|---|---|
| 0 | Under way using engine |
| 1 | At anchor |
| 2 | Not under command |
| 3 | Restricted in ability to manoeuvre |
| 4 | Constrained by her draught |
| 5 | Moored |
| 6 | Aground |
| 7 | Engaged in fishing |
| 8 | Under way sailing |
| 15 | Default / not defined (crew has not updated status) |
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Start a live lesson →Lesson 7 of 11 · 1 min
| Feature | Class A | Class B |
|---|---|---|
| Fitted to | SOLAS vessels; most commercial yachts | Leisure craft, small commercial |
| Transmit power | 12.5 W | 2 W (CS) — shorter range |
| Dynamic update rate | 2–10 s underway (faster when turning); 3 min at anchor | Typically 30 s underway — and Class B CS yields to Class A traffic in busy areas, so updates can be sparser still |
| Data set | Full static, voyage and dynamic data, including heading and rate of turn where sensors are connected | Reduced set; often no heading, no voyage data |
Say it out loud. Talk “Class A vs Class B” through with the live Tutor.
Start a live lesson →Lesson 8 of 11 · 1 min
| Layer | Content | Source |
|---|---|---|
| Static | MMSI, name, call sign, dimensions, type | Programmed at installation — errors here live for years; verify after any refit |
| Voyage | Draught, destination, ETA, persons on board | THE CREW — entered and updated by the OOW |
| Dynamic | Position, COG, SOG, heading, ROT — plus navigational status | Ship's sensors automatically — inherits every GPS and gyro error. The exception is navigational status: it travels with the dynamic data but the OOW sets it by hand |
Say it out loud. Talk “AIS data — static, dynamic and voyage” through with the live Tutor.
Start a live lesson →Lesson 9 of 11 · 1 min
Authorities increasingly transmit AIS AtoN: an AIS symbol marking a buoy or a danger. Three flavours matter:
| Type | Meaning |
|---|---|
| Physical AtoN | A real buoy fitted with an AIS transmitter — symbol and buoy coincide |
| Synthetic AtoN | A real buoy whose AIS signal is transmitted from a shore station — the buoy exists, the transmitter is elsewhere |
| Virtual AtoN | No physical buoy at all — a symbol marking a new wreck or danger before a buoy can be laid. Increasingly the FIRST marking of a new casualty, ahead of the emergency wreck buoy |
Say it out loud. Talk “AIS in SAR and AIS aids to navigation” through with the live Tutor.
Start a live lesson →Lesson 10 of 11 · 1 min
The AIS-SART does the radar SART's job digitally: activated in a survival craft, it transmits a standard AIS position with a distinctive SART symbol that plots on any AIS display in range — no radar interpretation needed. AIS MOB devices, worn on lifejackets, fire on immersion and drop a MOB symbol with position on the parent vessel's own display: for a yacht running tenders and watersports, they turn "he was there a minute ago" into a chartplotter target. Doctrine: an unexpected AIS-SART or MOB symbol is distress traffic — treat it as a Mayday: inform the MRCC, respond as the SAR topic demands, never assume a test.
Say it out loud. Talk “AIS principles and message types” through with the live Tutor.
Start a live lesson →Lesson 11 of 11 · 1 min
AIS transmits your position to everyone, friendly or otherwise. SOLAS requires it operating at sea — but the master may switch it off where continuous operation might compromise safety or security (piracy high-risk areas being the standard case), logging the decision and the reasoning, and restoring it as soon as the risk passes. What that clause does not cover: switching off to hide from harbour dues, race rivals, or the charter competition — and a gap in your AIS track is conspicuous evidence that invites exactly the questions you hoped to avoid.
Say it out loud. Talk “AIS principles and message types” through with the live Tutor.
Start a live lesson →Module 10
Scenario questions from the oral, with the reasoning after each one. The answers that look nearly right are there on purpose.
Module 10
Answer each one out loud before you open the model answer. If you can't say it, you don't know it yet.
No — that difference is information. COG is her track over the ground from GPS; heading is where her bow points from her gyro or compass. Fifteen degrees of difference means set and leeway — she is crabbing across a tide or wind. It is the same distinction as course to steer versus track made good in my own navigation, and reading it tells me about the current she is in — which is likely the current I am about to be in.
Not all vessels carry AIS — small craft, fishing vessels under 300 GT, and vessels not subject to SOLAS carriage requirements will not appear. Data can be incorrect — crews frequently input wrong draught, status, or destination. Position accuracy depends on the other vessel's GPS. AIS can be switched off deliberately. Class B transponders update only every 30 seconds which is inadequate at high closing speeds. AIS is not a collision avoidance system and does not feature in the COLREGS — all avoiding action must be based on radar and visual observation.
Static data: MMSI, IMO number, call sign, vessel name, type, dimensions. Dynamic data: position, COG, SOG, heading, rate of turn — updated automatically every 2–10 seconds for Class A — plus navigational status, which travels with the dynamic data but is set by hand by the OOW. Voyage data: draught, cargo type, destination, ETA, persons on board — entered and updated by the crew. The accuracy of voyage data depends entirely on the crew keeping it current.
Ready?
Sit a live oral on AIS.
The live Examiner asks the questions out loud, coaches you when you miss, and gives you a debrief.
Study aid built from primary sources. Verify regulations against the current editions before your oral. An official result needs an MCA assessment.