ONDECK
Academy
OOW Oral Prep
Module 10 — OOW 3000 GT Oral Preparation
AIS
Automatic Identification System — principles, carriage requirements, data transmitted, modes, limitations
What is AIS?
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.
Carriage Requirements SOLAS V Reg 19
| 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 |
Data Transmitted by AIS
Static Data (entered once)
- MMSI number (unique 9-digit identifier)
- IMO number
- Call sign
- Vessel name
- Type of vessel
- Length and beam
- Position of GPS antenna
Dynamic Data (updated automatically)
- Position (latitude and longitude)
- Time (UTC)
- COG (Course Over Ground)
- SOG (Speed Over Ground)
- Heading
- Rate of turn
- Navigational status (underway, at anchor, not under command, etc.)
Voyage-Related Data (updated by crew)
- Draught
- Type of cargo / hazardous cargo
- Destination
- ETA
- Number of persons on board
Class A vs Class B AIS
| 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 |
AIS Limitations — Critical for the Oral
AIS IS NOT A COLLISION AVOIDANCE SYSTEM. The COLREGS do not mention AIS. Collision avoidance is based on radar observation, visual lookout, and COLREGS compliance — not AIS data.
| 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 |
The rule: Use AIS to assist in building situational awareness and identifying vessels. Cross-check all AIS data against radar and visual observation. Never make a collision avoidance decision based on AIS alone.
AIS Navigational Status Codes
| 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) |
Status 15 (not defined) is extremely common — many crews never update the navigational status. Do not assume a vessel showing status 15 is actually underway using engine.
Class A vs Class B — What the Other Vessel Actually Has
| 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 |
Operational meaning: the 8-metre sportsboat on Class B may update every 30 seconds — at 25 knots that is 320 metres between position reports. Her AIS symbol is where she WAS. In close quarters the radar echo and the Mark One eyeball are the truth; the AIS label is the caption.
The Three Data Layers — and Whose Job Each One Is
| 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, navigational status | THE CREW — entered and updated by the OOW |
| Dynamic | Position, COG, SOG, heading, ROT | Ship's sensors automatically — inherits every GPS and gyro error |
Navigational status discipline: a vessel transmitting "at anchor" twelve hours into a passage tells every bridge in range to distrust the rest of her data — and it is the OOW who forgot to change it. Status is changed as the ship changes: underway using engine, at anchor, moored, NUC, RAM, constrained by draught. A false NUC/RAM status on AIS is the same offence as false lights: claiming a priority you are not entitled to.
AIS Aids to Navigation — Buoys That May Not Be There
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 |
A virtual AtoN on the display with empty water under the binoculars is not a fault — it is the system working. Treat it exactly as you would the physical mark it stands for, and expect it where Navtex has just warned of a new wreck.
AIS in Distress — SART's Digital Sibling and the MOB Device
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.
Switching It Off — The One Legitimate Case
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.
Additional Question
AIS shows a target's COG as 085° but her heading as 100°. Is the unit faulty?
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.
Oral Exam Practice Questions
Question 1
What are the limitations of AIS?
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.
Question 2
What information does AIS transmit?
Static data: MMSI, IMO number, call sign, vessel name, type, dimensions. Dynamic data: position, COG, SOG, heading, rate of turn, navigational status — updated automatically every 2–10 seconds for Class A. 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.