The first time you stand a watch in busy water with a good AIS display, the sea stops feeling anonymous. Every contact carries a name, a course, a speed, a destination. A radar echo that used to be a guess is now a 300-meter tanker bound for Rotterdam, doing 14 knots, drawing 14.5 meters. It feels like the bridge finally has eyes everywhere.
That feeling is exactly what gets officers into trouble. AIS is one of the most useful tools you will ever have, and it misleads you more often than any other instrument on the bridge, because almost everything it tells you about a ship’s voyage was typed in by a human who may have been tired, distracted, or simply wrong. Learning AIS properly means learning both halves: what it does brilliantly, and where it quietly fails.
What AIS Actually Is
The Automatic Identification System is a VHF broadcast system. Each fitted ship transmits a short digital message on two dedicated marine channels, AIS 1 (161.975 MHz, channel 87B) and AIS 2 (162.025 MHz, channel 88B), and listens for everyone else doing the same. The result is a shared, self-updating picture of who is around you, refreshed every few seconds.
It is not radar. Radar builds its picture from echoes it generates and receives itself, so it sees anything that reflects, transmitting or not. AIS only shows you ships that are switched on and sending. The two systems answer different questions, which is why the strongest watchkeepers run them together rather than trusting either alone.
Carriage is not optional for most commercial ships. Under SOLAS Chapter V, Regulation 19.2.4, AIS is mandatory on all ships of 300 gross tons and above on international voyages, cargo ships of 500 gross tons and above on domestic voyages, and all passenger ships regardless of size. The requirement has applied to every affected ship since the end of 2004, so on a normal commercial passage you can expect the large traffic around you to be transmitting.
How AIS Builds the Picture
Each station has to share the airwaves with every other station in VHF range without everyone talking over each other. Class A units solve this with SOTDMA, self-organizing time division multiple access, which divides each minute into 2,250 time slots and lets each transmitter reserve its own slot based on what its neighbors are doing. It is the reason AIS keeps working in crowded estuaries where hundreds of ships share the same frequency.
The class of a ship’s transponder tells you how much to trust the signal, so it pays to know the difference.
Class A and Class B
Class A is the mandatory SOLAS fit. It transmits at 12.5 watts and updates as often as every two to ten seconds while underway, slowing to roughly every three minutes at anchor. That high, maneuver-dependent reporting rate is what makes a Class A target reliable enough to plan around in close quarters, with a typical range of 20 to 40 nautical miles depending on antenna height.
Class B is the voluntary fit you find on yachts, fishing boats, and small workboats. It runs at 2 watts, updates far less often, every 30 seconds or slower, and usually reaches only 5 to 6 miles. It often omits voyage data altogether. The practical lesson is simple: a small Class B contact may appear late, update sluggishly, or vanish behind a sea or a structure, and plenty of small craft carry no AIS at all. Never let a clear AIS screen convince you the water is empty.
What Each Message Contains
An AIS message is built from three kinds of data, and they fail in very different ways.
Static data is the ship’s identity: its MMSI number, IMO number, call sign, name, type, and dimensions. This is set when the unit is commissioned and rarely changes, so it is usually reliable.
Dynamic data is the live navigational picture: position, course over ground, speed over ground, heading, rate of turn, and navigational status. Most of this comes straight from the ship’s GPS and compass, so it is generally trustworthy, with one important exception. The navigational status (under way, at anchor, restricted in ability to maneuver, constrained by draft, and so on) is set by hand, and a ship showing “under way using engine” while sitting at anchor is a common sight.
Voyage data is where the trouble lives. Draft, hazardous cargo, destination, and ETA are all entered manually by the crew. A wrong draft, a stale destination left over from the last voyage, or a blank ETA tells you nothing reliable. Treat voyage data as a useful hint, never as fact.
Using AIS for Collision Avoidance
AIS was designed for identification and tracking, not as a dedicated anti-collision system, but in practice it has become central to how officers assess risk. Where radar gives you a contact, AIS gives you a name and a steady course and speed, and a display that fuses the two will compute closest point of approach (CPA) and time to CPA (TCPA) from far cleaner data than radar plotting alone.
It also tempts officers into a dangerous habit. Because AIS shows a ship’s name, it is easy to call that ship on VHF and arrange a passing by agreement. This is how AIS-assisted collisions happen. The COLREGs decide who gives way and who stands on, and they do not change because two officers had a radio chat. Use AIS to identify and to monitor, never to negotiate your way around the rules. Verbal arrangements made over VHF have featured in more than one collision inquiry.
AIS on ECDIS and Radar
The real power of AIS shows when its targets are overlaid on your other displays. On ECDIS, AIS contacts sit directly on the electronic chart, so you can watch traffic move against the buoyed channel, the traffic separation scheme, and the depth contours all at once. On the radar, an AIS overlay lets you compare the transmitted position against the actual echo, and a mismatch is an instant flag that something is wrong, whether a timing lag, a GPS error, or a spoofed signal.
The discipline is to correlate, not to choose. When the AIS target and the radar echo agree, your confidence is high. When they disagree, you have learned something important, and the answer is to look harder, not to pick whichever screen you prefer.
Where AIS Helps Most: Fog and Identification
In restricted visibility, AIS earns its place. When a contact fades from sight and the radar echo weakens in heavy rain, the AIS position keeps arriving, letting you hold a track on a ship you cannot otherwise see. Paired with radar and the sound signals required in fog, it turns a tense blind passage into a managed one. Just remember it only covers the transmitting ships. The small wooden fishing boat with no transponder is exactly the contact fog hides, and exactly the one AIS will never show you.
For identification, AIS is unmatched. Knowing a contact is a fishing vessel, a ship constrained by her draft, or one restricted in her ability to maneuver tells you which rules apply long before you could read her lights or shapes, which sharpens every decision that follows.
Safety Beyond the Bridge
AIS technology now reaches past navigation into search and rescue. AIS-SART transponders, carried in survival craft, broadcast a distinctive position so rescuers can home in on a lifeboat or raft. AIS-MOB beacons clipped to lifejackets mark a person in the water directly on nearby AIS displays the moment they are activated, which can turn a frantic search into a direct recovery. AIS aids to navigation let buoys and lights transmit their own identity and position, and can even flag when a buoy has drifted off station.
The Limits You Must Respect
Everything good about AIS comes with a matching caution, and an officer who only knows the strengths is dangerous. The data is only as good as the human who entered it, so voyage fields and navigational status are routinely wrong. Not every ship transmits: small craft may carry nothing, and a master may legitimately switch AIS off where broadcasting his position would be a security risk, for example in waters prone to piracy. Signals can be lost behind land or large structures, and in conflict zones AIS spoofing, broadcasting a false identity or position, is a real and growing problem.
The deepest error, though, is none of these technical limits. It is treating AIS as the whole picture. AIS is one input to a watch built on radar, a proper visual lookout, and the judgment behind the rules of the road. Use it as the powerful supplement it is, and it will make you a sharper officer. Mistake it for the truth, and it will eventually let you down badly.
Frequently Asked Questions
Is AIS a legal requirement? Yes, for most commercial ships. SOLAS Chapter V, Regulation 19.2.4 requires AIS on all ships of 300 gross tons and above on international voyages, cargo ships of 500 gross tons and above on domestic voyages, and all passenger ships regardless of size. Smaller and recreational craft are generally not required to carry it, though many fit a voluntary Class B unit.
What is the difference between Class A and Class B AIS? Class A is the mandatory SOLAS fit: higher power, a faster update rate, and reliable slot reservation, so its targets are dependable in close quarters. Class B is the lower-power voluntary fit used by yachts and small craft, with slower updates, shorter range, and often no voyage data. A Class B target should be treated as less consistent and may appear late or not at all.
Can AIS replace radar? No. AIS only shows ships that are fitted and transmitting, while radar detects anything that returns an echo, transmitting or not. They answer different questions and are designed to be used together, alongside a visual lookout.
Why is AIS voyage information often wrong? Voyage data such as draft, destination, and ETA, along with navigational status, is entered manually by the crew and is easy to forget to update. A ship may still show last week’s destination, or an “under way” status while at anchor. Treat these fields as a hint, not as verified fact.
Can AIS be faked? Yes. AIS spoofing, broadcasting a false identity or position, occurs particularly in conflict zones and areas of illegal activity. This is one reason an AIS position should always be correlated against radar and visual observation rather than trusted on its own.