What AIS Actually Does

Automatic Identification System, usually shortened to AIS, allows equipped stations to broadcast short digital messages containing identity, position, movement and safety-related information. A ship can repeatedly say, in effect: this is my MMSI, this is my name, this is where I am, this is my course over ground, this is my speed over ground, this is my heading, and this is the navigational status I am declaring.

Other AIS-equipped vessels receive those messages automatically. Shore stations receive them. Vessel Traffic Services may receive them. Search and rescue aircraft and satellites may also receive AIS in the right circumstances. The result is a moving traffic picture that can be shown on an ECDIS, radar overlay, integrated bridge display, chartplotter, pilot tablet or authority monitoring system.

The key point is that AIS adds identity and declared movement to the marine picture. Radar may show an anonymous echo. AIS can show a named vessel, a Maritime Mobile Service Identity, a course, a speed and a predicted closest point of approach. That makes bridge communication faster and gives investigators a valuable timeline, but it does not make AIS infallible.

AIS Is Not Radar

This distinction matters. Radar is non-cooperative: your set transmits microwave energy and detects reflections from physical objects. The other vessel does not need to help you. AIS is cooperative: you see the target because a transmitter sends a message saying it is there.

Professional navigation therefore uses all available means appropriate to the circumstances. UK MCA guidance treats electronic systems as navigational aids and warns against over-reliance on a single source. AIS can improve situational awareness, but it must not be treated as a standalone collision avoidance answer.

How The Radio Network Works

AIS works over dedicated marine VHF channels. The two principal AIS channels are AIS 1 at 161.975 MHz and AIS 2 at 162.025 MHz. Instead of every ship speaking continuously, AIS uses time division multiple access. Each minute is divided into small radio time slots. Stations transmit short messages in those slots, allowing large numbers of vessels and shore stations to share the data link.

The current ITU technical recommendation is ITU-R M.1371-6, approved on 19 February 2026. It defines technical characteristics for VHF AIS using TDMA in the maritime mobile service. In plain English, the standard describes how the messages, timing, access methods and station types coexist on the same AIS data link.

SOTDMA, CSTDMA And Other Access Methods

The clever part of AIS is how stations organise access to the radio channel. Large commercial vessels using Class A AIS normally use SOTDMA, or Self-Organising Time Division Multiple Access. The AIS listens to the network, identifies which slots are being used, selects suitable slots and announces future slot use so nearby stations can organise around it.

Some Class B equipment uses CSTDMA, or Carrier Sense Time Division Multiple Access. This is closer to listening first and transmitting when the channel appears clear. It can work well for smaller craft, but it has less priority and lower performance in congested traffic compared with SOTDMA. Other AIS access methods include RATDMA for random access, ITDMA for initial access and changes, FATDMA for fixed allocations, and longer-range mechanisms used in satellite contexts.

What AIS Broadcasts

The Different Types Of AIS

Class A and Class B are only part of the system. AIS also includes shore base stations, AIS aids to navigation, search and rescue transmitters, man-overboard devices, SAR aircraft reports, inland AIS variants and satellite reception. The table below gives a practical reader-level comparison.

AIS, CPA And TCPA

Once a display knows your vessel's position and velocity and a target's position and velocity, it can calculate CPA, or Closest Point of Approach, and TCPA, or Time to Closest Point of Approach. These are predictions, not guarantees. If either vessel alters course or speed, the prediction changes. If the input data is wrong, the calculation can look precise while being dangerously misleading.

That is why heading and course are not the same thing. Heading is the direction the bow points. Course over ground is the direction the vessel is actually travelling across the surface. Tide, wind, current, sensor error or poor data entry can make those values differ in ways that matter at close range.

AIS As Evidence After A Maritime Incident

After a casualty, AIS can become part of the evidential timeline. Investigators may compare AIS against radar, voyage data recorder material, bridge audio, electronic chart history, engine logs, port records, weather, pilotage notes, tug movements, distress calls, photographs, satellite imagery and witness statements. The power is rarely one point on a track. The power is in the pattern: where a route changes, where a report goes quiet, where a destination looks stale, or where several sources disagree in the same minute.

Why AIS Can Be Wrong

AIS can look very precise, which is exactly why it needs care. Errors can include wrong MMSI, stale destination, wrong navigation status, incorrect vessel dimensions, poor antenna-offset data, faulty GNSS, faulty heading input, missing transmissions, out-of-range reception, satellite delay, public feed filtering, deliberate spoofing or a vessel lawfully turning AIS off for security reasons. A missing AIS target does not mean there is no vessel. A visible AIS target does not prove the full truth by itself.

AIS is not the truth. It is a broadcast. The truth begins when the broadcast is compared with everything else that survived.

Where AIS Is Going: VDES

AIS is increasingly discussed as part of the wider VHF Data Exchange System, or VDES. VDES is intended to expand maritime digital communications capacity by combining AIS, application specific messaging and higher-capacity terrestrial and satellite VHF data exchange. For readers, the useful idea is simple: AIS is the familiar automatic identity and position layer; VDES points towards a broader digital maritime communications architecture.

Why This Matters In The Collision Of Tides

In the series, the dangerous question is not only where a ship was. It is who decided what the record meant. AIS can support an official story, expose a missing minute, contradict a witness, or give an investigator the first small reason to doubt a clean explanation. A collision at sea creates urgency. AIS creates data. Power decides which parts of that data reach daylight.