AIS decoded: how ship tracking actually works (2026 reference)

Built for the people who read AIS every day but were never handed the spec: operators, analysts, port agents, fleet managers, compliance teams and the developers who build on vessel data. Everyone watches the little triangles move; almost nobody can decode what each field means, why a ship "disappears," or why the type says "cargo" when it's clearly a tanker. The full definition lives buried in ITU-R M.1371 and a scatter of IMO circulars. We assembled the working knowledge — every message type, every navigational status, the field-by-field anatomy of a position report, and how AIS lies — into one reference.

This is the technology behind the rynda live ports & vessel map: the same AIS your team sees on screen, explained down to the bit.

📖 See also: ECA / SECA zones reference · EU ETS & FuelEU cost reference

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Contents

• At a glance
• What AIS is and how it works
• Class A vs Class B and the other station types
• The message types (1–27)
• Anatomy of a position report
• Navigational status (0–15)
• Static & voyage data — the message 5 fields
• Ship type codes (0–99)
• Decoding the MMSI
• How the dimensions encode length and beam
• Reporting intervals
• Terrestrial vs satellite AIS
• When AIS lies — gaps, spoofing and identity
• Data-quality caveats
• For developers
• FAQ
• References

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At a glance

The short version, if you read nothing else:

• AIS is a self-reporting VHF broadcast system, not radar. Each ship transmits its own identity, position and movement on two VHF channels; anyone in range (or a satellite overhead) receives it.
• Two data streams, different cadence. Dynamic data (position, speed, course) updates every few seconds when underway. Static/voyage data (name, IMO, type, dimensions, destination, draught, ETA) is broadcast every few minutes and is typed in by the crew — which is why it's often stale or wrong.
• The message type number tells you what you're looking at. Types 1/2/3 are Class-A positions, 18/19 are Class-B, 5 is static/voyage, 21 is a navigation aid, 4 is a base station. Twenty-seven types in all.
• Navigational status is a manual field (0–15). "Under way," "at anchor," "moored," "not under command," "constrained by draught." It is set by the officer of the watch and is a frequent source of error — and a Port State Control (PSC) deficiency when wrong.
• The MMSI is not the ship. The first three digits (the MID) give the flag/country; the number can be reassigned when a ship changes flag, and special prefixes mark base stations, SAR aircraft, AtoN and distress beacons.
• A "dark" ship is usually a switched-off transponder or a spoofed position — both visible as patterns in the data (gaps, impossible jumps, duplicate identities), which is exactly what makes AIS analysis valuable.

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What AIS is and how it works

The Automatic Identification System is a VHF data broadcast. Every SOLAS (Safety of Life at Sea convention) ship (and many smaller craft) carries a transponder that:

• broadcasts its own data on two dedicated VHF channels — AIS 1 = 161.975 MHz (marine channel 87B) and AIS 2 = 162.025 MHz (channel 88B);
• listens to everyone else's broadcasts on the same channels;
• coordinates when it transmits so that many ships share the same frequency without talking over each other.

That time-sharing is the clever part. Class-A units use SOTDMA (Self-Organising Time Division Multiple Access): the VHF minute is divided into 2 250 time slots, and each transceiver reserves and announces the slots it will use, so neighbours avoid them. Class-B "CS" units use CSTDMA (Carrier-Sense TDMA): they listen for a free slot and grab it, yielding to Class A.

Range is roughly line-of-sight for VHF — typically 20–40 nautical miles ship-to-ship, more to a high shore antenna. Beyond the horizon there is no terrestrial reception, which is why satellite AIS exists (see satellite AIS).

AIS was mandated by SOLAS Chapter V after 2002 for ships ≥300 GT on international voyages, ≥500 GT not on international voyages, and all passenger ships. It exists first for collision avoidance and traffic management — the global tracking industry is a second-order use of a safety system.

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Class A vs Class B and the other station types

Not every triangle is the same kind of station. The transmitting device determines which message types you get and how often:

Practical consequences: a Class-B contact reports far less often and may omit IMO number, draught, destination and ETA entirely — so a fishing vessel or yacht looks "data-poor" next to a Class-A cargo ship. And an AtoN (message 21) is a buoy or a virtual mark, not a moving vessel — mislabelling one as a ship is a classic rookie error.

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The message types (1–27)

Every AIS transmission carries a message ID in its first field. This is the master key to reading AIS. The 27 defined types:

If you remember only a handful: 1/2/3 = big-ship position, 5 = big-ship details, 18/19 = small-craft position, 24 = small-craft details, 21 = a navigation aid, 4 = a shore station, 27 = the satellite-friendly position.

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Anatomy of a position report

A Class-A position report (message 1/2/3) is the single most-consumed AIS message. Its fields, decoded:

COG vs heading trips up newcomers constantly: COG is the direction the ship is actually moving (from GNSS — satellite positioning, i.e. GPS and its equivalents), heading is where the bow points (from the gyro). In a strong beam current or while turning they differ — and a drifting ship can have a valid heading with near-zero SOG.

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Navigational status (0–15)

A single manually-set field on Class-A ships. It is one of the most useful — and least reliable — values in AIS, because the officer of the watch types it in and updating it is easy to forget.

Why it matters:

• An incorrect nav status is a PSC deficiency risk — an anchored ship still broadcasting "under way" can attract an inspection note.
• Analysts use it to distinguish "genuinely stopped" (status 1/5) from "stopped but not updated" — a ship parked at anchor for two days still showing status 0 is a data-hygiene tell.
• Status 2 (NUC), 3 (RAM) and 4 (constrained by draught) change the collision-avoidance obligations of other ships, which is why they exist in a safety system in the first place.

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Static & voyage data — the message 5 fields

Message 5 (Class A) / message 24 (Class B) carries the crew-entered description of the ship and the voyage. Because it is typed in, treat it as asserted, not verified:

The dynamic stream (position/speed/course) comes straight from the ship's GNSS and is generally trustworthy; the static/voyage stream is only as good as the crew's last keystroke. Every serious AIS workflow cross-checks destination and draught against reality rather than trusting them.

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Ship type codes (0–99)

The ship type in message 5 is a two-digit code where the tens digit is the broad category and the units digit adds cargo/hazard detail:

Two everyday consequences: a chemical/oil tanker often shows as 80–89, and a "cargo" ship at 70 with hazard suffix tells you it may be carrying dangerous goods. But because the code is crew-entered, a tanker mistyped as "cargo (70)" is common — never treat the AIS type as authoritative; confirm against the IMO number and a registry.

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Decoding the MMSI

The Maritime Mobile Service Identity is a 9-digit number — a station identity, not a permanent ship identity. Its structure encodes a lot:

• Ship stations: MIDxxxxxx — the first three digits are the Maritime Identification Digits, i.e. the flag state. E.g. 232…/233…/234…/235… = United Kingdom, 273… = Russia, 477… = Hong Kong, 538… = Marshall Islands, 636… = Liberia, 563–567… = Singapore.
• Group of ships: leading 0 → 0MIDxxxx.
• Coast / base stations: leading 00 → 00MIDxxxx.
• SAR aircraft: 111MIDxxx.
• AtoN (navigation aids): 99MIDxxxx.
• Craft associated with a parent ship (lifeboats, tenders): 98MIDxxxx.
• Handheld VHF / DSC: 8MIDxxxxx.
• Distress beacons: AIS-SART 970xxxxxx, MOB 972xxxxxx, EPIRB-AIS 974xxxxxx.

The critical operational point: when a ship changes flag, its MMSI usually changes; its IMO number does not. So the MMSI is a fine live-tracking key but a poor historical one — to follow a hull across a re-flag or a sale, join on the IMO number. Reused and shared MMSIs are also a real phenomenon (and one of the tells in identity-manipulation analysis).

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How the dimensions encode length and beam

Message 5 doesn't send "LOA" and "beam" directly. It sends four distances from the GNSS antenna to the bow, stern and each side:

• A = antenna → bow
• B = antenna → stern
• C = antenna → port side
• D = antenna → starboard side

From which:

Length overall (LOA) = A + B
Beam                 = C + D

The offsets also tell you where on the ship the antenna sits, which matters for precise berthing/pilotage. Two traps: a Class-B "part A/B" split may leave dimensions zero/unknown, and some units report a reference point that isn't the true GNSS position, introducing a small position offset on very large ships.

Why is Class-B static data split into a Part A and Part B (message 24) at all? Because a single AIS time-slot can't carry all the static fields at once: Part A sends the vessel name, Part B sends the type, dimensions and call sign — transmitted separately, which is exactly why a Class-B contact so often shows a name but no dimensions (or vice versa) until both parts have been received.

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Reporting intervals

How often a ship transmits depends on its class and what it's doing. Approximate Class-A dynamic intervals (SOTDMA):

The takeaways: a fast, turning ship floods the channel with updates (down to ~2 s), while an anchored ship reports only every three minutes — so a "stale" position at anchor is normal, not a fault. And Class-B's slow cadence is why small craft trails look choppy compared with a Class-A liner.

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Terrestrial vs satellite AIS

• Terrestrial AIS — shore antennas and coastal receivers. High update rate, but only within VHF line-of-sight of land (tens of nm). Coverage is dense near coasts and empty mid-ocean.
• Satellite AIS (S-AIS) — low-earth-orbit satellites pick up AIS over open ocean. It fills the blue-water gap, but with two costs: latency (you see the position on the next satellite pass, minutes to hours later, not in real time) and message collision — in busy sea areas hundreds of ships share the same slots within a satellite's huge footprint, so many messages are lost. Message 27 exists specifically as a low-rate, long-range position report optimised for satellite pickup.

Practically: a mid-Atlantic position that's an hour old isn't necessarily "gone dark" — it may simply be waiting for the next satellite pass. Distinguishing satellite latency from deliberate darkness is a core analytical skill, and one the rynda vessel map is built to handle.

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When AIS lies — gaps, spoofing and identity

Because AIS is self-reported, it can be manipulated. The three broad failure modes, and how each looks in the data:

1. Going dark (transponder off). The ship simply stops transmitting. In the data it's a gap: a last-seen position, then silence, then (sometimes) a reappearance far away. A gap alone can be innocent — satellite latency, equipment failure, deep-ocean coverage holes — so gaps are judged by duration, location and context, not in isolation.
2. Position spoofing. The transmitted position is falsified — via a manipulated GNSS feed or an altered AIS message — so the ship appears somewhere it isn't. Tells include impossible jumps (hundreds of miles between consecutive reports), speeds inconsistent with the track, ships showing as "sailing on land," or circular/looping tracks generated by a simulator.
3. Identity manipulation. The MMSI, name, IMO or dimensions are changed or borrowed — two vessels broadcasting the same MMSI at once, an MMSI whose MID doesn't match the claimed flag, or a hull that changes name/identity between port calls.

None of this requires you to trust a single message. The patterns — a gap of the wrong length in the wrong place, a jump no ship could make, a duplicate identity — are what analysis surfaces. That is precisely why AIS remains valuable despite being spoofable: the manipulation leaves its own signature.

This section is deliberately descriptive — it explains how to read the data, not how to evade detection. For counterparty and vessel due-diligence, combine AIS behaviour with hull identity (IMO), ownership and sanctions screening rather than relying on the transponder alone.

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Data-quality caveats

• Static data is asserted, not verified — name, type, destination, draught and ETA are only as current as the crew's last entry.
• Nav status drifts — anchored ships showing "under way" and vice versa are common.
• MMSI ≠ ship — reused, shared and re-flagged identities break naïve historical joins. Use IMO for the hull.
• Coverage is uneven — dense at coasts, sparse mid-ocean; absence of data is not presence of a stationary ship.
• Timestamps matter — always read a position together with its age; a "current" position may be a satellite report from hours ago.

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For developers

The rynda vessel platform consumes exactly these AIS streams and exposes higher-level primitives you can build on.

• Live vessel map: the ports & vessel map renders live positions, resolves Class-A/B, and reconciles terrestrial vs satellite reports into coherent tracks.
• Identify on the durable key: store and join on the IMO number for the hull; keep MMSI as the live-radio key. This one rule prevents most tracking bugs across re-flags and sales.
• Route + ECA context: pair a track with POST /api/ports-map/route ({"ports":[...]} → eca_distance_nm, eca_zones, legs) to place AIS movement inside the regulatory geography — the bridge between "where the ship is" and "what rules apply." See the ECA reference.
• Read the type number first: branch on the message ID before parsing — 1/2/3 and 18/19 are positions; 5 and 24 are static; 21 is an AtoN, not a vessel; 4 is a base station. Mixing them up is the most common AIS parsing bug.

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FAQ and common confusions

Q: Why does a ship "disappear" from the map?
A: Most often it left terrestrial VHF range (mid-ocean) and you're waiting on the next satellite pass; sometimes the transponder is off. Judge by duration and location — a two-hour gap mid-Atlantic is routine; a two-hour gap off a busy coast is not.

Q: The AIS says "Cargo" but it's obviously a tanker. Why?
A: The ship-type code is typed in by the crew and is frequently wrong or generic. Never trust it — confirm the ship via its IMO number and a registry.

Q: What's the difference between COG and heading?
A: COG (course over ground) is the direction the ship is actually moving, from GNSS. Heading is where the bow points, from the gyro. Current and wind make them differ; a drifting ship can have a heading with near-zero speed.

Q: MMSI or IMO — which identifies the ship?
A: IMO is the permanent hull identity (never changes). MMSI is a radio/station identity that usually changes when the ship re-flags. Track live on MMSI; identify the hull over time on IMO.

Q: Is the destination field reliable?
A: No. It's free text set by the crew — "FOR ORDERS", port abbreviations, or a stale entry from three voyages ago are all common. Treat it as a hint, verify against actual movement.

Q: Why do small boats update so rarely?
A: They're usually Class B, which transmits at lower power and a slower cadence (roughly every 30 s to 3 min) and yields priority to Class-A ships. Choppy trails are expected.

Q: Can AIS positions be faked?
A: Yes — AIS is self-reported and can be spoofed. But manipulation leaves patterns (impossible jumps, ships "on land," duplicate MMSIs, wrong-length gaps) that analysis detects. That's why AIS stays useful despite being spoofable.

Q: What is message type 21 doing in my vessel feed?
A: That's an Aid to Navigation (a buoy, light, or a virtual mark) — not a moving ship. Filter it out of vessel logic or you'll get phantom "vessels" sitting on channel markers.

Q: Does every ship carry AIS?
A: SOLAS mandates it for ships ≥300 GT on international voyages, ≥500 GT otherwise, and all passenger ships. Many smaller vessels carry Class B voluntarily, but some craft carry none — absence from AIS doesn't prove absence of a ship.

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References

• ITU-R M.1371 — the technical standard defining AIS messages, fields and access schemes.
• IMO SOLAS Chapter V, Regulation 19 — the carriage requirement.
• IMO Resolution A.1106(29) — guidelines for the onboard operational use of AIS (including nav-status discipline).
• US Coast Guard Navigation Center (NavCen) — accessible AIS message and MMSI references.
• ITU — Maritime Identification Digits (MID) table for MMSI/flag decoding.
• rynda ports & vessel map — live AIS in production.

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Document version

• 2026-07 — first public version. Message types, navigational status, MMSI/ship-type decoding, dimension and interval references, and the patterns of AIS manipulation, in one frame. Field definitions follow ITU-R M.1371; always confirm against the current standard for edge cases.