MaritimeOctober 23, 2026· 5 min read· Last reviewed October 23, 2026

AIS: How Automatic Identification System Works on VHF and Why It Matters

How AIS broadcasts vessel identity and position over two marine VHF channels, what SOLAS requires, how the equipment classes differ, and where the system falls short.

#Maritime#AIS#VHF#SOLAS#IMO#Telemetry

AIS broadcasts a vessel's identity, position, course, speed and other navigation data over two dedicated marine VHF channels. Every vessel in range receives it, as does every shore station listening on the same frequencies.

It is autonomous and self-organising. There is no central server, no handshake and no subscription. A radio switches on, picks a slot, transmits, and listens again.

Legal basis and carriage requirement

The requirement sits in SOLAS Chapter V, Regulation 19.2.4, adopted in December 2002.

Mandatory carriage applies to:

Ships of 300 GT and upwards engaged on international voyages.
Cargo ships of 500 GT and upwards not engaged on international voyages.
Passenger ships of any size.

Implementation was phased from 1 July 2002 to 1 July 2008. The effective date most often cited for the main tranche is 31 December 2004.

Exempt are warships, naval auxiliaries and other government-owned or government-operated ships. Small vessels — leisure craft and fishing boats — are exempt unless a national authority requires otherwise.

RF and protocol parameters

AIS1 = 161.975 MHz (marine channel 87B).
AIS2 = 162.025 MHz (marine channel 88B).
Modulation: GMSK at 9,600 bit/s.
Access method: TDMA. One slot carries 256 bits, giving approximately 2,250 slots per minute per channel.

There are two density figures, and the distinction matters. The IMO performance standard requires a minimum of 2,000 reporting slots per minute, while the ITU universal AIS standard provides for 4,500 slots per minute.

Where both channels are unavailable in a region, a shore facility can direct a station to designated alternate channels. IMO notes this capability should be used only in urgent temporary situations, because of the adverse effects on AIS at sea.

The protocol defines 27 pre-defined message types, covering static and voyage data, dynamic data, scheduled and addressed messages, safety broadcasts, and interrogation and assignment replies.

Equipment classes

Class A — transmit power 12.5 W, SOTDMA access, full data set at roughly 2 to 10 second intervals depending on speed and manoeuvring. The mandatory equipment for SOLAS ships.
Class B+ — about 5 to 6 W, SOTDMA, roughly 5 second default interval. Voluntary, aimed at smaller craft wanting better visibility in busy traffic.
Class B — 2 W, CSTDMA, roughly 30 second default interval which a base station can change. Voluntary, and often fitted mainly to receive.

Typical onboard station

One or more VHF antennas.
One VHF transmitter.
Two multi-channel VHF receivers.
One channel 70 VHF receiver dedicated to channel management.
A central processing unit.
A GNSS receiver, providing timing and position redundancy.

The GNSS receiver does double duty. It supplies the vessel's own position for transmission, and it provides the TDMA clock — slot timing has to be synchronised tightly or transmissions collide.

What the messages carry

Static and voyage data: MMSI, IMO number, call sign, ship name, ship type, dimensions (length, breadth, draught), destination, ETA.

Dynamic data: position (latitude and longitude), speed over ground, course over ground, heading, navigational status, and a UTC timestamp.

CPA and TCPA

With a graphical display, the receiver computes Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA) from the received positions and the relative motion of the two vessels.

CPA is the closest distance their tracks will bring them together. TCPA is how long until that point. A small CPA with a short TCPA means a developing close-quarters situation.

These are a collision-avoidance aid, not a substitute for a proper look-out under the COLREGs. They describe what the tracks predict; they do not tell you what the other vessel is doing, whether it has seen you, or what it intends.

Limitations worth stating plainly

AIS is an aid to navigation. That is how it is officially classified. It was designed to improve situational awareness and safety, not to provide vessel tracking or surveillance. Treating it as a surveillance system misreads its purpose.
It is an open, unauthenticated broadcast. Identity and position data are effectively self-reported, so it can be spoofed.
It can be jammed. Deliberate jamming is a known and growing maritime-security concern, particularly where vessels transit contested waters.
Position accuracy depends on the vessel's own GNSS feed. AIS does not verify the position; it republishes whatever the transmitting station believes. A spoofed GNSS upstream produces a well-formed transmission carrying a wrong position.
Congestion degrades reception. In high-traffic areas such as straits and port approaches, collisions between transmissions cause messages to be lost, which is exactly where reliable information is most valuable.

VDES, the successor work

IMO's NCSR10 (2023) started work to amend SOLAS Chapters IV and V to introduce the VHF Data Exchange System (VDES). VDES would incorporate AIS and potentially supersede it as the mandatory carriage requirement, adding capacity alongside the existing navigation functions.

This is in progress, not yet in force. No amendment has entered into force, and vessels are required today to carry AIS under the existing provisions.

What a shore-side operator does with it

On the shore side, AIS data is aggregated across a region to build a vessel traffic picture: where ships are, where they are heading, where routes conflict. Operators spot anomalies — a vessel changing course unexpectedly, a ship stopping in a channel, an entry that never produces an exit — and integrate the picture with VTS. Historical tracks support route analysis and port planning.