What is AIS? Automatic Identification System (AIS) Overview, Types & How It Works

The Automatic Identification System (AIS) is a small VHF radio transceiver that automatically tells the world who you are, where you are, and where you’re heading. Whether you run a 1,000‑foot container ship or a 20‑foot center‑console, AIS lets you see – and be seen by – other boats, satellites, and shore stations, sharply cutting collision risk and giving rescuers a digital trail to follow.

What is AIS?

Automatic Identification System (AIS) is a digital “name tag” every large commercial vessel must broadcast. It rides on the same VHF band as your marine radio and sends two kinds of information:

• Dynamic Data: changes every few seconds (GPS position, speed, course, heading).
• Static Data: this information includes the vessel name, MMSI number, size, and destination and rarely changes. Ships exchange this data directly, ship‑to‑ship and ship‑to‑shore, 24 hours a day.

The International Maritime Organization wrote AIS into the SOLAS (Safety of Life at Sea) Convention, making it mandatory on passenger ships of any size and on most cargo ships above 300 gross tons by the end of 2004.

Why AIS Matters to Every Boater

Even if you never leave the inlet, AIS shrinks the blind spots left by radar and human vision.

• Prevents collisions by showing target names, CPA (closest‑point‑of‑approach) alerts, and speed vectors in real time.
• Speeds up rescues. Coast Guard watch‑standers can see your last broadcast even if they never catch your Mayday.
• Cuts stress. You know what the ferry up ahead is doing and she knows you’re there too.

How AIS works

Two tiny VHF channels do the heavy lifting

AIS uses Channel 87B (161.975 MHz) and Channel 88B (162.025 MHz). Every transceiver grabs a micro‑slice of time – one of 2,250 slots per minute – then blurts its message using 9,600 bps Gaussian Minimum Shift Keying (GMSK). Because the slots reshuffle (Time Division Multiple Access), hundreds of boats can talk at once without stepping on each other.

What gets sent

Your GPS feed tells the AIS where you are as onboard sensors add course, heading, and rate of turn; whereas the captain programs static facts once per voyage. The transceiver packages everything into a 168‑bit telegram and fires it out every 2–10 seconds underway for Class A – big ships, or every 30–180 seconds for Class B – recreational & light commercial. Shore stations and satellites scoop up the same messages.

Meet the Two Main Classes

Class B transponders are the faster‑growing segment because they give small‑boat captains the same situational awareness that commercial ships enjoy – without the big‑ship price tag.

Other AIS Types

• AIS Aids to Navigation (AtoN). Buoys and lights broadcast their own position – or appear virtually on your chart plotter.
• AIS SART & MOB beacons. A man‑overboard device screams your GPS fix to every AIS target within ~10 nm.
• Satellite AIS (S‑AIS). Low‑Earth‑orbit satellites extend reception thousands of miles offshore, feeding Coast Guards, insurers, and shipping analysts.

What Information Does AIS Broadcast?

How Far Can AIS Reach?

Terrestrial range is basically “radio line‑of‑sight.” From a sailboat mast you’ll see about 20 nautical miles; from a 150‑foot bridge wing the horizon stretches 40 nm or more. Satellites pick up the same 12.5‑watt whispers from 400 km overhead, but in dense lanes signals can collide, reducing hit‑rate.

Keeping the System Safe

AIS messages are not encrypted. Attackers can spoof, jam, or impersonate ships. However, countermeasures include:

• Plausibility filters that flag jumps in speed, course, or MMSI.
• Direction‑finding antennas that compare bearing angles.
• Experimental digital signatures now under review at the ITU and IEC.

Until encryption becomes standard, the best defense is layered sensors, radar, visual watch, and common sense.

The Future – VDES & Smarter Analytics

The VHF Data Exchange System (VDES) is the next‑gen upgrade. By adding new channels and burst data rates up to 307 kbps, VDES will carry software updates, weather files and even two‑way text without hijacking AIS safety slots. Machine‑learning models already crunch AIS tracks to forecast arrival times or spot illegal fishing; OECD and Global Fishing Watch dashboards make the insights public.

Installing AIS on your boat

Pick the right unit

Match the transceiver to your use case. Weekend coastal cruisers usually choose Class B SO (5 W, faster update). Offshore racers may spring for Class A to get turn‑rate data.

Mount the antenna high

AIS shares your VHF antenna or uses a dedicated whip. Height, not wattage, is king for range, so aim for the spreaders or radar arch.

Feed it GPS

Most black boxes include an internal GPS, but a dedicated antenna clears deck clutter and avoids metal shadowing.

Wire into your network

NMEA 2000 makes plug‑and‑play easy: one “T” drop to your backbone and every chart plotter, MFD, or tablet glimpses the same targets.

Program the static data

Enter your MMSI, boat length, call sign, and a default destination. Double check your information because the Coast Guard issues fines for bogus entries.

Test and sail

Power up, ask a buddy 5 miles away to confirm they see your MMSI, and that you’re broadcasting.

FAQs

Do I need a license?
In the U.S., recreational AIS transceivers don’t require an individual FCC license if they stay inside U.S. waters. Cross‑border cruisers still need an FCC ship station license.

Will AIS replace radar?
No. Radar sees squalls, channel markers, and wooden boats without electronics. AIS is a supplement, not a substitute.

What about privacy?
You can legally turn AIS off for security, but only if it doesn’t compromise safety. Commercial ships must log any shutdown in the bridge records.

Sea Tow Team

Sea Tow has been the premier leader in on-water boating assistance since 1983. We want to share news, press, tips and all things boating.