09Ships, AIS and channel 16

The short version: Large ships announce themselves on two VHF channels, 161.975 and 162.025 MHz, using AIS, the maritime cousin of the ADS-B signal that puts planes on flight trackers. An international treaty, SOLAS, requires AIS on most ships of 300 gross tonnage and up and on all passenger ships, and US rules extend it to many smaller commercial vessels. Ship-tracking sites such as MarineTraffic map those broadcasts using shore receivers mostly run by volunteers. For voice, channel 16 (156.800 MHz) is the worldwide distress, safety and calling channel, and in the US federal law lets anyone listen to marine radio, though rules differ elsewhere.

What is AIS?

What you see: A ship-tracking app showing a container ship, ferry or tug in the harbor with its name, destination and a little arrow for its heading.

AIS (Automatic Identification System) is a radio system in which a vessel announces itself on VHF. The US Coast Guard's channel table lists two channels for it: AIS 1 on 161.975 MHz and AIS 2 on 162.025 MHz, both labeled "Automatic Identification System (AIS)."¹⁴⁹ Like aircraft ADS-B (see aviation), it is a broadcast meant to be heard by anyone in range, which is why other ships, shore stations and hobbyists can all pick it up.

The idea is collision avoidance and traffic awareness: a ship's bridge sees nearby vessels by name, not only as anonymous radar blips. The same openness is what lets a website plot the world's shipping.

US rules recognize two equipment classes. Class A is the full-grade unit required on the vessels in the next table, and Class B is a lighter class that the rule allows on some fishing and smaller vessels.¹⁵⁰

VHF signals travel roughly line of sight, so a shore receiver hears ships out to its horizon and no farther.

In the US Coast Guard's summary, each AIS position report carries the ship's MMSI identity number, position, course and speed over ground and heading, and on Class A units also rate of turn and navigational status; a separate message every 6 minutes adds details such as name, call sign, ship type and dimensions, and on Class A the destination and ETA.¹⁵¹ A Class A unit transmits at 12.5 W and reports every 2 to 10 seconds under way, depending on speed and turning, and every 3 minutes when anchored or moored.¹⁵¹ The lighter Class B-CS units transmit at 2 W and report every 30 seconds.¹⁵¹

Which ships have to carry AIS?

What you see: Every big ship in a harbor shows up on a tracker; many small boats do not.

Internationally, the rule is the International Convention for the Safety of Life at Sea (SOLAS), Chapter V, Regulation 19.2.4, as compiled by the US Coast Guard:¹⁵²

New ships built on or after July 1, 2002 had to carry AIS from the start, and existing ships were phased in between 2003 and 2008.¹⁵² The Coast Guard compilation also contains a conflicting alternate deadline for the phase-in, so the exact dates for existing ships should be checked against the IMO's own text.¹⁵²

The passenger-ship line is the one with no size floor: under that wording, a small ferry on an international run falls under SOLAS whatever its tonnage.

What are the US rules for AIS?

What you see: Tugs, ferries, dredges and fishing boats on a tracker alongside the big ships.

US law goes further than SOLAS. The Maritime Transportation Security Act (46 USC 70114) requires AIS for self-propelled commercial vessels of at least 65 ft (about 20 m), among others.¹⁵² The Coast Guard's carriage rule, 33 CFR 164.46, sets out who needs a Class A unit:¹⁵⁰

Vessels that had previously been exempt had to install AIS by March 1, 2016.¹⁵⁰ That expansion is why a harbor tug or a large commuter ferry now appears on a tracking map in the US, and why a kayak, a sailboat or a small recreational motorboat usually does not: the rule is about commercial vessels, and recreational boats are not on the list.

How does MarineTraffic track ships?

What you see: A world map of colored ship icons, dense along coasts and thinner out at sea.

MarineTraffic's help center, hosted by Kpler, says the site gets AIS data through "MarineTraffic's network of coastal receiving stations," and that "The vast majority of these stations are operated by contributors," who can apply for a free receiver.¹⁵³ The model is the same one the flight trackers use: volunteers put up a VHF antenna and a receiver, the receiver listens on the two AIS channels, and the data goes to a central server that draws the map. Nothing is transmitted to the ships.

Because shore receivers reach only to the radio horizon, coverage is best where contributors live and along busy coasts. Out in the open ocean, a shore network cannot hear a ship at all.

Satellites fill that gap (more in satellites). The European Space Agency notes that Earth's curvature limits shore-based AIS to about 74 km from the coast, and that tracking ships with small satellites, which pick up AIS and relay it to ground stations, "solves this problem."¹⁵⁴

What is marine VHF channel 16?

What you see: A marine radio, a scanner or a harbor stream with a Coast Guard safety announcement or a captain calling another vessel and asking it to switch to another channel.

Marine VHF voice runs on numbered channels around 156-157 MHz. Channel 16 (156.800 MHz) is the "International Distress, Safety and Calling" channel.¹⁴⁹ Vessels hail each other on 16 and then move to a working channel so 16 stays clear for emergencies.

Source: USCG NAVCEN US VHF channel information.¹⁴⁹

Channel 13 is where ships coordinate passing and crossing, and in US waters ships longer than 20 m (about 66 ft) keep a listening watch on it.¹⁴⁹ The Coast Guard announces safety broadcasts on 16 and then reads them on channel 22A.¹⁴⁹ Channel 70 carries DSC (Digital Selective Calling), a digital signal with no voice, so on a scanner it sounds like a short data burst.¹⁴⁹

Can I listen to marine radio?

What you see: A handheld scanner at a waterfront, or an online stream of harbor traffic.

In the US, yes. The federal wiretap statute says it is not unlawful to intercept radio communications "transmitted by any marine or aeronautical communications system," and also those relating to "ships, aircraft, vehicles, or persons in distress."¹³⁹ The same exemption covers airband, which is why marine and aviation listening share a legal footing in the US. Police and fire radio follow different rules, covered in scanners.

Listening is not the same as transmitting. A scanner only receives; a marine VHF radio can also transmit, and transmitting is a separate matter with its own rules, outside the scope of this section.

In the UK, receiving is more restricted. Section 48 of the Wireless Telegraphy Act 2006 makes it an offense, "without lawful authority," to use a receiver to learn the contents of a message you are not meant to receive.¹²⁰ A 2012 petition to Parliament stated that listening to radio services you are not authorized to receive is an offense, while a 2014 Ofcom notice is silent on receive-only listening; see aviation for details.¹⁴²¹⁴³

Elsewhere in the world

What you see: The same AIS map working off the coast of Rotterdam, Singapore or Sydney.

SOLAS is an international convention, and its AIS rule is written for ships on international voyages, which is why the same big ships appear on trackers in every major port.¹⁵² Rules for smaller vessels on domestic voyages are set by each country's maritime authority. Channel 16 is labeled international in the US Coast Guard's own table.¹⁴⁹

The international channel plan comes from Appendix 18 of the ITU Radio Regulations, and the Coast Guard publishes a table adapted from it.¹⁵⁵ Some channels in that plan "are not allowed within U.S. territorial waters, but are allowed on the high seas," and the Coast Guard says the number of them reflects a shortage of VHF maritime spectrum in the US.¹⁵⁵

In Boston

What you see: A line of buoys in the water around Logan Airport, and police and Coast Guard boats patrolling near them.

Logan International sits on Boston Harbor, and the water around it is guarded: a 250-ft (about 76 m) security zone marked by 29 buoys, set up in 2002 and patrolled by the State Police, Boston Police, Environmental Police, the Coast Guard and harbormasters.¹⁴⁶

On a ship-tracking map, Boston Harbor follows the US carriage rules in the table above: commercial vessels of 65 ft or more, larger towing vessels and boats certificated for more than 150 passengers carry Class A AIS, so they appear, while most recreational boats do not.¹⁵⁰ On the radio side, the channel plan is the national one, with channel 16 for calling and distress and channel 13 for bridge-to-bridge talk among ships over 20 m.¹⁴⁹

Boston has no Coast Guard Vessel Traffic Service: as of October 2026 the Navigation Center lists 12 VTS areas, among them New York, Houston-Galveston and Puget Sound, and Boston is not one of them.¹⁵⁶

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