Is AIS the same as GPS?

No. GPS tells a ship where she is; AIS is how she tells everyone else. Her AIS takes a position from a position-fixing device such as a GPS receiver and sends it out by radio, with her name, speed, course and destination.

Does a ship have to carry AIS?

Only some. The international rules require it on ships of 300 gross tonnage and over on international voyages, on cargo ships of 500 and over at home and on every passenger ship, and a flag state may exempt some. A boat with none cannot be seen on any AIS map.

Can AIS be switched off?

Sometimes the rules allow it: a ship is meant to keep it running at all times, except where the protection of navigational information calls for something else. A unit can also fail or lose power. To a map, a ship with her AIS off looks the same as one out of range. Either way she is not on the map, and Shiptrail cannot tell which.

Why do two ships show the same MMSI?

An MMSI is meant to belong to one transmitter, but now and then two are found sharing a number. Shiptrail checks each new position against the last one, and holds back a position that is more than 10 nautical miles from the last one and further than a ship at 80 knots could have gone in the time, until a second position agrees with it 30 seconds or more later. With two transmitters on one number, each place can keep being confirmed, so the result can be one mark that hops between two places. Why a ship disappears has the rest of the jump check.

What AIS is: the radio that puts ships on a map

The short answer

AIS is the Automatic Identification System: a small radio transponder on a ship that keeps announcing, without anyone touching it, who she is, where she is, how fast she is going and where she is headed. Other ships hear it, so they know who is near them. Stations on shore hear it. And so does anyone with a receiver in range, which is why a map like Shiptrail can draw ships at all.

It is a safety system first, and drawing ships on a map is something it lets other people do. The rest of this page says what a ship sends, who has to carry it, how far it reaches and what it cannot tell you. If you want to find a ship rather than understand the radio, How to track a ship is the place to start.

What a ship sends, and how often

A ship's transponder sends three kinds of information, and they go out on different schedules.

The first says where she is: her position, speed, course and heading. This is the one that moves her mark. A large ship sends it every 2 to 10 seconds under way and every 3 minutes at anchor, and faster when she is going fast or turning, because a ship that is changing course needs her neighbours to keep up.

The second says who she is: her name, call sign, MMSI, IMO number, type and size. It hardly ever changes, so it goes out only about every six minutes. MMSI, IMO and call signs says what those three are.

The third says where she is going: her destination and the time she expects to arrive. The crew type these in by hand, and they ride along with her identity, in the same message, so they are sent about every six minutes too.

Class A and Class B

AIS comes in two kinds. Class A is the full-strength kind that large cargo and passenger ships must carry. Class B is the lighter kind on yachts, fishing boats and other small craft, which reports less often. A Class B unit sends about every 30 seconds when moving, against every 2 to 10 seconds for a large ship, and every 3 minutes when she is not moving faster than 2 knots.

Each ship's page has a row called AIS class that says which kind she has. Shiptrail works it out rather than being told, because only a Class A unit sends a status. When the feed that heard her passes on too little to go on, the row says Class A or B, and says it cannot tell them apart.

Who has to carry one, and who may switch it off

The international safety rules (SOLAS, regulation V/19) require AIS on ships of 300 gross tonnage and upwards on international voyages, on cargo ships of 500 and over that do not make international voyages, and on all passenger ships whatever their size. A flag state may exempt certain ships. Boats smaller than that can fit one too.

Then there is the question of keeping it on. The rule is that a ship fitted with AIS keeps it running at all times, except where international agreements, rules or standards provide for the protection of navigational information. So there are cases where switching it off is allowed. On a map, that looks the same as a ship that has sailed out of range, and you cannot tell the two apart from a dot.

How a ship is heard

AIS is sent by VHF radio, and VHF travels in straight lines. It does not bend round the curve of the Earth, so it reaches about as far as an antenna can see: a receiver on land hears ships within about 20 to 40 nautical miles.

That is the whole reason a coast is covered and the middle of an ocean is not. A ship is heard by the other ships near her, by stations on shore and by receivers on land. Once she is over the horizon from all of them, nobody hears her, and the open ocean is dark. Satellites can hear ships far out at sea, and Norway's feed carries some of their reports for Norwegian waters.

What AIS is not

AIS is not someone watching a ship from outside. It is a ship talking about herself, and it is only as good as what she says.

It does not measure where she is. The transponder takes her position from an electronic position-fixing device on board, such as a GPS receiver, and repeats it. If that instrument is wrong, AIS is wrong in the same way.

A destination and an arrival time are typed in by hand and nobody checks them, so a ship can leave a port still showing that port as her destination. And a ship that is not transmitting is not on the map at all: a boat with no transponder, or with hers switched off, cannot be seen. A boat that is missing from the map is not necessarily missing from the sea.

How Shiptrail uses it

Shiptrail does not run a receiver on every coast. It takes in what other people's receivers hear, from three sources: AISStream, which passes on what volunteers' receivers hear; BarentsWatch, whose data the Norwegian Coastal Administration collects, for Norwegian waters; and Digitraffic, from Fintraffic, for Finnish waters.

From those it keeps the newest report for each ship. She stays on the map for 3 hours after her last report if she was moving and 6 if she was stopped, and Shiptrail keeps a page for any ship heard in the past year. The limits are the radio's: receivers on land, ships near coasts, and the open ocean is dark. How to track a ship shows how to search for one, and Why a ship disappears says what happens when she goes quiet. Positions are not for navigation.

Questions people ask

Is AIS the same as GPS?: No. GPS tells a ship where she is; AIS is how she tells everyone else. Her AIS takes a position from a position-fixing device such as a GPS receiver and sends it out by radio, with her name, speed, course and destination.
Does a ship have to carry AIS?: Only some. The international rules require it on ships of 300 gross tonnage and over on international voyages, on cargo ships of 500 and over at home and on every passenger ship, and a flag state may exempt some. A boat with none cannot be seen on any AIS map.
Can AIS be switched off?: Sometimes the rules allow it: a ship is meant to keep it running at all times, except where the protection of navigational information calls for something else. A unit can also fail or lose power. To a map, a ship with her AIS off looks the same as one out of range. Either way she is not on the map, and Shiptrail cannot tell which.
Why do two ships show the same MMSI?: An MMSI is meant to belong to one transmitter, but now and then two are found sharing a number. Shiptrail checks each new position against the last one, and holds back a position that is more than 10 nautical miles from the last one and further than a ship at 80 knots could have gone in the time, until a second position agrees with it 30 seconds or more later. With two transmitters on one number, each place can keep being confirmed, so the result can be one mark that hops between two places. Why a ship disappears has the rest of the jump check.

Data sources and credits

Ship positions

AISStream, which relays what volunteer receivers hear.
Norwegian waters: Data delivered by BarentsWatch, collected by the Norwegian Coastal Administration, under the Norwegian Licence for Open Government Data (NLOD) 2.0.
Finnish waters: Source: Fintraffic / digitraffic.fi, license CC 4.0 BY.

The rest

Photographs: Wikimedia Commons, each credited to its author under its licence. Ship details: Wikidata and the US Coast Guard.
The map, the weather, the charts and the type are credited on About.