Most positions on AIS Fleet come from a vessel's AIS broadcast, received by a station and passed to our network. Some Saillogger boats also share positions directly over the internet. The source and the age of a report affect what the map can tell you.

What AIS is

AIS stands for Automatic Identification System. It is a radio signal that a vessel broadcasts continuously, saying who she is and where she is: her identity, her position, how fast she is going and which way. Any receiver in range can hear it, and that is the point of it. AIS exists so that ships can see each other, and so that coastguards and traffic services can see them, which makes it a safety system rather than a tracking system that vessels have been signed up to.

Two things follow from that, and both matter to you:

• The signal is broadcast without encryption. A compatible AIS receiver and antenna can receive it within range. It is intended to help nearby vessels and shore services identify traffic.
• Carrying AIS is required of some vessels and optional for others. Requirements depend on vessel type, size and voyage; the IMO AIS overview explains the international requirements. Many small craft do not carry a transponder, so they may have no reports on the map.

An MMSI is the nine-digit number assigned to a vessel's radio identity. Searching all nine digits is useful when several boats share a name. Incorrectly programmed radios can report the wrong number, so check the name and other details too. You will find a boat's MMSI on her radio licence and on many vessel-tracking sites.

Speed is shown in knots (nautical miles per hour). Course is the direction of travel over the ground, measured clockwise from north: 90 degrees is east. It can differ from the direction the bow points, especially in wind or current.

Who receives it

AIS is a line-of-sight radio signal. It travels roughly as far as you can see from the aerial, which is tens of miles from a good shore mast and much less from a boat.

AIS Fleet draws what has been reported to it by the networks it takes part in: stations ashore, receivers aboard other boats, and the receivers AIS Fleet members share themselves. Nobody has a receiver everywhere, so coverage looks like this:

• Thick along busy coasts, in ports, and through the cruising areas where boats with receivers are.
• Thin to nothing far offshore. In the middle of an ocean there is usually nobody within range to hear a vessel at all.

What a mark on the map means

A marker says: at the time she was last heard, this vessel reported being here. It does not say she is there now.

When she was last heard is the number to read. It is on every vessel's panel, and everything above it describes the moment of that report rather than this moment. A speed of twelve knots from nine hours ago is a fact about last night.

That is also why some vessels are on the chart looking perfectly current and are hours old. A boat at anchor with her instruments off, a boat out of range of every receiver, and a boat whose transponder is switched off all look identical from here: quiet. The panel tells you how long she has been quiet, and that is the honest answer we can give.

When something is not working runs through the reasons a vessel is missing, in the order they are likely.

What AIS Fleet does not know

• Where she is going. AIS carries a destination field, and it is typed in by hand aboard, so it is often stale, abbreviated or simply left from the last voyage. There is no ETA anywhere on AIS Fleet, and there will not be one we have invented.
• When she will arrive. No arrivals board, and no alert when a vessel comes within a distance of you. Following her is what turns her into a notification.
• Whether anyone aboard is all right. This is the important one. A position is a radio message from a machine. It tells you nothing about the people.

Is watching a vessel legal, and is it fair?

Watching is legal. Receiving and showing AIS is what every vessel-tracking service does: the signal is an unencrypted safety broadcast that the vessel is transmitting deliberately, in the open, for any receiver to hear.

That is not the same as blanket permission to republish it for any purpose. Local rules and the AIS Fleet Terms still apply, and fairness is a separate question worth answering honestly, because a boat is somebody's home as often as she is somebody's workplace.

• What AIS Fleet shows of a vessel is what she broadcasts: identity, position, course and speed. That is the same for a container ship and for a family's yacht.
• Saillogger extras are the owner's choice, every time you look. A boater who keeps a logbook with Saillogger decides whether their journal, their logged track and their photo of the boat travel with the position, and that decision is re-checked on every read rather than fixed on the day you followed her. Catalog photographs are contributed separately by members. Boats that log with Saillogger
• Following can be visible to a linked owner. An AIS Fleet member or Saillogger owner may receive a notice with your display name when you follow their boat.
• You can report unwanted attention. Your account and privacy explains follower controls, blocking and how to ask us to review a concern.

Can I share a screenshot?

Yes. A screenshot of the map, or of a vessel's position, is yours to post, within the Terms and any map attribution the chart carries.

Two things worth doing when you post one: keep the last-heard time visible, so an old position is not passed off as current, and think about whether sharing where a particular person is could put them at risk.

Photographs in the vessel catalog belong to their contributors. Viewing one does not grant permission to repost it; ask the photographer about reuse. The journal, logged track and personal photographs a Saillogger boater shares also need their permission before you pass them on.

Where AIS Fleet gets its own coverage

Some of the map is there because members put it there. A member with an AIS receiver, aboard or on a shore mast, can feed what it hears into the network, and the water right around them is often water nobody else covers. It is free on every plan and the receiver stays private to its owner.

If you have one, Share your AIS receiver is how. If you do not, this is simply why the map is better in some places than others.