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, by design. 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, not a tracking service that vessels sign up for.

Two things follow from that:

• 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.

A position report can arrive without a vessel's name. AIS Fleet keeps her last known name while updating her position.

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 the positions reported to it by stations ashore, receivers aboard other boats, and receivers that AIS Fleet members share. 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.

Who the map credits

The line under the vessel count starts with Data: and the name of our own network. The name is Saillogger when Saillogger sent a position for a vessel on screen, for the vessel you have open, or for a track on the map. In all other cases it is AIS Fleet. Then the line adds AISHub contributors, Fintraffic and BarentsWatch, each only when it sent one of those positions, for example Data: AIS Fleet, AISHub contributors. Each name means:

• Saillogger: boats on Saillogger and the AIS receivers aboard them.
• AIS Fleet: AIS Fleet's own network, including the AIS receivers that members share with it.
• AISHub contributors: the volunteers who share their receivers through AISHub.
• Fintraffic (CC BY 4.0): Finnish waters, from Fintraffic's Digitraffic service, under the CC BY 4.0 licence.
• BarentsWatch: Norwegian waters, delivered by BarentsWatch from data owned by the Norwegian Coastal Administration (Kystverket), under the NLOD licence.

To read each credit in full with its links on the website, select the information button at the end of the line, or on a touch screen tap the vessel count. In the app, tap the vessel count, then the Data: line in Vessels in view. A vessel's public page shows the same line in the corner of her map, and prints the Fintraffic and BarentsWatch notices in full below the map when they apply.

Names linger briefly as vessels leave view, so panning does not make the line flicker. When you open the panel with a keyboard, focus starts on Close. Press Tab to reach the links. Select Close or press Escape to close the panel and return to the control you used to open it.

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 describes her nine hours ago, not now.

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, which is as much as the reports can show.

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 and when she will arrive. AIS carries a destination and an ETA, and her crew types both in by hand, so either can be stale, abbreviated or left over from the last voyage. Her panel shows them as she sends them, and AIS Fleet does not estimate an arrival time of its own. There is no arrivals board: following her is what brings you notices about her.
• Whether anyone aboard is all right. This is the important one. A position is an automatic radio report from equipment aboard, and it tells you nothing about the people.

Is watching a vessel fair?

Receiving and showing AIS is what every vessel-tracking service does: the signal is an unencrypted safety broadcast that the vessel transmits openly, for any receiver to hear.

How AIS Fleet may be used is set out in the AIS Fleet Terms. Fairness matters too, 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?

Before posting a screenshot of the map or a vessel's position, check the AIS Fleet Terms, the map attribution and any third-party rights.

Two things worth doing when you post one: keep the last-heard time visible, so an old position is not mistaken for a current one, and consider 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 no other member sees the receiver.

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