How ship tracking works

From the bridge to your screen

• The transponder on board takes the position from the ship’s GPS and broadcasts it on the two AIS VHF channels, every 2 to 10 seconds under way and every 3 minutes at anchor for the Class A equipment large ships carry.
• A receiver within radio range decodes the signal. Coast guards, port authorities, companies and volunteers run receivers on shore; satellites carry them in orbit.
• A network collects the decoded messages from many receivers. Seadar takes open feeds (BarentsWatch in Norway, Digitraffic in Finland, AISstream) and its own receiver network, under each feed’s licence.
• The tracking service checks each message, attaches it to the right ship through its MMSI and the IMO number in its static data, discards impossible positions, and publishes the result.

Why some ships update every minute and others not for days

VHF radio travels roughly as far as the horizon between the two antennas. IMO guidelines put the typical range at sea at 20 to 30 nautical miles; a receiver on a hill or a tall building can hear ships well beyond that. So coverage is dense along busy coasts, in straits and in ports, and thin or absent in the middle of oceans.

Satellites fill the ocean gaps: a receiver in low orbit hears ships across a very wide area. Satellite AIS costs money, and a satellite can struggle in crowded waters where thousands of transmissions overlap. The large satellite AIS networks are commercial, which is why free trackers are strongest near coasts.

A gap in a ship’s track therefore usually means nobody was listening, not that the ship switched off. Seadar shows the age of every position (“received 12 min ago”) and draws only the track it actually received.

Why the destination and ETA are often wrong

Position, course and speed come from sensors. The destination and the estimated time of arrival are typed in by the crew at the start of a voyage and are only as good as the last update. The ETA field holds a month, day, hour and minute in UTC, with no year, and the destination is 20 characters of free text. IMO guidelines recommend writing the destination as a UN/LOCODE, such as NLRTM for Rotterdam, but many crews write a port name, an abbreviation or nothing useful at all.

So a tracking site can do two things with an ETA: show the crew’s ETA as reported, or estimate its own from the ship’s position, speed and the sea route. Seadar does both and labels them: the reported ETA is shown as the ship sent it, and Seadar’s own ETA estimate is set in italics with its method, because it is derived, not sourced.

How a port call is detected

Ships do not broadcast “I have arrived”. A tracker infers a port call from movement. Seadar opens a port call when a seagoing ship stays at 2 knots or less inside a port’s area for at least 15 minutes, and closes it when the ship is clearly outside that area again; its public pages count calls lasting at least 30 minutes. Harbour craft such as tugs, pilot boats and pleasure boats are excluded, because they live in port. The full method is on the methodology page.

What can go wrong

• Stale data: a ship out of range keeps its last position on the map until it is heard again. Always check the position’s age.
• Bad identity data: a transponder programmed with the wrong MMSI or a placeholder IMO number can make two ships look like one. Seadar keys ships on checksum-valid IMO numbers only.
• Deliberate gaps and false positions: some ships switch AIS off or transmit false positions to hide where they are. Unexplained gaps next to sanctions designations are a warning sign, never proof on their own.

You can extend coverage yourself: an AIS receiver costs about as much as a good pair of binoculars, and Seadar’s receiver programme explains how to set one up.

Questions and answers

Is ship tracking real time?

Close to it where coverage is good: a Class A ship under way reports every few seconds, and a receiver passes the message on within seconds. Where no receiver is in range, the last known position may be hours or days old.

Can every ship be tracked?

No. Only ships that transmit AIS within range of a receiver. Warships, many small boats and ships that switch off their transponders do not appear, or appear only now and then.

Why is a ship shown on land?

Usually a GPS or configuration fault on board, or a corrupted message. Seadar drops positions that imply an impossible speed from the previous one, but a single bad report can still slip through.

Sources

Primary sources for the facts in this guide. Figures labelled as Seadar data come from Seadar’s own records, with their method stated next to them.

1. IMO resolution A.1106(29): Revised guidelines for the onboard operational use of AIS
2. U.S. Coast Guard Navigation Center: How AIS works
3. ITU-R M.1371: Technical characteristics for an automatic identification system

Published Sep 28, 2026 · For information only. Not for navigation, and not a substitute for the official sources cited.