AIS, and what it cannot tell you about your container

The dot moving across the map on a container tracking page is a ship. It is not your fruit, it is not your container, and for a good part of the journey it is not even a measurement — it is a calculation about where a vessel probably is.

Understanding that gap is most of what an importer needs from AIS. Here is what the feed actually carries, and where it stops being about your cargo.

AIS — the Automatic Identification System — is a transponder that commercial vessels above a certain size are required to carry and broadcast under SOLAS. It transmits on two VHF channels, several times a minute, and any receiver in range can hear it.

It was built for collision avoidance, not cargo tracking. Ships broadcast so other ships can see them. That it also produces a global feed of vessel positions is a side effect — and the entire container-tracking industry is built on that side effect.

What is actually in the signal #

AIS sends two different kinds of message, and the distinction matters more than almost anything else on this page.

Dynamic data, broadcast every 2–10 seconds under way, comes from the ship’s own instruments:

Static and voyage data, broadcast every 6 minutes, is a mix of fixed identity and things a human typed in:

The crew-typed fields are the trap #

Destination and ETA look like the most useful fields in the whole feed, and they are the least trustworthy. They are free text, typed on the bridge, and nothing validates them.

You will see destinations abbreviated beyond recognition (ESALG, ALGECIRAS, ALG, SPAIN), left from the previous voyage, or simply blank. The ETA field is whatever someone entered on departure and often nobody updates. Navigational status is frequently left on “under way” while a vessel sits at anchor.

This is why an ETA sourced from AIS is worth less than one from the carrier, and much less than a discharge milestone. See ETA vs ETD.

Draught is more interesting than it looks: a laden container ship sits materially deeper than one in ballast, so reported draught is a rough read on whether a vessel is loaded — when the crew keeps it current.

How the signal reaches a map #

A position on your screen has already passed through five hands:

GNSS → AIS transponder → receiver → aggregator → tracking provider → us.

A receiver on the bridge fixes the ship’s position from GPS, Galileo, GLONASS or BeiDou. The transponder broadcasts it over VHF. Shore stations and satellites pick it up. Aggregators pool those receiver networks and sell the feed on. A tracking provider blends it with carrier data, fills the gaps and computes an ETA. We poll the provider.

Two consequences worth holding onto. The position is measured by the ship, not of the ship — nobody observes the vessel from outside, so corrupting GNSS at step one means every later step faithfully relays a wrong answer. And the smooth “live” dot is created near the end of that chain, by the provider, filling gaps the receivers left. It is the most processed thing on the page.

Why tracking positions are often wrong walks the chain step by step.

Terrestrial receivers — shore stations, roughly 40–60 nautical miles of range. Near ports, coverage is dense and near-continuous.

Satellite AIS — satellites pick up transmissions mid-ocean, but each only sees a patch of sea as it passes over, and in busy lanes so many vessels transmit at once that messages collide and are lost.

So a vessel approaching Rotterdam reports every couple of minutes. The same vessel mid-Pacific can go hours between confirmed fixes. Neither is a fault.

When the signal isn’t there — or isn’t true #

Coverage gaps are the ordinary case. These are the ones that surprise people.

Vessels switch AIS off deliberately. Transiting the Bab el-Mandeb strait and the southern Red Sea, ships have gone dark to avoid being identified and targeted — and others have used the free-text destination field as a broadcast message instead, transmitting crew nationality or ownership in the hope of not being attacked. A vessel disappearing from the map in that corridor is a normal precaution, not a tracking failure.

Going dark is also the standard technique for sanctions evasion, which is why “dark activity” is a category commercial AIS providers now sell analytics on.

Coverage around the Chinese coast largely vanished from commercial feeds. Terrestrial AIS data out of Chinese waters dropped sharply from late 2021, when domestic providers stopped supplying it to international aggregators following Chinese data-privacy legislation. Separately, GNSS interference and AIS spoofing have been reported around several Chinese ports for years — including vessels appearing to sail in tight circles on land. The net effect for an importer is the same: positions in that region are thinner and less reliable than the map’s confidence suggests.

AIS can be spoofed generally. The protocol has no authentication whatsoever. Anything you receive is what someone transmitted, and nothing proves the transmitter is the vessel it claims to be.

What this means for the dot on your map #

Because gaps are normal, most tracking products interpolate — placing the vessel along its expected route at its average speed. The result glides smoothly whether or not the ship is moving.

A moving dot is not evidence of a moving vessel. What matters is the timestamp of the last real fix. Trackberry leaves estimated positions off the map entirely and stands the marker on the newest genuine fix, for exactly this reason — why tracking positions are often wrong covers the trade in full.

The limit that matters most #

AIS tracks ships, not boxes. Your container has no transponder. Once it is lifted off at a transshipment hub, the feed is about a vessel that is no longer carrying your cargo — and a container crossing Panama by rail has no AIS at all.

For where your container is, carrier milestones are authoritative. AIS tells you where the ship is, which is a different question that happens to have the same answer most of the time.

FAQ #

Why does my vessel position not update for hours? #

Mid-ocean AIS depends on satellite passes rather than continuous shore coverage, so multi-hour gaps are normal. The fix time tells you how old the position really is — that number matters more than the dot.

Can I trust the ETA shown in AIS data? #

Not much. The ETA and destination fields are free text typed by the crew and frequently stale, abbreviated or blank. A carrier milestone or a discharge event is worth far more than an AIS-sourced ETA.

Why did my vessel disappear from the map in the Red Sea? #

Ships transiting Bab el-Mandeb have switched AIS off to avoid being identified and targeted. It is a deliberate precaution rather than a fault, and the signal typically returns once the vessel is clear of the corridor.

Is AIS position the same as my container’s position? #

Only while the container is on that vessel. After a transshipment, or on any inland leg, the vessel’s position says nothing about your cargo.

Can AIS data be faked? #

Yes. The protocol carries no authentication, so a transmission is only ever evidence that something transmitted. Spoofing is well documented, both for sanctions evasion and around certain ports.