Why tracking positions are often wrong

Open almost any container tracking page and you get a ship on a map, moving steadily along a line towards its destination. It looks like a live feed. It usually isn’t.

Most of what you are looking at is calculated. For a dry-goods importer that hardly matters — a container of furniture arriving four days late is an inconvenience. For a reefer of grapes it is the whole business: you have sold that fruit against a date, booked a cold store slot against it, and every day the shipment is quietly not where the map says costs shelf life you cannot get back.

So the question is not whether the dot is roughly right. It is which parts of that page are observed and which are inferred — because that is the difference between tracking that informs a decision and tracking that reassures you right up until the fruit doesn’t arrive.

How a position reaches your screen #

Before anything else, it helps to know how many hands a position passes through. Each one adds delay, and each one is somewhere the data can thin out or change meaning without saying so.

Two things follow from this, and both matter more than they sound.

The position is a measurement taken on the ship, not a measurement of the ship. Nobody is observing the vessel from outside. The vessel computes where it thinks it is from satellites and announces it. That is why GNSS interference is so effective: corrupt step 1 and every step after it faithfully relays a wrong answer.

“Live” almost always means step 5. The friendly moving dot is produced near the end of the chain, by a provider filling gaps left by steps 3 and 4. It is the most processed thing on the page and usually the least labelled.

An ETA computed from an interpolated position is an estimate built on an estimate — see why does my ETA keep changing?.

Carrier milestones — loaded, discharged, gate-out — do not come down this chain at all. They come from the shipping line’s own systems, which is why they are slower, sparser, and far more trustworthy about your actual container. AIS covers what the feed carries in detail.

Where the position comes from #

Vessels broadcast their position over AIS, a VHF transponder system built for collision avoidance. Shore stations pick it up within about 40–60 nautical miles of a coast. Beyond that, satellites do — but each satellite only sees a given patch of ocean as it passes over, and in busy sea lanes transmissions collide and are lost.

So coverage is excellent near ports and patchy in the middle of an ocean. A vessel approaching Rotterdam reports every couple of minutes. The same vessel mid-Pacific might go six hours between confirmed fixes.

What happens in the gaps #

A tracking product has three options when it has no recent fix, and only one of them looks good:

1. Show the last known position, hours old, and say so.
2. Show nothing.
3. Estimate where the vessel probably is now, and draw that.

Almost everyone picks the third. The estimate is straightforward — take the last fix, take the expected route, take the vessel’s average speed, and place it the appropriate distance along. Refresh it continuously and you get a dot that glides smoothly across the ocean.

It is a good guess most of the time. Ships travel in straight lines at constant speeds, and interpolation between two points on a great-circle route is usually close to correct.

When the guess fails #

Interpolation cannot know about anything that isn’t in the route and the speed. Specifically:

Slow steaming. Lines routinely cut speed to save fuel — the single largest variable cost of a voyage. A vessel doing 14 knots instead of 20 falls progressively behind an estimate built on its average.

Waiting at anchor. Congested ports queue vessels outside the breakwater, sometimes for days. The interpolated dot sails serenely into the port and sits at the berth; the actual ship is at anchor eight miles out with nothing happening.

Diversions. Weather routing, canal closures, security reroutes. The estimate follows the planned route; the vessel doesn’t.

The land bridge. Some Panama transits move containers by rail rather than through the locks. A train has no transponder, so there is no signal at all — and a system that treats silence as a fault reports a tracking outage where there is only a train.

In each case the failure is silent. The dot doesn’t flicker or turn amber. It keeps gliding.

When the position is right and the route is wrong #

Everything above is about a dot in the wrong place. The opposite failure is stranger, and it is the one that generates the most support tickets: the dot is exactly where the ship is, and the line it is supposed to be sitting on was never complete.

A tracking page draws the route your bill of lading describes — load port, any transshipment, discharge. The vessel is under no obligation to sail only that. It has its own rotation, and it can put into a port that has nothing to do with your cargo, work somebody else’s boxes, and carry on.

Here is the shape of it on a northbound South Pacific sailing. The paperwork says Callao, then Cartagena. On the way the vessel turned into the Gulf of Guayaquil, called, and came out again. Your container never left the ship, so nothing happened to it that was worth a milestone. The timeline shows nothing. The drawn line runs straight up the coast. And the marker — correctly placed, on a real fix — sits well inshore of that line, apparently off-route.

Nothing here is broken, which is what makes it so hard to read. The position is right. The schedule is right. The route is right about your cargo. What is missing is the vessel’s own itinerary, and that is the only thing that explains both the detour and the two days it cost you.

The tell is the shape of the departure. An interpolated dot drifts away from the line gradually and in the direction of travel, because the estimate is running fast or slow. A port call leaves the line sideways, stops, and comes back to the same point. If you see that, the question is not “is this position wrong” but “what is that place” — and the vessel’s own port calls, on any public AIS site, will name it in about thirty seconds.

And sometimes the signal is missing on purpose #

Interpolation assumes the gaps are accidents of coverage. Often they aren’t.

Vessels switch AIS off. Transiting Bab el-Mandeb 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 message board, broadcasting crew nationality or ownership rather than a port. A vessel vanishing from the map in that corridor is a deliberate precaution, not a tracking outage, and it will reappear once clear.

Whole regions thin out. Terrestrial AIS out of Chinese waters largely stopped reaching international aggregators from late 2021, after domestic providers withdrew it in the wake of Chinese data-privacy legislation. GNSS interference and spoofing have also been reported around several Chinese ports for years, including vessels that appear to sail in circles on land.

The protocol has no authentication at all. Anything you receive is simply what someone transmitted. Spoofing is well documented, and “dark activity” is now a product category commercial AIS providers sell analytics on.

None of this is exotic — it is ordinary background noise in a feed that was designed for ships to avoid each other, not for anyone to audit cargo. A tracking product that interpolates through it produces a confident line across a region where it has no idea what happened. AIS goes through what the feed does and doesn’t carry.

How to tell the difference #

Two habits, and they cost nothing:

Look at the fix time, not the position. Every honest tracking system records when a position was actually confirmed. If the last real fix is nine hours old, the position is a nine-hour extrapolation, whatever the map suggests. A smoothly moving dot with a stale fix time is the exact shape of the problem.

Trust milestones over geometry. A carrier milestone — loaded on vessel, discharged, gate-out — is a record of something that happened, reported by the party it happened to. A position is a calculation. When the two disagree, the milestone wins.

Check it yourself in two minutes #

You do not have to take anyone’s word for this, including ours. Public AIS sites — MarineTraffic, VesselFinder and others — show the same underlying feed the industry buys, and they will tell you things a polished tracking page often will not.

Take the vessel name from your bill of lading or arrival notice, and:

1. Search the vessel and confirm you have the right one. Names repeat and get reused, so check the IMO number against your paperwork before believing anything on the page. This is the step people skip, and it is the one that produces confident tracking of somebody else’s ship.
2. Find the timestamp on the last position. It is usually next to the position or under a “last report” label. This is the single most useful number on the page — everything else is derived from it.
3. Compare it to now. Minutes old near a coast is normal. Several hours mid-ocean is also normal. What matters is that you now know how much of what you are looking at is observation and how much is inference.
4. Look at speed and navigational status. A vessel showing 0 knots and “at anchor” is waiting for a berth, whatever any ETA says. That one line frequently explains a delay nobody has told you about yet.
5. Check the destination and ETA fields last, and trust them least. Both are free text typed on the bridge, and both are routinely stale or left over from the previous voyage.

Do this once on a shipment you already understand and the habit sticks. Do it on a shipment that has gone quiet and you will usually find the answer in under two minutes — which is faster than the email thread that would otherwise have produced it.

What we do instead #

Trackberry draws the observed track as a solid line with a dot at each confirmed fix, and the carrier’s route as a dashed line. The vessel marker stands on the newest position we could confirm as a genuine AIS fix — estimated positions are left off the map entirely, so the ship never runs ahead of its own track to a spot nobody observed.

The cost is that the marker is sometimes visibly behind where the vessel probably is. We think that is the right trade: a position that is honestly a few hours old is more useful than one that is confidently wrong, because you can reason about the first one.

Hovering any point gives you the time the vessel was actually confirmed there, and the freshness breakdown separates four different clocks — what the carrier published, when the tracking service last asked, when we last pulled, and when the vessel position last moved.

Where it stops being about your container #

None of this survives a transshipment. AIS reports the ship; your container has no transponder. Once the box is lifted off at a hub, the vessel feed is about a vessel that is no longer carrying your cargo — and it will continue to its destination, on schedule, reporting beautifully, while your fruit sits on the quay waiting for a connection.

This is the single most important limit to understand. Everything on the map is about the ship. Only the milestones are about the box.

FAQ #

Why does the vessel marker sit behind the route line? #

Because it stands only on positions confirmed by a real AIS fix, while the dashed route is the tracking service’s reconstruction of the intended path. If the last confirmed fix is six hours old, the marker stays there rather than gliding forward to a position nobody observed.

Is an interpolated position wrong? #

Not usually — it is a reasonable estimate, and most of the time it is close. It becomes wrong when the vessel does something the estimate cannot know about: slowing down, waiting at anchor, or diverting. The problem is that it fails silently.

How often does a vessel position actually update? #

Every few minutes near a coast, where shore stations pick up the signal. Mid-ocean it depends on satellite passes and can be several hours. Both are normal; the fix time is what tells you which you are looking at.

What should I trust when the map and the milestones disagree? #

The milestones. A carrier milestone records something that happened. A map position is a calculation about where a ship probably is, and it is about the ship rather than your container.