Why ships stand still in the middle of the sea
A ship that does not move on the map is usually not at rest — it is out of coverage. About how ship positions are actually collected.
Follow a ship on the map long enough, and sooner or later it stops. The symbol stays put for hours in the middle of the sea, or disappears entirely only to reappear a hundred kilometres further west. The ship has not stopped. It is the reception that has stopped working.
AIS is radio, not tracking
AIS stands for Automatic Identification System, and is essentially a VHF radio transmitter on board. The vessel broadcasts its own identity, position, course and speed at regular intervals. Anyone with the right receiver can listen. There is no central service that knows where the ships are — the picture is pieced together by everyone who manages to hear the signals.
That has two consequences that explain almost everything odd you see on a ship map:
- Range. VHF in practice follows the line of sight. From a land station the signal typically reaches a few dozen nautical miles, depending on antenna height and weather. Beyond that, no one on land hears anything at all.
- Who is listening. Where there is no receiver, there is no data. Satellites pick up AIS over the open sea, but only pass by at intervals, and they struggle in dense traffic where many ships transmit at the same time.
Why the coast is dense and the sea is empty
Along the Norwegian coast, coverage is very good. The Norwegian Coastal Administration's network of land stations, provided through BarentsWatch, gives an almost continuous picture from Svinesund to Grense Jakobselv. Kartradar combines it with aisstream.io, which collects from receivers all over the world.
Out in the Norwegian Sea, between Iceland and the Faroe Islands, or north of Svalbard, the picture changes. There is no land station within range, and the picture depends on satellite passes. A ship can then sit still on the map for an hour or more, before jumping ahead to where it actually is. The jump is not an error in the position — it is the time between two observations becoming visible.
Why small boats are missing
AIS comes in two classes. Class A is mandatory for larger vessels in international traffic, transmits at high power and updates frequently, down to every other second when under way. Class B is meant for smaller vessels, transmits more weakly and less often, and is largely voluntary. Leisure boats without AIS simply do not exist on any ship map, no matter how good the coverage is.
That is also why a ship map can never be used to judge whether the water is clear. It shows who is transmitting, not who is there.
Not everything that sends AIS is a ship
This surprised us too. AIS is also used by objects that are not vessels at all, and they come in the same data stream as the ships. The identity lies in the MMSI number, nine digits where the first digits reveal what kind of object it is:
| Starts with | What it actually is |
|---|---|
| 111 | Search and rescue aircraft |
| 99 | Aid to navigation: buoy, light or virtual sea mark |
| 970, 972, 974 | Emergency beacons, including man overboard |
| 00 | Coast radio station, which stands on land |
| 8 | Handheld VHF |
| 98 | Auxiliary craft associated with a parent ship |
We discovered this when a "ship" on the map turned out to be moving straight across mountains and fjords in a star pattern, out and back from the same point 93 times. It was the Norwegian rescue aircraft with MMSI 111257014. The track was entirely correct. It was the classification that was wrong: an aircraft can fly over land, a ship cannot.
Kartradar now filters out everything that is not a vessel, with one deliberate exception. Numbers starting with 98 are kept, because auxiliary craft, workboats and MOB boats really are craft on the water. The rescue aircraft do not disappear from the map — they are shown where they belong, in the air layer, via the aircraft's own system.
When name and destination are nonsense
The ship's name, destination and expected arrival are not measured by anyone. They are typed in by the crew and updated when someone remembers. That is why you see ships heading for "FISHING GROUNDS", destinations that are not places, or arrival times that passed last week. Position, course and speed, on the other hand, come from the instruments and can be trusted.
How to read it on the map
A ship that has not moved for a long time is usually not at rest: it is out of coverage. Between two received messages, the symbol glides smoothly along its last known course and speed so the movement does not stutter. That is smoothing of the last measurement, not new information. The time of the last received message is shown in the info panel, and that is the number that tells you how much you can trust the position right now.