A ship disappears from an AIS map for one of three reasons: nobody is receiving its signal (a coverage gap), the crew has deliberately switched off or throttled the transponder (a so-called dark vessel), or the data being broadcast is false (spoofing). A gap in the track does not tell you which of the three happened. To judge that, you look at where the track breaks, what kind of vessel it is and what was being reported nearby at the same time — on the Yemen and Red Sea event map, for instance.
How does AIS work, and why does it have gaps at all?
AIS, the Automatic Identification System, is a VHF transmitter that periodically broadcasts a ship's position, course, speed, its MMSI identifier and a handful of descriptive fields such as name, vessel type and declared destination. It was designed as a collision-avoidance tool: nearby ships see each other and coast stations see traffic along the shore. Global tracking is a by-product — somebody has to receive the signal and pass it on.
Reception happens in two ways. Terrestrial stations, including thousands of hobbyist receivers feeding public tracking sites, hear ships within radio line of sight, typically up to a few tens of kilometres from the coast. On the open ocean the signal is collected by satellites, which pass over a given area intermittently and never provide a continuous picture. On top of that comes the split between Class A and Class B: large SOLAS vessels transmit more often and with more power, while fishing boats and yachts transmit less often and more weakly. The mechanics are covered in our guide to how AIS works.
Innocent gaps: range, satellites and congestion
Most "disappearances" have nothing to do with intent. A ship heading offshore simply leaves the range of terrestrial receivers, and its next position appears only when a satellite passes or when it approaches another coast. In very busy waters — around major ports, for example — many transmitters overlap and a satellite receiver drops some of them. Terrestrial receivers can also be offline, overloaded or simply absent from a stretch of coastline: the coverage of public trackers is uneven and reflects where people with antennas live rather than where ships sail.
In practice this means tracks routinely break on ocean crossings, along sparsely populated coasts and in regions where few services operate their own receivers. Such gaps look calm: the ship vanishes in one place and reappears in another, roughly where its course and speed would have taken it.
When does a crew switch AIS off on purpose?
The SOLAS convention requires AIS to be in operation at all times, but it allows an exception: the master may switch it off if they judge that continuous operation compromises the safety or security of the ship. Operators use that right in piracy areas and conflict zones. After the Houthi attacks on shipping began in late 2023, some vessels transiting the Bab el-Mandeb strait and the Gulf of Aden throttled or switched off AIS for the passage, while others typed messages aimed at potential attackers into the destination field. Similar practices have been reported in the Persian Gulf and around the Strait of Hormuz, where tankers have been seized and harassed.
For an analyst this is a useful clue: a gap that starts just before a threat zone and ends just after it is far more likely to be a crew decision than a coverage problem. The context — attacks, warnings, incidents — can be followed on the Yemen map and the Iran map, and the background is explained in our guide to the Houthi attacks in the Red Sea.
Spoofing and false positions
The third scenario is data that is being broadcast but is not true, and it pays to separate two phenomena. The first is jamming or spoofing of satellite navigation (GNSS): the ship's receiver is fed a wrong position and honestly broadcasts it. The symptoms are tracks that circle in one spot, jump by hundreds of kilometres or run across dry land — often near ports, airports and military sites, where such interference is reported most. The second is manipulation of AIS itself: broadcasting a false MMSI, impersonating another vessel, using a borrowed name or transmitting an invented route. Here the intent is unambiguous and usually tied to hiding real activity.
Both are detected the same way: by comparing what is declared with what is physically possible. A tanker does not cover 300 kilometres in an hour, a container ship does not sit in the middle of an airport, and an IMO number tied to one hull should not surface under a different name on the other side of the world.
The shadow fleet and ship-to-ship transfers
The term "shadow fleet" describes vessels — mostly ageing tankers — that carry sanctioned cargoes and go to some lengths to stay out of sight: they sail under flags with weak oversight, change names and owners frequently, limit their AIS broadcasts and transfer oil from ship to ship on the open sea, away from ports. To someone watching a map, the typical picture is a track that breaks near known transfer areas, a long silence, and then a reappearance with a different draught than before the gap. Not every such gap means sanctions evasion, but a repeated pattern from one hull is a reason to look at its history.
Warships broadcast selectively
SOLAS does not apply to warships, so they are under no obligation to transmit AIS. In practice they do so selectively: on during transits of busy straits or port visits, when navigational safety outweighs discretion, and off during operations. Naval vessels also appear under generic names or with positions reported late. The absence of warships on a map in a tense region therefore tells you nothing — just as the absence of military aircraft on an ADS-B map tells you nothing, as we explain in why planes disappear from radar.
How to read a gap in a track: a checklist
- Where did the track break? Far offshore — probably range. Just before a threat zone or a known transfer area — probably a crew decision.
- What does the return look like? A position consistent with the earlier course and speed points to an ordinary reception gap. A jump, a changed draught or a new name calls for further analysis.
- What type of vessel is it? Warships and naval auxiliaries may not transmit at all; yachts and fishing boats transmit weakly and less often.
- Are the positions physically possible? Tracks across land, circles in one spot and impossible speeds are the classic signs of GNSS interference or manipulation.
- What was happening around it? Set the gap against reports from the same day: attacks, navigational warnings, seizures, exercises.
- Do other sources see the same thing? Compare several trackers — we review them in our piece on the best ship tracking services.
What the battleMap tracker shows, and how to pair it with the event map
The battleMap live tracker displays vessel positions from public AIS feeds alongside air traffic from ADS-B and MLAT networks. It is always a subset of real traffic: what public receivers hear, not everything that sails. We do not label vessels as "dark" or "suspicious" — we show the last known position and the time it was received, and leave the interpretation to the reader. The most useful view is the two layers together: AIS tracks from the tracker and geolocated reports from the regional maps, such as Yemen or Iran, which refresh roughly every five minutes. When ships go quiet en masse near an incident and the reports speak of an attack or a warning, both layers are telling the same story. The rules we follow when collecting and describing data are set out in our methodology and data sources page.
Key takeaways
- A gap in an AIS track has three possible causes: no reception, deliberate switch-off and false data. On its own it proves nothing.
- Gaps are normal on the open sea and along poorly covered coasts; the suspicious ones coincide with threat zones and transfer areas.
- A master may lawfully switch AIS off when its operation endangers the ship — common practice in conflict zones.
- Spoofing is recognised by physically impossible tracks and mismatched identifiers.
- Warships broadcast selectively, so their absence from the map is not information.
- Public trackers, battleMap included, show a subset of traffic; the value lies in pairing tracks with reports from the same time and place.