AIS receiver range and coverage limits

Coverage is the constraint that shapes everything on this site: which vessels appear, how often they update, and what a “last received 18 hours ago” timestamp actually means. AIS coverage explained covers where the map works and where it fails. This guide goes behind that to the specific numbers that set the limits: the range formula, the transmit power differences between vessel classes, and what the four open feeds this site draws from actually cover.

The range formula

AIS transmits on two VHF channels: 161.975 MHz and 162.025 MHz. VHF is line-of-sight; the signal does not follow the earth’s curve, and there is no relay at sea.

The standard approximation for the radio horizon between two antennas:

d (NM) ≈ 2.23 × (√h₁ + √h₂)

where h₁ is the vessel’s antenna height above sea level in metres, and h₂ is the shore station’s height in metres. Some worked examples:

Vessel antenna	Shore station	Range
10 m (small yacht)	30 m (low coastal site)	~17 NM
20 m (coastal freighter)	50 m (port mast)	~26 NM
30 m (large container ship)	100 m (headland)	~34 NM
40 m (large tanker)	150 m (tall coastal station)	~41 NM

These are geometric ceilings. Real-world conditions, including obstacles, sea state, and receiver noise floor, typically reduce reliable coverage below these figures. Atmospheric ducting, where a temperature inversion traps the signal, can occasionally push range past 60 NM, but this is a weather effect, not something infrastructure can depend on.

How AIS works has the full physics background.

Class A and Class B: different power, different reach

The range formula treats antenna heights as fixed, but transmit power also matters. SOLAS mandates Class A transponders on most commercial vessels. Class B transponders are used by recreational craft and smaller commercial boats that fall below the SOLAS threshold.

Class A transponders transmit at 12.5 W. Class B transponders transmit at 2 W; the newer Class B/SO variant transmits at 5 W. The power difference translates to roughly 8 dB, or 2.5 times the signal strength, for Class A over standard Class B.

In practice:

• Class A vessels are reliably heard at 25 to 40 NM from a competent shore station, depending on antenna heights.
• Class B (2 W) vessels are typically heard at 10 to 20 NM vessel-to-shore.
• Class B/SO (5 W) vessels sit in between, roughly 15 to 30 NM.

This is why recreational vessel coverage is patchy even in regions with good commercial-vessel coverage. A marina in the English Channel will show every container ship passing outside, but the yacht that left the same morning may drop out within a few miles. It is not the range that differs so much as the transmit power and, usually, a lower antenna on a shorter mast.

Slot congestion: a different kind of coverage failure

AIS uses TDMA, dividing each minute into 2,250 time slots on each channel. Class A transponders self-organise using SOTDMA, selecting and reserving a slot in advance. Class B transponders use CSMA, waiting until they sense a free slot before transmitting. When the channel is full, Class B transmissions are deferred.

In a major port with hundreds of Class A vessels at berth, the time slots can fill. Class B traffic drops out not because of range but because the protocol deprioritises it. A yacht in the middle of a busy anchorage may appear on a quiet evening and vanish at a busy morning peak, purely from congestion.

This matters for interpreting vessel pages. A small craft that drops out in a harbour approach, surrounded by working Class A vessels, is likely a congestion gap, not a feed failure or equipment fault.

What the open feeds actually cover

This site draws from four sources. Each covers a different geography.

AISStream.io aggregates community receivers from contributors around the world. Coverage is densest in Europe, North America, East Asia, and Australia. It thins considerably across Central America, most of sub-Saharan Africa, and parts of Southeast Asia. Mid-ocean coverage is effectively absent.

Digitraffic (Fintraffic) provides Finnish and Baltic coverage. Finnish coastal waters and the major Baltic shipping lanes receive consistent, high-density coverage from a maintained national network.

Kystverket/BarentsWatch covers the Norwegian EEZ plus Svalbard and Jan Mayen. Norway operates one of the world’s denser national receiver networks; this feed extends usable coverage into the Barents Sea and above 70 degrees North, an area that would otherwise be a gap on community networks.

Danish Maritime Authority covers Danish waters, including the Danish Straits through which most Baltic-North Sea traffic flows.

The four feeds overlap substantially in the North Sea and western Baltic. Outside those core areas, coverage depends primarily on AISStream’s community network, which varies by contributor density. See the sources page for full attribution.

Why coverage boundaries are irregular

A reasonable assumption is that coverage extends in a smooth band out from a coastline. It does not. Shore receiver placement follows infrastructure: port authority buildings, coast guard stations, VTS (Vessel Traffic Service) installations, and individual contributors running software-defined radio receivers. The result is uneven.

A receiver on a headland can hear 35 NM offshore. The bay immediately behind it, screened by terrain, might be covered only to 10 NM. A stretch of coast without any installation shows gaps close inshore. Fjords, estuaries, and harbours surrounded by hills often have localised blind spots even when the open water nearby is well covered.

This is why the coverage boundary on any map does not follow coastlines cleanly. It follows receivers, and receivers follow human decisions about where to put them.

What “last received” means for a specific vessel

Every position on this site carries a timestamp: when a shore receiver last decoded a message from that vessel. Not when the vessel last transmitted.

An AIS transponder broadcasts continuously, every few seconds, whether or not anyone is listening. A vessel that stopped appearing in the feed 18 hours ago has not necessarily stopped transmitting. It has almost certainly crossed outside the range of every receiver in our network.

This is why vessel profiles for ships that mainly operate offshore are sparse. The track shows coastal segments: approach, departure, any port calls within covered areas. The open-water portion of the voyage is invisible to a terrestrial network. The vessel is still out there; the receivers are not.

When the ship returns to a covered area, the position updates. The track resumes. The timestamp jumps forward. Nothing unusual happened.

For the cases where something unusual did happen, why ships go dark covers deliberate deactivation, equipment failure, and position spoofing.

Frequently asked questions

What is the maximum range of AIS?

Under standard conditions, the geometric horizon sets the ceiling. A large commercial vessel with a 30 m antenna talking to a 100 m coastal station can be heard at roughly 34 NM. Add atmospheric ducting and occasionally higher, but that is inconsistent. For planning purposes, 40 NM is a reasonable upper bound for reliable terrestrial coverage from a single station.

Why can Class B vessels disappear in busy harbours?

AIS channels have a finite number of time slots per minute. Class A transponders reserve slots. Class B transponders wait for free slots using carrier-sense access and are deprioritised when channels fill. In a major port approach with heavy Class A traffic, Class B vessels can drop out of coverage entirely without leaving range. This is a protocol limit, not a fault.

Do all countries have the same AIS coverage?

No. Coverage density follows infrastructure. Norway, the UK, the Netherlands, Finland, the US East Coast, Japan, and the Singapore Strait have dense, maintained networks. Large sections of West Africa, parts of South America, and most remote island chains have sparse or no terrestrial coverage. The sources page explains which feeds this site uses and what each covers.

How do I know if poor coverage explains a missing vessel?

Check the last known position and heading. A ship heading offshore that stopped updating past a certain point almost certainly crossed a coverage boundary. A ship that stopped updating while remaining close to a well-covered coast is a different case: look for equipment failure, a feed outage, or slot congestion. If every neighbouring vessel kept reporting while one specific ship went quiet, the cause is more likely with that ship than with the network.

Does weather affect AIS range?

Rarely in a predictable way. Heavy precipitation attenuates VHF slightly but not enough to explain most coverage losses. The more common weather effect is ducting, which extends range rather than reducing it: a temperature inversion can push signals well past the normal horizon, causing vessels to appear from unexpected distances. Ducting is a meteorological anomaly, not a coverage tool.