What AIS is, technically
AIS is a VHF, self-organizing, ship-to-ship and ship-to-shore broadcast reporting system, standardized in Recommendation ITU-R M.1371. It runs on two dedicated 25 kHz channels in the maritime mobile band: 161.975 MHz and 162.025 MHz. Modulation is GMSK/FM at 9,600 bit/s. Stations alternate between the two channels, which gives redundancy and doubles capacity.
Time is divided into frames of one minute, each split into 2,250 slots, so a channel offers 2,250 transmission opportunities per minute and the pair offers 4,500. Slot timing is referenced to UTC, normally from a GNSS receiver inside the AIS unit. That dependency is worth pausing on: AIS slot synchronization relies on satellite navigation, so jamming and spoofing touch the maritime domain too, not only the aviation one. It is one of several reasons these two layers belong on the same map.
Access is by SOTDMA for Class A: a station listens to the slot map, reserves future slots by announcing them in its own transmissions, and the network organizes itself with no master. Because reservations are broadcast, congestion degrades gracefully, with stations reusing the most distant occupied slots so that the nearest and most relevant targets survive. Class B uses CSTDMA, listening for a free slot immediately before transmitting and deferring if it is busy. That makes Class B polite and cheap, and also makes it the first casualty of a crowded channel.
Class A and Class B are not equally fresh
This is the single most important thing to know when reading vessel positions on a map, and most renderings ignore it.
| Class A | Class B "CS" | |
|---|---|---|
| Access scheme | SOTDMA, reserves slots | CSTDMA, defers |
| Transmit power | 12.5 W | 2 W |
| Under congestion | wins | loses |
| Under way, 0 to 14 kt | every 10 s | every 30 s |
| Under way and turning | every 2 to 3⅓ s | no change |
| Nearly stopped | every 3 min | every 3 min |
So a Class A ship under way updates every 2 to 10 seconds and its track is smooth enough to interpolate safely. A Class B leisure craft updates every 30 seconds at best, every 3 minutes when nearly stopped, and may be lost entirely in a crowded harbor. The two should not be read as though they had equal freshness.
Carriage is mandatory under SOLAS Chapter V for ships of 300 gross tonnage and above on international voyages, cargo ships of 500 gross tonnage and above otherwise, and all passenger ships regardless of size. Everything else, including most fishing vessels and leisure craft, is voluntary. Absence from this layer says nothing about absence of a vessel.
What a vessel transmits, and which fields lie
A position report carries the MMSI, a 9-digit identity whose first three digits identify the flag administration; navigational status; rate of turn; speed over ground; position; course over ground; true heading; and a timestamp. Static data, including name, ship type, dimensions and destination, arrives in separate messages.
Several of those fields are routinely misread:
- Navigational status is set by hand. Under way, at anchor, moored, engaged in fishing, and so on are typed by the crew, so the value is frequently stale. The exception that matters is status 14, which marks an active AIS-SART, man-overboard or EPIRB-AIS device, making a distress beacon machine-detectable in the same stream as ordinary traffic.
- Heading is not course. True heading comes from a compass; course over ground comes from the position solution. A vessel crabbing in a current shows a heading that differs from its track. A value of 511 means heading is not available, which is extremely common on Class B and on anything without a compass interface. A renderer that does not handle 511 draws hulls pointing at an impossible bearing.
- Destination and ETA are free text typed by the crew, and unreliable by nature.
- MMSI is reprogrammable, set at installation. That is the root of both benign misconfiguration and deliberate identity spoofing.
In this layer, vessels from the Finnish feed often appear without names, because its locations endpoint carries position, course and speed only, with names on a separate endpoint.
Why the coverage stops at the coast
AIS at 162 MHz propagates by line of sight. Transmit power barely matters compared with antenna height, because the limit is the geometric radio horizon. A ship antenna 15 m up and a shore antenna on a 50 m mast give roughly 24 NM between them. Typical figures, and they should be read as typical rather than as a specification, are around 20 NM ship to ship and 40 to 60 NM or more from a well-sited shore station. Ducting occasionally produces receptions far beyond that; terrain shadows produce gaps well inside it.
So a terrestrial network sees coastal waters, straits, fjords and approaches, and goes dark offshore. Satellite AIS exists, but a spacecraft sees a footprint containing far more traffic than 2,250 slots per minute can carry, so dense areas suffer collisions and revisit is measured in hours. This layer uses terrestrial national feeds only. It is not global vessel tracking and does not pretend to be.
The deeper reason the layer is regional is not technical. AIS is broadcast in the clear, so anyone in range can receive it. What is scarce is the network: a national shore-station chain with the funding, siting and backhaul to cover a coast continuously. Those are built by state maritime authorities, and whether the aggregate is published openly is a policy choice. Most administrations keep it internal or license it commercially. A minority publish a live open feed, and that is why this layer covers Norway and Finland rather than the world. Both also withhold parts of the picture by design, so small craft and some government vessels are not present.
Why an aviation site carries a ship layer
Cross-domain correlation. Air and surface activity are not independent, and co-located co-timed tracks are the observable signature of operations invisible in either domain alone: a helicopter transiting to a named vessel, a patrol aircraft flying a search pattern over a specific hull, two platforms holding a constant relative bearing. Pairing the two also gives a free consistency check. A helicopter whose ADS-B position puts it nowhere near the only vessel it could be working with is a candidate for a position problem, which links this layer straight back to GNSS interference.
Search and rescue. This is in the standard, not in marketing. SAR aircraft are a defined AIS platform class with their own message type and a nominal 10 second reporting interval, reducible to 2 seconds in the area of an operation, so an airborne SAR asset appears on the maritime picture. AIS-SART and man-overboard devices announce themselves through navigational status 14. And the SOLAS duty to assist means the vessel picture around a ditching is directly actionable: the nearest hulls, their speeds and their status are what a coordinator needs first. AeroScope is not a SAR system and should not be used as one. It just makes that picture visible.
Offshore helicopter operations. Offshore rotary-wing flying is the one aviation activity whose landing sites are mostly in no airport database at all. Helidecks sit on drilling rigs, production platforms, construction vessels and wind-farm service vessels, and many of those decks move. The destination of an offshore sector may be a hull that was 20 NM away yesterday. AIS is the only open source giving a moving deck's present position, course and speed, and heading matters too, since a helideck's usable approach sector is defined relative to the hull. The geography of these two feeds fits that use well: the Norwegian coast and North Sea, and the Gulf of Finland and Bothnian Bay.
What this layer does not tell you
Every data source has an edge. These are the ones that matter for reading this layer correctly.
- AIS is a cooperative broadcast with no authentication whatsoever. Identity, position, destination and status are all self-declared and reprogrammable, and a transponder can simply be switched off. AIS tells you about cooperative traffic. It is not a sensor.
- Coverage is Norwegian and Finnish waters only, because those are the national authorities that publish an open live feed. This is not global vessel tracking.
- Terrestrial AIS is line-of-sight limited, so the picture ends tens of nautical miles offshore and has terrain shadows well inside that.
- Both source feeds exclude categories of vessel by policy. Small fishing and recreational craft in particular are not present.
- Class B positions are many times staler than Class A, and are the first to be lost in a congested channel.
- Navigational status, destination and ETA are typed by the crew and are frequently stale or wrong. Heading is often reported as unavailable.
AeroScope is an situational awareness and research tool. Nothing here is approved for air traffic control, navigation, flight planning or collision avoidance. For operational decisions, use the authoritative source named above.