Explainer
What Is AIS Ship Tracking? How Vessel Tracking Works
AIS is the VHF radio system ships use to broadcast who they are and where they are going. Here is how those broadcasts become dots on a live ship map, and what they cannot tell you.
Key takeaways
- AIS (Automatic Identification System) is a VHF transponder system that broadcasts a ship's identity, position, course and speed on 161.975 and 162.025 MHz.
- SOLAS requires AIS on ships of 300 GT and up on international voyages, cargo ships of 500 GT and up on domestic voyages, and all passenger ships.
- Every AIS unit is identified by a 9-digit MMSI; the separate 7-digit IMO number stays with the hull for life.
- Shore receivers hear ships out to roughly 20 to 40 nautical miles, so open-ocean coverage depends on satellite AIS.
- AIS is self-reported: positions can be switched off, delayed or manipulated, so treat every track as a claim to check.
AIS ship tracking is the practice of collecting the radio broadcasts that ships send from their Automatic Identification System transponders and plotting them on a map. Every few seconds an AIS-equipped vessel announces its identity, position, course and speed on two VHF channels, and anyone with a receiver, including shore stations and satellites, can pick those messages up.
That is the whole trick behind every public ship tracker you have used. The website does not see the ship; it decodes what the ship says about itself. Knowing how those messages are built, how often they are sent and who is listening explains why some ships update every few seconds while others vanish for hours.
This guide covers what AIS is, who must carry it, the difference between Class A and Class B, what the MMSI and IMO numbers mean, how terrestrial and satellite AIS differ, and the limits you should keep in mind, including spoofing and "dark" ships. If you want to try it as you read, open the live ship tracker in another tab.
What is AIS?
The Automatic Identification System (AIS) is a shipboard broadcast system that works like a transponder in the VHF maritime band. The US Coast Guard Navigation Center describes it as a way for vessels to display each other's identity, position, course and speed on electronic charts, and notes that it can update as often as every two seconds (USCG NAVCEN).
AIS was designed for collision avoidance and traffic management, not for websites. Ships exchange data with each other and with shore authorities so a bridge officer can see the name and intentions of a vessel behind a headland before it shows up on radar. Public tracking sites are a side effect: the same open broadcasts can be received by hobbyists, companies and space agencies.
The two AIS frequencies
AIS uses two simplex VHF channels set aside for it: 161.975 MHz (AIS 1) and 162.025 MHz (AIS 2), each 25 kHz wide (47 CFR 80.393). Transponders share those channels using a time-slot scheme called SOTDMA (self-organizing time division multiple access), in which each unit reserves slots and announces its plans so nearby units avoid talking over it (USCG NAVCEN).
Who has to carry AIS?
Under SOLAS, the international convention on safety of life at sea, AIS must be fitted on all ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 GT and upwards not on international voyages, and all passenger ships regardless of size. The requirement applied to all such ships by 31 December 2004 (IMO).
National rules add more. In US navigable waters, most commercial self-propelled vessels had to carry AIS by 1 March 2016, including commercial fishing vessels of 65 feet or more (USCG NAVCEN FAQ). Many yachts and small boats fit AIS voluntarily for safety, which is why you also see pleasure craft on public maps.
How AIS ship tracking works, step by step
- The ship's GPS (or the AIS unit's own receiver) provides position, speed over ground and course over ground.
- The transponder combines that with stored identity data (name, MMSI, call sign, ship type) and voyage data typed in by the crew (destination, ETA, draught).
- It broadcasts short digital messages on AIS 1 and AIS 2 in its reserved time slots.
- Other ships, shore base stations, volunteer receivers and satellites decode the messages.
- Aggregators merge those receptions, remove duplicates and publish a map that refreshes every few seconds to minutes.
AIS Class A vs Class B: what is the difference?
Class A is the full-specification equipment required on SOLAS ships. Class B is a cheaper, lower-power option used mostly by leisure craft and small commercial boats. The Canadian Coast Guard's e-Navigation portal summarizes the practical differences: Class A reports more often, carries voyage data and can send safety messages, while Class B reports less often, at lower power, and cannot transmit voyage-related information (Canadian Coast Guard).
| Feature | Class A | Class B |
|---|---|---|
| Who uses it | SOLAS ships, most commercial vessels | Leisure craft, small workboats |
| Position report while underway | Every 2 to 10 seconds | Less often than Class A |
| Position report at anchor or moored | Every 3 minutes | Every 3 minutes or less |
| Static and voyage data | Every 6 minutes (Message 5) | Every 6 minutes (Message 24, no voyage data) |
| Transmit power | 12.5 W | Lower than Class A |
| Access scheme | SOTDMA | SOTDMA or CSTDMA (carrier sense) |
Sources: USCG Class A reports, USCG Class B reports, Canadian Coast Guard.
For a map user, the practical result is simple. A large tanker or container ship usually moves smoothly, while a sailboat with Class B may jump between positions and may never show a destination.
What is an MMSI number, and how is it different from an IMO number?
Every AIS unit is programmed with a Maritime Mobile Service Identity (MMSI), a 9-digit number regulated internationally by the ITU and nationally by regulators such as the FCC (USCG NAVCEN). The first three digits of a ship's MMSI are the Maritime Identification Digits that point to the flag administration. A valid ship MMSI starts with a digit from 2 to 7, and US-assigned MMSIs start with 338, 366, 367, 368 or 369 (USCG NAVCEN FAQ).
The IMO number is different. It is "IMO" followed by seven digits, assigned to the hull of seagoing merchant ships of 100 GT and above, and it is never reassigned to another ship even when the vessel changes name or flag (IMO). An MMSI can change when a ship re-flags; the IMO number should not. When you research a vessel's history, the IMO number is the more stable key. Our guide on how to track a ship walks through searching by each identifier.
What do AIS messages contain?
AIS defines 27 message types. You only need a handful to read a ship map (USCG NAVCEN message list):
| Message | What it carries | Sent by |
|---|---|---|
| 1, 2, 3 | Position report: lat/lon, speed, course, heading, navigational status | Class A |
| 4 | Base station position and UTC time | Shore stations |
| 5 | Static and voyage data: IMO number, call sign, name, ship type, dimensions, draught, destination, ETA | Class A |
| 18, 19 | Standard and extended position report | Class B |
| 21 | Aid-to-navigation report (buoys, lighthouses, virtual marks) | AtoN units |
| 24 | Static data in two parts: name, then type, call sign and dimensions | Class B |
| 27 | Compressed long-range position report designed for satellite reception | Class A and B |
Fields worth knowing
- Navigational status: a 4-bit code set by the crew. 0 is "under way using engine," 1 is "at anchor," 5 is "moored," 7 is "engaged in fishing," 8 is "under way sailing" and 15 is "undefined" (USCG NAVCEN).
- Speed over ground (SOG): in tenths of a knot; a raw value of 1023 means "not available."
- True heading: 0 to 359 degrees; 511 means "not available," which is why some ships show a course but no heading.
- Destination and ETA: free text typed by the crew, up to 20 characters for destination. Treat it as a hint. Crews forget to update it, abbreviate port names and sometimes enter nothing at all.
How far can AIS be received?
AIS is VHF, so it travels roughly line of sight. The Canadian Coast Guard puts typical range at 20 to 40 nautical miles (about 37 to 74 km), depending on antenna height, weather and terrain, with elevated shore stations reaching farther (Canadian Coast Guard). Past that distance a coastal network simply cannot hear the ship.
Terrestrial AIS
Terrestrial AIS is collected by shore receivers run by coast guards, companies and volunteers. It is dense and fast near busy coasts, ports and straits, and sparse or absent in mid-ocean. Free networks and the AIS layer on GodsView are built on terrestrial reception, so coastal waters are well covered and the middle of the Pacific is not.
Satellite AIS
Satellite AIS (S-AIS) puts receivers in low Earth orbit. AIS signals travel only a short distance horizontally but go much farther vertically, which lets a satellite hear ships far from any coast. The catch is that a satellite sees many self-organizing cells at once, so messages from ships that never coordinated with each other collide. ESA's Columbus AIS experiment on the International Space Station, which began in June 2010, was built to show that receivers could untangle those collisions (ESA). Message 27 exists specifically as a short, satellite-friendly position report.
Commercial satellite AIS is usually sold as part of paid plans. If you compare services, check whether "global" coverage includes satellite positions or only terrestrial ones; our MarineTraffic alternatives comparison lists which plans include it.
Can AIS be turned off or faked?
Yes, and this is the most important thing to understand before drawing conclusions from a ship map.
Switching AIS off
SOLAS says ships must keep AIS operating at all times, except where international rules allow navigational information to be protected (IMO). The US Coast Guard also notes that AIS may be switched off if continual operation would compromise the vessel's safety or security, for example in waters with a piracy threat (USCG NAVCEN FAQ). So a gap in a track is not, by itself, proof of wrongdoing.
Some gaps are deliberate for other reasons. A study by Global Fishing Watch, NOAA Fisheries and UC Santa Cruz, published in Science Advances in November 2022, identified more than 55,000 suspected intentional AIS disabling events by fishing vessels between 2017 and 2019 (Global Fishing Watch). Ships with their transponders off are often called dark ships.
Spoofing and manipulation
Because AIS is self-reported, the content can be wrong. A 2020 US sanctions advisory from the State Department, Treasury's OFAC and the Coast Guard lists disabling or manipulating AIS among the deceptive practices used to evade sanctions, alongside ship-to-ship transfers, falsified documents and flag hopping (OFAC advisory). Manipulation can mean broadcasting a false position, a borrowed identity or a misleading destination.
Satellite navigation interference adds another layer. If a ship's GPS is jammed or spoofed, its AIS will faithfully broadcast a wrong position. Our explainer on GPS jamming and spoofing covers how that shows up on maps.
See GPS interference zones on the map
What AIS data cannot tell you
| Question | Can AIS answer it? | Why |
|---|---|---|
| Where is a cooperating ship near a coast right now? | Usually | Dense terrestrial reception, frequent reports |
| Where is a ship in mid-ocean right now? | Only with satellite AIS | Terrestrial range ends at roughly 20 to 40 nm |
| What cargo is on board? | No | AIS carries ship type and draught, not cargo manifests |
| Is the destination accurate? | Not reliably | Free text entered by the crew |
| Is a warship or a sanctioned tanker visible? | Often not | Warships are exempt from SOLAS chapter V (UK MCA, MGN 610); other ships may go dark |
| Who owns the ship? | No | Ownership comes from registries, not AIS |
Two further cautions. First, aggregators differ in how long they keep showing a ship after its last message, so a "live" dot may be several hours old; always check the last-seen time. Second, AIS is an aid to navigation, not a safety service for people ashore. For maritime emergencies, rely on coast guards and official notices.
Is AIS tracking legal?
AIS is an open broadcast intended to be received, and in most countries receiving and displaying it is lawful. The IMO has nonetheless voiced concern since 2004 that publishing AIS data on the internet could affect ship and port security (IMO), and privacy, harassment and data-protection laws still apply to how you use what you see. Read is flight and ship tracking legal? for a fuller overview; it is informational, not legal advice.
How GodsView uses AIS
GodsView AI shows ships from aisstream.io, a free terrestrial AIS stream, refreshed about every 30 seconds. The ship layer is part of the free tier alongside flights, satellites, earthquakes, fires and weather, and you need only a free account, with no API key or payment, to view it. Our methodology page lists every data source and refresh rate.
Pro adds tools for following ships over time: vessel arrival and departure alerts, watch-area (geofence) alerts, fishing vessels from Global Fishing Watch, a ports layer, and the option to follow a ship on the 3D globe. Pro is $10.99/month, $99.99/year or $199.99 lifetime (prices as of October 2026). Teams watching routes and ports can start from the maritime awareness and logistics live map pages.
Because the stream is terrestrial, GodsView coverage is strongest near coasts, ports and narrow straits. That makes it a good fit for watching shipping chokepoints such as the Red Sea or the Strait of Malacca, and a weaker fit for tracking a bulk carrier across the open Indian Ocean.
The short version
AIS is a VHF transponder system that makes ships announce themselves several times a minute. Class A ships do it often and in detail; Class B boats do it less often. Shore receivers hear them out to a few dozen nautical miles and satellites fill in the oceans. Everything on a ship map is self-reported, so confirm unusual tracks with a second source before you draw a conclusion. The same discipline applies to aircraft; see how ADS-B flight tracking works for the aviation equivalent.
Frequently asked questions
What does AIS stand for in shipping?
AIS stands for Automatic Identification System. It is a VHF radio transponder system that lets ships broadcast their identity, position, course, speed and voyage details to nearby ships, shore stations and satellites. SOLAS requires it on ships of 300 gross tonnage and up on international voyages, cargo ships of 500 GT and up on domestic voyages, and all passenger ships.
Is AIS ship tracking real time?
Close to it near coasts. A Class A ship underway reports every 2 to 10 seconds, and public maps typically refresh every few seconds to a minute; GodsView refreshes ships about every 30 seconds. In mid-ocean, positions depend on satellite AIS and can be minutes or hours old, so always check the last-seen time on a vessel before trusting its position.
Why can't I see a ship on the AIS map?
The most common reasons are that the ship is outside terrestrial receiver range, its transponder is off or faulty, it is a small boat or warship that does not carry AIS, or it changed its MMSI. Search by IMO number as well as name, and try again later. A missing ship is not proof that it has gone dark deliberately.
What is the difference between MMSI and IMO number?
The MMSI is a 9-digit radio identity programmed into the AIS unit; its first three digits indicate the flag, and it can change when a ship re-flags. The IMO number is a 7-digit number assigned to the hull of seagoing merchant ships of 100 GT and above and is never reassigned, so it is the better key for vessel history.
Can ships fake their AIS position?
Yes. AIS is self-reported, so a ship can broadcast a false position or identity, and GPS spoofing can make a working transponder report the wrong place. A 2020 US sanctions advisory lists AIS manipulation among deceptive shipping practices. Cross-check suspicious tracks with satellite imagery, port records or other sources before drawing conclusions. See our GPS jamming and spoofing guide.
Sources
- IMO — AIS transponders and SOLAS carriage requirements
- USCG NAVCEN — AIS overview
- USCG NAVCEN — Class A AIS position reports
- USCG NAVCEN — AIS messages
- USCG NAVCEN — AIS frequently asked questions
- 47 CFR 80.393 — AIS frequencies
- Canadian Coast Guard — How AIS works on board ships
- IMO — Ship identification number scheme
- ESA — Space Station keeps watch on world's sea traffic
- Global Fishing Watch — Hotspots of unseen fishing vessels
Facts checked on against each provider's own site.