How Does AIS Tracking Work?

Open any live ship map and you will see thousands of vessels moving in near real time. Behind every one of those icons is the same technology: the automatic identification system, or AIS. So, how does AIS tracking work? In short, each ship broadcasts its identity and position over VHF radio, receivers on shore and in orbit pick those messages up, and tracking platforms decode and plot them on a map.

This guide walks through each step of that journey.

Step 1: The ship's AIS transponder collects its data

Every AIS-equipped ship carries a transponder on the bridge. It pulls together three kinds of information:

• Static data entered once at installation: MMSI, IMO number, ship name, call sign, ship type and dimensions
• Dynamic data fed automatically by the ship's sensors: GPS position, speed over ground, course over ground, heading and rate of turn
• Voyage data updated by the crew: destination, ETA, draught and navigational status (for example "under way using engine" or "at anchor")

SOLAS requires AIS on ships of 300 gross tonnage and above on international voyages, cargo ships of 500 GT and above, and all passenger ships. These are known as Class A transponders. Smaller vessels such as yachts and fishing boats often fit cheaper Class B units voluntarily.

Step 2: The transponder broadcasts on VHF radio

The transponder transmits short digital packets on two dedicated marine VHF channels: 161.975 MHz (AIS 1) and 162.025 MHz (AIS 2). Using two channels reduces interference and improves reliability.

Thousands of ships share the same frequencies without talking over each other thanks to a scheme called TDMA (time-division multiple access). Each minute is divided into 2,250 time slots per channel. A Class A transponder reserves its own slots in advance and announces them, so neighboring ships avoid them. This self-organizing system is why AIS keeps working even in crowded ports and straits.

Step 3: How often a ship reports

A moving ship reports more often than one tied up at a berth. For Class A transponders the standard intervals are:

Intervals shorten further when the ship is changing course. Class B units report less frequently, typically every 30 seconds when moving.

Step 4: Receivers pick up the signal

AIS messages are received in three ways.

• Other ships. Vessels within VHF range see each other on their radar or electronic chart displays. This is the original collision-avoidance purpose of AIS.
• Terrestrial receivers. Coast stations, port authorities and networks of volunteer receivers on shore capture signals within roughly 20 to 40 nautical miles, depending on antenna height and terrain.
• Satellite AIS (S-AIS). Satellites in low Earth orbit listen for AIS messages over open ocean, far beyond the reach of shore stations. This is what makes global AIS ship tracking possible.

Step 5: Data is decoded, merged and mapped

Raw AIS arrives as encoded sentences that mean little to a human reader. A tracking platform decodes each message, matches it to the right vessel using its MMSI, removes duplicates from overlapping receivers, filters out obvious errors, and plots the result on a map.

The best platforms then go further, enriching positions with port calls, voyage history and analytics. That is the difference between a dot on a map and genuine live AIS tracking that a business can act on.

What AIS tracking can and cannot tell you

AIS vessel tracking is powerful, but it has limits worth understanding:

• Coverage gaps. Satellite revisit times mean ocean positions can be minutes to hours apart, while coastal positions are usually near continuous.
• Manual fields. Destination, ETA and draught are typed in by the crew and are sometimes out of date.
• Switched-off transponders. Ships may legally turn AIS off for security reasons, and some do so to hide their activity. Sudden gaps in a track are worth investigating.
• Spoofing. False positions can be broadcast deliberately, so unusual tracks should be checked against other sources.

Read with these caveats in mind, AIS data is still the richest open source of vessel movement information in the world.

Live AIS tracking with Navtrac

Navtrac takes care of steps four and five for you. It brings real-time AIS data into an interactive map, so you can monitor ships, ports and maritime traffic as it happens, and uses predictive analytics to help you plan routes and make data-driven decisions.

Whether you are a ship operator, a charterer or a port agent, try Navtrac to see live AIS tracking for any vessel in the world.

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