How to track a cruise ship's live location (IMO and MMSI explained)

Maybe friends or relatives at home want to watch your ship sail out of port. Maybe you want to know whether you'll dock on time before your flight home, or you're just curious where the ship is right now. You can follow almost any cruise ship's position live, as long as you know which ship you're looking for. Searching by name gets you most of the way. The IMO and MMSI numbers get you the rest.

How live ship tracking works: AIS

Ocean-going cruise ships carry an AIS (Automatic Identification System) transponder. It broadcasts the ship's identity, position, speed, course and the destination entered by the crew. The broadcasts go out every few seconds while the ship is moving and less often at anchor.

These signals are picked up in two ways:

• Ground-based receivers near coasts and ports. Near land, positions are usually only seconds to minutes old.
• Satellites, for ships far offshore. Mid-ocean positions are relayed when a satellite passes overhead, so they can be anywhere from minutes to a few hours old.

The one thing to remember: always check the timestamp. A ship that seems "stuck" mid-Atlantic is often just between satellite reports.

The three ways to identify a ship

1. Ship name

This is the easiest place to start. Type the ship's name into a tracker and pick the right result. The catch is that names aren't unique. Ships get renamed when they're sold, and different vessels sometimes share or reuse a name. Look at the ship type ("passenger"), size and flag to make sure you've got the right one.

2. IMO number: the ship's permanent ID

The IMO number is "IMO" followed by seven digits, and the last digit is a check digit. According to the International Maritime Organization:

• It's assigned once, when the ship is built, and stays with the ship for life, even if it's renamed, sold or moved to another country's flag.
• It's never reassigned to another ship.
• It's required for passenger ships of 100 gross tonnage and up, and cargo ships of 300 gross tonnage and up, under SOLAS regulation XI-1/3.
• It must be permanently marked on the hull or superstructure. Passenger ships also carry it on a horizontal surface that's visible from the air.

If you want to be sure you're tracking the right ship, the IMO number is the safest choice.

3. MMSI number: the ship's radio identity

The MMSI (Maritime Mobile Service Identity) is a nine-digit number that identifies the ship's radio station, including its AIS transponder. Under the ITU's rules, a ship's MMSI starts with three Maritime Identification Digits (MID), which show the country the ship is registered with. The other six digits identify the ship.

Because the MID is tied to the flag, a ship's MMSI can change when it changes flag, while its IMO number stays the same. The MMSI is useful because it's exactly what the transponder broadcasts. Treat it as current, not permanent.

Where to find your ship's IMO and MMSI numbers

• Vessel tracking sites. Search by name, open the ship's page, and both numbers are usually listed.
• The cruise line's ship fact sheet or press materials sometimes list the IMO number.
• The IMO's public GISIS ship database lets you look up IMO numbers. You'll need to register for a free account.
• The ship itself. The IMO number is painted on the hull.

Once you have the IMO number, write it on your trip notes. It works on any tracker.

Step by step: find your cruise ship right now

1. Open a vessel tracker and search by ship name, IMO number or MMSI.
2. Confirm it's the right ship: passenger vessel, the right size, the right flag.
3. Check the position timestamp before you rely on it.
4. Look at the reported destination and ETA. Remember that the crew enters these by hand, so they can be out of date.
5. To plan around arrival (a flight home, a pickup), check again on the morning you dock. Then check your cruise line's own updates, because they're the official source for port times.

Tips for cruisers

• Make it a game. Traveling with kids or friends? Take turns finding the ship each morning and guessing the next port.
• Share the ship's name and IMO number with people at home so they can follow along without asking you for updates.
• Don't overshare your plans online. Posting your exact ship and dates publicly tells people when your house is empty.
• Ferries work the same way. Most large passenger ferries broadcast AIS too.

Track the cruise alongside the rest of the trip

Most cruise holidays aren't just a cruise. There's a flight to the port, maybe a train or a hotel night, and a flight home. Voyager, the all-in-one travel tracker, lets you add a cruise by ship name, IMO or MMSI. It tracks the cruise alongside your flights, trains, buses and ferries in one dashboard, with delay and gate alerts on the flying days. Afterwards, the trip can feed your Logbook and Atlas.

Voyager is free during beta. Start free or try the demo, no account needed.

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