How we track ships

The Automatic Identification System (AIS) is a radio tracking standard required by the International Maritime Organization under SOLAS regulation V/19.2.4. Every passenger vessel and every commercial ship over 300 gross tons must carry an active Class A AIS transponder.

A transponder broadcasts a short VHF packet every 2 to 10 seconds (faster when underway, slower at anchor). It carries the ship's MMSI, its GPS position, speed over ground, heading, destination and ETA. The packets are not encrypted; any receiver in range can decode them.

Cruise ships broadcast at the 2-second interval while underway and about every 3 minutes when moored, which makes their positions among the most reliably updated on any AIS network.

We receive position data from two AIS networks: Marinesia, our primary feed, and AISStream, which fills gaps for ships Marinesia has not reported recently. Shore-based AIS receivers typically reach 40 to 50 nautical miles, so coverage is thinner in open ocean.

The pipeline asks the feeds only about the ships in our cruise-ship registry, a curated list of 380+ ships keyed by MMSI and IMO number. Every fix a feed returns is stored as broadcast in ship_positions: ship, time, latitude, longitude, speed, heading, destination and which feed it came from. When a feed returns nothing for a ship, we record that too, so a page can say how long it has been since the last real fix. We do not smooth, estimate or fill in positions; we keep the full time series, not only the latest fix.

How fresh positions are: the position job runs every 10 minutes and refreshes a rotating share of the fleet each time, within our providers' rate limits. A ship's latest fix is often minutes old near busy ports and can be several hours old at sea. Each ship page shows when its last fix was recorded.

A port visit is inferred from position history, not from a manifest or booking system. The rules:

1. Each port has an arrival radius set by its type: 5 km for ports of call and private islands, 15 km for cruise ports, 20 km for large turnaround ports such as Miami. A position is matched to the single nearest port within that radius (its geofence), never to two ports at once.
2. A new visit opens only when the ship is doing under 3 knots. A ship steaming past a port is not a call.
3. Later positions inside the radius extend the open visit for up to 6 hours between fixes.
4. When a ship has been outside the radius for more than 3 hours, the visit closes and the last matched time is recorded as the end.
5. A ship anchored at one line's private island is never credited with a visit to a neighboring line's island.

These rules are reliable for major ports. They can under-count at ports with unusual layouts, or where a ship anchors offshore rather than berthing. Because fixes can arrive hours apart, the recorded end of a visit runs late, which is why we publish arrival counts and not time in port.

We pre-compute aggregates for each port and ship over 30, 90 and 365 days: tracked visits, unique ships, unique cruise lines, day-of-week distribution and a month-by-month breakdown. These power the Busiest Ports ranking, the port seasonality pages and the monthly port traffic reports.

Rollups are recomputed nightly and carry an updated_at stamp, which the pages show. The 90-day window is the default for rankings because it smooths seasonal spikes while staying responsive to schedule changes.

What a visit means: one visit is one ship at one port on one call. A ship that calls at Cozumel 10 times in 90 days adds 10 visits to Cozumel's total. The unique-ships count removes duplicates.

• July 2, 2026: arrival radius set by port type (5, 15 or 20 km), one nearest port per position, private-island rule, and the under-3-knots rule to open a call.
• July 3, 2026: port calls recorded for the whole tracked fleet and every active port. Earlier months are not comparable and are shown as not tracked.
• August 3, 2026: a second AIS position source added. Changes between July and August partly reflect better coverage.

When a ship page says “in our tracking data”, the position, port-visit count or route figure comes from our own AIS database: records we collect, store and derive ourselves from the raw transponder feed. It is not taken from a third-party directory, a cruise line press release or a scraped timetable.

Our ship_positions table stores every validated fix we receive for the 380+ ships we track. When a ship page says “last seen near Cozumel on June 12, 2026 at 17:42 UTC”, that statement is backed by a row in our database, not an estimate.

Stale but real: when no fresh fix is available, for example a ship that has been out of receiver range for days, the page shows the most recent position we hold, labelled “last seen” with its date, never “live now”. We never show an estimated or interpolated position as if it were a live fix.