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To trace a vessel, start with its MMSI, verify the match with the IMO number and call sign, then read the provider’s historical track in UTC order. Treat every plotted point as a received AIS report—not a continuous recording—and treat “last known position” as the latest report the provider received, not proof that the vessel is still there.
What AIS shows—and what it does not
Automatic Identification System (AIS) is a maritime safety communications system. The U.S. Coast Guard describes it as a system that provides vessel identity, type, position, course, speed, navigational status and other safety information to equipped shore stations, ships and aircraft. It also receives comparable information, monitors and tracks ships, and exchanges data with shore facilities. The International Maritime Organization (IMO) describes AIS transponders as automatically providing position, identification and other ship information to ships and coastal authorities.
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The U.S. Coast Guard’s formal wording is: “Automatic Identification Systems or AIS means a maritime navigation safety communications system standardized by the International Telecommunication Union (ITU), adopted by the International Maritime Organization (IMO)…”
That definition matters when you open an online tracker. AIS is a stream of transmitted reports received by a terrestrial station, satellite network or both. It is not a black-box voyage recorder that continuously stores every metre of a ship’s movement. A public map can only show reports that reached that service and were retained in its archive.
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- Identity: MMSI, vessel name, call sign and usually an IMO number.
- Navigation: position, course over ground, speed over ground and navigational status.
- Voyage details: ship type, dimensions, draught, destination and estimated time of arrival (ETA), when transmitted.
- Timing: the UTC time associated with each received report.
If the map draws a line between two reports, the line is a visual connection between samples. It does not prove the vessel followed that exact curve, maintained a constant speed, or remained visible between the two timestamps.
Step 1: identify the correct vessel
Search by MMSI first
The Maritime Mobile Service Identity (MMSI) is the most practical first lookup key. Enter the nine-digit MMSI in the tracker’s vessel search rather than relying only on a name. Names can be duplicated, abbreviated, transliterated or changed during a ship’s career.
Cross-check independent identifiers
Before reading a route, compare the search result with the vessel’s:
- IMO number
- Call sign
- Displayed name
- Ship type
- Dimensions
- Displayed flag
The IMO number is especially useful because it remains the same when a ship changes registration flag. The name, flag, destination and other voyage fields can change. If two records have similar names, reject a match when the MMSI, IMO number, dimensions or call sign do not agree.
Check the timestamp before trusting the record
Write down the latest report time in UTC and note how old it is. A record with the right name but an old timestamp may be a historical match rather than the vessel’s current movement. If the service offers a vessel photo or dimensions, use those as an additional visual check, but do not substitute a photo for an identifier.
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Step 2: open and read the historical route
Use the provider’s history or playback view
- Open the vessel profile that matches the MMSI and cross-checked identifiers.
- Select the historical-track, voyage-history or playback function. The exact label differs by provider.
- Set the date range and confirm that the displayed timezone is UTC, or convert every timestamp to UTC yourself.
- Read reports from oldest to newest. Do not infer direction from the map line alone.
- For each meaningful change, record timestamp, latitude and longitude, course over ground, speed over ground and navigational status.
- Compare destination and ETA fields with the vessel’s movement, but treat them as transmitted voyage data rather than a guaranteed plan.
Historical playback may animate a route, but the underlying evidence is still a sequence of received reports. Zoom in on harbours, turns and apparent detours; these areas often expose long gaps or provider interpolation that is hidden at a continent-level view.
Read the report fields together
| Field | How to use it | What it cannot prove |
|---|---|---|
| Position | Establishes the reported latitude and longitude at the report time. | The ship’s exact path between reports. |
| Course over ground (COG) | Shows the reported direction of movement over the ground. | That the bow was pointed exactly along that direction or that the course remained constant. |
| Speed over ground (SOG) | Helps test whether successive positions and timestamps are plausible. | That the value was error-free or sustained through a gap. |
| Navigational status | Provides the status transmitted by the vessel, such as underway or at anchor. | Independent confirmation of what the crew or vessel was actually doing. |
| Destination and ETA | Indicate the voyage information entered into the AIS system. | A binding destination, arrival time or proof that the ship followed the declared route. |
Recognize sampled tracks and interpolation
A plotted line is trustworthy only to the extent that its surrounding reports are available and plausible. A sudden long segment, a speed that would require an impossible acceleration, or a line that appears to cross land is a reason to inspect the raw timestamps and points. The provider may be connecting sparse reports, displaying a corrupted coordinate, or interpolating a line for visual continuity. Mark the interval as uncertain instead of presenting the drawn line as an observed path.
How often AIS reports are transmitted
Class A reporting
Current U.S. Coast Guard guidance states that Class A position reports are sent every 2–10 seconds while underway, depending on speed and course changes. At anchor or moored, Class A position reports are sent every three minutes or less. Static and voyage information is sent every six minutes.
Class B reporting
Class B transponders transmit fewer fields and report less frequently. MarineTraffic documentation describes a minimum interval of 30 seconds for Class B transmissions. The exact detail visible in a public archive also depends on reception and on which messages the provider stores.
Why the interval changes your interpretation
A fast-moving Class A ship can produce a comparatively dense track, while a Class B vessel may have visibly wider gaps. Even a Class A ship can disappear from a terrestrial network outside coastal coverage. Do not compare the apparent precision of two tracks without checking their transponder class, report interval and receiving network.
What “last known position” means
“Last known position” means the latest AIS report that the selected provider received and retained for that vessel. It is a statement about the provider’s data, not a guarantee that the vessel is still at that coordinate.
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Assess the age of the position
- Fresh report: The timestamp is recent and successive reports are arriving at the expected interval.
- Ageing report: The timestamp is older than the vessel’s expected reporting interval; describe it as stale or unconfirmed.
- Old report: The provider has not received a newer message for a prolonged period; do not use the point as a present-location statement.
There is no universal age threshold that makes a position “current.” A few minutes may be significant for a fast vessel in dense coverage; a longer interval may be normal for a small Class B vessel or a ship outside terrestrial reception. The timestamp, vessel class and coverage context must be considered together.
Why an AIS track can stop
Coverage limits
Terrestrial AIS receivers generally cover coastal and inland areas, not every ocean route. Satellite reception extends the possible area but can still be intermittent, and public services may combine networks with different coverage and retention policies. A vessel may continue sailing while no report reaches the service you are viewing.
Reporting and equipment problems
A gap can result from an AIS unit being powered down, damaged, incorrectly configured or unable to transmit. Transmission errors can also create unusable messages. A map that shows a final point does not tell you which of these causes occurred.
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Intentional silence and security
AIS may be intentionally silenced in circumstances permitted by maritime rules or operational practice. The IMO warns that publishing AIS data can raise ship and port security concerns and recognizes circumstances in which navigational information may be protected. Do not assume that a missing track indicates an accident, a change of ownership or a secret destination.
Provider-side causes
The provider may have a processing outage, a delayed feed, a retention limit or a policy that removes certain reports. Check whether other nearby vessels are updating. If many tracks stop at the same time, a network or provider issue is more likely than a vessel-specific failure.
A practical gap diagnosis
- Record the last received timestamp and position.
- Check whether the vessel’s transponder class and normal report interval are known.
- Compare the gap with the map’s terrestrial and satellite coverage.
- Look for later reports in another legitimate AIS service, noting that services may receive different messages.
- Inspect the first point after the gap for an implausible jump or speed.
- Report the interval as unknown rather than filling it with a straight-line assumption.
How to judge a route segment
Calculate whether the movement is plausible
Compare the distance between consecutive coordinates with the elapsed UTC time. A rough speed check can identify bad data: if the implied speed is far above the vessel’s type and the transmitted SOG, inspect for a missing report, swapped coordinates or an interpolation artifact. This is a consistency check, not proof of the true speed.
Watch for land crossings and sharp jumps
A line crossing an island, harbour wall or continent usually means the displayed segment is not a measured path. Open the point list, not just the map, and identify the two reports that created the segment. If one timestamp is far older than the next, label the route between them as a gap.
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Separate navigation status from movement
A status such as “at anchor” is a transmitted declaration. Compare it with the cluster of positions and SOG values, but do not treat the status as an independent sensor. A vessel can drift, swing at anchor or transmit stale status while its position changes.
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Choosing an AIS tracking source
No single public service has the same coverage, archive or display rules everywhere. Compare the service on the dimensions below before relying on it for a route reconstruction.
| Comparison point | Questions to ask | What is established universally? |
|---|---|---|
| Terrestrial versus satellite coverage | Where are receivers located, and are satellite reports included? | Not universal; coverage is provider- and location-specific. |
| Live display versus playback | Can you view only the newest report, or replay an archived voyage? | Not every service offers both. |
| Retention period | How far back can the selected vessel be queried? | No universal retention period is established for all public providers. |
| Timestamp and timezone | Are report times shown in UTC, local time or a converted display? | Always verify the service’s label and conversion. |
| Class A and Class B fields | Does the archive preserve ship type, dimensions, draught, destination and ETA, or only position? | Field completeness varies by message and provider. |
| Gap handling | Does the map expose missing points or draw connecting/interpolated lines? | Visual lines should be treated as samples unless raw reports are available. |
| Privacy and security | Are some vessels, ports or reports restricted? | IMO recognizes legitimate security concerns and protected information. |
When documenting a route for someone else, name the provider, the query time and the UTC timestamp of the last report. That makes your conclusion reproducible without implying that another service would show identical data.
Using AIS equipment aboard a vessel
A marine AIS receiver can receive and display nearby ship symbols and information such as name, course, speed, classification, call sign, registration number and MMSI. An AIS transponder also transmits the vessel’s own information to other equipped ships and receiving stations, subject to the vessel’s equipment, power, configuration and operating rules.
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Onboard hardware and a public historical website solve different problems. A receiver or transponder can show or transmit nearby AIS messages; it does not automatically create a searchable web archive. To build a history, you need a logging system with storage, reliable timekeeping and a way to preserve the received messages. A public provider may still have gaps where your receiver had no coverage or where the provider did not retain the report.
A repeatable workflow for a defensible answer
- Define the question. Decide whether you need the current feed, a past route, the last received point or a list of port calls.
- Identify the vessel. Search by MMSI, then verify IMO number, call sign, name, type, dimensions and flag.
- Set the time basis. Convert displayed times to UTC and record the conversion.
- Export or note raw points. Capture timestamps, coordinates, COG, SOG, navigational status, destination and ETA where available.
- Inspect gaps. Mark long intervals, impossible speeds, land crossings and sudden coordinate jumps.
- Qualify the conclusion. Describe the route as a sequence of received reports and identify every interval that is uncertain.
- State the endpoint precisely. Use “last received AIS position at [UTC time]” and include the provider used.
Common mistakes and how to avoid them
- Searching only by name: Use MMSI and confirm the IMO number or call sign.
- Calling the map line the exact route: Treat it as a connection between sampled reports.
- Ignoring UTC: A timezone mismatch can make a route appear to arrive before it departed.
- Assuming a stopped track means a stopped ship: Check coverage, report interval, equipment and provider status.
- Assuming “last known” means current: Quote the timestamp and its age.
- Mixing transponder classes: Expect Class B to provide fewer fields and less frequent reports than Class A.
- Using destination as a confirmed arrival: Destination and ETA are voyage fields entered into AIS and can be stale or changed.
- Assuming every provider has the same archive: Retention and reception are provider-specific.
What you can responsibly conclude
A well-supported statement identifies the vessel, names the provider, gives the UTC time and describes the observed reports rather than claiming continuous knowledge. For example: “The MMSI and IMO number match the vessel record. The provider received Class A positions from 08:00 to 10:14 UTC, with a 47-minute gap. The latest retained report was at 10:14 UTC; the vessel’s position after that time is not established by this track.”
That format preserves what AIS can establish—identity and received navigation messages—while keeping the uncertainty visible where coverage, transmission, interpolation or security restrictions limit the record.
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