To track a ship’s past route, start with its Maritime Mobile Service Identity (MMSI), confirm the match with the IMO number and call sign, then open a provider’s historical-track or voyage-playback view. Read every report in UTC order and treat the plotted line as a series of received samples, not a continuous recording. A vessel’s “last known position” is only the most recent report the service received; its timestamp and the receiving network determine how useful that position is.
The method below explains how to identify the right vessel, judge route gaps and jumps, understand Class A and Class B reporting, and decide whether a stopped track means a coverage problem, equipment issue, deliberate silence or simply an old timestamp.
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What AIS history actually represents
Automatic Identification System (AIS) is a maritime navigation-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 vessels, 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.
That definition matters when you read a web map. AIS is not a continuous black-box recording of everything a ship did. A transponder sends discrete reports, and a shore, satellite or other receiving network captures some of them. A tracking service stores the reports it receives and may draw lines between those points. The line is therefore a visualization of sampled observations. It does not prove the vessel followed every metre of that line, nor that the service saw every report transmitted.
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Two consequences follow:
- A route can contain gaps even when the vessel was moving normally.
- A displayed endpoint can be old, even if the map still shows the ship symbol at that location.
Use the timestamp attached to each report, not the apparent position of a symbol, as the starting point for every conclusion.
Step 1: Identify the correct vessel before reading its track
Name searches are convenient but unreliable. Ships can share a name, change names, or appear with spelling and formatting variations. The MMSI is the practical first lookup key because it identifies the transmitting station in the AIS network. After finding a candidate, verify it against independent fields shown by the provider.
Fields to cross-check
- MMSI: Confirm that the number in the result is the one you were given or the one associated with your investigation.
- IMO number: This is the strongest long-term identity check. The IMO number remains the same when a ship changes registration flag, while the name and voyage fields can change.
- Call sign: Compare the radio call sign with the record you expect.
- Ship type: A tanker, container ship, tug, passenger vessel or other type should fit the context.
- Dimensions and draught: Length, beam, antenna position and draught can reveal that a similarly named ship is the wrong one.
- Displayed flag: Treat the flag as a current registration field, not a permanent identity. A change of flag does not change the IMO number.
- Timestamp: Check when the identity and position data were received. A profile with an old timestamp may not describe the vessel’s present voyage.
When multiple vessels share a name, do not proceed until the MMSI and at least one additional stable identifier, preferably the IMO number, agree. Record the exact MMSI, IMO number, call sign and the UTC time of the first observation so another person can reproduce the lookup.
Step 2: Open and read a historical route
Tracking services use different labels, but the relevant feature is usually called historical track, history, voyage playback or past positions. Select the vessel profile, choose the desired date and time range, and display the points rather than relying only on the animated line.
- Set the time zone to UTC. If the interface uses local time, convert it before comparing reports, port calls or other records. Write down the conversion rather than mixing local and UTC times in the same timeline.
- Read points chronologically. Start with the earliest received report and move forward. Note the timestamp, latitude and longitude, course over ground (COG), speed over ground (SOG) and navigational status for each significant change.
- Review voyage fields. Destination, estimated time of arrival (ETA), draught and ship type can help distinguish a routine transit from an arrival, departure or data error. Voyage fields describe what was entered or transmitted; they are not proof that the ship reached the stated destination.
- Measure the time between reports. A short interval supports a more detailed reconstruction. A long interval means the map is connecting two observations across an unknown portion of the voyage.
- Inspect suspicious segments. If a line crosses land, makes an implausibly sharp jump or implies an impossible speed, open the individual reports and look for a missing interval, bad coordinates, an encoding problem or provider interpolation.
- Save the evidence. Export or screenshot the point list if the service permits it, and include the provider name, vessel identifiers, date range and UTC timestamps in your notes. A moving web display can change as new reports arrive.
How often reports are expected
Reporting frequency varies by AIS class, navigational status, speed and course changes. Current U.S. Coast Guard guidance gives these figures for Class A position reports:
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| Situation or data type | Published reporting interval | What it means for a history |
|---|---|---|
| Class A, underway | Every 2–10 seconds, depending on speed and course changes | Potentially dense position sampling, although reception and storage can still create gaps. |
| Class A, anchored or moored | Every three minutes or less | Expect fewer points than during a fast transit, but a multi-hour silence still needs investigation. |
| Class A, static and voyage information | Every six minutes | Identity and voyage fields can refresh separately from rapid position reports. |
| Class B | Less frequent and fewer fields than Class A; provider documentation describes a 30-second minimum interval | Track detail and available metadata can be substantially thinner. |
The intervals are transmission rules, not a promise that a public website will display every message. Receiver range, satellite visibility, network processing and the provider’s storage policy all affect the history you see. Static and voyage fields documented by MarineTraffic include the IMO number, call sign, name, ship type, dimensions, draught, destination and ETA; its documentation also says those fields are transmitted every six minutes. Class B transponders transmit fewer fields and report less frequently.
How to interpret a route line without overclaiming
Each plotted point is a received AIS report. The segment between point A and point B is an inference that connects samples. It is not evidence that the vessel maintained a constant heading or speed between them.
Use the numbers together
- Timestamp: Establishes when the network received or recorded the report. Prefer the provider’s explicit received time when it supplies one.
- Position: Check latitude and longitude for continuity. A small change over a short interval is ordinary; a large displacement in a few seconds is a warning sign.
- COG: Shows the reported direction over the ground, not necessarily the ship’s bow heading in wind or current.
- SOG: Compare the speed with the vessel type, navigational status and elapsed time. An isolated extreme value may be a bad report rather than a real acceleration.
- Navigational status: “At anchor,” “moored” or another status can explain sparse movement, but status is transmitted data and can be stale or incorrectly set.
- Destination and ETA: Useful context, not a guarantee. Crews can enter a planned destination and later change course or update the field.
Recognize interpolation and missing reports
A provider may draw a smooth line through widely separated points. Look for a time gap before trusting the apparent track. For example, if the last point before a gap is at 12:00 UTC and the next is at 18:00 UTC, the map cannot tell you where the vessel was at 15:00 UTC. It can show only the two endpoints and whatever assumptions the display uses to connect them.
A long segment that cuts across land, reverses direction instantly or implies a speed inconsistent with the vessel should trigger a data check. Open the raw points, verify that the time zone is UTC, and compare the reported coordinates and timestamps. If the provider offers a setting to hide interpolated segments or display only received positions, use it for analysis. If it does not, label the segment as an inferred connection in your notes.
What “last known position” means
“Last known position” means the latest AIS report that the selected provider received and retained for that vessel. It does not mean the vessel is still there. The first question to ask is: When was that report received, in UTC?
Use precise wording such as “last received AIS position at 14:32 UTC on 17 October 2026.” Avoid writing “the vessel is currently at” unless the report is demonstrably fresh and you understand the provider’s coverage in that area. A symbol that remains on a map without a recent timestamp is a stale record, not live confirmation.
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- Elapsed reporting interval: A vessel at anchor or a Class B vessel may transmit less often than an underway Class A vessel.
- Receiver coverage: Terrestrial receivers have geographic limits, and satellite reception can vary with antenna visibility, traffic and network availability.
- Network or provider processing: A report can be transmitted but arrive late, be filtered, or not be retained in the public history.
- Equipment or power problems: A transponder, antenna, onboard network or electrical system can fail.
- Transmission errors: Corrupted or malformed data may be rejected or displayed incorrectly.
- Intentional AIS silence: A crew or authority may stop transmitting under operational or security circumstances.
The IMO warns that publishing AIS data can create ship and port security concerns and recognizes circumstances in which navigational information may be protected. Therefore, a silent track is not proof of an accident, and a visible track is not proof that the vessel is safe or presently at the displayed coordinates.
Why an AIS track stops, and how to investigate
Start with the timestamp
Write down the age of the last report. A few minutes may be normal for a vessel that has slowed, anchored or changed reporting class. Hours or days require a broader check. Do not infer the cause from the age alone.
Check whether the vessel changed environment
Determine whether the final point is near the edge of terrestrial coverage, behind an island or in an area where the provider relies on satellites. Compare another receiving network if available, while remembering that two public services can have different antennas, satellites, filters and retention policies.
Compare identity and voyage fields
Look for a recent change in destination, ETA, draught, navigational status or ship type. A profile that suddenly switches identity fields may indicate a transponder configuration issue or a mismatched vessel record. Recheck the MMSI and IMO number before treating it as a real voyage event.
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Look for a later report
Search the same MMSI over a wider date range and inspect nearby ports or approaches. A later point proves that the network heard the vessel again, but it does not reveal what happened during the gap.
Do not treat silence as a location
The final received point is the endpoint of the available record, not an estimated present position. If you publish or share the result, state the UTC timestamp and the provider used, and describe the track as unavailable after that time.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing a source for AIS history
No single public service has a universal retention period or identical coverage. Compare the service characteristics that affect your question rather than assuming that the most polished map has the most complete record.
| Comparison point | Questions to ask | Why it matters |
|---|---|---|
| Terrestrial versus satellite coverage | Which receiving network covers the sea area and port? Are satellite reports included? | A route can disappear outside antenna range even when the transponder is operating. |
| Live display versus historical playback | Does the service show only current points, or can you select a date and replay a voyage? | A live map cannot answer a past-route question by itself. |
| Retention period | How far back are reports available for this vessel and account level? | Retention differs by provider; verify it directly for the service you use. |
| Timestamp and time zone | Are report times explicit, and can you view them in UTC? | Without a clear time base, speed and gap calculations can be wrong. |
| Class A/Class B fields | Does the service show MMSI, IMO, call sign, COG, SOG, status, destination and ETA for this class? | Fewer fields make identity checks and route interpretation harder. |
| Gap handling | Does the map distinguish received points from interpolated lines? | A smooth line can conceal hours of unknown movement. |
| Privacy and security restrictions | Are some vessels, areas or reports delayed, suppressed or protected? | Missing public data may be deliberate rather than technical. |
When comparing two services, align the vessel identifiers and UTC time range first. If one shows a later point, that may reflect better local reception or a different update pipeline rather than a contradiction in the ship’s AIS transmission.
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A marine AIS receiver can receive nearby transmissions and display 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 identity and navigation data to other equipped ships, aircraft and shore stations.
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Hardware and a public historical archive are different things. A receiver can show what its antenna hears and, with suitable software and storage, create a local log. It does not automatically provide a retrospective web archive of every voyage, nor does it recover reports that were never received. Before relying on a local setup, confirm antenna placement, power, GPS input, software logging and the retention capacity of the storage system.
A practical, repeatable workflow
- Define the question. Decide whether you need identity confirmation, a route for a specific date, the final received position, or an explanation for a reporting gap.
- Collect identifiers. Start with MMSI; record the IMO number, call sign, name, type and displayed flag.
- Set UTC. Convert any local event times before opening the history.
- Choose the history view. Select the provider’s date range and playback or historical-track function.
- Read raw points. Capture timestamp, coordinates, COG, SOG, navigational status and relevant voyage fields.
- Mark gaps. Calculate or note the time between consecutive reports instead of assuming the line is continuous.
- Check plausibility. Investigate land crossings, sudden jumps, impossible speeds and identity-field changes.
- State the endpoint accurately. Report the last received AIS position with its UTC timestamp and provider, not an unqualified current location.
- Record uncertainty. Identify where coverage, interpolation, retention or security restrictions limit what can be concluded.
Worked interpretation example
Suppose a history shows a Class A cargo vessel at 08:00 UTC, another point at 08:00:10, and then no point until 14:00 UTC. The first ten-second interval is consistent with the published underway range, but the six-hour interval is not evidence that the vessel remained at the last coordinate. The correct description is that the provider received a point at 08:00:10 and the next retained point at 14:00 UTC, with the vessel’s path between those reports unresolved.
If the final 14:00 point is followed by a map label saying “last known position,” publish the time and age of that report. If the vessel later appears at 16:30 UTC on another network, that establishes a later received report, not a continuous record of the missing two and a half hours. If no later report appears, possible explanations include coverage loss, an equipment or power problem, transmission errors or intentional AIS silence; the data alone cannot choose among them.
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Common mistakes to avoid
- Searching by name alone when several vessels share it.
- Treating the IMO number as changeable when the flag changes; the IMO number remains the same.
- Reading local display time as UTC without checking the interface.
- Assuming every point on a smooth line was directly received.
- Calling a stale endpoint the vessel’s current location.
- Interpreting an AIS silence as proof of an emergency.
- Comparing Class A detail with Class B detail as though their reporting rules were identical.
- Assuming a receiver creates a historical archive without configured logging and storage.
- Quoting a provider’s retention period as universal; retention is service-specific.
Used carefully, AIS history can establish which vessel was observed, where and when the network received reports, and how the reported course and speed changed. It cannot fill every gap, guarantee a present location, or reveal movement that no receiving network captured.
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