A ship disappearing from a public AIS map usually means the data feed has stopped, not that the vessel has vanished or is in distress. First save the ship’s last reliable record, verify its identity by MMSI or IMO number, compare independent receivers, and check for coverage gaps, equipment faults, authorized silence and GPS interference. If safety is a realistic concern, contact the operator, the relevant maritime authority or a rescue coordination centre rather than relying on a web map.
The procedure below separates what a member of the public can check from information available only to operators and authorities.
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What AIS shows—and what a blank map does not prove
Automatic Identification System (AIS) is a radio-based maritime safety system. For an equipped vessel, it can transmit identity, vessel type, position, course, speed and navigational status to other ships, shore stations and aircraft. It also receives reports, supports tracking and exchanges data with shore facilities.
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A public tracking website is only displaying the reports that reach its own receivers or data partners. When the map stops updating, the missing display may be caused by the receiver network, the vessel, the navigation signal or the website’s processing. A blank track is therefore evidence of a data gap, not proof that the ship has sunk, been hijacked or stopped moving.
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“AIS transponders are designed to be capable of providing position, identification and other information about the ship to other ships and to coastal authorities automatically.” — International Maritime Organization
Why a track can disappear
- Reception loss: Terrestrial AIS is line-of-sight VHF radio. Offshore distance, islands, cliffs, buildings and receiver outages can leave a ship outside the receiving network even while its transponder is working.
- Transponder or power failure: The AIS unit may lose electrical power, develop an antenna fault or disconnect from the vessel’s onboard network.
- Deliberate, permitted silence: U.S. Coast Guard rules generally require covered vessels to keep AIS operating while navigating or at anchor, but allow a temporary shutdown when continuous operation could compromise safety or security. The event and reason must be logged and reported.
- GPS interference: Jamming or spoofing can make the reported position inaccurate or create an apparent gap. The Maritime Administration warns that interference can also affect navigation, timing and satellite communications equipment.
- Data latency or processing problems: A provider may receive a report late, merge duplicate identities or temporarily lose a feed. Two displays can consequently show different last-seen times.
Six-step check when a ship stops updating
- Capture the last good record. Save the vessel name, MMSI, IMO number, call sign if shown, latitude and longitude, timestamp, course, speed, navigational status, destination and the tracking provider. Take a screenshot as well as copying the text. AIS dynamic reports contain position and movement fields; static and voyage information arrive separately, so record both kinds when available.
- Search by an identifier, not just the name. Vessel names can be duplicated, changed or entered incorrectly. MMSI (Maritime Mobile Service Identity) and IMO numbers are more reliable keys. The U.S. Coast Guard’s NAVCEN Vessel Information Verification Service can confirm recent static broadcasts and expose discrepancies, but it is not a public live-feed replacement.
- Compare independent reception. Check another AIS display or receiver. If a satellite-AIS layer is available, compare it with terrestrial data. Note each source’s reception time, not merely the time shown for the vessel. A difference can reflect station coverage or provider latency rather than a real movement.
- Estimate a possible position, cautiously. Project the last credible course and speed only as a search estimate. For example, a vessel reported at 15 knots for four hours could cover about 60 nautical miles in still conditions, but turns, speed changes, current, an earlier transmission failure or a bad GPS fix can make the result substantially wrong. Label the result as an estimate, never as a confirmed location.
- Check local notices and interference reports. Review Coast Guard, Vessel Traffic Service (VTS), port and navigation-safety notices for the area. Look for reported GPS jamming or spoofing, which can produce false tracks as well as gaps.
- Escalate when there is a credible safety concern. Contact the vessel’s operator, agent, destination port, pilot station, flag-state authority, nearest maritime rescue coordination centre or Coast Guard. Give them the IMO/MMSI, last known coordinates and time, course, speed and every source you checked.
How to judge the last AIS report
Read the timestamp and reception source together
“Last update” can mean the time the vessel generated a report, the time a receiver heard it or the time a website processed it. Keep all available times. A satellite provider may deliver a report after a delay; a shore receiver may show a near-real-time reception but lose the ship when it moves beyond radio range. A newer timestamp from one system is not automatically more accurate than an older timestamp from another.
Use the report fields as a consistency check
- Position: Check whether the latitude and longitude are plausible for the route and water depth.
- Course and speed: Compare the heading with the plotted line. A sudden impossible turn or speed can indicate a stale, duplicated or spoofed report.
- Navigational status: “At anchor,” “moored” or “not under command” changes how far the vessel could have travelled after the last report.
- Destination and voyage data: These are static or voyage-related fields and may remain unchanged after a route alteration, so do not treat them as a current position.
- Identity: Match the MMSI and IMO number across sources. A name-only match can attach reports to the wrong vessel.
Understand normal reporting intervals
Underway AIS position reports are commonly transmitted every 2–10 seconds, while anchored reports are commonly sent about every three minutes. These are typical intervals, not a guarantee that a public website will display every message. Receiver coverage, message collisions, satellite passes and provider processing can create longer visible gaps.
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| Method | Coverage | Must the vessel be transmitting? | Identity and time quality | Access and limitations |
|---|---|---|---|---|
| Another terrestrial AIS receiver | Coastal and inland areas within VHF radio range | Yes | Often close to the receiver’s hearing time; verify the station and timestamp | Useful for a local coverage hole, but cannot reveal a non-transmitting ship or one beyond radio horizon |
| Satellite AIS | Offshore and ocean areas covered by the provider’s satellites | Usually yes; it captures Class A and Class B transmissions | Provider reception time and latency must be retained with every observation | Performance is affected by antenna visibility, message collisions, satellite/provider coverage and delayed delivery |
| Long-Range Identification and Tracking (LRIT) | Global satellite system for qualifying ships | The ship must be an eligible LRIT participant and connected to the authorized system | Authority-controlled reports rather than a public live map | Ordinary readers generally need an operator or government authority to request data |
| Operator, port, pilot or VTS records | Operational area served by that organization | No public AIS display is required; the organization may have other records | Can be more current than an aggregator, depending on the organization | Access is discretionary and normally requires the vessel’s identifiers and a legitimate reason |
Satellite AIS: useful offshore, not a magic locator
Commercial satellite receivers can hear AIS transmissions far beyond the range of a coastal antenna. They may capture Class A or Class B messages while a ship is in the open ocean. However, satellites can receive many overlapping messages at once, and antenna visibility, provider coverage and processing queues affect what is delivered. Always preserve the provider’s reception timestamp, the displayed vessel timestamp and, when shown, the receiving satellite or source.
Satellite AIS cannot identify a vessel that is not transmitting AIS. A switched-off, unpowered or failed transponder leaves no AIS message for the satellite to collect. It can also miss a transmitting ship during a coverage or collision problem, so one absent satellite report is not conclusive. Use a second satellite source, a later pass or non-AIS operational records when the situation warrants.
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LRIT is separate from public AIS tracking
The IMO describes LRIT as a global satellite identification and tracking system for qualifying ships under the SOLAS framework. The U.S. Coast Guard’s LRIT overview says its National Data Centre monitors ships of IMO member states that are 300 gross tons or more, are on international voyages and are bound for a U.S. port or within 1,000 nautical miles of the U.S. coast.
LRIT data is exchanged through authorized data centres. It is not a public replacement for every missing AIS feed, and a casual web search will not normally expose it. A vessel operator, flag administration, port-state authority or other authorized body must request or access the information through the appropriate channel. If you are reporting a potential emergency, give the authority the ship’s IMO number and last known time so it can determine whether an LRIT request is appropriate.
When AIS silence is allowed—and what should be recorded
U.S. Coast Guard rules require covered vessels to operate AIS while navigating or at anchor, subject to a safety or security exception. A master may temporarily silence the system when continuous operation would compromise the vessel’s safety or security. The time and reason should be entered in the official log and reported to the nearest Captain of the Port or Vessel Traffic Center.
“The time and reason for the silent period should be recorded in the ship’s official log and reported to the nearest Captain of the Port or Vessel Traffic Center (VTC).” — U.S. Coast Guard
A deliberate silent period therefore does not, by itself, indicate an emergency or illegal activity. A public viewer normally cannot tell whether silence was authorized, so that question belongs with the operator, Captain of the Port, VTS or flag-state authority.
Which ships are expected to carry AIS?
IMO SOLAS carriage requirements cover ships of 300 gross tons and above on international voyages, cargo ships of 500 gross tons and above on non-international voyages, and all passenger ships. The requirement took effect by 31 December 2004. National rules, vessel size, route and exemptions can change what is required in a particular case. A small recreational boat or a vessel operating under a local exemption may never have appeared on AIS, so the absence of a track is not evidence that its transmitter failed.
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Contact the right party with a concise evidence package
Start with the vessel’s company or agent when you have contact details. The destination port, pilot station or VTS may know about an arrival, diversion, anchorage or radio report that has not reached public aggregators. For a foreign-flagged ship, the flag-state authority can coordinate with the operator. For a vessel near the United States, contact the appropriate Coast Guard sector or rescue coordination centre.
Send one clear message containing:
- Vessel name plus IMO and MMSI numbers
- Last verified latitude, longitude and timestamp, including time zone or UTC
- Last course, speed and navigational status
- Destination, departure point and intended route if known
- The AIS services or receivers checked, with their last-seen times
- Any signs of distress, unusual course changes, GPS interference or overdue arrival
U.S. reporting duty for responsible parties
Under 46 U.S.C. §2306, a responsible U.S. vessel owner, charterer, managing operator or agent who believes the vessel may be lost or imperiled must immediately notify the Coast Guard and use all available means to determine its status. This obligation applies to the responsible maritime parties; it is not a requirement for a member of the public to prove an emergency before making a good-faith safety call.
If someone may be in immediate danger, use the relevant emergency or maritime rescue channel rather than waiting for another website to update. Provide the last known position and time even when the position is several hours old.
Estimating movement without overstating certainty
A simple dead-reckoning estimate multiplies the last reported speed in knots by elapsed hours to obtain nautical miles, then projects that distance along the last credible course. It is useful for deciding which port, coastline or search authority to contact, but it is not a track. Currents, weather, manoeuvres, traffic separation schemes, anchoring and an earlier loss of AIS all widen the uncertainty.
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Use a range when possible: calculate the distance at the last reported speed, then a slower and faster case that reflects the vessel’s type and status. Mark the starting report as “last known,” draw the projection as an estimate and preserve the original coordinates separately. Never present an estimated point to responders as if it were a new AIS observation.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Troubleshooting by symptom
Only one website is blank
Check a second independent display and search by MMSI or IMO. If another source has recent reports, the first provider likely has a receiver, feed or processing problem. Keep both timestamps because they document the discrepancy.
Every public source stopped at the same time near the coast
Consider a transponder power or antenna fault, a local network outage, a permitted silent period or regional GPS interference. Contact the operator or VTS, and compare with official navigation-safety notices.
Terrestrial AIS stopped offshore but satellite AIS has a later message
The ship probably moved beyond the coastal receiver’s radio horizon. Treat the satellite reception time as a separate observation and check whether the two identity numbers match exactly.
Positions jump or show impossible speeds
Do not extrapolate from those points. Check for GPS spoofing or jamming notices, duplicate vessel names and stale reports. Use the last physically plausible position and report the anomalous data to the provider and the relevant authority.
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The ship is overdue but has never appeared on AIS
Confirm whether the vessel type, size and voyage required AIS. Small craft, exempt vessels, an unpowered unit or a transmitter that never worked may have no public track. Move directly to the operator, port, flag-state authority or rescue coordination centre rather than treating the absence of a first report as a disappearance.
Equipment for monitoring a local area
A marine AIS receiver is the most direct hardware option for receiving nearby transmissions. It still depends on VHF line-of-sight, antenna placement, power and a functioning transmitter aboard the ship. An integrated VHF marine radio with AIS can be practical for vessel operators who also need voice communications, but it has the same coverage limitations as a standalone receiver. Neither device can reveal a vessel that is silent or outside radio range.
For offshore monitoring, a satellite-AIS subscription can add coverage, subject to the provider’s satellite footprint, message-collision handling, latency and terms of access. Treat subscription data as an observation with a stated source and time, not as an infallible continuous trail.
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A practical decision framework
- Need to know whether the map is wrong? Compare a second terrestrial source and verify the MMSI or IMO number.
- Need an offshore update? Check a satellite-AIS source and retain its reception and processing times.
- Need information on a qualifying SOLAS ship? Ask the operator or an authorized authority whether LRIT data can be obtained.
- Need operational confirmation? Contact the company, agent, port, pilot station or VTS with the evidence package.
- Could anyone be in danger? Escalate immediately to the nearest maritime rescue coordination centre or Coast Guard; do not wait for a public tracker to recover.
The most reliable result comes from combining identity-verified reports, independent timestamps and authoritative operational contacts. A missing AIS signal narrows what can be observed publicly, but it does not by itself establish the ship’s condition.
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