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A missing AIS position is a data gap, not proof that a ship has vanished or is in distress. Save the vessel’s last verified MMSI or IMO number, time, coordinates, course and speed; check another terrestrial or satellite AIS source; then contact the operator, port or maritime authority. If there is a credible safety concern, report it immediately to the appropriate rescue coordination centre or Coast Guard. AIS only shows what a vessel’s transponder transmitted and what a receiver successfully captured.
What a blank AIS display actually means
Automatic Identification System (AIS) is a radio-based maritime safety system. An equipped vessel broadcasts identity, vessel type, position, course, speed and navigational status to other ships, shore stations and aircraft. Shore facilities can also send and receive reports through the system. A public tracking map is therefore a presentation of received AIS messages, not a continuous satellite view of every ship.
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When the map stops updating, one of several things may have happened:
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors- Reception ended. The ship moved beyond a terrestrial receiver’s radio range, entered a fjord, went behind an island or other terrain, or encountered a gap in the provider’s network.
- The transmitter or its power failed. A damaged antenna, transponder fault, onboard network problem or loss of electrical power can stop reports even while the vessel continues safely.
- The crew intentionally entered a silent period. U.S. Coast Guard rules require covered vessels to keep AIS operating while navigating or at anchor, but permit a temporary shutdown when continuous operation could compromise safety or security. The time and reason must be entered in the official log and reported to the nearest Captain of the Port or Vessel Traffic Center.
- Position data became unreliable. GPS jamming or spoofing can produce false coordinates, jumps or gaps and can also affect timing and satellite communications equipment.
- The public feed is delayed or filtered. Different aggregators receive different stations, process messages on different schedules and may display an old position while a newer message is still being validated.
AIS position reports normally contain latitude, longitude, time, course, speed and navigational status. Dynamic reports from an underway vessel commonly arrive every 2–10 seconds; an anchored vessel may report about every three minutes. Static and voyage information, such as name, call sign, dimensions and destination, is transmitted separately, so a map can retain an old identity record after current positions stop.
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Start with a disciplined six-step check
Do these steps before assuming an emergency. The objective is to preserve the last reliable observation, rule out an identification error and determine whether the gap is local, technical or intentional.
- Capture the last good record. Write down the displayed vessel name, MMSI, IMO number, call sign if shown, last latitude and longitude, timestamp including time zone, course, speed, navigational status, destination and the tracking service or receiver that supplied the data. Take a screenshot, but keep the raw numbers in a separate note.
- Search by a unique identifier. Vessel names can be duplicated, changed, abbreviated or encoded incorrectly. Use the MMSI first and the IMO number second. The U.S. Coast Guard’s Vessel Information Verification Service can confirm recent static broadcasts and expose mismatches, but it is not a public live-feed replacement.
- Compare an independent receiver. Check a second AIS display or a locally installed receiver. If one source has a recent message and another does not, the difference is usually receiver coverage, processing or latency rather than a physical disappearance.
- Look for a satellite-AIS observation. Where available, compare the provider’s satellite layer with terrestrial data. Record the reception source and timestamp; a satellite message can be older or newer than the position shown on a web map.
- Check interference and maritime notices. Review local Coast Guard or Vessel Traffic Service notices and current GPS-interference reports. A spoofed position can look plausible, so an apparent track should not be treated as proof of the ship’s actual location without corroboration.
- Escalate if safety is plausible. Contact the vessel operator, flag-state authority, nearest maritime rescue coordination centre or Coast Guard. In the United States, 46 U.S.C. §2306 requires a responsible owner, charterer, managing operator or agent who believes a vessel may be lost or imperiled to notify the Coast Guard immediately and use all available means to determine its status.
Estimate a possible position without presenting it as fact
A dead-reckoning estimate can help you decide whom to call, but it cannot restore a missing AIS signal. Use the last credible position, course and speed, then project the vessel’s travel for the elapsed time. In simple terms, distance in nautical miles is speed in knots multiplied by hours. A ship making 12 knots for two hours could cover about 24 nautical miles along its reported course.
Label the result as an estimate and include a broad uncertainty area. The ship may have altered course, reduced speed, stopped, entered a traffic separation scheme, encountered weather or lost AIS before the last position displayed by the provider. Reported GPS coordinates may also be wrong during interference. Do not use a projected point as an exact search location or as evidence that a vessel is safe.
What can still work after terrestrial AIS ends?
| Method | Coverage | Must the vessel be transmitting? | Identity and timestamp considerations | Access and cost |
|---|---|---|---|---|
| Another terrestrial AIS receiver | Only within that receiver’s VHF radio coverage; terrain and network gaps matter. | Yes. A receiver cannot reveal a non-transmitting vessel. | Compare MMSI or IMO and the receiver’s reception time. Different stations can show different last-message times. | Local hardware or a public feed may be available; pricing is not established here. |
| Satellite AIS | Offshore and beyond shore-station range, subject to satellite coverage, antenna performance and message collisions. | Yes. It detects Class A or Class B AIS transmissions; it does not locate a silent or failed transponder. | Keep the satellite provider, reception source and timestamp with every observation. Latency can vary. | Usually supplied through a commercial service; subscription terms and prices vary. |
| Long-Range Identification and Tracking (LRIT) | Global satellite identification and tracking for qualifying ships under the IMO system. | The ship must be an eligible LRIT participant and its LRIT equipment must report. | Data is exchanged through authorized data centres rather than ordinary public maps. | Normally available to governments, flag administrations, ports or authorized operators, not to casual public users. |
| Operator, agent, port, pilot station or VTS records | Operational knowledge may cover areas where public reception is absent. | No public AIS signal is required, although the organization must have another current record. | Provide IMO/MMSI and the last known time so staff can match the correct vessel. | Contact access depends on the organization and incident; no universal public fee is established. |
Another terrestrial receiver
A second shore receiver is the fastest way to test whether the first provider simply lost coverage. It helps only when the vessel remains within VHF range and continues to transmit. A receiver on the opposite side of a headland or at a higher elevation may see messages that a city-based station misses. If both receivers stop at nearly the same time, investigate the vessel’s equipment, a deliberate silent period or regional interference rather than assuming one website is broken.
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Satellite AIS
Commercial satellite receivers can capture AIS broadcasts offshore, but the result is not a guaranteed continuous track. Satellite antennas may receive many ships at once, causing message collisions; provider coverage, processing rules and network latency also affect the displayed time. Compare the ship’s MMSI or IMO number and inspect the reception timestamp and source. Satellite AIS cannot identify a vessel that is not transmitting, and it may miss a message even when the transponder is working.
LRIT
LRIT is separate from AIS. The International Maritime Organization describes it as a global satellite identification and tracking system for qualifying SOLAS ships. The U.S. Coast Guard says its National Data Centre monitors IMO member-state ships of 300 gross tons or more on international voyages that are bound for a U.S. port or within 1,000 nautical miles of the U.S. coast. LRIT information moves through authorized data centres, so an ordinary reader generally needs the operator or a maritime authority to request it. LRIT is not a public substitute for every missing AIS feed and does not cover every vessel.
Operator and port records
A ship’s company, local agent, destination port, pilot station or Vessel Traffic Service may have a later departure, arrival, pilotage or communications record than a public aggregator. Give them the vessel’s IMO number or MMSI, name, last known position and exact time. Ask whether the vessel has checked in, changed destination, entered a restricted area or reported equipment trouble. They may not disclose operational details to an unrelated caller, but precise identifiers make a safety referral more useful.
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When a missing signal becomes a safety concern
Treat the gap as more urgent when it coincides with a distress call, an abrupt stop in a hazardous area, a sudden course or speed anomaly, severe weather, known navigation interference, a collision report, or a vessel that fails an expected port arrival. A routine offshore coverage gap, by itself, is not evidence of danger.
Use the most direct authority for the ship’s location: the nearest maritime rescue coordination centre, Coast Guard, port authority or flag-state contact. Pass the preserved record rather than a screenshot alone: MMSI, IMO number, vessel name, last coordinates, UTC timestamp, course, speed, destination, source and any earlier anomalies. Explain what you know and what is only an estimate. Do not delay a genuine safety report while waiting for a second tracking website to update.
Why GPS interference can make a track misleading
GPS interference does more than create a blank map. Jamming can prevent a receiver from obtaining a position; spoofing can provide a false position that the AIS transponder then broadcasts as if it were real. Interference can also affect navigation, timing and satellite communications equipment. A sequence of smooth-looking points is not automatically trustworthy if nearby vessels show simultaneous jumps, multiple ships report impossible positions or official notices describe interference.
When interference is suspected, preserve the original timestamps and compare several independent vessels and sources. Report the discrepancy to the relevant Coast Guard or Vessel Traffic Service. Avoid publishing an estimated location as a confirmed one, especially near a conflict zone, port security area or search operation.
Which vessels are expected to carry AIS?
IMO SOLAS carriage rules 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. Smaller domestic vessels, recreational craft and vessels outside a particular carriage rule may not appear at all, even when a local receiver is functioning.
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For vessels that are covered by U.S. Coast Guard rules, AIS is generally required to operate while navigating or at anchor. A temporary safety- or security-related shutdown is allowed, but the event and reason should be recorded in the official log and reported to the nearest Captain of the Port or Vessel Traffic Center. The rule explains why a legitimate ship can disappear without implying that its crew abandoned the vessel.
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Dedicated marine AIS receiver
A marine AIS receiver is the most direct hardware option for a person who needs local reception. It receives VHF AIS messages and normally sends decoded targets to navigation software or a chart plotter. Its useful range depends on antenna height, antenna quality, cable loss, local terrain and the transmitting vessel’s equipment. It cannot see beyond radio horizon, recover a silent transponder or replace an emergency reporting channel.
VHF marine radio with integrated AIS
An integrated VHF radio with AIS can reduce the number of devices on a vessel and display nearby targets alongside voice communications. It is a practical secondary choice for operators who already need a compliant VHF radio. Confirm that the installation includes a suitable antenna arrangement, GPS input and a way to view or export target data. Integration does not change the fundamental limits of VHF range or AIS transmission.
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Satellite-AIS and vessel-tracking subscriptions can extend offshore visibility, but evaluate them by coverage area, message latency, identity matching, historical retention and whether the service discloses the reception source. A lower update rate or delayed feed may be adequate for broad voyage awareness but unsuitable for time-critical safety decisions. Pricing and availability depend on the provider and plan.
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Troubleshoot the most common scenarios
The ship disappears just beyond the coast
Expect a terrestrial coverage boundary. Check a receiver farther along the route or a satellite-AIS layer, then compare the last message time with the provider’s stated refresh behavior. If no source has a later message, contact the operator or port rather than repeatedly refreshing one map.
The name remains, but the position is old
The static identity record may have been retained after dynamic reports stopped. Search the MMSI and IMO number, inspect the age of the last position and do not infer that the ship is still at the displayed point.
Two services show different positions
Record each source’s timestamp and reception type. One may be a recent terrestrial message while the other is a delayed satellite observation. If both claim the same timestamp but disagree materially, check the identifier for a duplicate or encoding error and look for GPS-interference notices.
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Consider a swapped or duplicated MMSI, stale data joined to the wrong vessel, GPS spoofing or a provider processing error. Verify the IMO number, compare neighboring traffic and contact the provider or maritime authority. Do not connect the points into a continuous voyage until the discrepancy is resolved.
The vessel is late but still transmitting
Use its current course, speed and navigational status to estimate arrival, but remember that AIS is not a schedule. Weather, pilot availability, port congestion, traffic separation schemes and operational stops can change the voyage. Contact the destination port or agent for an authoritative arrival update.
A compact incident checklist
- Vessel name, MMSI, IMO number and call sign.
- Last latitude, longitude, UTC timestamp, course, speed and navigational status.
- Destination, draught or voyage details if displayed.
- Name of each AIS provider or receiver, plus terrestrial or satellite reception source.
- Screenshots and the unedited numeric record.
- Weather, navigational warnings, Coast Guard or VTS notices and GPS-interference reports.
- Names, phone numbers and times of operator, port or authority contacts.
- A clear distinction between observed data and any projected position.
Bottom line
When AIS disappears, first preserve and verify the last record, then test independent terrestrial and satellite sources. Use dead reckoning only as a clearly marked estimate. Satellite AIS can extend coverage while a vessel is transmitting; LRIT may provide an authority-controlled path for eligible SOLAS ships; operator, port and VTS records can be more current than public maps. Escalate immediately when the circumstances suggest danger, because a blank web display alone cannot tell you whether the cause is coverage, equipment, deliberate silence or interference.
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