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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitchesAIS does not track every ship everywhere. Automatic Identification System (AIS) is a cooperative VHF radio broadcast: a vessel’s transponder sends identity, position, course, speed and voyage information for other equipped ships, shore stations and aircraft to receive. Terrestrial AIS is limited mainly to VHF line of sight. Satellite AIS extends reception over the open ocean, but only for vessels that are transmitting and only when a satellite can receive and process the message. A blank AIS display therefore never proves that a ship is absent.
What AIS is and why the distinction matters
AIS is a safety and traffic-awareness system, not an independent camera in space. Under the International Maritime Organization (IMO) framework, an appropriately equipped ship automatically broadcasts information that can identify it and describe its movement. Other ships, shore stations and aircraft receive those broadcasts and display them on navigation or monitoring systems.
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The radio link uses the VHF maritime mobile band and time-division multiple access (TDMA), as specified in ITU-R M.1371-6 (2026). Each message is a report from a cooperating transmitter. If the transmitter is switched off, defective, incorrectly configured or out of reception range, the receiving system has no new AIS message to display.
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“Satellite tracking” is a broader phrase than AIS. It can mean satellites listening for AIS messages, the IMO’s separate Long-Range Identification and Tracking (LRIT) regime, or satellite radar and optical imaging. Those systems answer different questions and should not be treated as interchangeable.
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What information an AIS transponder broadcasts
ITU-R M.1371-6 groups AIS data into static, dynamic and voyage-related information. The exact fields shown by a service depend on the transponder, the receiving system and how a provider processes the data.
| Information group | Typical examples | What it helps you determine |
|---|---|---|
| Identity and static data | MMSI identity, vessel name, vessel type and other configured particulars | Which transmitting unit is reporting and what kind of vessel it claims to be |
| Dynamic data | Position, UTC time, course over ground, speed over ground, heading where available and navigational status | Where the vessel reported itself and how it was moving at that report time |
| Voyage-related data | Configured voyage details such as destination or navigational information | Context for the voyage, subject to the accuracy of entries made by the crew or operator |
AIS data is therefore both a location report and an identity claim. A map can show a vessel name and MMSI, but those fields are not a substitute for independent verification: entries can be stale, incomplete or wrong.
How often Class A AIS reports
U.S. Coast Guard Navigation Center guidance states that a Class A unit can send a position report every 2–10 seconds while moving, with the interval changing according to speed and course changes. When the vessel is at anchor or moored, the position report is sent every three minutes or less. Static and voyage-related information is sent every six minutes in the schedule defined by ITU-R M.1371-6.
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Those are reporting intervals for the transmitter, not a promise that a public map will refresh at the same rate. A receiver may miss a message, a satellite may not be in a useful listening geometry, and a data service may queue, filter or delay what it receives.
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What terrestrial AIS can see
Terrestrial AIS means a shore station, ship or aircraft receives the VHF broadcast directly. VHF is essentially line of sight, so reception is strongest around ports, coasts, busy approaches and shipping lanes where receiver infrastructure exists.
Range is influenced by antenna height on both ends, islands and terrain, local radio conditions, receiver placement and the number of vessels sharing the radio channels. A high shore antenna can hear farther than a low installation, while a vessel hidden behind land may disappear despite being nearby. Dense traffic can also make simultaneous messages harder to separate.
Terrestrial AIS is consequently excellent for local navigation and traffic awareness. A bridge team can use frequent reports from nearby vessels to support collision-avoidance decisions, while a harbor or coastal authority can monitor traffic inside its receiving footprint. It is not a guaranteed nationwide or global sensor.
What a terrestrial blank means
If a vessel is missing from a coastal AIS display, the most limited explanation is that it is outside the receiver’s VHF coverage. Other explanations include an obstructed path, a receiver outage, a faulty or misconfigured transponder, a vessel that is not required to carry the relevant equipment, or an intentional transmission shutdown permitted under specific rules. The absence of a symbol is an observation about the receiving system, not proof about the vessel’s physical presence.
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What satellite AIS adds
Satellite AIS (Sat-AIS) places an AIS receiver in orbit. The satellite listens for the same VHF messages that a terrestrial station would receive and forwards them for processing. The important change is geographic reach: an ocean area without shore stations can still produce AIS data when a satellite passes within a useful reception geometry.
Sat-AIS does not make a non-cooperative vessel visible. The target still has to transmit an AIS message, and the message must be received, decoded and delivered by the satellite and its ground segment. Satellite geometry and revisit determine when an area can be heard. A vessel may move a considerable distance between usable passes, so the resulting track can look intermittent rather than continuous.
Open-ocean traffic also creates a technical limit. Many vessels transmit in the same region, and overlapping signals can collide. Providers may apply filtering or other processing to resolve reports, and commercial or public services can introduce additional latency. A satellite-AIS position is therefore a received AIS report, not proof that every vessel in the surrounding ocean was detected.
Sat-AIS versus a live global map
A live-looking interface can hide several delays: the time the vessel transmitted, the time a satellite heard it, the time the data reached a ground station, and the time a provider processed and published it. Check the report timestamp and the service’s stated update behavior before treating a plotted position as current. “Seen by satellite” means “an AIS message was received,” not “the satellite continuously watched this ship.”
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Satellite tracking is a family of different systems
| System | How it obtains information | Best suited to | Important limits |
|---|---|---|---|
| Terrestrial AIS | Shore, ship or aircraft station receives a vessel’s VHF AIS broadcast | Low-latency harbor, coastal and ship-to-ship traffic awareness | Normally limited to line of sight and areas with receiver coverage; requires an AIS transmission |
| Satellite AIS | An orbital receiver captures AIS broadcasts and sends them for processing | Broader ocean monitoring of vessels that transmit AIS | Satellite geometry, revisit, message collisions, processing and service latency create gaps; requires an AIS transmission |
| LRIT | The IMO long-range identification and tracking carriage/reporting regime supplies reports through designated authorities and data centers | Regulated global identification and tracking for shipping security, safety and marine-environment protection | It is not an open, high-rate collision-avoidance feed; access and reporting context differ from public AIS services |
| Other satellite remote sensing | Radar or optical instruments detect physical objects rather than decoding AIS messages | Finding or characterizing objects that may not be transmitting AIS | Results are imagery or derived detections, not the identity and navigation fields contained in an AIS report; performance depends on the sensor and conditions |
IMO describes LRIT as a system for the global identification and tracking of ships to enhance shipping security and support safety and marine-environment protection. LRIT should therefore be selected for its regulated purpose, not assumed to be a public replacement for a coastal AIS screen.
What each system can and cannot tell you
- Can terrestrial AIS identify a transmitting vessel? Yes, when a usable message is received; identity comes from AIS fields such as MMSI and vessel name.
- Can satellite AIS identify one far offshore? Yes, if the vessel transmits and a satellite receives the message, subject to reception and processing limits.
- Can either AIS system see a vessel that never transmits? No. AIS is cooperative. A different sensing method would be required to detect a physical object independently of its AIS broadcast.
- Can AIS provide frequent maneuver data? Terrestrial reception near the vessel is the best fit. Class A may report every 2–10 seconds while moving, but satellite reception and provider processing usually make Sat-AIS less continuous.
- Does LRIT provide the same public, high-rate feed? No. LRIT is a separate regulated regime with a different access and reporting purpose.
- Does a missing position prove absence? No. It only proves that the chosen system has no usable report to display for that time and place.
Why a ship can disappear from AIS
Interpret a gap as a diagnostic question, not a conclusion. Several causes can produce the same blank map:
- Coverage limit: The vessel left a terrestrial receiver’s line of sight, or the relevant satellite has not had a useful pass.
- Obstruction or radio conditions: Islands, terrain, antenna height and changing propagation can block or weaken a VHF signal.
- Message collision: Heavy traffic can cause overlapping AIS transmissions that are not all decoded.
- Processing or service delay: A provider may filter, queue or delay reports, making the latest displayed point older than expected.
- Equipment problem: The transponder, antenna, GNSS input, power supply or configuration may be faulty.
- Stale or incorrect data: Static and voyage fields can remain unchanged or be entered incorrectly even while the ship moves.
- Transmission stopped: A crew or operator may turn AIS off where applicable rules allow it. IMO security guidance recognizes shutdown in specified high-threat circumstances.
- Applicability: The vessel may be exempt from a carriage requirement or may not be equipped for the service you are using.
To investigate, first compare the report timestamp with the current time. Then check whether the vessel was expected to be inside terrestrial coverage or within a satellite service’s reception pattern. Compare more than one receiving source where lawful and available, and look for a last-known track rather than treating a single missing point as an event. If the stakes are navigational or safety-critical, use approved bridge, coastal-authority or emergency channels instead of relying on a public map.
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Choose terrestrial AIS for local decisions
Use terrestrial AIS when the question is “What is moving around this harbor, approach or coast right now?” Its direct VHF reception offers the lowest practical latency where coverage is good. It is the natural layer for ship-to-ship awareness, port operations and coastal traffic monitoring.
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Choose satellite AIS for ocean-scale awareness
Use Sat-AIS when the question is “Which transmitting vessels are reporting across a large ocean area?” It fills geographic gaps beyond shore receivers and can support fleet awareness, maritime-domain analysis and broad traffic studies. Plan for gaps, variable latency and incomplete detection in dense traffic rather than promising an uninterrupted track.
Choose LRIT for the regulated global regime
Use LRIT when the requirement is the IMO framework for global identification and tracking in support of shipping security, safety and marine-environment protection. Confirm the applicable flag-state, authorized-data-center and access rules; an LRIT feed is not simply another public AIS website.
Use remote sensing when cooperation cannot be assumed
Radar or optical satellite imagery can address a different problem: whether a physical object is present even when no AIS message is available. The available evidence here does not establish universal detection rates or revisit times for those sensors, so do not describe imagery as equivalent to AIS identity data. An image may show an object without telling you its MMSI, course or voyage details.
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- Define the area: Is the use case a harbor, a national coast, a shipping lane or the open ocean?
- Define the target: Must every physical vessel be detectable, or only vessels that cooperate by transmitting AIS?
- Check the timestamp: Distinguish the vessel’s report time from the time the service displayed it.
- Check fields: Confirm that MMSI, vessel name, position, course, speed, heading, navigational status and voyage data are actually supplied.
- Check continuity: Ask how the provider handles satellite passes, message collisions, filtering and delayed reports.
- Check purpose and access: Decide whether you need navigation awareness, commercial monitoring, or the regulated LRIT regime.
- Plan for gaps: Document what secondary source or operational procedure is used when the map is blank.
Onboard equipment: marine AIS transponders
The physical product that directly enables AIS reporting is a marine AIS transponder. Class A and Class B hardware serve different use cases and have different reporting behavior; the applicable carriage rules and vessel category determine which is appropriate. Do not select a unit solely because a consumer map can display it.
Installation normally involves a suitable VHF antenna system, a GNSS position source, power, correctly configured identity and voyage fields, and compliance checks for the vessel and flag-state requirements. A mis-entered MMSI or an antenna fault can make a functioning installation appear absent or create misleading identity data. Commissioning should verify the transmitted identity, position and report timing with an approved test or monitoring procedure.
A transponder also does not guarantee worldwide visibility. A Class A unit may transmit at the intervals specified for its navigation state, but terrestrial receivers still need line of sight and satellite services still depend on reception, geometry and processing. Hardware, receiving infrastructure and the data service are three separate links in the chain.
Bottom line for interpreting an AIS map
Read every plotted point as: “This AIS-equipped transmitter reported these fields, and this receiving system obtained and published them.” Terrestrial AIS is the strongest tool for immediate local traffic awareness. Satellite AIS extends the same cooperative data model across oceans but introduces reception gaps and latency. LRIT is a separate regulated tracking regime, while radar and optical imagery detect objects without necessarily identifying them through AIS. When a ship disappears, investigate coverage, timing, equipment, processing and lawful transmission status before concluding that the ship itself has gone.
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