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Short answer: AIS is a cooperative VHF radio broadcast, not a global imaging system. A vessel’s transponder sends identity, position, course, speed and voyage information. Terrestrial receivers hear those messages near coasts, ports and shipping lanes; satellite AIS (Sat-AIS) receivers can hear them offshore. Neither system can show a vessel that is not transmitting a usable AIS signal, and a blank map does not prove that a ship is absent.
For regulated global identification and tracking, LRIT is a separate International Maritime Organization (IMO) regime. It is not simply a higher-powered or more continuous version of AIS. The right choice depends on whether you need low-latency local maneuver information, broad ocean awareness of transmitting ships, or an authorized long-range reporting service.
What AIS actually is
The Automatic Identification System (AIS) is a maritime safety communications system. An appropriately equipped ship broadcasts digital messages over the VHF maritime mobile band. Other ships, shore stations and aircraft with compatible receivers decode those messages and place the reported information on a navigation display or tracking service.
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The information in an AIS message
A standard AIS installation can send several categories of information:
- Identity: the Maritime Mobile Service Identity (MMSI), and often the vessel name and call sign.
- Static particulars: vessel type and dimensions, including antenna reference points entered during installation.
- Dynamic data: latitude and longitude, UTC time, course over ground, speed over ground, heading when available and navigational status.
- Voyage-related data: destination, estimated time of arrival, draught and other fields, depending on the equipment and what the crew has entered.
The quality of a display depends on both the radio message and the underlying sensors and entries. A faulty GNSS source can make a position wrong; a crew member can leave an old destination in the voyage field; and a transponder can continue broadcasting an outdated static field while its dynamic position changes normally.
How often Class A reports
The U.S. Coast Guard Navigation Center says a Class A transponder sends position reports every 2–10 seconds while moving, with the interval varying according to speed and course changes. At anchor or moored, it reports every three minutes or less. Static and voyage-related information is transmitted every six minutes in the ITU-R M.1371-6 schedule. These are reporting rules for the transmitter, not a promise that a particular receiver or website will display every message immediately.
| Message category | Typical content | Class A reporting behavior | What a viewer should infer |
|---|---|---|---|
| Dynamic position | Position, time, course, speed, heading and navigational status | Every 2–10 seconds while moving; every three minutes or less at anchor or moored, according to U.S. Coast Guard guidance | Frequent reports are possible when a receiver is continuously in range, but reception and service processing can add gaps or delay. |
| Static vessel data | MMSI, name, call sign, type and dimensions | Every six minutes in the ITU-R M.1371-6 schedule | Identity and dimensions can be stale or incorrectly entered even when the position is current. |
| Voyage data | Destination, estimated arrival, draught and related fields | Every six minutes in the ITU-R M.1371-6 schedule | These fields describe what was entered, not an independently verified voyage plan. |
ITU-R M.1371-6 also defines a long-range satellite AIS Message 27 format. That format helps satellites receive AIS from beyond coastal coverage; it does not change AIS into a sensor that can detect non-transmitting vessels.
What terrestrial AIS can see
Terrestrial AIS means VHF reception by a shore station, another vessel or an aircraft. VHF is normally line-of-sight, so the practical range is governed by antenna height, terrain, islands, atmospheric and radio conditions, transmitter power, receiver sensitivity and local interference. Coverage is usually strongest in ports, approaches, busy coasts and established shipping lanes where receiving infrastructure has been installed.
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Why range is geographic rather than global
A shore receiver can only decode a signal that reaches its antenna with enough quality. A ship behind an island may be invisible even though it is close. A low antenna on a small vessel has less horizon than a tall shore mast. Two receivers in the same region can show different traffic if one has a better antenna site or a different data feed. Aircraft and neighboring ships can extend reception temporarily, but that is still a local VHF link, not guaranteed worldwide coverage.
Terrestrial AIS is consequently excellent for immediate traffic awareness. A bridge team can see nearby equipped ships, compare their reported course and speed, and use identity information when communicating. A port operator can monitor approaches with low latency. Neither user should treat the edge of a coverage map as a hard boundary: reception can fade gradually, return unexpectedly, or be blocked by a physical obstruction.
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Conditions required for a target to appear
- The vessel must carry an AIS transmitter appropriate to its operation and have it powered and configured.
- The transmitter must be sending a usable message, and its GNSS and other connected sensors must be functioning.
- A receiver must be within practical VHF range and have a working antenna, receiver chain and data connection.
- The receiving service must accept, process and display the message without a material software, filtering or network delay.
If any one of these conditions fails, a terrestrial map can be blank even while the vessel is operating normally.
What satellite AIS adds
Satellite AIS places a receiver in orbit. ESA describes ordinary AIS as a short-range coastal tracking system and satellite AIS as a way to extend reception over ocean areas without shore stations. The satellite hears the same type of AIS broadcast that a ship or shore station would hear, then forwards the observation to a ground system for processing and distribution.
Offshore reach, with non-continuous observation
Sat-AIS can add positions far beyond the horizon of a coastal antenna. Its coverage is especially useful for broad-area awareness, route analysis and finding transmitting vessels in remote ocean regions. A satellite, however, is not above every point continuously. Orbital geometry, the satellite’s pass schedule, antenna characteristics, ground-station connectivity and provider processing determine when a message is received and when it reaches a user.
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Satellite reception is also affected by traffic density. In a busy sea area, many ships may transmit into the same reception footprint at once. Their messages can collide, reducing the number that a satellite decodes. A service provider may filter duplicates, suppress implausible points, combine feeds or delay delivery. The result is a useful wide-area data set with variable update latency, not an uninterrupted track for every equipped vessel.
What Sat-AIS does not do
- It does not reveal a vessel that has no AIS transmitter or is not transmitting.
- It does not guarantee that every AIS message sent below a satellite is decoded.
- It does not provide a continuous video-like path between two received positions.
- It does not independently verify that the reported identity, destination or position is correct.
- It does not make AIS equivalent to radar, optical imagery or other remote-sensing systems.
A satellite-AIS point should therefore be read as “a satellite received this AIS report at this time,” subject to the provider’s processing and timing. It should not be read as “every vessel in this area was searched and this one was physically confirmed.”
AIS, Sat-AIS and LRIT are different systems
“Satellite vessel tracking” is an umbrella phrase. Use the specific label because each system has a different purpose, transmission model and access context.
| Capability | Terrestrial AIS | Satellite AIS (Sat-AIS) | LRIT |
|---|---|---|---|
| What it receives | VHF AIS messages at a shore, ship or aircraft receiver | VHF AIS messages received by a satellite and forwarded through a service | Reports generated within the IMO Long-Range Identification and Tracking regime |
| Identity of a transmitting vessel | Yes, from AIS identity fields | Yes, when the satellite receives the message | Yes, within the LRIT regime |
| Offshore operation | Usually limited by line of sight and receiver locations | Often available offshore, subject to satellite reception, geometry and processing | Designed for global long-range reporting |
| Local maneuver detail | Best fit where a receiver has continuous coverage; Class A can report every 2–10 seconds while moving | Usually less continuous because satellite passes, message collisions and processing create gaps | Not designed as a public, high-rate collision-avoidance feed |
| Requires the vessel to cooperate | Yes, for an AIS message to exist | Yes, for an AIS message to exist | Uses the applicable LRIT carriage and reporting regime |
| Does no displayed position prove absence? | No | No | No; interpretation depends on reporting context and system status |
LRIT’s role
IMO describes LRIT as providing the global identification and tracking of ships to enhance shipping security and support safety and marine-environment protection. That regulated purpose distinguishes LRIT from an open AIS web map. LRIT should be selected when an authorized organization needs the LRIT reporting regime, not merely when a user wants a larger public map.
IMO’s AIS guidance says: “Ships fitted with AIS shall maintain AIS in operation at all times except where international agreements, rules or standards provide for the protection of navigational information.” That exception matters in high-threat circumstances, and it means an AIS silence can be deliberate and lawful under the applicable rules. It is not evidence by itself that a ship has sunk, stopped or left an area.
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Why a ship disappears from an AIS map
A missing point has several possible explanations. Treat it as a data-quality or coverage question first, not as a physical-event conclusion.
Coverage and reception causes
- Outside terrestrial range: the vessel moved beyond the nearest shore, ship or aircraft receiver.
- Obstruction or radio conditions: islands, terrain, antenna height and changing propagation reduced the signal.
- No satellite pass or usable geometry: a satellite has not yet had a practical opportunity to receive the message.
- Dense traffic: simultaneous transmissions collided in a satellite or local reception area.
Equipment and configuration causes
- The AIS unit is powered down, in standby, disconnected from its antenna or affected by an electrical fault.
- The GNSS input or another sensor has failed, producing unusable or rejected data.
- Static information is wrong, the MMSI was entered incorrectly, or a vessel changed equipment without a corresponding database update.
- The vessel is exempt from carrying AIS or is operating under rules that permit transmission to stop.
Data-service causes
- The provider has not received the latest message, has filtered it, or is showing a delayed position.
- Different services use different receiver networks, update policies and quality controls, so one map can show a vessel that another does not.
- The display may retain an old point for a while, making a stale track look current unless its timestamp is checked.
A practical diagnostic sequence
- Read the timestamp first. Distinguish a recent report from a point that is hours or days old.
- Identify the feed type. Check whether the display is using a local receiver, a terrestrial network, Sat-AIS or a blended service.
- Check nearby coverage. Look for coastlines, islands, antenna sites, satellite-pass limitations and known reception gaps.
- Compare identity fields. Search by MMSI as well as vessel name, and check whether a duplicate or misspelled identity is being used.
- Check the vessel’s operating context. An anchored vessel reports less often than a moving Class A vessel; a high-threat voyage may involve permitted AIS shutdown.
- Seek an independent operational source when safety matters. Contact the relevant authority, operator or established maritime reporting channel rather than inferring an emergency from a blank map.
This sequence narrows the likely cause without claiming more certainty than the data supports. A prolonged gap can justify escalation, but the gap alone cannot establish the vessel’s physical condition or location.
Choosing the right tracking approach
| Need | Best fit | Why | Important limitation |
|---|---|---|---|
| Immediate awareness of nearby traffic in a harbor or coastal route | Terrestrial AIS | Low-latency VHF reception and frequent Class A position reports when continuously in range | Coverage ends or degrades outside receiver line of sight. |
| Wide-area monitoring of equipped ships on ocean routes | Satellite AIS | Receives AIS beyond shore-station horizons | Passes, collisions, filtering and processing create gaps and variable latency. |
| Regulated global identification and tracking | LRIT | Built for the IMO long-range reporting purpose | It is not a public, high-rate collision-avoidance display. |
| Physical detection of a non-transmitting object | Another remote-sensing method, evaluated separately | Does not depend on an AIS message being broadcast | The available specifications, revisit and identification performance vary by sensor and are not equivalent to AIS message data. |
When comparing services, ask for the actual coverage map and the meaning of “real time.” Also ask how often positions are expected, how timestamps are assigned, whether data is filtered or delayed, which identity and voyage fields are supplied, how long gaps are retained, and whether the target class of vessel is required and configured to transmit.
What vessel operators need to know
A marine AIS transponder is the direct onboard product for sending AIS data. Class A and Class B units serve different operating contexts and have different reporting behavior; the correct class depends on the vessel, its legal obligations and the intended navigation role. Installation normally requires a suitable VHF antenna system, a reliable GNSS source, correct MMSI and static-data entry, and integration checks for heading and other connected sensors.
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- Use an antenna, cabling, connectors and mounting position appropriate for marine VHF service.
- Enter the MMSI, vessel name, call sign, dimensions and type accurately; verify every field after configuration.
- Confirm that GNSS position and UTC time are valid and that heading or other sensor inputs are mapped correctly where fitted.
- Test transmission and reception in accordance with the equipment manual and the relevant authority’s compliance procedure.
- Record software, configuration and service details so a later identity or data problem can be traced.
A transponder improves visibility only when it is powered, correctly configured and legally operated. It cannot make a vessel visible to a receiver that is out of range, a satellite that has not yet passed, or a service that has not decoded the message.
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When AIS may be switched off
Routine operation is the norm. IMO’s stated rule is that fitted ships should maintain AIS in operation except where international agreements, rules or standards provide for protection of navigational information. Masters should follow the applicable flag-state, coastal-state, company and safety procedures, document the reason when required, and restore transmission when the authorized situation ends. Observers should treat a shutdown as one possible explanation for a gap, not as proof of wrongdoing.
How to read an AIS track without overclaiming
Use the timestamp, source and report type on every point. A coastal Class A feed that updates every few seconds can support close-quarters traffic awareness; a Sat-AIS service with intermittent reception can support route-level analysis but may not show turns as they happened. Static and voyage fields deserve separate scrutiny because they are transmitted on a different schedule and can remain incorrect after a change.
Do not draw a straight line through a gap and label the intervening path as observed. Mark it as unobserved time. Do not interpret the last point as the current position unless the timestamp and service latency justify that conclusion. When two services disagree, compare their receiver coverage, timestamps, identity matching and filtering rules before deciding that one vessel has “moved” or “disappeared.”
Finally, keep the system’s purpose in view. AIS supports navigation and situational awareness through cooperative reports. Sat-AIS extends the audience for those reports. LRIT serves a regulated long-range identification and tracking function. None of the three, by itself, is a universal guarantee that every ship everywhere is continuously visible.
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