AIS Ship Tracking: Complete Guide to How It Works, Regulations, and Dark Vessel Detection
- Capt. Anuj Chopra
- 22 hours ago
- 28 min read
Quick Answer: AIS (Automatic Identification System) is a VHF radio-based transponder system that broadcasts a vessel's identity, position, course, speed, and voyage data automatically to other ships and shore stations. Mandatory under SOLAS Chapter V for most commercial vessels, it is the primary data source for vessel tracking, port management, collision avoidance, and maritime compliance.
Between January 2024 and July 2025, Kpler identified 261 vessels that spoofed their AIS position before being sanctioned, the single largest behavioural category among all pre-sanction deceptive practices. The tanker Skipper was broadcasting a position near Guyana when satellite imagery placed it loading Venezuelan crude at the José terminal, hundreds of miles away. US special operations forces subsequently seized the vessel from the aircraft carrier USS Gerald R. Ford.
These are not fringe events. They represent the operational frontier of a system that was designed for collision avoidance in 1994 and now underpins global trade intelligence, sanctions enforcement, port management, and commodity flow analysis. AIS is the most consequential data source in commercial maritime, and the one most systematically manipulated by the operators who have the most to hide.
This guide covers how AIS works at the technical level, what data it transmits and what it cannot, the difference between terrestrial and satellite coverage, who is legally required to carry it, how dark vessels and spoofing work, and how operators, charterers, and compliance teams use AIS data correctly, including its limitations.
Who this guide is for: Ship operators, navigators, freight forwarders, charterers, maritime compliance officers, trade finance teams, and cargo owners tracking vessel movements.
What Is AIS and Why It Was Created
Before AIS, a vessel traffic service operator managing a busy port approach relied on two sources of situational awareness: radar, which shows position but not identity, and manual radio reporting, which provides identity but depends on crews to call in consistently. Neither is adequate when dozens of vessels are converging on a single anchorage in reduced visibility.
The idea of automating vessel identification and position reporting emerged from work at the International Association of Lighthouse Authorities (IALA) in the late 1980s. The International Telecommunication Union formalised the technical standard in ITU-R Recommendation M.1371. The IMO incorporated mandatory AIS carriage into SOLAS Chapter V, Safety of Navigation, with requirements phased in from 2002.
The original purpose was narrow: give vessel traffic services and nearby ships a real-time picture of who was where, to reduce collision risk and improve port approach management. The system worked well enough for this purpose that the data it generated attracted commercial interest. By the late 2000s, shipping intelligence firms had begun aggregating AIS signals from shore receivers into databases covering vessel movements at coastal scale. By the mid-2010s, satellite AIS constellations were providing global coverage. By 2025, the same system that was conceived for harbour approach safety was being used to track commodity flows, detect sanctions evasion, price freight derivatives, and monitor deceptive shipping operations in near real-time.
More than 400,000 AIS devices broadcast vessel location, identity, and movement data continuously. Platforms including MarineTraffic, Kpler, Windward, and Lloyd's List Intelligence aggregate this data into commercial intelligence products used across shipping, finance, insurance, and government.
How AIS Works: The Technical Mechanics
AIS operates on two dedicated VHF frequencies: AIS 1 at 161.975 MHz and AIS 2 at 162.025 MHz. These frequencies sit within the maritime VHF band, meaning AIS propagation behaves similarly to VHF radio: line-of-sight, with a practical range in open water of 15 to 20 nautical miles from vessel to vessel and up to 40 nautical miles from vessel to an elevated shore station.
TDMA: sharing the channel
The fundamental technical challenge of AIS is that thousands of vessels need to broadcast on the same two frequencies simultaneously without their signals colliding. The solution is Time Division Multiple Access (TDMA). The AIS time frame is divided into 2,250 slots per minute. Each transmitting station claims one or more slots and broadcasts during its allocated time. The slot allocation is self-organised: Class A stations use Self-Organising TDMA (SOTDMA), in which each transponder listens to the channel, identifies which slots are in use, and selects free slots autonomously. This allows the network to scale without central coordination.
Transmission rates
Class A transponders transmit at rates that vary with vessel speed and manoeuvring state. A vessel underway at more than 23 knots transmits every 2 seconds. Between 14 and 23 knots, every 4 seconds. Between 3 and 14 knots, every 8 seconds. Vessels travelling at under 3 knots transmit every 3 minutes. A vessel at anchor transmits every 3 minutes. This dynamic rate ensures that fast-moving vessels in traffic-dense situations update their position more frequently than anchored vessels in quiet water.
Class B transponders, used on smaller vessels below the mandatory SOLAS threshold, transmit at a minimum interval of 30 seconds. Class B+ variants with 5W power and SOTDMA capability operate at closer to Class A rates.
GPS integration
Position data in AIS transmissions is derived from the vessel's GPS receiver, either integrated within the AIS transponder unit or connected via NMEA 0183 or NMEA 2000 data bus from an external GPS/GNSS receiver. Where differential GNSS (DGNSS) is available, position accuracy in coastal waters improves to sub-meter precision.
AIS transponders also receive heading data from the vessel's gyrocompass (connected via NMEA interface), rate of turn from an ROT indicator where fitted, and navigation status from the officer of the watch via manual selection on the transponder panel. The accuracy of static and voyage data fields, vessel name, destination, ETA, cargo type, depends entirely on the crew keeping them current.
ECDIS and radar overlay
On a modern bridge, AIS data is displayed as an overlay on the Electronic Chart Display and Information System (ECDIS) and on the radar. Each AIS target appears as a symbol whose size and orientation reflect the vessel's actual dimensions and heading. Hovering over or selecting a target reveals the full AIS data set: name, MMSI, IMO number, type, flag, speed, course, heading, navigation status, destination, and ETA. Closest Point of Approach (CPA) and Time to CPA (TCPA) can be computed automatically, providing the watch officer with collision avoidance information that supplements radar.
AIS supplements radar; it does not replace it. AIS relies on targets transmitting accurately and honestly. Radar detects physical objects regardless of whether they are transmitting anything.
AIS Message Types: What Data Is Actually Broadcast
AIS transmits several distinct message types. Understanding which fields are which is important for both navigators using the data and commercial users interpreting it.
Dynamic Data
Dynamic data is transmitted automatically and continuously, updated at the transmission intervals described above.
MMSI (Maritime Mobile Service Identity): A unique 9-digit number identifying the vessel's radio station. Assigned by the national telecommunications authority when the vessel registers its radio equipment. Not the same as the IMO number, MMSI can change when a vessel changes flag, while the IMO number is permanent. Fleet identifiers and coast guard stations also have MMSIs that follow a different numbering convention.
Position: Latitude and longitude, expressed to four decimal minutes precision in standard Class A transmissions. Updated at the dynamic transmission interval.
Speed Over Ground (SOG): Vessel speed in knots, to 0.1-knot precision. This is speed relative to the earth, not through water. In strong currents, SOG and speed through water diverge.
Course Over Ground (COG): The vessel's actual track across the earth's surface, to 0.1-degree precision. Differs from heading in cross-current conditions.
True Heading: The direction the vessel's bow is pointing, sourced from the gyrocompass. Not always transmitted if the gyrocompass is not connected to the AIS system.
Rate of Turn (ROT): The rate of change of heading, in degrees per minute. Indicates whether the vessel is in a turn. Available only where a ROT indicator is connected.
Navigation Status: A manually set field indicating the vessel's operational condition. Fifteen defined values include: underway using engine, at anchor, not under command, restricted in ability to manoeuvre, constrained by draught, moored, aground. The navigation status is set by the OOW and must be updated as conditions change. An incorrect navigation status, most commonly "at anchor" when the vessel is underway, or "underway using engine" when moored, is both a safety issue and a PSC deficiency risk.
Timestamp: The UTC second within the minute when the GPS position was generated.
Static Data
Static data is programmed into the AIS transponder and updated only when vessel particulars change. It does not update automatically.
IMO Number: The permanent 7-digit identifier assigned to a vessel at construction by Lloyd's Register on behalf of IMO. It never changes regardless of name, flag, or ownership changes. The IMO number is the most reliable identifier for tracking a vessel's history across its operational life.
Vessel Name: As registered. Not always consistent with the name in classification society records or P&I entry documentation, particularly for vessels that have recently changed names.
Call Sign: The vessel's radio call sign as assigned by the flag state telecommunications authority.
Type of Ship: A numeric code from the ITU vessel type table. Covers 100+ vessel type categories. The accuracy of this field depends on correct entry at registration and updating when the vessel changes from, say, a tanker to a bulk carrier after conversion.
Dimensions: Length and beam to the nearest metre, and the position of the GPS antenna relative to the vessel's bow and port side. Used in ECDIS to display the vessel's actual footprint rather than a generic symbol.
Voyage-Related Data
Voyage data is entered by the bridge team at the start of each voyage and should be updated when relevant parameters change.
Draught: Current maximum draught. This field is frequently inaccurate, either not updated from the previous voyage, or deliberately entered incorrectly to conceal cargo load.
Cargo Type / Hazardous Cargo Indicator: An indicator of whether dangerous goods or marine pollutants are carried. Related to but not a substitute for the IMDG Code dangerous goods declaration.
Destination: The vessel's next port of call, as a text field. Not standardised, entries include everything from full port names to UN/LOCODE abbreviations to cryptic entries. Frequently not updated.
ETA: Estimated time of arrival at the declared destination, in UTC. The accuracy of this field is variable and depends on how diligently the bridge team updates it. Commercial platforms use AIS SOG and COG data to calculate their own ETA estimates rather than relying on the vessel's declared ETA.
AIS Classes: Class A, Class B, and Shore Infrastructure
Class A
Class A AIS transponders are the mandatory standard for SOLAS-regulated commercial vessels. They transmit the full message set including dynamic, static, and voyage data at the dynamic rates described above. Class A units operate at 12.5W transmit power, giving them reliable range in the VHF maritime band. They use SOTDMA, which self-organises slot allocation across the network.
The SOLAS mandatory carriage requirements for Class A are:
All cargo ships of 300 GT and above on international voyages
All cargo ships of 500 GT and above on domestic voyages (flag state implementation may vary)
All passenger ships regardless of size or GT
Class A units are typically integrated into the vessel's bridge electronics suite, connected to the gyrocompass, GNSS receiver, ECDIS, and radar via NMEA interfaces.
Class B
Class B transponders are designed for vessels below the mandatory SOLAS threshold, smaller commercial vessels, fishing vessels, recreational craft, workboats, and river traffic. They transmit at 2W (standard) or 5W (Class B+ SOTDMA variants) and use CSTDMA (Carrier Sense TDMA) rather than SOTDMA. Class B transmissions do not include rate of turn or the full navigation status set.
The practical consequence is that Class B vessels appear on AIS displays but with less frequent updates and less complete data than Class A vessels. In areas with dense Class A traffic, satellite AIS receivers may struggle to separate Class B signals from the background noise, making Class B tracking less reliable.
Class B coverage is expanding as fishing vessel tracking regulations develop globally. The United States requires all fishing vessels over 65 feet (approximately 20 metres) to transmit AIS while operating in US waters.
Shore Infrastructure and Special Purpose AIS
AIS Base Stations: Shore-based VHF transceivers operated by coast guards, port authorities, and vessel traffic services. They receive AIS transmissions from vessels and relay them to VTS displays. They also transmit weather and navigational warnings as AIS broadcasts.
AIS AtoN (Aids to Navigation): Virtual and physical AIS-equipped buoys and beacons. Virtual AtoN are positions broadcast by shore stations to mark navigational hazards or temporary marks, they have no physical presence on the water.
AIS SART (Search and Rescue Transponder): A personal distress device that, when activated, transmits an AIS signal identifiable as a distress position. Appearing on nearby vessels' AIS displays as a distinctive SART symbol.
AIS MOB (Man Overboard): A personal AIS device worn by crew, activated manually or automatically on water contact, broadcasting the crew member's position to the parent vessel.
SOLAS Chapter V: Who Must Carry AIS and What It Requires
SOLAS Regulation V/19 (Carriage Requirements for Shipborne Navigational Systems and Equipment) is the primary legal instrument mandating AIS carriage. Its requirements apply to all ships on international voyages.
Mandatory AIS vessels
Under Regulation V/19.2.4:
All ships of 300 GT and above on international voyages
Cargo ships of 500 GT and above not on international voyages (domestic trading; flag state implementation varies)
All passenger ships irrespective of size
The practical effect is that the overwhelming majority of commercial oceangoing vessels carry Class A AIS. Vessels below 300 GT on international voyages and below 500 GT on domestic voyages are not required to carry AIS under SOLAS, though many do and national regulations may impose requirements independently.
What "always transmitting" means
SOLAS Regulation V/19.2.4.5 requires that AIS shall be in operation at all times, unless the Master determines, on the basis of professional judgment, that it is necessary to switch off AIS to ensure the safety or security of the vessel. The most commonly cited legitimate basis for switching off AIS is the risk of providing positioning information to pirates in piracy-prone waters. A vessel transiting the Gulf of Guinea or the Strait of Malacca may legitimately reduce AIS transmission to avoid identifying its position to potential attackers.
Any AIS blackout, for whatever reason, must be logged in the vessel's record with the justification and the time at which AIS was restored. The IMO position is that ships deliberately avoiding detection are subject to thorough PSC inspection. An unexplained AIS gap, one not logged with a legitimate justification, is a deficiency.
PSC inspection and AIS
Port state control inspectors check AIS compliance in two ways. First, the vessel's GMDSS radio log and AIS documentation confirm that the transponder is surveyed, calibrated, and certified. Second, the AIS operating log (where the transponder maintains one) shows the transmission history, including any gaps. A vessel that arrived in port with an unexplained AIS gap in the preceding 24 hours is likely to receive scrutiny. A vessel with a pattern of unexplained gaps across multiple voyages is a PSC red flag.
Terrestrial AIS vs. Satellite AIS: Coverage, Gaps, and What Each Captures
The difference between terrestrial and satellite AIS is not just range, it determines what proportion of the global fleet is visible at any moment, and how significant the gaps in coverage are for commercial users.
Terrestrial AIS (T-AIS)
Terrestrial AIS relies on shore-based VHF receiver networks operated by coast guards, port authorities, vessel traffic services, and commercial data aggregators who install receivers at coastal locations globally. The effective reception range of a single shore station is 15 to 40 nautical miles, depending on antenna height and topography.
T-AIS coverage is dense in areas with high maritime traffic and well-resourced maritime infrastructure: the English Channel and North Sea, the US East Coast and Gulf Coast, the Strait of Malacca and Singapore Strait, the Japanese archipelago, and the approaches to major European ports. It is sparse or absent in the mid-Pacific, mid-Atlantic, the Arctic, and large sections of the Indian Ocean coastline.
For commercial maritime operations, T-AIS provides the highest-quality data in coastal and port waters, low latency (seconds to a few minutes), complete message sets, reliable signal strength. For mid-ocean tracking, it provides nothing.
Satellite AIS (S-AIS)
Satellite AIS is passive reception of VHF AIS transmissions by low-Earth-orbit (LEO) satellites. As a satellite passes over a region of ocean, it receives AIS transmissions from all vessels within its footprint, typically hundreds to a few thousand nautical miles across, depending on orbit altitude.
S-AIS extends AIS coverage globally, including mid-ocean routes, polar passages, and coastlines without terrestrial receiver infrastructure. The leading commercial S-AIS providers, Spire Global (which acquired exactEarth), Orbcomm, and Planet Labs, operate constellations of dozens to hundreds of satellites that collectively provide global revisit times measured in minutes to hours.
The packet collision problem
S-AIS has a fundamental technical limitation in high-density areas. When many vessels transmit simultaneously and a satellite receives signals from across a large footprint, multiple AIS transmissions arriving at the satellite in the same time slot interfere with each other, "packet collision." The satellite receives a garbled signal and cannot decode either transmission.
The areas where this is most problematic are exactly the areas with the highest maritime traffic: the English Channel, the Singapore Strait, major port approaches. In these regions, terrestrial AIS is already available and provides better data quality, so the packet collision problem in S-AIS is a theoretical concern but not a practical gap. The genuine S-AIS value is in the mid-ocean areas where T-AIS has no coverage and packet collision rates are low.
Latency
S-AIS latency depends on how recently a satellite passed over the region in question. For a constellation with good coverage, position updates may be available within 5 to 15 minutes. For areas at the edge of coverage windows, latency can reach 30 minutes or more. For tracking fast-moving vessels making course changes in real time, S-AIS latency is noticeable. For retrospective analysis of trade patterns and voyage histories, it is inconsequential.
T-AIS and S-AIS fusion
Commercial maritime intelligence platforms combine terrestrial and satellite AIS into a single feed. Where T-AIS is available, it takes precedence, more recent, higher quality. Where T-AIS coverage ends, S-AIS fills the gap. The result is near-continuous tracking for the majority of the world fleet, with residual gaps in polar and remote ocean areas. Platforms like Kpler, MarineTraffic, and Lloyd's List Intelligence all operate fused T-AIS + S-AIS networks, supplemented in some cases by vessel-borne receivers (ships acting as relay nodes in areas where satellite reception is degraded).
AIS Spoofing, Dark Vessels, and Deceptive Shipping Practices
AIS manipulation is not a theoretical vulnerability. It is active, systematic, and growing, driven primarily by the sanctions evasion operations of Russia, Iran, and Venezuela, and by the expanding shadow fleet that serves those operations.
Going Dark: AIS Switch-Off
The simplest form of AIS evasion is switching the transponder off. The vessel disappears from AIS tracking platforms. When it reappears, its previous and subsequent positions are known; the gap is the unexplained interval.
Going dark is not always deceptive. Piracy avoidance is a documented legitimate reason. But when AIS gaps correlate systematically with proximity to sanctioned ports, known STS transfer zones, or the approaches to Bandar Abbas, Kharg Island, or Venezuelan coastal terminals, the inference is clear.
A 2026 investigative analysis found that Russia-linked vessels had over six times more AIS gaps than a comparable random sample of vessels from four European nations. This gap frequency is not explained by piracy avoidance, it is explained by the desire to conduct operations that should not be visible.
The IMO position, while not an enforcement mechanism in itself, is clear: any blackout must be logged. A vessel that cannot provide a logged justification for an AIS gap to a PSC inspector is in a difficult position.
AIS Spoofing: Active Position Falsification
Spoofing is more technically sophisticated than going dark and harder to detect without cross-reference to independent data. It involves broadcasting false AIS data, typically a fabricated GPS position, while the vessel is actually elsewhere.
Circle spoofing is the most easily detected pattern: the vessel's AIS broadcasts a perfect circular holding pattern, a signature of automated position generation software that creates fake coordinates to make the vessel appear busy in safe waters while it conducts an illicit operation elsewhere. The geometric regularity of the circle is immediately anomalous, no vessel actually moves in a perfect circle.
Location spoofing places the vessel at a false position that looks plausible. The Skipper case illustrated this: the vessel broadcast a position near Guyana, a realistic location for a tanker in the region, while satellite imagery confirmed it was actually at the José terminal in Venezuela. Detection required cross-referencing AIS data against SAR (Synthetic Aperture Radar) satellite imagery, not something that happened manually but through automated platform analysis.
Identity spoofing involves broadcasting another vessel's MMSI, either a legitimate vessel's MMSI to create confusion, or the MMSI of a scrapped vessel (the "zombie vessel" technique). Zombie spoofing allows a sanctioned tanker to appear in databases as a decommissioned vessel, effectively delisting itself from active monitoring. Detection requires cross-referencing AIS identity data against vessel registry records, IMO number databases, and classification society records.
Flag hopping is a related technique: rapidly changing flag registration between permissive registries to outpace sanctions blacklists that target specific flag states. A vessel sanctioned under Flag A changes to Flag B within days. Without tracking registration history, it appears clean under its new flag.
Kpler's data for January 2024 through July 2025 identified 261 vessels that spoofed their AIS before being sanctioned. The pattern is consistent: spoofing begins, algorithmic analysis detects anomalies, detection drives sanctions designation, sanctions follow the spoofer. The detection speed advantage for authorities has grown as automated cross-referencing between AIS, satellite imagery, and commercial databases has become faster.
STS Transfers and AIS Manipulation
Ship-to-Ship (STS) transfers, transferring cargo between two vessels at sea, are a legitimate operation used routinely for lightering, offshore bunkering, and cargo blending. They become deceptive when used to obscure the origin of sanctioned cargo.
A dark STS transfer involves both vessels switching off AIS before meeting, conducting the transfer at sea, and both resuming transmission afterward. The originating vessel, carrying, say, sanctioned Russian crude, disappears from tracking. The receiving vessel takes the cargo and subsequently delivers it to a legitimate buyer whose due diligence shows clean vessel history for the receiving ship, not the original source.
Detection of dark STS requires combining multiple data sources: AIS gap analysis for the originating vessel, SAR satellite imagery of the known STS transfer zones (the Ceuta anchorage, the waters off Malaysia and Indonesia, the Gulf of Oman approaches), and cargo and customs declaration cross-referencing at the receiving port.
In 2024, more than half the vessels engaged in illicit STS activity were not yet under existing sanctions, per Kpler's grey fleet analysis. They operated in the enforcement gap between detectable suspicious behaviour and official designation.
How AIS Data Is Used Commercially
Port Operations and Vessel Traffic Management
Vessel Traffic Services in major ports use real-time T-AIS as the primary traffic management tool. Pilots use it to confirm vessel identity and position before boarding. Terminal operators access AIS ETA data to schedule berth allocation, crane deployment, and truck gate openings. Port congestion monitoring tools, including those described in our guide to port congestion, count vessels at anchor and calculate wait times directly from AIS positional data.
Just-in-Time (JIT) arrival programmes, which have reduced anchorage waiting and bunker consumption at major European ports, function by sharing AIS speed-over-ground data with vessels en route, allowing operators to reduce speed to arrive at a pre-assigned slot rather than rushing to join an anchorage queue.
Chartering and Commercial Operations
From the operations room of a shipping company, AIS provides what was previously available only via the Master's noon report: current position, speed, course, ETA. For operations managers running a fleet of 50 vessels across multiple trade routes, AIS replaces a significant amount of manual position reporting.
Specific commercial uses:
NOR verification: Charterers use AIS to confirm that a vessel tendering a Notice of Readiness is genuinely at the agreed port or anchorage, not en route and tendering prematurely. The AIS position record at the time of NOR tendering is frequently produced in laytime and demurrage disputes.
Speed warranty monitoring: Time charter parties typically include a speed warranty, the vessel is warranted to achieve a minimum speed in specified sea conditions. Operations teams monitor AIS speed-over-ground data over the voyage period to detect underperformance and build the evidentiary base for off-hire or warranty claims.
Cargo ETA tools: Freight forwarders, importers, and logistics operators use AIS-powered ETA calculation tools to plan container pickup, warehouse receiving, and delivery scheduling. The calculated ETA from speed and distance is typically more reliable than the vessel's own declared AIS ETA field, which is often stale.
Maritime Compliance and Sanctions Screening
This is the fastest-growing commercial use of AIS data and the one with the most direct legal and financial stakes for users.
OFAC published guidance in April 2025 on detecting and mitigating Iranian oil sanctions evasion. The guidance explicitly lists AIS manipulation, dark periods, spoofing, MMSI inconsistencies, as key red flags that financial institutions, insurers, and commodity traders must monitor. The "reasonable care" standard in OFAC enforcement means that relying on a vessel's self-reported AIS data without cross-checking against independent sources is insufficient due diligence for entities exposed to sanctions risk.
The compliance workflow for maritime sanctions screening now involves:
Step 1, Vessel identity verification: Cross-reference AIS MMSI against IMO number, vessel name, and classification society records. Flag any discrepancy between what the AIS reports and what registry records show.
Step 2, AIS history review: Examine the vessel's AIS track over a defined lookback period (typically 90 to 180 days). Flag unexplained dark periods, port calls in sanctioned jurisdictions, and proximity to known STS transfer zones.
Step 3, Spoofing indicator analysis: Check for circle patterns, physically impossible speed changes, position jumps inconsistent with vessel speed, or MMSI identity anomalies.
Step 4, Satellite cross-reference: For high-risk vessels or high-value transactions, cross-reference AIS position against SAR or optical satellite imagery for the flagged dark period.
Step 5, Ownership screening: Cross-reference the vessel's beneficial ownership chain against OFAC SDN list, EU consolidated list, and other applicable sanctions registries.
Commercial platforms, Kpler, Windward, Lloyd's List Intelligence Seasearcher, and Pole Star PurpleTRAC, automate most of this workflow, flagging vessels with anomalous AIS behaviour for human review. Financial institutions processing trade finance, commodity traders buying cargoes from vessels they do not own, and P&I clubs underwriting coverage for vessels in high-risk trades all use these platforms.
Trade Intelligence and Commodity Flow Analysis
At scale, AIS position data becomes a global trade flow database. Every port call, the vessel's arrival and departure times, the port's identity, and the vessel's type and flag, is a data point. Aggregated across thousands of vessels and ports, this data reveals commodity trade flows in near real-time, before official customs and trade statistics are published.
Commodity analysts use AIS data to track:
Iron ore loadings from Brazilian and Australian ports (Capesize port calls)
US grain export pace from Gulf Coast terminals (Panamax and Supramax calls at New Orleans, Houston)
Chinese coal import volumes (vessel arrivals at Qingdao, Tianjin, Rizhao)
Russian crude export flows (tanker loadings from Baltic and Arctic terminals)
The Baltic Dry Index (BDI) has a leading-indicator relationship with AIS anchorage wait data at major dry bulk ports, an expanding anchorage queue at Qingdao typically precedes a BDI move by 2 to 4 weeks as vessels clear and the tightened available supply is reflected in freight fixtures.
For energy market analysts, AIS data on tanker loading from Iranian, Venezuelan, and Russian export terminals provides visibility into sanctioned commodity flows that official data obscures. This is the intelligence that platforms like TankerTrackers.com and Kpler use to estimate sanctioned crude output and export volumes, data that moves energy markets.
AIS Limitations: What Commercial Users Need to Know
Despite its breadth, AIS data has significant limitations that anyone using it for commercial or compliance purposes must understand.
Static data accuracy
The vessel name, IMO number, vessel type code, call sign, and dimensions in AIS static data are self-reported and programmed by the vessel's crew or operator. They are frequently wrong. Vessels that have changed names without updating their AIS static data continue to broadcast the old name. Vessels that have changed flag without updating call signs broadcast inconsistent identity data. Vessels that misrepresent their vessel type, declaring as a general cargo vessel when operating as a tanker, distort automated type-based filtering.
The IMO number is the most reliable field, because it does not change, and cross-referencing MMSI against IMO number against class society records quickly reveals discrepancies. MMSI is less reliable, it can change and is issued by national authorities whose records are not always synchronised.
The spoofing reliability problem
The fundamental epistemological problem with AIS for compliance purposes is that it is a self-reporting system. A vessel that is willing to manipulate its AIS data does so easily, and the manipulation may not be immediately detectable in the AIS feed alone. Commercial users who rely solely on AIS position data for vessel due diligence are relying on the honesty of the operators most motivated to deceive them.
The solution is not to abandon AIS, it is to use it as one layer in a multi-source intelligence framework. AIS + satellite imagery + ownership registry + port arrival records + cargo declaration data together provide a substantially more manipulation-resistant picture than AIS alone.
What AIS cannot tell you
AIS does not transmit: the actual cargo on board, the actual loaded draught (the declared draught field is frequently wrong), the vessel's beneficial ownership, the cargo's origin or destination beyond what is declared in the voyage data field, or whether the vessel's certificates and class surveys are current.
A vessel can broadcast a clean AIS track, no dark periods, no spoofing indicators, consistent position history, while carrying sanctioned cargo, being owned by a sanctioned entity through multiple holding company layers, and operating with expired surveys. AIS is a navigation and tracking system; it is not a due diligence database.
Coverage gaps and interpolation
In mid-ocean areas with no T-AIS coverage and infrequent S-AIS passes, there can be gaps of several hours in a vessel's AIS track. Commercial platforms fill these gaps with interpolation, estimating the vessel's position between known fixes based on the last known speed and course. This interpolation is accurate for vessels maintaining steady speed and course, and unreliable for vessels that changed course during the gap.
AIS and the Shadow Fleet: 2024-25 in Context
The shadow fleet, the loose collection of vessels operating outside transparent ownership, insurance, and regulatory structures to move sanctioned commodities, has grown substantially since 2022. Estimates vary by definition, but Kpler and Windward both place the number of vessels actively engaged in deceptive practices at 500 to 700, with a high-risk pipeline of a further 370 vessels identified as likely future designations based on behavioural patterns.
The three primary commodity flows driving shadow fleet AIS manipulation:
Russian crude and refined products: Following EU and G7 price cap measures on Russian crude from December 2022 and refined products from February 2023, a large proportion of Russian oil exports shifted to vessels operating under obscure flag registry and ownership structures. These vessels routinely use AIS dark periods and spoofing to conceal their port calls at Russian Baltic, Arctic, and Pacific terminals.
Iranian oil: Sanctioned Iranian crude has been transported by a fleet of vessels that has used every available AIS manipulation technique, including the zombie vessel MMSI laundering described above. OFAC's April 2025 guidance on Iranian oil sanctions specifically identified AIS manipulation as a central enforcement challenge.
Venezuelan crude: The Skipper seizure in December 2025 illustrated both the sophistication of Venezuelan crude evasion and the capability of US maritime enforcement to respond when intelligence is sufficient. Circle spoofing patterns, satellite imagery correlation, and predictive analytics combined to identify the vessel's operations months before the seizure.
The compliance arms race between evasion and detection has intensified. As satellite imagery cross-referencing became routine for high-risk vessel screening in 2023 and 2024, operators began experimenting with GPS jamming, transmitting radio frequency interference to prevent legitimate GPS reception and cause AIS transponders to broadcast a null or erroneous position. Jamming patterns were identified in Nakhodka Bay (Russia's Pacific crude export hub) in Q3 2025 and off Qatar. The response from intelligence platforms has been to add RF emission detection and electronic intelligence (ELINT) capabilities to their data fusion architecture.
Predictive analytics represents the next frontier. Kpler's grey fleet analysis found that 34% of high-risk vessels face sanctions within 250 days of first exhibiting suspicious AIS behaviour. Platforms are now marketing sanction probability scoring based on AIS behaviour patterns, allowing compliance teams to avoid vessels that have not yet been designated but show the behavioural signatures of vessels that subsequently are.
Frequently Asked Questions
What does AIS stand for and what is it used for?
AIS stands for Automatic Identification System. It is a VHF radio-based transponder system that automatically broadcasts a vessel's identity, position, course, speed, and voyage data. Its primary uses are collision avoidance (vessels and VTS seeing nearby traffic), port management (traffic management and berth scheduling), and increasingly, commercial intelligence applications including cargo tracking, trade flow analysis, and sanctions compliance screening.
Which vessels are legally required to carry AIS?
Under SOLAS Chapter V, Regulation V/19: all cargo ships of 300 GT and above on international voyages, all cargo ships of 500 GT and above on domestic voyages, and all passenger ships regardless of size. These vessels must carry Class A AIS transponders. Many countries impose additional carriage requirements for fishing vessels and smaller commercial craft.
What is the difference between Class A and Class B AIS?
Class A is mandatory for SOLAS-regulated commercial vessels, cargo ships above 300 GT on international voyages and all passenger ships. Class A transmits the full AIS message set at dynamic rates (every 2-10 seconds when underway) at 12.5W power. Class B is used by smaller vessels below the mandatory threshold. It transmits at lower power (2-5W) and at longer intervals (minimum 30 seconds), with a reduced message set. Class B position data is less frequently updated and may be less reliably received, particularly from satellites in high-traffic areas.
What is satellite AIS and how does it differ from terrestrial AIS?
Terrestrial AIS (T-AIS) is received by shore-based VHF stations, covering coastal waters up to approximately 40 nautical miles from land. Satellite AIS (S-AIS) is received by LEO satellites passively capturing VHF transmissions from ships at sea. S-AIS provides global coverage including mid-ocean routes but with higher latency (minutes to 30+ minutes) and reduced accuracy in high-traffic areas due to packet collision. Commercial maritime intelligence platforms combine both for continuous global tracking.
What is AIS spoofing and how is it detected?
AIS spoofing involves transmitting false AIS data, typically a fabricated GPS position, while the vessel is actually elsewhere. Common techniques include circle spoofing (geometric circular patterns in the AIS track), location spoofing (plausible false positions), and identity spoofing (broadcasting another vessel's MMSI). Detection methods include: satellite imagery cross-referencing (SAR imagery places the vessel where AIS does not); physically impossible movement detection (speeds exceeding vessel capability, instantaneous position jumps); and AIS signal consistency analysis (patterns inconsistent with real vessel behaviour).
When is it legal to turn off AIS?
Under SOLAS Regulation V/19.2.4.5, the Master may switch off AIS when professional judgment indicates it is necessary for the safety or security of the vessel. The most common legitimate basis is piracy risk in high-risk waters. Any AIS switch-off must be logged in the vessel's record with the justification and restored as soon as practicable. Unexplained AIS gaps are a PSC inspection trigger.
What is MMSI and how does it differ from an IMO number?
MMSI (Maritime Mobile Service Identity) is a 9-digit number identifying a vessel's radio station, assigned by the national telecommunications authority. It can change when a vessel changes flag. The IMO number is a permanent 7-digit identifier assigned at construction that never changes regardless of name, flag, or ownership changes. For vessel identity verification in compliance work, the IMO number is the more reliable identifier; MMSI inconsistency relative to IMO number is a red flag.
How do compliance teams use AIS data for sanctions screening?
Compliance teams use AIS data to detect: AIS dark periods in proximity to sanctioned ports or STS transfer zones; spoofing indicators including circle patterns and impossible movements; MMSI-IMO number inconsistencies suggesting identity manipulation; and unusual port call patterns inconsistent with the vessel's declared trade. This AIS analysis is combined with vessel registry cross-referencing, ownership due diligence, and in high-risk cases, satellite imagery verification. OFAC's April 2025 guidance on Iranian oil sanctions explicitly identified AIS manipulation as a key red flag for financial institutions and commodity traders.
What is a dark STS transfer and how is it detected?
A dark STS (Ship-to-Ship) transfer involves both vessels switching off AIS before meeting at sea to transfer cargo, typically sanctioned crude oil, and resuming transmission afterward. The originating vessel carrying sanctioned cargo disappears from tracking; the receiving vessel shows clean history. Detection uses: AIS gap analysis for both vessels, SAR satellite imagery of known STS zones, and cargo declaration cross-referencing at the receiving port. In 2024, over half the vessels engaged in illicit STS activity were not yet formally sanctioned.
What are the main limitations of AIS data for commercial use?
Key limitations: static data (vessel name, type, IMO) is self-reported and frequently incorrect or outdated; dynamic position data is only as accurate as the vessel's intention to transmit honestly; coverage gaps exist in mid-ocean and polar areas even with satellite AIS; AIS cannot report cargo type or quantity, actual draught, or beneficial ownership. For compliance purposes, AIS must be cross-referenced against satellite imagery, registry records, and ownership databases rather than treated as a standalone reliable source.
How does the shadow fleet use AIS manipulation?
Shadow fleet vessels use a range of techniques: AIS switch-off before port calls at sanctioned terminals; location spoofing to broadcast a false position while conducting illicit operations; zombie vessel MMSI laundering (programming scrapped vessel identifiers into active tankers); flag hopping between permissive registries; and dark STS transfers. Between January 2024 and July 2025, Kpler identified 261 vessels that spoofed AIS before being sanctioned, spoofing was the single largest pre-sanction behavioural category.
Can AIS data predict which vessels will be sanctioned?
Predictive analytics platforms use AIS behavioural patterns, dark periods, spoofing events, proximity to sanctioned ports, STS activity, to calculate sanction probability scores for individual vessels. Kpler's grey fleet analysis found that approximately 34% of vessels exhibiting high-risk AIS behaviour face sanctions within 250 days. As of 2025, over 370 vessels were identified as at high risk of future designation based on current behavioural patterns. This predictive capability allows compliance teams to avoid counterparty exposure to vessels that have not yet been officially designated.
Glossary
AIS (Automatic Identification System): A VHF radio transponder system that automatically broadcasts vessel identity, position, course, speed, and voyage data. Mandatory under SOLAS Chapter V for most commercial vessels.
AIS AtoN (Aid to Navigation): An AIS-equipped navigational mark, physical or virtual, broadcasting position and identification data to nearby vessels.
AIS Base Station: A shore-based VHF transceiver operated by port authorities, coast guards, or VTS, receiving vessel AIS data and broadcasting navigational information.
AIS SART: A Search and Rescue Transponder transmitting an AIS distress signal when activated, identifying a vessel or person in distress on nearby AIS displays.
Class A AIS: The mandatory transponder standard for SOLAS-regulated commercial vessels. Full message set, 12.5W transmit power, SOTDMA, dynamic transmission at 2-10 second intervals when underway.
Class B AIS: AIS transponders for vessels below the mandatory SOLAS threshold. Lower power (2-5W), 30-second minimum transmission interval, reduced message set.
Circle Spoofing: An AIS spoofing pattern where the vessel's broadcast position traces a perfect geometric circle, a signature of automated position-generation software creating fake coordinates.
COG (Course Over Ground): A vessel's actual track across the earth's surface, expressed in degrees. Differs from heading when tidal currents or wind push the vessel sideways.
CPA (Closest Point of Approach): The minimum distance that will exist between two vessels based on their current courses and speeds. Calculated automatically on AIS-enabled bridge systems.
CSTDMA (Carrier Sense TDMA): The multiple-access protocol used by Class B AIS, where the unit listens for a clear channel before transmitting. Less sophisticated than SOTDMA.
Dark Vessel: A vessel that has switched off its AIS transponder, making it invisible to AIS tracking systems.
Dark STS Transfer: A Ship-to-Ship cargo transfer conducted with both vessels' AIS switched off, used to obscure the origin of sanctioned cargo.
DGNSS (Differential GNSS): An augmented GPS system providing sub-meter position accuracy in coastal waters where differential correction signals are available.
ECDIS (Electronic Chart Display and Information System): The bridge system on which AIS data is overlaid on electronic navigational charts, providing integrated situational awareness.
Flag Hopping: Rapidly changing vessel registration between permissive flag registries to outpace sanctions blacklists targeting specific flag states.
IMO Number: A permanent 7-digit vessel identifier assigned at construction by Lloyd's Register on behalf of IMO. Does not change with flag, name, or ownership changes.
Location Spoofing: Transmitting a false GPS position in AIS data to place the vessel electronically in one location while it is physically elsewhere.
MMSI (Maritime Mobile Service Identity): A unique 9-digit number identifying a vessel's radio station. Assigned by the national telecommunications authority. May change when a vessel changes flag.
Navigation Status: A manually set AIS field indicating a vessel's operational condition, underway, at anchor, not under command, restricted in manoeuvring, moored, aground, or other defined states.
NMEA 0183 / NMEA 2000: Marine electronics data bus standards used to connect AIS transponders to GPS receivers, gyrocompasses, and ECDIS systems.
Packet Collision: Interference in satellite AIS reception when multiple vessels transmit simultaneously within a satellite's large coverage footprint, preventing the satellite from decoding individual signals.
ROT (Rate of Turn): The rate at which a vessel is changing heading, in degrees per minute. Transmitted in AIS dynamic data where an ROT sensor is connected.
S-AIS (Satellite AIS): AIS signals received by LEO satellites, providing global vessel tracking coverage beyond the range of terrestrial shore stations.
Shadow Fleet / Grey Fleet: Vessels operating outside transparent ownership, insurance, and regulatory structures to move sanctioned commodities, using AIS manipulation and other deceptive practices.
SOG (Speed Over Ground): A vessel's speed relative to the earth's surface, as measured by GPS. Differs from speed through water in the presence of currents.
SOTDMA (Self-Organising TDMA): The multiple-access protocol used by Class A AIS, where transponders autonomously coordinate their transmission slots without central allocation.
Spoofing: Transmitting false AIS data, position, identity, or navigation status, to deceive AIS tracking systems.
T-AIS (Terrestrial AIS): AIS signals received by shore-based VHF receivers, covering coastal waters within VHF radio range of the antenna.
TCPA (Time to Closest Point of Approach): The time until two vessels on their current courses will reach their closest point of approach. Used in collision avoidance assessment.
TDMA (Time Division Multiple Access): A channel-sharing protocol that divides transmission time into slots, allowing multiple stations to share the same frequency without continuous collision.
VTS (Vessel Traffic Service): A maritime equivalent of air traffic control, managing vessel traffic in busy ports and waterways using AIS and radar.
Zombie Vessel / MMSI Laundering: Programming the MMSI of a scrapped or decommissioned vessel into an active tanker to disguise the active vessel's identity in AIS databases.
References
IMO SOLAS Chapter V, Regulation V/19: https://www.imorules.com/SOLAS_REGV.A.19.html https://media.liscr.com/marketing/liscr/media/liscr/online%20library/maritime/solas%20v_reg19.pdf
ITU-R Recommendation M.1371-5: https://www.itu.int/rec/R-REC-M.1371 https://www.e-navigation.nl/sites/default/files/R-REC-M.1371-5-201402-I!!PDF-E_1.pdf
Kpler AIS Spoofing Blog (November 2025): https://www.kpler.com/blog/ais-spoofing-fast-track-to-sanctions
Kpler Grey Fleet Whitepaper (March 2025): https://www.kpler.com/whitepapers/sanctions-evasion-sea-grey-fleet https://www.kpler.com/sanctions-evasion-at-sea-the-expanding-role-of-the-grey-fleet-download
OFAC Guidance (April 2025): https://ofac.treasury.gov/media/934236/download?inline
Windward 2025 Maritime Stress Test: https://windward.ai/insights/
Lloyd's List Intelligence AIS Guide (May 2026): https://www.lloydslistintelligence.com/resources/blog/essential-guide-automatic-identification-system-ais-signals
SatNews Dark Fleet Tanker Skipper Article (December 2025): https://satnews.com/2025/12/11/u-s-forces-seize-dark-fleet-tanker-skipper-in-caribbean-raid-following-ais-spoofing-detection/
USCG AIS Overview: https://www.navcen.uscg.gov/automatic-identification-system-overview
Global Fishing Watch AIS Explanation: https://globalfishingwatch.org/faqs/what-is-ais/ https://globalfishingwatch.org/our-technology/
Spire Global Satellite AIS: https://spire.com/maritime/
BIMCO AIS Guidance: https://www.bimco.org/contractual-affairs/bimco-clauses/current-clauses/ais_switch_off_clause_2021/
Compliance Disclaimer: Shipfinex FZCO operates under a VARA In-Principle Approval (IPA/26/01/002) issued by the Virtual Assets Regulatory Authority, Dubai. The information in this article is published for general maritime industry education and does not constitute legal advice, regulatory guidance, or a solicitation to invest in any financial product. Readers should consult qualified legal and compliance professionals for advice applicable to their specific circumstances. Nothing in this article constitutes an offer or invitation to deal in virtual assets or securities.
Capt. Anuj Chopra
Advisor / Contributing Author
Capt. Anuj Chopra ExC FNI FICS is a maritime industry executive with over 40 years of experience. As former VP Americas at RightShip and co-founder of ESGplus LLC, he specialises in maritime risk, ESG, and environmental compliance. He is an Adjunct Professor at the University of Houston and Fellow of both The Nautical Institute and the Institute of Chartered Shipbrokers.