What AIS Is and Why It Exists
The Automatic Identification System -- AIS -- is a VHF radio broadcast protocol that commercial vessels use to continuously transmit their identity, position, speed, and course to nearby ships, shore stations, and satellites. Every vessel in the world fitting certain size thresholds is legally required to carry and operate an AIS transponder. The result is a real-time, global map of commercial maritime traffic that anyone with a receiver or a data subscription can read.
AIS was not designed for public transparency. It was designed for collision avoidance. Before AIS, ships navigating congested waters depended on radar, radio calls, and visual observation to track traffic around them. Radar is effective but limited: it shows targets without identifying them. AIS added identity to the picture. A watch officer could now see not just that a large vessel was approaching at 15 knots on a crossing track, but that it was the Ever Given, a 400-meter containership, currently loaded, with a draught of 14.5 meters. That is actionable information. Collision avoidance improved substantially after mandatory AIS rollout.
The International Maritime Organization mandated AIS in SOLAS (Safety of Life at Sea) regulations starting in 2002, with full implementation phased through 2004 for larger vessels. The specific thresholds are: all vessels of 300 gross tonnes or more engaged on international voyages, all cargo vessels of 500 gross tonnes or more regardless of voyage type, and all passenger ships regardless of size. Vessels below these thresholds -- small fishing boats, recreational craft, most river barges -- are not required to carry AIS, though many do voluntarily. The commercial vessels that matter most for global trade -- containerships, tankers, bulk carriers, LNG carriers -- are all above the 300 GT threshold by a wide margin. A Panamax containership is typically 50,000 to 80,000 GT. A VLCC (Very Large Crude Carrier) is 250,000 to 320,000 GT. The vessels that move global commodity volumes are all broadcasting, all the time.
How AIS Works: The Technical Mechanics
AIS transponders operate on two dedicated VHF channels: 161.975 MHz (Channel 87B) and 162.025 MHz (Channel 88B). Transmissions use TDMA -- Time Division Multiple Access -- which divides each channel into 2,250 time slots per minute and allows multiple vessels to broadcast simultaneously without interfering with each other. Each vessel's transponder is assigned slots dynamically through a self-organizing algorithm (SOTDMA, or Self-Organized TDMA), so the channel manages high-traffic congestion automatically.
Class A transponders, required for vessels subject to the SOLAS mandate, broadcast at up to 12.5 watts and send position reports every 2 to 10 seconds when a vessel is underway, depending on speed. A vessel doing more than 23 knots updates every 2 seconds. A vessel anchored or moored reports every 3 minutes. Class B transponders, used by smaller voluntary vessels, are lower power (2 watts) and update less frequently -- typically every 30 seconds to 3 minutes.
AIS messages come in several types. Type 1, 2, and 3 are position reports carrying MMSI, navigation status, speed, heading, position, and course. Type 5 is the static voyage data message -- transmitted every 6 minutes -- carrying vessel name, callsign, IMO number, ship type, dimensions, draught, and destination. Type 18 and 19 are the Class B equivalents. Type 21 carries aids-to-navigation (buoys, lighthouses). Type 24 is a simplified Class B identifier. Understanding message types matters when parsing raw AIS streams: position and identity data arrive on different schedules and must be joined by MMSI and timestamp to reconstruct a complete vessel picture.
VHF radio is line-of-sight. In open ocean, that gives a range of roughly 15 to 40 nautical miles between two vessels at comparable heights. From a shore station antenna mounted on a coastal hilltop, range extends to 40 to 60 nautical miles. In sheltered harbors and straits, buildings and terrain reduce range considerably. The practical implication: shore-based AIS receivers can achieve nearly complete coverage of vessels in coastal waters and major ports, but coverage drops to zero within a few dozen miles of the coastline in the open ocean.
Shore-Based AIS vs. Satellite AIS
The distinction between shore-based and satellite AIS defines the geographic reach of any vessel tracking platform. Understanding both -- and where each fails -- is essential for interpreting the coverage limitations in any AIS-derived dataset.
AIS Coverage Comparison
- ✓ Near-complete coverage within 40--60 nm of coastline
- ✓ Sub-second latency from vessel to receiver to database
- ✓ Very high message density -- captures every position update
- ✓ No collision decoding issues in normal traffic
- ✗ Zero coverage beyond coastal range
- ✗ Coverage gaps in remote coastlines with no receiver networks
- ✓ Global coverage including open ocean
- ✓ Can track vessels transiting between continents
- ✗ Latency of 5 to 30 minutes depending on satellite pass timing
- ✗ Message collision in high-density areas (chokepoints)
- ✗ Position update rate limited to satellite pass frequency
- ✗ Premium cost -- commercial providers charge per vessel or per message
Shore-based AIS emerged first, built by a global network of hobbyist and commercial receivers sharing data through aggregators like MarineTraffic and VesselFinder. These networks are remarkably dense in busy shipping corridors: the English Channel, the Singapore Strait, the approaches to Rotterdam and Los Angeles, the Suez Canal zone. Commercial operators and coast guard agencies have layered professional antennas on top of hobbyist coverage, creating redundant, high-availability networks in the corridors that matter most for global trade.
Satellite AIS was developed to fill the open ocean gap. A low-orbit satellite with an AIS receiver passes overhead and picks up broadcasts from vessels below. Early satellite AIS was unreliable in high-traffic areas because the satellite was hearing hundreds of vessels simultaneously, and TDMA -- designed for vessels hearing each other -- breaks down when the receiver is 700 km overhead and can simultaneously receive thousands of transmissions that were time-slotted to avoid terrestrial collisions. Modern satellite AIS systems use multi-channel receivers, advanced signal processing, and deconfliction algorithms to recover most messages even in dense traffic areas. Providers like Orbcomm, exactEarth (acquired by Spire Global), and Spire Global now offer near-complete global coverage with position update rates of 30 minutes or better for most commercial vessels.
For chokepoint monitoring, the distinction matters less than you might expect. The Suez Canal, the Strait of Hormuz, the Strait of Malacca, and the Bab-el-Mandeb are all coastal passages. They fall squarely within shore-based AIS coverage. The Suez Canal Authority operates its own comprehensive AIS infrastructure. Shore stations in Oman, the UAE, and Egypt provide excellent coverage of the Hormuz approaches. Singapore maintains one of the world's densest AIS receiver networks. Where satellite AIS matters most for Risk and Route is in tracking vessels mid-voyage: knowing that a tanker that departed Ras Tanura five days ago is currently 300 miles south of Sri Lanka and on course for Singapore, rather than having diverted to China. That voyage-level context requires satellite AIS.
The Data Fields: What Each Vessel Broadcasts
An AIS transponder does not broadcast one type of message. It broadcasts a structured series of message types on a fixed schedule. The fields below are drawn from the IMO standard (Resolution MSC.74(69) Annex 3) and the ITU-R M.1371 technical specification. Not all fields are reliable in practice -- several are manually entered and frequently stale or blank. The table below notes reliability.
The unique numeric identifier assigned to a vessel's radio station. Functions like a phone number for the ship. Registered through national maritime authorities and the ITU. A vessel may have multiple MMSIs if it operates under different flags across its life, which creates identity-matching challenges.
Derived from the vessel's onboard GPS. Position accuracy is typically within 10 meters for modern receivers. The AIS standard requires Class A transponders to broadcast position every 2 to 10 seconds when underway.
The actual direction of movement relative to the Earth's surface, accounting for wind and current drift. Distinct from heading, which is where the bow points. The difference between COG and heading reveals drift -- significant in strong currents like the Malacca Strait.
Actual speed across the ocean floor. When SOG drops sharply in a chokepoint approach, it signals congestion, weather, or waiting at anchor. Sustained low SOG across a transit corridor is one of the clearest signatures of supply chain stress in AIS data.
The direction the bow is pointing, as measured by the onboard compass or gyroscope. Combined with COG, heading reveals set and drift. A vessel with heading 090 and COG 095 is being pushed five degrees southward by current.
A two-digit code classifying the vessel: 70–79 for cargo ships, 80–89 for tankers, 60–69 for passenger vessels, 30 for fishing, and so on. Essential for filtering AIS streams by commodity type -- you cannot distinguish oil tankers from container ships by MMSI alone.
Set by the vessel operator and transmitted as part of the static data message. Names are not unique -- multiple vessels can share the same name. Callsigns are unique per ITU registration. Cross-referencing MMSI against IMO number is the most reliable identity anchor.
A permanent, hull-assigned identifier that does not change when a vessel is re-flagged, renamed, or sold. The IMO number is the gold standard for vessel identity matching across historical datasets. Not all vessels carry IMO numbers (fishing vessels and small craft typically do not).
Entered manually by the captain or operator. This is the most unreliable field in the AIS dataset. Vessels frequently leave it blank, enter an abbreviated or misspelled port name, or fail to update it after a route change. Risk and Route treats destination fields as advisory rather than authoritative.
Also manually entered, also frequently stale or blank. More useful as a sanity check on voyage progression than as a reliable scheduling input. Machine-computed ETAs derived from SOG and great-circle distance are generally more accurate than the broadcast field.
How deep the vessel sits in the water, updated manually. Laden tankers sit lower than ballast tankers; a fully loaded VLCC draws around 22 meters. Draught data, when reliable, allows inference about cargo load -- a critical input for estimating oil volumes transiting the Strait of Hormuz.
Operational state: 0 = underway using engine, 1 = at anchor, 2 = not under command, 5 = moored, and so on. Status 1 (at anchor) in chokepoint approaches is a direct signal of a waiting queue. Risk and Route counts anchored vessels within defined geographic bounding boxes to compute queue length metrics.
Two fields deserve special emphasis because they are both critical and frequently misunderstood by non-specialists. The first is the difference between SOG and heading. Speed over ground and heading are not the same thing. A vessel heading directly into a strong current may have a heading of due east and a COG of northeast -- the current is pushing it off track. Transit speed calculations based on heading instead of SOG will be wrong in any waterway with significant current, including the Hormuz Strait and the northern approaches to Malacca.
The second is the MMSI/IMO distinction. MMSI numbers can be recycled or reassigned. A vessel reflagged from Panama to the Marshall Islands will receive a new MMSI but keep its IMO number. Any analysis tracking vessels across time -- voyage history, behavioral patterns, ownership chains -- must anchor on IMO number, not MMSI. Using MMSI as the primary key in a vessel database is one of the most common errors in amateur AIS analysis.
How Risk and Route Uses AIS Data
Risk and Route ingests AIS data through multiple feeds, primarily aisstream.io for real-time streaming and commercial data subscriptions for historical replay and satellite coverage. The platform processes AIS data for a single analytical purpose: monitoring the five major chokepoints and deriving the metrics that appear on the dashboard.
The methodology begins with geographic bounding boxes. Each chokepoint is defined by a polygon that encompasses the relevant transit corridor, the anchorage zones where vessels wait, and the approach lanes on both sides. Vessels within a bounding box are classified into three states: transiting (underway, SOG above threshold, trajectory consistent with transit direction), waiting (anchored or very slow, inside the approach zone), or passing through (anomalous -- may be fishing, naval, or turning back).
Derived Metrics: From Raw AIS to Dashboard Numbers
Count of distinct MMSIs (filtered by IMO number when available) within the chokepoint bounding box over a rolling 24-hour window. Filtered by ship type to separate tankers, container ships, and bulk carriers. A rise in vessel count without a corresponding rise in transits signals queue formation.
Median SOG of vessels classified as "transiting" within the corridor, expressed as a percentage of the 90-day historical median for that corridor and vessel type. An index of 100 is normal. An index below 80 signals congestion or adverse conditions. Below 60 is a significant disruption signal.
Count of vessels in navigation status 1 (at anchor) or with SOG below 0.5 knots within defined waiting zones at each chokepoint approach. Queue length is the most operationally significant metric: a growing queue means vessels are waiting for convoy assignment (Suez), draft restrictions (Panama), or security clearance (Hormuz). Each additional vessel in queue represents roughly 24 to 72 hours of delay depending on the chokepoint.
Number of vessels that complete a full transit -- entering the bounding box on one side and exiting on the other -- within a calendar day. Smoothed over a 7-day rolling window to reduce noise from convoy timing and weather. Compared to the same-day figure from the prior year to produce a year-on-year change rate.
Share of transiting vessels by ship type code (tanker vs. container vs. bulk). Shifts in cargo mix signal changes in commodity demand or diversion patterns. During the 2023--2024 Red Sea crisis, the tanker share of Suez transits rose as container lines diverted around the Cape while oil continued to transit, changing the traffic mix significantly.
These metrics are computed on a rolling basis as new AIS position reports arrive and are displayed on the Risk and Route chokepoint dashboard alongside freight rate data and news alerts. When a metric crosses a threshold -- transit speed below 75% of normal, or queue length more than 40% above the 30-day average -- the system flags it for editorial review. A human editor assesses whether the anomaly is operationally significant before it appears in published analysis.
Limitations: Where AIS Data Fails
AIS is a powerful dataset, but it has structural limitations that affect any analysis built on it. Responsible use requires understanding each one.
1. Coastal Coverage Only for Shore-Based Feeds
A vessel transiting the Indian Ocean between Hormuz and Malacca will disappear from shore-based AIS networks for most of that voyage. The gap is roughly 2,500 nautical miles of open ocean where only satellite receivers can hear the transponder. For monitoring the chokepoints themselves, this is not a problem -- the straits are coastal. But for tracking voyage completion, verifying cargo delivery, or detecting mid-ocean course changes (diversion events), satellite AIS is necessary. Shore-based AIS alone cannot confirm that a vessel that left Hormuz actually arrived at its declared destination in Singapore. It can confirm it passed through the Singapore Strait, but the middle leg is a satellite-only window.
2. Message Gaps and Update Rate Variability
Even in well-covered coastal waters, individual position reports are dropped. VHF radio is subject to interference, propagation anomalies, and receiver saturation. In the Singapore Strait -- one of the world's busiest waterways -- AIS channel loading can approach saturation, and TDMA collision rates rise. A vessel that should be broadcasting every 10 seconds may only appear in an aggregated database every 60 seconds if its messages are lost or deprioritized by the receiver. This creates apparent speed anomalies in computed tracks: a vessel that jumps from one position to another in a single database record, with an implied speed of 40 knots, has simply had intermediate messages dropped. Risk and Route applies a trajectory smoothing algorithm that flags and interpolates such jumps rather than treating them as real accelerations.
3. AIS Spoofing and Dark Vessels
AIS was designed for safety, not surveillance. A vessel operator can turn off the transponder. They can also broadcast false position data. Both practices are technically straightforward and increasingly documented. Turning off AIS -- going "dark" -- is detectable only if satellite images or radar can confirm the vessel's actual position. Broadcasting a false position -- spoofing -- is harder to detect because the false broadcast looks identical to a genuine one in the raw data feed.
Spoofing and dark vessel behavior are concentrated in specific contexts: vessels evading sanctions, vessels engaged in ship-to-ship transfers (typically for sanctioned cargo), vessels in conflict zones seeking to reduce targeting risk, and vessels involved in illegal fishing. For the commercial vessels that dominate chokepoint traffic -- major carrier containerships, VLCC tankers operated by established shipping companies -- AIS manipulation is rare and reputationally costly. But for vessels in sanction-adjacent trades (Iranian crude, Russian oil above the price cap, North Korean coal), dark AIS events are a routine operational tactic. Risk and Route does not attempt to track sanctioned fleets in detail; those analyses require satellite SAR (Synthetic Aperture Radar) imagery combined with AIS, a capability reserved for specialized intelligence providers.
AIS Anomaly Signatures
- ▲ Teleportation: Position jumps hundreds of miles between consecutive reports without plausible transit time. Indicates spoofing to a false location or a database artifact from MMSI reuse.
- ▲ GPS jamming clusters: Multiple vessels simultaneously reporting positions at fixed GPS reference points (e.g., airports, military facilities). Indicates GPS jamming in the area -- the transponders lock onto false signals. Observed repeatedly in the Persian Gulf and Black Sea since 2020.
- ▲ AIS gap followed by different port arrival: A vessel goes dark for 2 to 6 weeks and reappears at a port inconsistent with its last declared destination. Classic signature of a ship-to-ship transfer or sanctions evasion voyage.
- ▲ Duplicate MMSI: Two vessels simultaneously broadcasting the same MMSI from different positions. Indicates a cloned transponder -- one vessel has copied another's identity to mask its own.
4. Data Quality and Stale Static Fields
The manually entered fields -- destination, ETA, draught, vessel name -- are the least reliable in the AIS dataset. A survey of major AIS aggregators consistently finds that 15% to 25% of vessels have blank destination fields, and another 10% to 20% have destinations that do not match their actual trajectory. Draught is often updated only at port departure and not corrected after cargo adjustments at sea. Vessel names in AIS broadcasts are not verified against any registry; a vessel can broadcast any name it chooses. Risk and Route cross-references AIS static data against the IMO Global Integrated Shipping Information System (GISIS) and Lloyd's Register for name and type verification before using vessel identity in published analysis.
5. AIS Is a Lagging Indicator for Cargo Volumes
AIS counts vessels. It does not directly measure cargo. A containership broadcasting on AIS could be fully loaded, partially loaded, or in ballast. Counting AIS transits through Hormuz gives you the number of tanker transits; it does not directly give you the volume of oil moved. Converting vessel counts to commodity flows requires pairing AIS data with vessel specifications (deadweight tonnage, typical load factors), draught observations, and cargo declarations from port systems. Risk and Route uses vessel specifications from the IHS Markit (now S&P Global) vessel database to convert tanker transit counts into approximate barrel equivalents, but these are estimates with meaningful uncertainty ranges, not precision measurements.
Key Takeaways
- AIS is a VHF radio broadcast protocol required by IMO SOLAS regulations for vessels of 300 GT or more on international voyages. Every significant commercial vessel -- containerships, tankers, bulk carriers -- is broadcasting continuously.
- Each AIS broadcast includes MMSI (radio identity), GPS position, speed over ground, course over ground, heading, navigation status, and -- on a separate 6-minute cycle -- static data including vessel name, IMO number, ship type, draught, and destination.
- Shore-based AIS provides near-complete, low-latency coverage within 40 to 60 nautical miles of coastline. Satellite AIS provides global coverage with 5- to 30-minute latency. For chokepoint monitoring, shore-based coverage is adequate. For tracking mid-ocean voyages, satellite AIS is required.
- Risk and Route derives five metrics from AIS: vessel count, transit speed index, queue length, daily transit count, and cargo type mix. Each is computed from position reports within defined geographic bounding boxes around the five major chokepoints.
- AIS has four significant limitations: no open-ocean coverage from shore-based systems; message gaps from channel saturation; deliberate manipulation (transponder off or spoofed position); and unreliable manually entered fields (destination, ETA, draught).
- AIS counts vessels, not cargo. Converting transit counts to commodity volumes requires pairing AIS data with vessel specifications and load factor estimates. These conversions carry meaningful uncertainty.
- Use MMSI as a session key, not a vessel identity key. Use IMO number for persistent vessel identity across time. MMSI is recycled and reassigned; IMO number is permanent to the hull.