AIS data 101 for traders

What AIS is

AIS — Automatic Identification System — is a VHF radio protocol developed in the late 1990s primarily for collision avoidance. Every ship over 300 gross tonnes engaged in international voyages is required by the SOLAS treaty to carry an AIS transponder that broadcasts the vessel's identity, position, course, speed, and other navigational data every few seconds. Smaller vessels carry it voluntarily or by national regulation.

The signal is open: any nearby AIS receiver — a coastal antenna, another ship, or a low-earth-orbit satellite — can pick it up and decode it. There is no central authority that receives or distributes AIS data. The market for AIS data is the market for aggregation: running enough receivers to have global coverage, cleaning the data, classifying vessels, and reselling.

What each AIS message contains

Two message types matter for traders:

• Position reports (Type 1-3, broadcast every 2-10 seconds when underway, every 3 minutes at anchor): MMSI identifier, latitude/longitude, course over ground, speed over ground, heading, rate of turn, and navigational status (underway, at anchor, moored, aground, etc.).
• Static and voyage data (Type 5, broadcast every 6 minutes): the vessel's name, IMO number, ship type, dimensions, draught at the moment of transmission, destination port, and estimated time of arrival. This is the slot where vessel class (tanker, container, LNG, bulk, passenger) actually shows up.

Critically: the static data is self-reported by the vessel's master. There is no verification mechanism. A tanker captain typing the wrong ship type into the transponder is a real, common cause of misclassification.

Terrestrial vs satellite AIS

Terrestrial AIS uses land-based VHF receivers along coastlines. Range is roughly 40-60 nautical miles, depending on antenna height and atmospheric conditions. It's cheap, real-time, and dense in port areas — but useless for open-water tracking. Every major port and coastline has terrestrial coverage. Hormuz, Bab el-Mandeb, the Singapore Strait, and the Suez approaches are well-covered terrestrially because of their proximity to land.

Satellite AIS uses LEO satellites equipped with VHF receivers. Coverage is global but per-vessel revisit time is roughly 30-90 minutes. Satellite AIS is essential for tracking vessels mid-ocean — Indian Ocean, mid-Atlantic, Pacific. It's also significantly more expensive: there are only ~4 commercial satellite AIS providers globally (Spire, exactEarth/ORBCOMM, MarineTraffic infrastructure, Saab/Kongsberg), and most data resellers license from one or more of them.

For port-centric analysis (vessel counts at Houston, Singapore, Rotterdam, etc.), terrestrial is sufficient. For chokepoint analysis where vessels may be partially out at sea, you need satellite. For open-ocean route tracking — say, Russian crude flows to India around the Cape of Good Hope — satellite is mandatory.

What AIS can't tell you

• What's actually in the hold. AIS broadcasts ship type, not cargo. Cargo information comes from manifests, bills of lading, port records — all of which are not publicly available in real time. The closest AIS proxy for "is this tanker laden or ballast" is reported draught, which is master-supplied and often unreliable.
• Trade intent. A tanker reporting destination "ROTTERDAM" might actually discharge at Antwerp or change destination mid-voyage. AIS destination fields are notoriously unreliable.
• Anything when AIS is off. Vessels can legally turn off AIS in specific circumstances (entering certain ports, transiting piracy zones). Some vessels — sanctioned Iranian, Russian, and Venezuelan tankers especially — turn it off illegally to hide activity. AIS gaps are the dark fleet signal.
• Spoofing. A vessel can broadcast false MMSI, false position, or false identity. Sophisticated spoofing has been documented for sanctioned trades. Cross-checking AIS against satellite imagery is the only fully reliable defense, and it's expensive.

Why vessel classifications are sometimes "Unknown"

On any AIS data feed, a meaningful percentage of vessels — sometimes 30-60% — will be classified as "Unknown" ship type. This isn't a data bug. It's an artifact of how AIS works.

Position reports (Type 1-3) are broadcast every few seconds, but static data with ship type (Type 5) is only broadcast every 6 minutes. A receiver that picks up several position reports but no static data within its window simply doesn't know what the vessel is. Small vessels and harbor traffic often broadcast static data even less frequently. Older transponders have firmware bugs. Some vessels never set the ship-type field at all.

Commercial AIS aggregators reduce the unknown rate by cross-referencing MMSI against ship registry databases (IHS Markit, Lloyd's Register, Equasis). HarborSignal's current feed relies primarily on the in-band ship-type field, which is why our unknown rate is higher than premium services. The data we do have classified is accurate; the unknown rate just reflects coverage gaps rather than wrong information.

Practical implications for trading

AIS is a real-time approximation of physical reality, not a perfect feed. Use it for:

• Anomaly detection. Compare current vessel counts to a rolling baseline. Spikes and drops are real signals even if absolute counts are imperfect.
• Trend confirmation. Sustained rises in tanker traffic at an export hub, corroborated by EIA inventory drawdowns, is a strong joint signal.
• Chokepoint stress. Transit count drops at Hormuz or Bab el-Mandeb are immediately tradeable on Brent and tanker equities.

Don't use it for:

• Absolute throughput numbers. Use port authority statistics or EIA data for that.
• Cargo composition. AIS doesn't know.
• Sanctioned vessel tracking without cross-references. AIS gaps are clues, not conclusions.

What HarborSignal shows

Live AIS-derived vessel counts and class breakdowns are on every port detail page (e.g. Houston, Singapore). The Map view shows individual vessel positions with trail history. The Hormuz page includes a 24-hour AIS gap counter as a proxy for dark fleet activity. The data is cross-checked daily against VesselFinder's published arrival counts — a separate AIS aggregator — to validate accuracy.