What AIS transmits
Almost every ship-tracking website, sanctions-screening tool and port-arrival prediction rests on the same underlying data source: the Automatic Identification System. AIS was designed for collision avoidance between ships within VHF range. Everything else it now does is a repurposing, and understanding the difference explains most of the data's quirks.
| Data type | Contents | Update rate |
|---|---|---|
| Static | MMSI, IMO number, call sign, name, type, dimensions | Every few minutes |
| Dynamic | Position, course over ground, speed over ground, heading, rate of turn, navigational status | Seconds to minutes, depending on speed and status |
| Voyage-related | Draught, destination, ETA, cargo type, persons on board | Manually entered; updated as required |
The third category is the important caveat: voyage data is typed in by the crew. Destination fields contain abbreviations, old entries, jokes and blanks. Any analysis built on AIS destination data without cleaning is building on sand.
Terrestrial versus satellite
| Terrestrial AIS | Satellite AIS | |
|---|---|---|
| Range | VHF line of sight, typically up to ~40 nm from a receiver | Global |
| Latency | Near real time | Minutes to hours depending on constellation |
| Coverage gaps | Beyond coastal receivers | Fewer, but message collision in dense areas |
| Cost | Low | Higher |
Satellite reception has a specific limitation: in congested regions, many ships transmit on the same channels within the satellite's large footprint, and messages collide. Coverage of busy areas can therefore be worse per ship than in open ocean.
Where AIS is unreliable — and why it matters
Switched off. AIS may legitimately be switched off where its continued operation would compromise the safety or security of the ship, and this should be logged with reasons. Prolonged unexplained gaps are a recognised sanctions red flag.
Spoofed. Position data can be falsified, producing tracks that are physically implausible — a vessel apparently in two places, or moving faster than it can steam.
Identity manipulation. MMSI reuse, name changes and mismatches between MMSI and IMO number.
Manual field errors. Draught and destination frequently stale or wrong.
For compliance purposes this means AIS is a screening tool, not proof. It flags anomalies that require investigation; it does not by itself establish what a vessel did.
Vessel Traffic Services
Distinct from AIS, VTS are shore-based services in ports and confined waters that monitor and, in some cases, direct traffic. Ships entering a VTS area must report and comply with instructions. VTS uses AIS alongside radar and voice communications.
Confusing AIS with VTS is common; the difference is that AIS is a ship-to-ship and ship-to-shore broadcast, while VTS is an authority providing a service and, at higher levels, giving instructions.
Practical uses in ship management
| Use | Notes |
|---|---|
| Fleet position monitoring | Straightforward; the most common use |
| ETA prediction and port planning | Improve with historic port-time data |
| Counterparty screening | Check vessel behaviour before STS or bunkering |
| Congestion analysis | Waiting time at anchorages, useful for CII planning |
| Emissions estimation | Third-party models use AIS; accuracy varies |
| Voyage classification support | Helpful, but the authoritative record is the ship's own data |
| Own-fleet AIS continuity monitoring | Detect and document legitimate gaps |
That last row is underused. Monitoring your own vessels' AIS continuity, and recording any legitimate transmission stoppage at the time with the reason, protects the fleet against later questions from insurers, banks or regulators.
AIS carriage requirements are set by SOLAS Chapter V. Reliability chart is an indicative model. Reviewed by the Zeaclub Editorial Team, 24 August 2026.
Frequently asked questions
What is AIS?
The Automatic Identification System — a VHF broadcast from ships carrying identity, position, course, speed and voyage information, required by SOLAS for most commercial vessels above defined thresholds.
Can ships legally turn off AIS?
It may be switched off where its operation would compromise the ship's safety or security; the decision and reason should be recorded in the log. Routine or prolonged unexplained gaps attract scrutiny.
Is ship-tracking data accurate?
Position and speed are generally reliable. Draught, destination and ETA are manually entered and frequently stale or wrong. Identity data can be manipulated.
What is the difference between AIS and VTS?
AIS is an automatic broadcast from the ship. VTS is a shore-based traffic service that monitors and can direct vessels in defined areas, using AIS among other tools.