Understanding where AIS data is available, its accuracy, and the limitations you should consider when building maritime applications.
AIS data is collected through two primary methods, each with distinct characteristics.
Coastal areas & busy shipping lanes
Range
Up to ~50km (27 nm)
Latency
Near real-time
Cost
Lower
Worldwide including open ocean
Range
Global
Latency
Minutes to hours
Cost
Higher
Data Docked provides both terrestrial and satellite AIS data combined into a single unified feed, giving you the best of both worlds for comprehensive vessel tracking.
How often vessels transmit AIS data depends on their speed, whether they're turning, and their AIS class.
| Condition | Class A | Class B |
|---|---|---|
| At anchor or moored | 3 minutes | 3 minutes |
| Speed 0-2 knots | 10 seconds | 3 minutes |
| Speed 2-14 knots | 10 seconds | 30 seconds |
| Speed 14-23 knots | 6 seconds | 30 seconds |
| Speed >23 knots | 2 seconds | 30 seconds |
| Changing course | 2 seconds | 30 seconds |
Class A transponders are required for SOLAS vessels. Class B is typically used by smaller commercial vessels and recreational craft.
Understanding these limitations is crucial for building reliable maritime applications.
Vessels can disable their AIS transponders, intentionally or unintentionally. This is called "going dark" and is common in illegal fishing, sanctions evasion, and piracy.
AIS is only mandatory for SOLAS vessels (300+ GT international, 500+ GT domestic) and passenger vessels. Many small craft, fishing boats, and leisure vessels don't carry AIS.
Vessel information (name, destination, ETA) is manually entered by crew and often contains errors, typos, or outdated information. Navigation status is frequently incorrect.
AIS signals can be spoofed to show false positions or vessel identities. While detectable with analysis, this remains a security concern in some regions.
Terrestrial AIS only works near shore. Satellite AIS has coverage gaps due to satellite orbits, with potential delays of hours between position updates in open ocean.
In very busy areas like anchorages, too many vessels transmitting can cause message collisions and data loss. Weather and atmospheric conditions can also affect reception.
Not all vessels are required to carry AIS transponders. Understanding the requirements helps set proper expectations.
Recommendations for building robust applications that account for AIS limitations.
Cross-reference vessel names, IMO numbers, and dimensions with authoritative databases like IHS Markit or Lloyd's.
Use speed, position changes, and proximity to ports/anchorages to infer actual vessel state.
Implement interpolation for position gaps and alert users when data is stale or missing.
Terrestrial data is more reliable near coast; satellite data fills gaps but may have delays.
Sudden position jumps, impossible speeds, or vessels appearing inland may indicate spoofing or equipment issues.
Access combined terrestrial and satellite AIS data through our unified API. Start with 20 free credits.