AIS Coverage & Limitations
Understanding where AIS data is available, its accuracy, and the limitations you should consider when building maritime applications.
Terrestrial vs Satellite AIS
AIS data is collected through two primary methods, each with distinct characteristics.
Terrestrial AIS
Coastal areas & busy shipping lanes
Range
15-50 nautical miles
Latency
< 1 second
Cost
Lower
Advantages
- Real-time data
- High accuracy
- Low latency
- Complete message reception
Limitations
- Limited to coastal areas
- No coverage in open ocean
- Depends on station density
Satellite AIS (S-AIS)
Worldwide including open ocean
Range
Global
Latency
Minutes to hours
Cost
Higher
Advantages
- Global coverage
- Open ocean tracking
- No shore infrastructure needed
Limitations
- Higher latency
- Message collision in dense areas
- Higher cost
- Update gaps
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.
AIS Transmission Frequency
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.
Known Limitations
Understanding these limitations is crucial for building reliable maritime applications.
AIS Can Be Turned Off
Vessels can disable their AIS transponders, intentionally or unintentionally. This is called "going dark" and is common in illegal fishing, sanctions evasion, and piracy.
Not All Vessels Required
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.
Data Quality Issues
Vessel information (name, destination, ETA) is manually entered by crew and often contains errors, typos, or outdated information. Navigation status is frequently incorrect.
Spoofing & Manipulation
AIS signals can be spoofed to show false positions or vessel identities. While detectable with analysis, this remains a security concern in some regions.
Coverage Gaps
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.
Signal Interference
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.
Which Vessels Carry AIS?
Not all vessels are required to carry AIS transponders. Understanding the requirements helps set proper expectations.
Required by SOLAS
- All ships of 300 GT and upwards on international voyages
- Cargo ships of 500 GT and upwards not on international voyages
- All passenger ships regardless of size
Required by Flag State
- Fishing vessels (varies by country, often >15m)
- Domestic commercial vessels
- High-speed craft
Not Required
- Recreational vessels (may carry voluntarily)
- Military vessels (exempt from SOLAS)
- Small fishing boats under national thresholds
- Government vessels on non-commercial service
Best Practices for Using AIS Data
Recommendations for building robust applications that account for AIS limitations.
Validate static vessel data
Cross-reference vessel names, IMO numbers, and dimensions with authoritative databases like IHS Markit or Lloyd's.
Don't rely solely on navigation status
Use speed, position changes, and proximity to ports/anchorages to infer actual vessel state.
Handle data gaps gracefully
Implement interpolation for position gaps and alert users when data is stale or missing.
Consider the data source
Terrestrial data is more reliable near coast; satellite data fills gaps but may have delays.
Watch for anomalies
Sudden position jumps, impossible speeds, or vessels appearing inland may indicate spoofing or equipment issues.
Get Comprehensive AIS Coverage
Access combined terrestrial and satellite AIS data through our unified API. Start with 20 free credits.