Quick Summary: AIS is an automatic vessel identification and tracking system that improves safety at sea by broadcasting a ship’s position, movement, and key information to nearby vessels and coastal authorities. This guide explains how AIS works and how mariners use it for navigation and collision avoidance.
What AIS Is and Why Mariners Rely on It
The Automatic Identification System, commonly called AIS, is one of the most transformative safety tools on the modern bridge. It provides a continuous stream of information about vessels nearby, including their identity, position, course, speed, and navigational status. Unlike radar, which interprets echoes, AIS broadcasts data directly from one ship to another, making it highly accurate for tracking traffic.
Although AIS does not replace radar or visual watchkeeping, it enhances both. It helps mariners identify ships long before they come into view and understand their intentions with far greater clarity. In congested waters, AIS offers a level of transparency that was unimaginable a few decades ago, allowing navigators to anticipate risks and coordinate movements with confidence.
How AIS Works
AIS functions through a combination of GPS signals, VHF radio frequencies, and transponders installed on vessels. Each AIS-equipped ship broadcasts its own data while receiving the data of others. This creates a shared information network that updates in near real time.
Types of AIS Data
AIS broadcasts two categories of information:
- Static data:
- Ship name
- Call sign
- IMO number
- Type of vessel
- Dimensions
- Antenna location
- Dynamic data:
- Position (latitude and longitude)
- Course over ground
- Speed over ground
- Heading
- Navigational status
- Rate of turn
Together, these data points paint a detailed picture of each vessel’s behavior and movement.
AIS Frequencies and Range
AIS operates on VHF marine channels, allowing signals to travel over 20–40 nautical miles depending on antenna height and weather conditions. Shore-based AIS stations extend this range along coastlines, while satellite AIS systems allow global tracking even in mid-ocean.
AIS Equipment and How It Is Used on Ships
Most merchant ships carry mandatory AIS equipment based on SOLAS requirements. Smaller vessels often use Class B units, while larger commercial vessels use Class A units, which broadcast more frequently and at higher power.
Class A AIS
Used by:
- Tankers
- Cargo ships
- Passenger vessels
- Large offshore units
These units broadcast at short intervals, ensuring highly accurate position updates.
Class B AIS
Used by:
- Yachts
- Fishing vessels
- Smaller commercial craft
These units transmit less frequently and with lower power but still enhance situational awareness significantly.
AIS Display Systems
AIS information appears on:
- ECDIS
- Radar overlays
- Standalone AIS terminals
- Integrated bridge systems
The navigator can see nearby vessels as symbols with accompanying data such as name, speed, heading, CPA, and TCPA.
AIS for Collision Avoidance
Although AIS was not specifically designed as a collision-avoidance tool, it has become indispensable in congested waters.
Identifying Vessels
Radar may show a target as a simple echo. AIS reveals its name, type, size, and navigational status. This allows navigators to contact the ship directly using VHF, reducing misunderstandings during passing arrangements.
Predicting Movement
AIS provides the course and speed of a target with great accuracy. This information enhances the navigator’s ability to assess risk of collision, especially when combined with CPA and TCPA calculations.
CPA and TCPA
AIS systems often display a predicted closest point of approach and the time at which it will occur. This helps navigators take early and decisive action according to COLREGS.
Limitations
AIS supports collision avoidance but does not replace visual lookout or radar. AIS is only one piece of the navigator’s decision-making tools.
AIS and ECDIS Integration
Many vessels display AIS targets directly on ECDIS. This integration creates powerful situational awareness by combining chart data with real-time vessel movement.
Benefits of AIS on ECDIS
- Visualizing traffic in narrow channels
- Monitoring crossing, overtaking, and head-on situations
- Identifying ships in anchorages or port approaches
- Confirming whether observed vessels match charted traffic lanes
Overlays and Layers
Radar overlays, AIS targets, and ENC data together provide a full picture of the maritime environment. A skilled navigator uses all layers without relying on one exclusively.
AIS in Restricted Visibility
AIS becomes especially valuable when visibility drops due to fog, heavy rain, or darkness.
Persistent Tracking
Even when a vessel disappears from sight or radar echoes weaken, AIS continues to broadcast its position clearly.
Decision-Making in Fog
Navigators compare AIS data with radar information to validate movement patterns. This dual-system approach prevents errors when one system struggles due to weather.
AIS for Vessel Identification
One of AIS’s greatest advantages is vessel identification. Before AIS, mariners often relied on binoculars, radar reflections, or call signs obtained through VHF. Now, identification is immediate.
Why Identification Matters
- Helps determine vessel intentions
- Confirms whether a ship is fishing, constrained, or restricted in ability to maneuver
- Simplifies communication for collision-avoidance arrangements
- Allows early planning of passing distances
Limitations of AIS
Despite its usefulness, AIS is not perfect. Mariners must understand its limitations to use it safely.
Human Input Errors
AIS relies on manually entered data such as voyage information and navigational status. Incorrect entries can create serious misunderstandings on the bridge.
Signal Loss
VHF range limits may cause temporary loss of targets, especially behind large structures or in heavy seas.
AIS Spoofing
Some vessels may intentionally broadcast false information, particularly in conflict zones or areas prone to illegal activity.
Over-Reliance
The most common error is treating AIS as a complete solution. It is a tool, not a substitute for radar, visual lookout, or seamanship.
AIS in Search and Rescue
AIS plays an important role in emergency operations.
AIS-SART Devices
Search and rescue transponders use AIS signals to broadcast their position, allowing ships to locate lifeboats, life rafts, or persons in distress with remarkable accuracy.
MOB Devices
Some vessels and companies equip crew with personal AIS MOB beacons. When activated, they mark a man overboard’s location on nearby AIS displays, greatly improving recovery chances.
Future of AIS Technology
AIS continues to evolve with satellite integration, improved coverage, and advanced analytics. Future developments may include:
- Global real-time vessel layer overlays
- Enhanced anti-spoofing measures
- Automatic voyage reporting systems
- Hybrid radar-AIS tracking algorithms
AIS remains a cornerstone of modern navigation, and future generations of mariners will depend on it even more.
Conclusion
AIS has become an essential navigation tool aboard modern ships. It improves situational awareness, enhances collision avoidance, and strengthens communication between vessels. When combined with radar, ECDIS, and traditional seamanship, AIS provides a powerful layer of safety and confidence for mariners navigating busy waterways or open seas. Understanding AIS—its strengths, limitations, and proper use—is an essential skill for every watchkeeper on today’s bridge.
