How to Use AIS: Automatic Identification System for Sailors

The Automatic Identification System (AIS) is a VHF-based tracking and identification system that has become one of the most valuable collision-avoidance and situational-awareness tools available to recreational sailors. Originally developed for large commercial vessels under SOLAS regulations, AIS now appears on everything from offshore cruising yachts to daysailers, either as a receive-only receiver or a full transponder. This article explains what AIS is, how it works, how to use it effectively on a coastal passage, and where its limits lie.

Main Explanation

What it is

AIS is an automated system that uses VHF radio frequencies to broadcast a vessel’s identity, position, course, and speed, and to receive the same information from other AIS-equipped vessels and shore stations. It is not tied to any single manufacturer or boat brand; it is an international standard, with carriage requirements set by the International Maritime Organization (IMO) under SOLAS for cargo ships over 300 gross tons and all passenger vessels. For recreational boats, AIS is optional but increasingly common. Two main hardware types exist: Class A transponders (required on SOLAS vessels, higher power, more frequent reporting) and Class B transponders (designed for recreational and smaller commercial craft, lower power, slightly less frequent reporting). A third option is a receive-only AIS, which listens to AIS traffic but does not transmit your own vessel’s data. According to Sailboats.fr, AIS can ‘see’ around corners and over the horizon, picking up vessels whose signals bounce off the atmosphere or relay through satellite networks, giving sailors a layer of awareness that radar alone cannot match.

Why it matters

In busy shipping lanes, restricted visibility, or nighttime coastal passages, AIS reduces the uncertainty of ‘what is that ship doing?’ Instead of guessing a vessel’s name, course, speed, and closest point of approach (CPA), AIS displays it directly on a chartplotter or dedicated screen. RepairYachts notes that for under $1,500 installed, a cruiser can see every commercial ship and AIS-equipped vessel within VHF range, with exact heading, speed, name, and CPA calculations laid on top of the chart. This matters because poor decisions at sea often stem from incomplete or outdated information. AIS provides standardized, real-time data that is independent of a single school, brand, or manual. It helps sailors evaluate traffic, make early course changes, and call a specific vessel by name or MMSI on VHF—something that is far more effective than a generic ‘cargo vessel astern’ call.

How it works

An AIS transponder takes position data from a GPS receiver, combines it with static information (MMSI number, vessel name, type, dimensions) and voyage-related data (destination, draft, status), and broadcasts it over two dedicated VHF channels. Other AIS units receive these broadcasts and plot the targets on a chartplotter, multifunction display, or mobile app. Key data fields include MMSI, position, speed over ground (SOG), course over ground (COG), rate of turn, and CPA/TCPA (time to closest point of approach). Modern units can set alarms for CPA/TCPA thresholds, anchor watch, and lost targets. In practice, a sailor integrates AIS with the chartplotter, sets a guard zone or alarm, and uses the target list to identify and call vessels. AIS is not a navigation system; it is a collision-avoidance and identification aid. It works alongside radar, visual lookout, and VHF radio.

How to do it

For a typical coastal passage, use AIS as part of a layered watch routine. Before departure, confirm the AIS unit has a valid GPS fix and that your MMSI and vessel data are correctly programmed. Set CPA and TCPA alarms to values appropriate for the conditions—for example, 0.5 nm and 12 minutes in open water, tighter in confined waters. While underway, monitor the AIS target list and chartplotter overlay. When a target appears, note its MMSI, name, course, and speed. If a risk of collision exists, use the target’s name or MMSI to call on VHF channel 16 or the working channel. Cross-check the AIS picture with radar and a visual bearing. If you have a Class B transponder, remember that other vessels can see you, but not all will; never assume you are visible. Log any close-quarters situations and review the AIS data after the passage to improve your decision-making.

When not to do it

AIS has clear limits. Not all vessels carry AIS—small fishing boats, some recreational craft, and vessels with faulty or switched-off units may not appear. AIS data can be stale or incorrect if a vessel’s GPS is degraded or its voyage data is not updated. VHF range is typically line-of-sight, so AIS may not show vessels behind a headland or beyond about 20–30 nm depending on antenna height. AIS is not a substitute for a proper visual lookout under COLREGS Rule 5, nor for radar in poor visibility. Do not rely solely on AIS for navigation, weather routing, or evaluating your boat’s structural capabilities. For those tasks, use official charts, weather forecasts, a certified marine surveyor, or professional training. A human weather router or sailing instructor remains more appropriate for complex routing or skill development.

Visual

Common Mistakes

Assuming every vessel appears on AIS

Consequence: You may miss small fishing boats, recreational craft without AIS, or vessels with AIS switched off. Correct approach: Always maintain a visual lookout and use radar if available; treat AIS as one layer, not the whole picture.

Over-relying on AIS and neglecting visual lookout or radar

Consequence: AIS can fail, lose GPS, or show stale data; a target may not be where the screen says. Correct approach: Cross-check AIS targets with visual bearings and radar; follow COLREGS Rule 5.

Setting CPA/TCPA alarms too low or ignoring them

Consequence: You may not have enough time to maneuver in a crossing situation. Correct approach: Set alarms conservatively for the conditions and respond early; do not silence alarms without checking the target.

Using a receive-only AIS and assuming others see you

Consequence: Other vessels cannot see your AIS signal, so they may not know you are there. Correct approach: If you want to be seen, install a Class B transponder; otherwise, assume you are invisible to AIS-equipped ships.

Not updating voyage data or MMSI

Consequence: Your vessel may display incorrect name, destination, or status, confusing other vessels and reducing the usefulness of AIS. Correct approach: Program the MMSI and static data correctly during installation; update voyage data before each passage.

Confusing AIS with a navigation system

Consequence: AIS does not provide depth, charted hazards, or weather; using it as primary navigation can lead to grounding or other incidents. Correct approach: Use AIS only for traffic identification and collision avoidance; navigate with official charts and a properly calibrated chartplotter.

Safety Note

AIS is a powerful aid, but it must fit into a broader safety framework. Always cross-check critical data—weather, tides, navigation—with official sources such as national weather services, coast guard broadcasts, and up-to-date charts. Never use a tablet or phone as your only navigation tool; keep a dedicated chartplotter and paper charts as backup. Take a certified safety and seamanship course, and practice using AIS in good conditions before relying on it in fog or heavy traffic. Remember that AIS does not relieve you of the obligation to maintain a proper lookout under COLREGS Rule 5, nor does it replace sound judgment. If you are unsure about a crossing situation, slow down, alter course early, and communicate by VHF. AIS is a tool to help you make better decisions—it is not a substitute for training, experience, and vigilance.