How Reliable Are AIS Collision‑Avoidance Alarms in Practice?

AIS collision‑avoidance alarms are undeniably useful, but they’re nowhere near infallible. They’re a decision‑support tool, not a guardian angel. When you understand how the alarms are generated—and where the weak points lie—you can tune your system to give you timely, meaningful alerts without being driven mad by nuisance beeps.

Understanding How AIS Collision Alarms Actually Work
Why AIS Alarms Generate False or Missed Alerts
The CPA/TCPA Settings Cruisers Should Adjust
Vessel‑Type Filtering and “Dangerous Target” Logic
Offshore vs Coastal: How to Tune AIS Alerts for Each Environment
AIS vs Radar: Strengths, Weaknesses, and When Each Wins
Practical Watchkeeping Tips to Reduce Alarm Fatigue
Key Takeaways for Cruising Sailors

Understanding How AIS Collision Alarms Actually Work

AIS collision‑avoidance alarms are built around two simple calculations:

CPA (Closest Point of Approach) — the predicted minimum distance between your vessel and another
TCPA (Time to CPA) — how long until that closest point occurs

Your AIS or plotter continuously projects both vessels’ current course and speed forward in time. If the predicted CPA falls below your chosen threshold and the TCPA is within your alert window, the system triggers an alarm.

This sounds wonderfully precise, but it relies on one huge assumption: that both vessels will continue on their current course and speed.

In the real world, that assumption is often wrong.

Why AIS Alarms Generate False or Missed Alerts

AIS alarms fail for three main reasons:

1. The other vessel’s data is imperfect

AIS transmissions can be delayed, incomplete, or simply wrong. Common issues include:

GPS position jumps
Incorrect heading or rate‑of‑turn data
Slow update rates on Class B units
Antenna shadowing on large ships

If the input is flawed, the CPA/TCPA calculation will be flawed.

2. Course and speed change constantly

A ship altering course by just a few degrees can flip a “safe” target into a “dangerous” one—or vice versa. Fishing boats, tugs, ferries, and yachts under sail are especially unpredictable.

3. Default thresholds rarely suit real cruising

Manufacturers tend to set conservative defaults to avoid liability. That means:

Offshore: alarms trigger far too early
Coastal/traffic separation zones: alarms trigger far too late

Unless you tune the thresholds, you’ll either get alarm fatigue or miss the one alert that matters.

The CPA/TCPA Settings Cruisers Should Adjust

These are the settings that make the biggest difference to real‑world reliability.

CPA (Closest Point of Approach)

A good starting point:

Offshore: 1–2 NM
Coastal: 0.5–1 NM
Confined waters: 0.2–0.5 NM

TCPA (Time to CPA)

This determines how early the alarm fires.

Offshore: 20–30 minutes
Coastal: 10–15 minutes
Confined waters: 5–10 minutes

Alarm behaviour

Most systems let you choose:

Visual only
Visual + audible
Continuous audible until acknowledged

For passagemaking, I prefer visual + audible, but not continuous. You want to be alerted—not tortured.

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Vessel‑Type Filtering and “Dangerous Target” Logic

Modern AIS units allow you to filter alerts by vessel type. This is one of the most effective ways to reduce nuisance alarms.

Useful filters include:

Ignore Class B targets (in busy anchorages or marinas)
Prioritise Class A ships (offshore and in shipping lanes)
Suppress alerts for anchored or moored vessels
Highlight fast‑moving craft (ferries, pilot boats, fast fishing vessels)

“Dangerous Target” logic

Some AIS units apply additional logic beyond CPA/TCPA, such as:

Rate‑of‑turn
Speed thresholds
Whether the vessel is overtaking or crossing
Whether the CPA is decreasing or increasing

Enabling this logic dramatically reduces false alarms from vessels that look threatening but are actually diverging.

Offshore vs Coastal: How to Tune AIS Alerts for Each Environment

Offshore Sailing

Offshore, you want early warning without constant nuisance alarms.

Recommended approach:

Setting	Recommended Offshore Value
CPA	1–2 NM
TCPA	20–30 minutes
Vessel filtering	Prioritise Class A ships
Alarm type	Visual + audible

Coastal or Confined Waters

Here you need precision, not early warning.

Setting	Recommended Coastal Value
CPA	0.5–1 NM
TCPA	10–15 minutes
Vessel filtering	Highlight fast movers
Alarm type	Visual + audible

AIS vs Radar: Strengths, Weaknesses, and When Each Wins

AIS is brilliant for identifying ships, predicting CPA, and understanding traffic patterns. But it has blind spots.

AIS strengths

Identifies vessels by name, MMSI, size, and type
Predicts CPA/TCPA
Works in fog, rain, and darkness
Excellent for long‑range awareness

AIS weaknesses

Not all vessels transmit AIS
Data can be wrong or delayed
Small craft often have poor antenna installations
Course changes invalidate predictions instantly

Radar strengths

Sees everything, including unlit or non‑AIS vessels
Shows real‑time movement
Works even when AIS fails

Radar weaknesses

Requires interpretation
Clutter in rain or heavy seas
Range and resolution depend on scanner size

The golden rule

AIS tells you who and what; radar tells you where and now. Neither replaces a proper visual watch.

Practical Watchkeeping Tips to Reduce Alarm Fatigue

Treat every AIS alarm as a prompt to look, not to panic

Cross‑check with radar when available

Don’t rely on AIS alone for collision avoidance

Keep CPA/TCPA thresholds appropriate to the environment

Use vessel‑type filters intelligently

Remember that fishing boats, tugs, and yachts under sail are the least predictable targets

Key Takeaways for Cruising Sailors

AIS collision‑avoidance alarms are a powerful aid, but they’re only as good as the data they receive and the settings you choose. With sensible tuning—especially CPA/TCPA thresholds and vessel‑type filtering—you can dramatically reduce nuisance alerts while still getting timely warnings when they matter.

AIS should enhance your situational awareness, not replace it. The best defence against collision remains the same as ever: a proper watch, a clear head, and a willingness to look outside the boat.

This article was written by Dick McClary, RYA Yachtmaster and author of the RYA publications 'Offshore Sailing' and 'Fishing Afloat', member of The Yachting Journalists Association (YJA), erstwhile member of the Ocean Cruising Club (OCC) and owner/administrator of 'Liveaboard Sailboats for Sale'.