Module 19 - Electronic Navigation

AIS CPA and Limitations

What you will learn

Use AIS identity and motion data as supplementary evidence while preserving lookout, radar assessment and Rule 7 responsibility.

AIS can add identity, reported position and motion information, but it is not a complete traffic picture and the received data are only as good as the transmitting vessel's sensors and entries.

AIS broadcasts a vessel's identity, position, course, speed, and other data. Plotters can calculate CPA and TCPA from AIS targets, making AIS extremely useful for identifying ships and planning early avoiding action.

AIS has limitations. Not all vessels carry it, not all units transmit accurate data, small craft may use lower-power Class B units, and data can be delayed or filtered. AIS is GPS-derived, so it shares GNSS vulnerabilities. It must complement lookout and radar, not replace them.

Challenge an attractive AIS target

An AIS target displays a reassuring name, destination and CPA, while radar shows a different relative track and another small echo has no AIS label.

1. Keep the lookout
Continue visual, auditory and radar observation; not every vessel carries or transmits AIS.
2. Check the data
Separate automatically sensed position and motion from manually entered status, destination or dimensions.
3. Compare motion
Compare AIS, radar and visual trends, accounting for update rates, heading, course and target manoeuvres.
4. Apply Rule 7
Use all available appropriate means and do not make AIS the sole collision-risk system.

Sense check: A named target can still transmit wrong data, and an unnamed radar or visual target can still be the most immediate risk.

Common mistake or limitation

AIS is not a complete picture because some craft are not fitted, may be exempt or may have AIS switched off.
Manually entered status, destination and dimensions may be stale or wrong.
AIS position, course and speed depend on the target's sensors and can carry their errors.
AIS does not remove the Rule 7 duty to use all available appropriate means.

Recap

Use AIS to supplement identification and motion assessment.
Expect missing, delayed, stale or erroneous target information.
Compare AIS with radar, visual and other safety-relevant evidence.
Never make AIS the sole collision-avoidance information system.

Optional quick check

Section 9 of 18

A radar echo has no AIS target. What is the safest interpretation?

Sources and factual review

Reviewed 20 August 2026 - Compass Revision editorial review.

Continue studying Electronic Navigation

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