Module 15 - Restricted Visibility
Using radar in restricted visibility
What you will learn
Set up, interpret and monitor radar as systematic range-and-bearing evidence while accounting for range scale, clutter, blind sectors, acquisition time and own-ship input errors.
Radar supports early detection, navigation and collision-risk assessment only when the operator understands the current picture and keeps testing it. A crisp number is not automatically an accurate solution.
Before depending on the display, check that the radar is transmitting and appropriately tuned, the picture is aligned, heading and speed inputs are suitable and the operator understands the presentation and stabilisation. Adjust gain, sea and rain controls without erasing weak targets. Use longer ranges periodically for early detection and suitable shorter ranges for close contacts; one range scale can hide either the developing picture or nearby weak echoes.
A single range and bearing gives no relative track. Make a succession of systematic observations over known time or use a properly operating tracking aid. ARPA or ATA needs acquisition and smoothing time—MCA guidance notes that full smoothed accuracy can take up to about three minutes—and wrong heading, speed, target association or a target manoeuvre can make the displayed vector, CPA and TCPA misleading. Keep the raw echoes and trends under review.
Sea clutter, rain clutter, shadow or blind sectors, small target size, heel and rough conditions can hide real contacts. A correctly installed radar reflector or target enhancer can improve a small yacht's probability of detection but cannot guarantee it; SOLAS V requires a reflector or other means on vessels under 150 GT where practicable. Continue lookout, sound signals, safe speed and Rule 19 decisions even when the yacht carries one.
| Control or feature | Use | Failure to guard against |
|---|---|---|
| Long and short ranges | Early detection and close weak-target search | One scale hiding part of the picture |
| Gain and clutter controls | Make useful echoes visible among noise | Over-suppression or masked targets |
| EBL, VRM and systematic observations | Track bearing and range change over time | Conclusion from one snapshot |
| ARPA or ATA | Automate target tracking and CPA/TCPA calculation | Acquisition, input, swap and manoeuvre errors |
| Radar reflector or enhancer | Improve probability of another radar detecting the yacht | Assuming guaranteed return in heel, rain or sea clutter |
Challenge an attractive ARPA solution
A newly acquired contact shows CPA 0.8 M after forty seconds, while the raw echo closes quickly and the heading sensor has just restarted after a power interruption.
1. Distrust premature precision
Treat the early vector as unstable until acquisition time and valid own-ship inputs support it.
2. Observe the raw evidence
Take repeated ranges and bearings and check the echo on an appropriate longer and shorter range.
3. Check the inputs
Verify heading, speed, alignment, target association and whether clutter controls are suppressing another contact.
4. Protect the decision
Use safe speed and sea room while the solution matures; do not wait for a neat vector if the raw trend already indicates risk.
Sense check: A larger displayed CPA is not reassuring when the sensor chain or target track that produced it is still unreliable.
Common mistake or limitation
- Leaving radar on one range scale and assuming the same settings reveal both distant developing traffic and weak close targets.
- Treating the first ARPA vector as settled instead of allowing acquisition time and checking own heading, speed and target association.
- Assuming clutter controls can remove rain or sea returns without also weakening a real small target.
- Treating a radar reflector as a guarantee that another vessel will detect and correctly interpret the yacht.
Recap
- Tune and align the radar, use appropriate ranges and understand the current sensor inputs.
- Risk comes from systematic observations, not one echo or an immature digital vector.
- Clutter, blind sectors and weak targets remain; a reflector improves probability rather than guaranteeing detection.
Optional quick check
Section 6 of 8
An ARPA target acquired forty seconds ago shows a comfortable CPA after the heading sensor restarted. What should the navigator do?
Sources and factual review
Reviewed 2 September 2026 - Compass Revision editorial review.
Practice restricted-visibility decisions
The complete module connects radar evidence, Rule 19, fog signals, AIS limits and practical precautions.