Using radar in restricted visibility

Radar is a valuable source of range-and-bearing evidence in fog, but it only helps when it is switched on, set up and interpreted properly. Check gain, sea clutter, rain clutter, range scale, heading input and alignment before depending on the picture. Scan at a longer range to detect developing situations, then use a suitable shorter range for close contacts; one range scale alone can hide important information.

The display is centred on your vessel. Range rings at fixed intervals (for example 0.5, 1, 2, 4 NM) let you estimate the distance to a target. A bearing cursor or Electronic Bearing Line (EBL) allows you to take the bearing of a contact, and a Variable Range Marker (VRM) gives precise range measurements. Together these let you plot the position and track the movement of other vessels.

Radar has important limitations. Sea clutter — caused by wave reflections close to your vessel — can obscure small targets at short range. Rain clutter produces areas of false echoes on the display, masking real contacts behind a rain shower. Blind sectors may exist behind the mast, rigging, or funnel. Small wooden or GRP vessels without radar reflectors can be very difficult to detect.

A radar reflector can increase your vessel's radar cross-section and improve the chance of being detected by other vessels' radar. Small craft should carry or fit an effective reflector suitable for the vessel, especially for poor visibility and night passages.

ARPA (Automatic Radar Plotting Aid) can track selected contacts and calculate course, speed, CPA (Closest Point of Approach), and TCPA (Time to CPA), but acquisition, heading, speed or target-swap errors can make its output misleading. Rule 7 requires proper use of radar equipment when fitted and operational, including long-range scanning and radar plotting or equivalent systematic observation. Use timed observations, verify the resulting trend and monitor an avoiding action until the vessels are finally past and clear.

Radar does not turn a restricted-visibility encounter into an ordinary sighted crossing. When vessels are not in sight, Rule 19 applies. Avoid assumptions from scant information, use safe speed and engines ready for immediate manoeuvre, listen for fog signals, and combine radar with every other available source rather than steering from a single CPA number.