Radar in Restricted Visibility

Radar is one of the most valuable tools in fog. It transmits microwave pulses and displays returning echoes on a screen, showing the range and bearing of targets such as other vessels, land masses, and large buoys. Understanding the basics of radar interpretation is essential at Coastal Skipper level, even if you are not a radar expert.

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 hoisted in your rigging dramatically increases your vessel's radar cross-section, making you visible to other vessels' radar from much further away. Every small craft should carry and display one in restricted visibility.

ARPA (Automatic Radar Plotting Aid) is fitted on many larger yachts and commercial vessels. It automatically tracks radar contacts and calculates their course, speed, CPA (Closest Point of Approach), and TCPA (Time to CPA). Even without ARPA, you can manually plot radar contacts on a plotting sheet at regular intervals to determine their relative motion, assess risk of collision, and decide on avoiding action.