Some things produce echoes we really don’t want. Sea water, for instance, is a superb reflector, so the center of the radar screen is often packed with big bright blobs caused by radar energy being reflected from waves around the boat.
The sea-clutter control can remove these blobs, but it’s a crude tool. Remember, your radar is only receiving echoes; it doesn’t know what creates them. So the sea-clutter control works by suppressing all echoes near the center of the screen. If turned up too high, it will remove boats, buoys, and even land.
As a rule of thumb, turn down the sea-clutter control to its minimum. (This is always my first move in my “beat the system” game because at least I can then be sure that it is not doing any harm.) If sea clutter becomes a problem, you can increase it a little at a time but leave at least a trace of clutter at the center.
Rain-clutter controls are more sophisticated, but nearly as problematic. They remove the smudgy contacts from rain clouds by weakening the drawn-out echoes the clouds produce. But these controls also weaken the echoes from genuine targets, especially the drawn-out echoes received from sloping coastlines.
Some radars even have two rain-clutter controls, in which case one usually affects only the area around the middle of the screen while the other (often called “FTC”) affects the whole screen. Whether you have one rain-clutter control or two—or even the increasingly old-fashioned “on/off” control—the rule of thumb is the same as for sea clutter: turn it off when you don’t need it and use it as sparingly as possible when you do.
The last clutter control is different. Usually called “interference rejection,” it uses clever processing to reject visual clutter that can occur when your radar receives other radar transmissions. It doesn’t really enhance your radar image, so most recent radars have it switched on all the time and give you no option to switch it off. Even if you can switch it off, there’s no point.
This article originally appeared in the August 2009 issue of Power & Motoryacht magazine.