Cruising

Can You See Me Now?

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Many cruisers believe it’s best to locate a radar reflector as high as possible, perhaps even at the top of the mast, for better detection. However, the key issue for a radar reflector is the water-surface reflection, which affects the strength of the reflected signal. When the reflection is increased, the reflector is more apt to be seen. If reflection is reduced, a reflector could become invisible to radar.

How large a reflector appears to a marine radar depends on three things: the height of the radar antenna, the height of the reflector, and the distance between the two. For example, a 40-foot sailboat with no reflector should be detected by a low-power radar on a small powerboat at 0.7 mile, by a mast-mounted low-power radar on a sailboat at 2 miles, and by a ship with high-power radar at 6 miles. A typical reflector mounted 16 feet above the water increases these detection ranges to 1.7 miles, 4.6 miles, and 11.5 miles, respectively.

When approaching the small powerboat, the target sailboat passes through alternating zones in which it is visible or invisible to the low-power radar. If the target’s reflector is mounted at the masthead—say, 50 feet above the surface of the water—the detection range increases from 3.5 miles to 5.4 miles. But the reflector can become invisible at a range between 2.2 and 2.7 miles. The high-power radar on the ship would first see the target sailboat at about 15 miles, but not between 9.5 and 11 miles out or between 6.6 and 7 miles out. Both blind spots, or zones, would be eliminated if the target’s reflector was just 16 feet above the water.

While a masthead-mounted reflector does increase maximum detection range, it can also make a boat invisible to a high-power radar over 25 percent of the total detection area created by a reflector mounted 16 feet above the water. In other words, mounting a reflector higher increases the maximum range at which it can be detected, but the reflector will be invisible to a radar in significantly wide areas. Being detected at long range for a short period of time is nice, but being invisible periodically at shorter ranges might be detrimental in a closing situation. This is why mounting a reflector 16 feet above the water produces the best combination of maximum detection range and minimum blind zones. Phillip Gallman

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