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How far should my radar be able to see? K.C. via e-mail
There are a number of factors affecting a radar’s effective range, but the primary one is the same “horizon” business that limits our regular vision. While it’s obvious that the horizon gets further away the higher your eye is, there are some specifics to that principal that are useful to boaters—salty, too—because the open-water horizon is mathematically definable. For instance, if your eye is ten feet above sea level, the horizon is 3.7 NM away under clear atmospheric conditions. Hence, as an island you’re steaming toward rises above the horizon, the moment you see waves lapping the beach, you’re 3.7 miles away. This works for the top of the island, too, which is why its height is usually on the chart. It, like you, has its own horizon distance, which you can get from a table or calculate (1.17 times the square root of the height). If that island is 100 feet high, its horizon distance is 11.7 miles. If you add the two horizon distances—3.7 + 11.7 = 14.4 miles—you’ll get the distance where that island top should first peek over your horizon, assuming clear conditions and good eyes.

Radar “sees” essentially the same way, with maximum distance limited by the combined height of the antenna and the target, except that radars can actually see a bit over the visible horizon due to a difference in how microwaves and visible light waves bend. According to Furuno, you can add 15 percent to standard horizon-distance table values or use 1.23 in the formula. Of course you are also limited by how powerful the radar scanner is, not to mention how well the target reflects back microwave pulses. However, if your 48-mile-rated antenna was 100 feet high (big boat!), it would still only be able to see a good 100-foot-high target about 24 miles away due to the horizon effect (12.3+12.3=24.6 miles). That’s why long ranges are mostly useful for tracking rain clouds and high-flying birds.

While we’re on the subject, note that the visibility distance given on charts for navigation lights only describes the brightness of the lights (in normal conditions). You may need to know the heights of the light and your eye to figure out how far away you can see it. In fact, if you do that calculation and catch the light right on the horizon plus get its bearing, you can plot your position without electronics. It’s called “bobbing a light,” and it’s very salty. Finally, the horizon-distance table I’ve been referring to is in American Practical Navigator (a.k.a. Bowditch), and we’ll put a copy of it, along with this Q&A, on our Web site. —B.E.

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