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88 | BoatU.S. Magazine OCTOBER | NOVEMBER 2016
sight,” so the height of your antenna and the height of the target
are most often the limiting factors that determine range. Ready
for a little math? Here’s the equation:
1.2NM x (square root of antenna height in feet) + 1.2NM x
(square root of target height in feet) = Range
An example (don’t worry, we’ll keep the math simple for
now): Your radar sits 16 feet above the water on your boat’s
hard-top, and the vessel you’re looking for stands 16 feet above
the water’s surface. That’s 1. 2 x 4 + 1. 2 x 4, or 4. 8 + 4. 8. No mat-
ter how expensive and powerful your radar may be, it will never
see this other boat until it’s within 9. 6 nautical miles. Period.
If you’re feeling a bit confused at this point, consider that
we haven’t used one single acronym yet. And when it comes
to radar (which, incidentally, stands for RAdio Detection And
Ranging,) there are plenty. Here are the important ones:
Closest Point of Approach. This is the point at which
your boat and a target will be the closest, assuming neither
changes course nor speed.
Electronic Bearing Line. The EBL on a radar allows
you to accurately navigate with a radar and to determine the
exact bearing to different targets.
Mini-Automatic Radar Plotting Aid. MARPA
functions help identify and track a target’s speed, bearing,
CPA, and TCPA and often allows you to associate these with
a proximity alarm.
Time to Closest Point of Approach. TCPA describes
how long it will be before your boat and a target reach CPA,
assuming neither changes course nor speed.
Variable Range Mark. This is exactly what it sounds
like: a marker that enables you to determine the range to different targets.
All of this radar knowledge is great, but about now, there are
undoubtedly people rolling their eyes and groaning. All they
really want to know is how to look at that LCD screen and
distill what’s a channel marker, what’s another boat, and what’s
land. For you folks, investing in a system that overlays your
radar returns on your chartplotter screen is probably a good
move. (See photo page 87.) It eliminates an awful lot of the
guesswork, as long as the overlay doesn’t add to the confusion.
Work with one to see it if works with you.
Beyond that, there are several things to keep in mind. First
off, before you try discriminating between those blips and
blobs, zoom in as much as possible. For most of us, the majority of the time what we’re really concerned about lies within a
mile or two of our boat. Looking at the radar set to a farther
range only reduces the size of the returns you’re concerned
about and adds unnecessary information. Long ranges, however, can detect squalls and enable you to cruise around them,
can detect landfalls, and have other uses, so don’t just set it for
Secondly, don’t view radar on a split screen but instead give