From DIY Boat Owner Magazine
Get A Fix On Radar
By David Anderson
Practice makes perfect when it comes to accurately
interpreting range and bearing of landmarks and boats
on your radar screen. If you have radar onboard, follow
these tips to maximize your navigational skills
earning to take full advantage of radar means you’ll need to understand how it works, how to use its basic functions, and how to incorporate that knowledge into routine navigation. Radar “sees” by sending out microwave pulses, and detecting the pulses that are reflected
back from objects, called “targets,” around your boat. What it is not
is a television camera. On the radar screen, the user sees only blips or echoes of
the targets, not realistic representations. Consequently, it takes practice to read a
radar screen and to interpret what’s really out there. And, arguably, an accurate
understanding of the images you see on-screen, especially the moving targets, is
the most important aspect of using radar.
Teach yourself to better read those blips by practicing in good visibility.
Compare how nature, your charts, and/or chart plotter, and the radar image fit
together when you can see what’s around you with the naked eye. You’ll find
that there’s usually quite a lot missing in the radar image, owing to the two-dimensional illumination of the surroundings with your boat at the center of
Just how far away will your radar be able to see those targets? Antenna
and target heights are the key. Don’t judge by the unit’s maximum-range scale,
because when a target is over the “radar horizon,” you won’t see it, no matter
how much power you’re broadcasting. Radar range is slightly farther than visual
or geographic range due to the refraction of microwaves, but it still can’t see over
the radar horizon, which can be calculated as follows:
[ 1.2NM x √ antenna height (in feet)] + [ 1.2NM x √ target height (in feet)]
For example, if your antenna is mounted at a height of 12 feet above the
water and you’re looking for a vessel that is 25 feet high, the formula to deter-
mine radar range will be [ 1. 2 x 3] + [ 1. 2 x 5] = 3. 6 + 6 = 9. 6. Until it’s within
9. 6 nautical miles — even if you have a unit that ranges out to 24 or 36 miles
— the target won’t appear on-screen.
Broadband radar offers amazing close-range abilities; the
pilings seen onscreen are just a few feet from the boat.