Marine Electronics 101: How to Use Radar

It's nice to have radar. It's even nicer knowing how it works and how to use it correctly. Practice in fair weather and daylight so you know what your radar is telling you in the dark.

23rd September 2012.
By Lenny Rudow

You might have radar on your boat, but do you know how to use it to its fullest abilities? Can you tell the difference between a boat and a buoy, or a rain storm and a sand spit? All of those blips and blobs can be deceiving, so every good mariner should have an understanding of exactly what radar is and how it works.


Understanding how your radar works will help you use it more effectively.

Picture yourself standing in a dark field, holding a powerful spotlight parallel to the ground. Shining it into the blackness, you see nothing but the beam as you turn in a circle. Then, suddenly, it illuminates a tree 50 feet away. But if you keep turning you can only spot the tree for a fraction of a second.

That brief moment of illumination is similar to the way a radar picks out targets—except it’s using a microwave pulse, instead of light, to illuminate solid items.

Be it an open-array or an enclosed dome, the radar antenna rotates just as you spun around with the flashlight (though a lot faster), shooting out rapid pulses of energy. Reflections from solid targets are then received by the antenna, which alerts the radar’s “brain” to their existence. A mathematical calculation (which uses the amount of time the pulse took to be reflected back) then takes place to calculate an exact distance to the target.

One exception is Broadband radar, now in its fourth generation. Instead of radiating microwave pulses with a magnetron, Broadband sends out a continuous transmission wave. This wave increases in frequency as it moves away from the dome, hits a target, and is reflected back. The difference between frequency in the transmitted and returned waves is how the unit determines target distance.

After determining distance the radar’s brain knows where to paint a blip on the display screen, in the corresponding location. Large pulse reflections are painted with big blips, and weak ones get small blips. But generally speaking, size differences on a radar’s LCD screen are a poor indicator of real-world size. The shape, material, and height of the target all have an impact on how large it appears on-screen, and in many cases, a 100-foot motor yacht will look smaller than a 50-foot sailboat. And small fiberglass boats with rounded lines and little metal may create a reflection so weak that your radar barely picks it up—or misses it altogether.


Once the radar antenna “sees” a target, it can display the strength of the return –which is not necessarily the same as the object’s size – on-screen.

Everything else being equal, more powerful radar can provide better returns from farther away. But in fact, power is just one factor. Contrary to popular belief, the amount of power your radar zaps a target with isn’t the sole determinant of how good a return it will get; beam width is another key factor. Thin beam widths can pick out weaker targets from farther off than wide beam widths can.

Let’s go back to that dark field, but this time with a flashlight that can be adjusted to spotlight or floodlight. On the floodlight setting, it can’t cut through the darkness nearly as far as it does when set to the thin, tight beam of the spotlight. Radar beams work the same way. A wide beam width (six degrees, for example) won’t penetrate nearly as far or pick up nearly as much as a tight two-degree beam. That’s why anglers, who may use a radar to search for birds (which indicate feeding fish below) often choose units with beam widths as low as 1.2 degrees, so they can detect those birds from miles away.

Wattage is, of course, still an important factor. Just like that flashlight, more power will reach farther into the distance. Low-end units that put out a mere two kilowatts can’t be expected to “see” much farther than 16 to 20 miles, and weak returns like a small fiberglass boat aren’t likely to show up from more than a mile or two away. Four-kilowatt units usually fall into the 32-mile range and will see weak returns a bit farther, while more powerful units can see farther still.

There’s one more factor that needs to be taken into account when determining just how far your radar will be able to spot things: the curvature of the earth. Because we can’t bend the horizon or the microwave beams we send out over it, the combination of the height of your radar antenna and the height of the target you’re looking for will always limit just how far your radar can see. Here’s the formula that expresses the relationship between the height of the target and height of the radar antenna:

(1.22 nautical miles x square root of height of radar) + (1.22 nautical miles x square root height of target)

No matter how powerful your radar may be, and no matter how narrow its beam width, it will never be able to see beyond this mathematical calculation.

Okay, now you should have a firm grasp of how radar works. Ready to tune it up, and tune in to what’s out there? Well I have some great news for you: Today’s radars have such advanced software that in most cases you can set them to full-auto mode and simply look at the screen.

That said, when you’re looking for more distant, weaker targets, increasing gain will help. As you boost gain, watch the screen closely to differentiate between single speckles (clutter) and multi-pixel returns (actual targets). And in rainy conditions, you’ll often want to arm the anti-clutter function to prevent the screen from lighting up. The same goes for rough seas; when you get blips and blobs appearing and disappearing, mostly to the windward side of the boat, you’ll know you’re picking up waves.


Most modern units have such advanced software that "auto" mode will give a clear picture of what lies around you.

Another tidbit of good news: Objects that are meant to be clearly visible to radar make clean, crisp, well-defined targets. Large buoys marking shipping lanes are a perfect example. On most units these consistently show up as clean, tight, circular returns with few cluttering pixels around them. Poor targets, on the other hand, show up as different-sized and -shaped blobs with every sweep of the antenna. Barely perceptible targets in motion, such as birds or small buoys in a large sea, will jump into and out of view. To differentiate these from clutter, you’ll need to pay very close attention for several minutes and identify which smudges and dots appear, disappear, and reappear again in the same location. As you do so, you also have to take the course and speed of your boat, and potentially that of the target, into account.

Most modern units allow you to “tag” moving targets and some will show you their speed and direction of travel if your unit has ARPA (automatic radar plotting aid). But if you have an older or a less expensive radar, you’ll have to figure out this information the old-fashioned way. Watch the target for five minutes, and plot its progress. Let’s say it moved one mile. Divide this distance by five minutes, and you get 0.2. That’s the boat’s speed in miles per minute. Now multiply by 60, and you’ll know it’s moving at 12 knots (or mph, depending on your settings). There are other ways to get to the same answer, but all of them involve a bit of arithmetic.

The boat’s course has to be determined relative to your own. The simplest way to do this is to put your own boat on a 90-degree angle to the target, look at your course line, and add 90 degrees when approaching from port or subtract 90 degrees when approaching from starboard to get its course over the ground (COG). Allow a few minutes to pass and repeat the process several times, to get a firm idea of which way the target is heading. Remember, it’s the relative bearing to the target that counts. If the relative bearing stays the same and the range is decreasing, then the potential for a collision is increasing.

No matter who has the right of way, be ready to alter your own course and speed — and there’s no harm in trying to get in touch with the target by VHF.

Which radar is best for your boat? Check out our article, Choosing the Right Radar.

Once you have your unit picked out and installed, the best way to become proficient with it is to practice. When you go for a cruise, even if it’s sunny and clear, turn the radar on and glance at the screen now and again. You’ll soon begin to match up those blips and blobs with boats and markers, and before you know it, you’ll understand what you’re looking at on the LCD long before those boats and markers come into view.

- Lenny Rudow


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About the author:

Lenny Rudow

Lenny Rudow is Senior Editor for Dominion Marine Media, including and With over two decades of experience in marine journalism, he has contributed to publications including Boating Magazine, Marlin Magazine, Boating World, Saltwater Sportsman, Texas Fish & Game, and many others. Lenny is a graduate of the Westlawn School of Yacht Design who has won 28 BWI and OWAA writing awards.
Connect with Lenny Rudow on Google+

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