Learning To Love Your Boat Radar

By Lenny Rudow

Radar can seem daunting to a newcomer. Here's a brief introduction on all you need to know to get you started.

Radar in use aboard

Whether there's a pea-soup fog or night has fallen, there's no substitute for radar when it comes to operating a boat in reduced visibility. Even on clear, sunny days, radar can be a huge advantage, letting you "see" for miles into the distance.

But radar is expensive and complex, right? These days, no, not so much. Today's units are light-years ahead of those found aboard recreational vessels just a decade or so ago. They're easier to use, more sensitive, and less expensive. Yet still, many mariners who haven't used radar are a bit apprehensive about looking at all those blips and blobs and decoding exactly what they mean.

Here's How Radar Works

Before we delve into using radar, let's make sure you have a solid understanding of the basics. In a nutshell, radar sends out a transmission in the form of a high-frequency radio wave and "listens" for it to be bounced back by a solid object. Most traditional radar units send out this transmission in a burst of power, then calculate the time delay of any returned signals to calculate distance to the target. As a general rule, this type of radar provides the best long-range abilities. Unfortunately, that big burst of power creates something called a "main bang" 360 degrees around your boat. This is a visionless dead-zone that can cover 100 feet or more. So while long-range performance is excellent, very short ranges are hampered.

Instead of using strong bursts of power, some newer solid-state radar units instead calculate the difference between transmitted and received frequencies. The advantage is better target discrimination at short range; there's no big burst, so there's no main bang. Their range, however, is often more limited than that of traditional radar.

The latest and greatest units may combine these two technologies, and some also apply Doppler enhancements. Remember learning about the Doppler effect in high school? As an ambulance gets closer and closer, the frequency of its siren sounds higher and higher, and as it gets farther away, the frequency sounds lower and lower. Many of the latest marine radar use this same principle to help determine the speed and hazard-level of moving targets.

Powerful Returns

The strength of a radar's return depends on a number of variables, including the target's material, shape, and size. That's why some items (such as channel markers, which are designed to maximize radar returns) may appear to be bigger on radar than a boat 10 times their size. This is also why small fiberglass boats may not show up on some radar at all, or may show up only at very close range. Your radar's beam width also has a big impact on how it sees things. The narrower the beam, the more gain (intensity) it has, and the more range it will have at a given power level. Beam width is determined by antenna size, which is why larger, open-array units generally have much narrower beam widths, and hence more maximum range, than small, enclosed-dome antennas.

What's most important to recognize about radar range, however, is that beam width, power, and every other factor gets trumped by the curvature of the Earth. Radar is "line-of-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 matter 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:

  • CPA: 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.
  • EBL: 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.
  • MARPA: 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.
  • TCPA: 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.
  • VRM: 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 below.) 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.

Broadband radar overlayIt's much easier to tell what you're seeing on screen when your radar is overlaid on your chartplotter. The red areas show the radar "echo" and clearly delineate the coastal outline.

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 short ranges.

Secondly, don't view radar on a split screen but instead give it as much LCD territory as possible. The bigger the picture you're looking at, the easier it will be to figure out what's what. And when you really need radar, looking at something like the fishfinder should be the least of your concerns, so dedicate that entire MFD screen to what counts. Overlays of information can cause problems or solve them, so as we said before, see what works for you.

Finally, remember that some thoughtful interpretation is often necessary. Three strong returns that remain static and are lined up neatly in a row are likely to be a series of channel markers; weak returns that come and go are often poor targets like small fiberglass boats; and two targets keeping pace close to each other could be a tug and its barge. Accurately reading returns like these requires a different sort of algorithm — the one that's in your own brain.

Garmin's Fantom with Doppler technologySome newer radars, like Garmin's Fantom, apply Doppler technology to accurately track moving targets.

Yes, it will take some practice to accurately and proficiently determine what's on the LCD screen. But we do have one very big piece of good news for you: Today's modern radars have such advanced processing powers that you'll rarely need to adjust anything. Gone are the days of constantly fiddling with sea state and clutter adjustments to get a clear picture on the screen. Leave your unit on auto mode and in most situations, it can do a better job than you or I at presenting the best possible picture. And whether you're trying to navigate through a pea-soup fog or the inky darkness, that one fact alone will make using your radar far easier than ever before.

Never overestimate radar, or any other equipment, however. For example, most radars will not see through significant amounts of rain, and you may find yourself running blind if you've only relied on radar as you approach the storm. Also, it's very important to practice steering to radar. Refresh rates of the best screens are less than what we're accustomed to with our vision. And loss of horizon, shorelines, and other external data can drastically affect orientation, distance perception, turn rates, and other things. Practice running on radar alone when it's safe, in good weather, to learn what it can be like in pea soup. You may be very surprised. 

— Published: October/November 2016

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