Navico’s whispering sonar

Why shout when you can whisper?

How Navico’s new breed of whispering sonar promises clearer pictures at greater depths.

Three years ago, Navico introduced something that it described as a “Broadband Sounder.” Depending on which of the Navico brands you preferred, it came in either a gray (Lowrance) or black (Simrad) box that was wired in between the sonar transducer and a multifunction display and was claimed to produce a clearer picture, better deep-water penetration, and better performance at high speed. And all this came from a stated output of 250 watts (RMS), which is more in line with what you’d expect of a hundred-dollar supermarket fishfinder than from Simrad’s flagship sonar.

But the funny thing is that despite the low power —or perhaps because of it—the Broad-band Sounder worked. Its low power cut interference and clutter before they were created, its long pulses more than compensated for the lack of power, and its clever digital signal processing more than made up for the lack of discrimination that we might have expected from long pulses. The Broadband Sounder collected plaudits from pundits as well as trophies from trade shows and user groups such as ICAST and NMEA.

But there was just one little gripe:

“Long pulse” doesn’t mean “broadband.”

Operating on three frequencies doesn’t mean “broadband.”

And neither does “digital processing.”

So was the name just a marketing ploy, like the quasi-scientific gobbledegook in advertisements for wrinkle creams? Surely not! In fact, the original Broadband Sounder—now called the BSM-1—is being joined by a bigger, heavier, and more rugged-looking new stablemate. The BSM-2 has an output that’s still only 250 watts RMS (2 kW peak-to-peak), but Navico boasts that it can stay locked onto the seabed, even in water that is two miles deep—that’s a jaw-dropping 10,000 feet.

What makes this possible is that the BSM-2 really is a broadband sounder, in just the same sense that Navico’s BR24 (introduced two years ago) is a broadband radar: It uses enormously long pulses of sound energy to make up for its lack of raw power.

Conventional sonar works by transmitting short clicks of very high frequency sound energy and listening for the echoes that are reflected from the bottom, from structure, or from fish. Longer pulses carry more energy than short clicks, so they are better at penetrating to great depths. But there is a price to pay for extended range, because when a long pulse hits two objects that are close together, such as a fish or a piece of structure, the two echoes can easily merge and appear as a single, undistinguished lump.

Each “click” of a conventional sonar lasts a fraction of a millisecond and is either on a single frequency or on two or three separate frequencies. By contrast, the BSM-2 transmits 70-millisecond frequency-modulated “chirps.” Stripped of its geek-speak, what that means is that instead of clicking, the BSM-2 squeaks. Each squeak is a hundred times longer than the clicks of a conventional sonar, and instead of being only at

a fixed frequency, it changes from beginning to end. (Indeed, if you could slow it down and lower the pitch enough to be able to hear it, it would sound rather like the call of a whippoorwill.) So when the echoes return to the transducer, the BSM-2 is able to recognize them as two distinct echoes because although they overlap, they are chirping slightly out of synch with each other.

It seems odd, at first sight, that Navico should introduce its new breed of super-sonar so soon after Structurescan, last year’s new breed of super-sonar. But the two new sonars are really quite different and complement, rather than compete with, each other. Structurescan uses very high frequencies to show great detail, but its range is limited. And one of the unfortunate side effects of showing great detail is that small things (including fish) tend to appear as small blobs, rather than as distinctive and conspicuous fish arches. The BSM-2, on the other hand, produces a picture much more like that of a conventional fishfinder. It’s cleaner and more consistent than a conventional sonar, and yet old hands will find it instantly recognizable while newcomers will find it easy to get a picture without a lot of tweaking and fiddling.

BSM-2 isn’t a low-budget technology. Performance that is said to be five times better than that of the BSM-1 is matched by a price tag that at $2,495 is three times higher, while broadband transducers start at a few hundred dollars each and run on up to a couple of thousand or so.

The pictures will tell you whether it’s worth it: check out those depths!

Simrad (603) 324-2024.
Lowrance (800) 324-1354.

This article originally appeared in the December 2010 issue of Power & Motoryacht magazine.

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