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CHIRPs and Pings

See the fish better than ever before with advanced finders

August 3, 2011
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marlin chirp

marlin chirp

Blind men make great anglers, because they feel every move a fish makes on the end of the line, but blind captains? Not such a good idea. Yet many of us are operating more or less in the dark, once we go beyond 200 fathoms or so. Most recreational fish finders, helpful though they may be, aren’t up to the task when you need to pinpoint a pinnacle that rises up from 5,000 feet; they’re little good when you’d like to spot that ball of squid (much less tell the difference between predator and prey) thousands of feet below as you prep for a daytime sword drop; and they rarely do the job when you’d like to get a clear reading of the bottom structure before deep-dropping for tilefish and grouper. With an electronics upgrade, however, you can open your eyes and peer into the depths — clear down to 10,000 feet — without having to focus more on your fish finder than on your fishing.

Yes, you can already dial in some high-end units to see down to 10,000 feet or so. Thanks to broadband transducers and the frequency-tuning abilities of some fish finders, you can choose your kilohertz on the spot and get down to the bottom in deep water, or dial in the ability to spot specific species of game fish. But as anyone who’s done so already knows, it’s a brain-cramping operation that requires your complete focus when you could be scanning the horizon or watching the baits. And once you do start changing frequencies, there’s always some level of doubt as to the results you’re getting. You wonder: Are there really no schools of bait down there, or am I just not seeing them because I’ve set the finder to ping at 36 kHz instead of 42? What we need is a fish finder with the ability to scan the depths at both of those settings — and many others — all at the same time, without any extra input from us.

This year, two companies at the Miami Boat Show, Garmin and Simrad, demonstrated finders that do just that. Better still, if you already have multi­function display systems from these manufacturers, making the upgrade to these units can be as simple as swapping black boxes, plugging in a few wires and mounting a new transducer.

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Chirping
In the case of both the Garmin and the Simrad, the technological advancement that’s key is the ability to look and listen through a spectrum of frequencies at the same time, instead of probing the depths with single or dual frequencies. It’s been used in high-end commercial sonar for decades. It’s called chirp (or CHIRP), though looking up the acronym — compressed high intensity radar pulse — doesn’t help much, because the technique was first used in advanced military radars. But the term makes sense when you realize that a long, frequency-modulated acoustic pulse really does sound more like a chirp than a short, single-frequency ping. And the concept may seem as sweet as singing birds when you see how much more information chirps can bring back to your fish finder.

The big advantage of chirp sonar is that each pulse sent out by the unit sweeps through a range of frequencies, which allows the electronic brains to then interpret targets with multiple-frequency returns. In other words, it doesn’t just see a return made with one kilohertz’s ping — it sees many at the same time. In the world above the water, this would be akin to looking at an object in the distance with your naked eye, a pair of 7x binoculars, light-gathering goggles, a pair of 12x binoculars, an infrared camera and a telescope, all at the same time — and without any one view interfering with another. With that kind of visual acuity and the brainpower to interpret all of those images at once, you’d know if the blob bobbing around 1,000 yards away was a sea turtle or sargassum weed at a glance. Similarly, these new chirp units can tell if they’re looking at a grass shrimp or a ghost echo through a mile or more of water.

It’s helpful that CHIRP is what Simrad actually calls the chirp technology built into its new BSM-2 Broadband Sounder, while Garmin is marketing the same feature in its new GSD 26 digital sonar as Spread Spectrum Technology (SST). Technically speaking, chirp is a form of spread spectrum, but the term is more commonly applied to communications. Not that Simrad is innocent when it comes to confusing terminology. Now is a good time to note that the term broadband, used for several Simrad fish finders (and one radar model), is unrelated to what Airmar means by broadband transducers. The latter offers wide bands of available frequencies, which are thus essential for either chirp or custom kilohertz features, but only the BSM-2 version of broadband sonar offers these features. Other broadband sonar versions do seem to achieve fairly deep readings and high resolution, but they do it using signal-processing magic, not bandwidth.

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It’s an especially good idea to get the terminology straight now, as it may well get even more complicated since Furuno, Raymarine and Geonav are all reportedly working on their own versions of chirp fish finding. And maybe we’re lucky then, in that today there are only two chirping black boxes to look at and compare in terms of details and functionality.
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Eye Candy**
The $2,495 BSM-2 can network with Simrad’s NSE and NSO systems — as well as the just-announced NSS touch-screen series — while the $1,999 GSD 26 will work with any of Garmin’s GPSMAP 4000, 5000, 6000 and 7000 multifunction displays. Transducers are not included, and they’re not trivial. These advanced systems naturally need specialized mouths and ears to shout and hear with — lots of them. Airmar’s next-generation broadband transducers, like the big-daddy R209LH used on both Miami demo boats, do the trick. The R209LH can be tuned to — or can chirp through — any frequency in a “low” 28 to 60 kHz band or a “high” 130 to 210 kHz band (which could be replaced by a “medium” 80 to 135 kHz band in a coming LM model). To perform such frequency agility, a transducer like this uses a huge array of precise piezo-ceramic elements to act as those mouths and ears. And that means that size matters. The R209LH, which costs about $2,600, is a really big transducer, nearly 2 feet long, over 6½ inches wide, over 4 inches deep and weighing in at 46 pounds. And note that both units boast dual transceivers, which means you can add a second transducer to cover any frequencies or beam widths.

So, what are the differences between the new Garmin and Simrad fish finders? With the Garmin GSD 26, you can create presets that chirp through custom frequency bands, as long as they’re available in the transducer, while the BSM-2 simply offers the maximum available ranges. (The Airmar transducers, incidentally, automatically tell the black boxes exactly what they can do.) Garmin also has a neat feature that smoothes out screen transitions as the GSD 26 automatically changes depth ranges. Meanwhile, the BSM-2 lets a fisherman scroll back through soundings that have gone off screen, which is great for marking fish or structures that you may have missed in the heat of battle. The Garmin can also pump out up to 3 kW of power, with adjustable output from 300 watts on up, on single frequencies, while a characteristic of Simrad broadband is that it only uses about 250 watts, putting it into longer pulses to “whisper” through the depths. Since chirp pulses are always long, Garmin uses about 600 watts in Spread Spectrum mode.

Whether you call it CHIRP or SST, a very important part of the technology is all the analysis that goes on inside the black-box processor. In other words, these units have advanced software that helps them interpret the multiple returns. Any new software, of course, goes through a process of debugging and refinement.

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In any case, the improvements of these new units are immediately noticeable, and they go way beyond merely seeing deeper than you ever have before. Target discrimination is thoroughly enhanced; Garmin doesn’t nail down a numeric claim, but Simrad says it’s up to five times better. After gazing at the screens, we don’t have a way to objectively specify a figure, but we can say without pause that it is noticeably better, particularly when it comes to telling the difference between bait and predators. Bait balls, for example, don’t appear as mere balls; we could actually see individual fish in the school. The difference is significant — so significant, in fact, that even a blind man could see it.

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