Here, fishy, fishy, fishy

It's been said that in any given body of water, 90% of the fish inhabit just 10% of the volume. Sonar, of course, helps keep fishermen away from where the fish aren't.

These days, anglers rely on sonar gear, often called echo sounders, depth finders, or fishfinders, to not only find the best place to trawl but also to keep from running aground. But these aren't your fathers' depth finders. New-generation units offer more sensing capability, greater computer processing power, and clearer, more-informative displays than ever before.

In most cases, fish are displayed as lines or arches. Fish arches are created as the fish moves through the transducer's coneshaped scanning area. To print as a perfect arch, a fish must enter the edge of the cone area, remain at constant depth as the cone travels, and pass out of the cone area at the same depth. The wider the cone angle, the bigger the arch. If the fish changes depth, passes through only the edge of the cone, or wanders around under the boat before swimming off, it won't print as a perfect arch. Some fishfinders offer an option where fish are displayed as actual fish icons; the bigger the icon, the bigger the fish. Usually, purists prefer to see arches. However, as one expert points out, arching a point target such as a fish, can cause other structures under the boat to arch, covering up depressions or holes that might be holding fish.

Flashers versus LCDs
Early depth finders featured simple technology and were referred to as flasher units. Here, the display consisted of a motor attached to a 4-in.-diameter disk with a light-bulb on it. The motor spun the disk and whenever the transmitter would fire, the lightbulb lit at the "zero" depth. It would flash again when the echo returned. For example, on a 0-to-100-ft depth scale, a flash after the disk had moved half of one revolution would indicate a 50-ft depth.

Some anglers still use flashers but most prefer looking at a picture on an LCD, says Computrol Inc., maker of Bottom Line sonar products. The vertical columns of information that cross an LCD on a Bottom Line fishfinder correspond to the separate revolutions of a flasher.

To mark target depth, LCDs darken pixels in a column that remains visible until the column scrolls off the screen. In contrast to flashers, which show changes in echo depth with each flash, LCD units average and filter several soundings before generating a display. Years ago, this caused a delay between when anglers passed a target and when the LCD displayed it. However, today's units offer more computer processing power to eliminate that lag.

Storing the data
Lowrance Electronics Inc., Tulsa, is pushing the technology envelope using postagestamp-sized Multi-Media Cards (MMC) to store sonar data. Two new units, the LCX16CI (color illuminated) and LCX-15MT (monochrome transflective) and CT (color transflective), digitally record and playback sonar graphs and GPS trip details on the depth finder or a home PC. Moreover, users can download custom mapping to the cards and upload these maps to the units.

The MMCs are available in 8, 16, 32, and 64 Mbytes. Says Kim Mitchell a Lowrance project engineer, "Maps in the GPS unit aren't new. But before, users were limited to 2 or 4 Mbytes of memory. Now, users have a minimum of 8 Mbytes to store maps, sonar data, waypoints, routes, and other points of interest. It allows much more map detail and wider coverage." Another plus is the X-16's high-resolution display. The big-screen (6.38-in.), 640 480-pixel, liquid-crystal TFT display shows 64 colors simultaneously. The display reportedly can be seen in direct sunlight, at night, or in low lighting thanks to cold-cathode backlighting, says the company. "In display, contrast and viewing angle are very important," explains Mitchell. "It's best to have a wide viewing angle and good contrast so that the display is viewable under different light conditions."

Users can also customize GPS map and sonar displays with different colors to identify game fish, baitfish, contour, structure, and background. A dual-frequency transducer on both the X-16 and X-15 allows 50 or 200-kHz operation, or both, simultaneously viewed in a split-screen. "For freshwater and shallow water, 200-kHz is the most widely used frequency," says Mitchell. "When you start getting into deeper water, such as saltwater or the Great Lakes, most sport-fishing sonars use 50 kHz because the lower frequency penetrates the water better than the higher frequency."

Lowrance engineers are constantly working with different transducers to accommodate the many different boat styles and fishing preferences. For example, both the X-16 and X-15 handle depths of more than 3,000 ft with a dual-frequency transducer that delivers 1-kW rms (8-kW peak-topeak) with a 37° cone at 50 kHz, and 500-W rms (4-kW peak-topeak) with a 12° cone at 200 kHz.

A transducer with a wide-angle cone scans more water and can find fish and structures faster. On the flip side, it may look at two or three stumps on the bottom and lump their reading together making it impossible to see the stump with a fish next to it. Conversely, a narrow-angle cone detects small structure details that might draw fish. The more concentrated sound output of narrow beams also reaches greater depths.

Covering more water
Bottom Line products span both worlds using sidefinding technology. Sidefinders are similar to basic fishfinder sonar except that they augment a straight-down beam with a multiple-beam transducer that sends and receives soundwaves to the side of a boat. Rotating the transducer lets fishermen find fish in a 360° area surrounding the boat.

Sophisticated filtering software eliminates bottom structure and echoes from the surface, showing only fish over 7-in. long. The viewing screen pinpoints exactly where the transducer is pointed, and the sidefinding transducer maintains a narrow 9° cone of soundwaves, further pinpointing location. The narrow beam provides high-density detail, improving echo identification, and its high concentration of sound waves reach greater depths and distances than wider beams.

Typically a bottom-finding unit that emits a 20° cone angle provides about 10 ft of search radius in 30 ft of water. The Bottom Line models, with sidefinder technology, can increase that search radius to 480 ft.

Coordinating sonar beams
Most sonar experts agree that many narrow sonar beams are better than one wide beam. Techsonic Industries Inc., maker of Humminbird, is a big believer in this philosophy, offering units with as many as six-beam elements inside one transducer housing. Says David Betts, Techsonic research and development manager, "With a single-beam depth finder, positioning a fish is difficult because a user can't tell where it is within the beam. A typical sonar scenario involves detection, localization, and classification: First is knowing the object is there, second is knowing where it is, and third is determining what it is. With single-beam depth finders, a small fish in the beam's center and a large fish on the outside of the beam will return the same amplitude. But with two coaxial beams, for instance, the wide beam determines how big the fish are within the narrow beam. Users can do a correction to properly size the fish within the narrow beam. With Techsonic's six-beam depth finder, a target or a fish can be in more than one beam at any given time and we can then use that to properly identify the size of the fish."

Fish-identification algorithms help determine if an object is actually a fish. Using the amplitude of the return and the elongation — how much the return is spread out over time — the units can tell if an object meets the criteria to be a fish. Separating fish from one another or from the bottom is best accomplished by transmitting short pulses, says Betts. "A long transmit pulse won't tell anglers if fish are individuals or if what they are seeing on the display is one big fish. The length of time that a pulse is transmitted, its frequency, the amount of filtering done, and signal-processing aspects determines how well a unit can separate fish from each other or from the bottom."

Another identification method involves time-variable gain corrections. "There must be a correction for square loss spreading and attenuation in the water to make a fish 100 ft below your transducer show up as the same size as one 10 ft below," says Betts. "We use log amplifiers to get a wide dynamic range so the receiver can handle very small to large signals in one look."

Seeing in color
All this complex data must be displayed simply on the LCD. Most fishfinders have black-and-white transflective displays. Sunlight or other ambient lighting serves as the light source. However, color technology is emerging. Raymarine, a major producer of sonar and other marine equipment, is one manufacturer incorporating color LCDs.

One problem with early color LCDs is that they didn't appear as sharp as blackand-white versions. As explained by Morten Andreasen, product line manager at Raymarine, "In color units, sunlight actually absorbs into the pixels because sunlight itself is a composite of the red, green, and blue colors used to create the images on color screens. Thus, the image is diluted and only a portion of the sunlight is reflected back to the user's eyes."

To combat this effect, Raymarine backlights the display. A special filter eliminates glare from covers used for waterproofing. Andreasen claims it is the first LCD fishermen can truly see in sunlight.

Raymarine's daylight-viewable, color L760 (7-in. display) fishfinder and the new 1250 (10-in. display) model both incorporate a 32-bit ARM Risc processor running at 57 mHz to handle image processing and acoustic noise cancellation. The dual-frequency 760 puts out 600 W of power and works at depths to 2,700 ft. Both units incorporate GPS, a chart plotter, and a host of other high-tech features such as full or split-screen display, user-defined windows for digital information, adjustable color thresholds, and the ability to put a waypoint on a fish target using the cursor.

The units also use a Raymarinedeveloped interface called HighSpeed Bus technology that makes it possible with a simple cord, to link more than one display on the bridge. Lack of standards has made this difficult, says Andreasen. So, "We developed our own: One is the high-speed bus for broadband data requiring a lot of bandwidth such as radar and fishfinders, and the other is SeaTalk, a plug-and-play system for connecting various displays without worrying about physical connection issues."

Fishfinders of the future
Wireless technology and more functional MMIs are the next step, according to Andreasen. "People would like to access any information from their boat, be it Internet access right there on the echosounder or radar display, and many other broadband technologies," he says. "High-Speed Bus technology lets us take the signal in and distribute it, but the big problem is how to get from land-based lines onto a boat. We aren't the only ones working on it. The most likely scenario is that the computer industry will solve this problem for us."

Another hurdle is keeping the user interface simple and easy to use. "Most displays have small, closely spaced buttons. When the boat is shaking and vibrating, it's very difficult to press just one," says Andreasen. "We need to address that as well as how the software is layered for operation. In other words, putting the most commonly used features on the surface and burying more obscure data deeper in the software. When looking through a marine-electronics catalog, one might see many different approaches to solving these fundamental problems. As we did with interfacing peripheral equipment, we will make our own open standard to which everybody can adapt."