Article by Capt. Steve Katz
Getting Sideways
If consumer side scanning fishing sonar wasn’t so popular there wouldn’t have been so many patent lawsuits between Navico (Simrad, Lowrance B&G) and Garmin resulting in a licensing agreement allowing Garmin to use Navico’s technology.
They must be on to something! What is imaging side scan sonar and why might I want it?
A traditional transducer has an “element”, a piezo ceramic disc that both sends and receives acoustic waves into the water. The differences in the return signal are an indication of the objects in the water below the transducer.
Typically, a transducer will send the signal directly downward into the water, perpendicular to the water’s surface. The signal is cone shaped and gets wider as the water deepens, allowing you to “see” a wide area beneath the boat.
Some transducers have multiple elements, operating at different frequencies in a single housing, such as the typical 50/200kHz transducer. A CHIRP transducer operates in a similar fashion, though the frequency is much broader, resulting in better definition of the sonar image.
One thing in common these time honored transducers have is that they are designed to send their signal into the water perpendicular to the surface. This allows the fish finder to show you a historical view of the bottom and what lies between the bottom and your boat.
Not too long ago, engineer’s at all the major marine electronics companies started to experiment by changing the angle of the elements in the transducer housing. Since transducers are not transparent, we can’t see what’s inside, but we can learn how it works.
The engineers also started to experiment with different frequencies. As many captains have learned, the low frequency sounder works better in deeper water, with limited detail, and a high frequency setting works better in shallow water and shows more detail.
By redesigning the typical round elements into a bar shape and moving the transmitting angle upwards, the sonar can now “see” to the side of the boat with a narrow fan shaped sonar beam. This sounder technology is often referred to generically as Imaging sonar. Each manufacturer seems to have a different and often confusing name for their products: Garmin calls their imaging sonar; ClearVu, SideVu, Ultra High-Definition (UHD) scanning sonar and Panoptix. Simrad/Lowrance referees to their imaging sonar as; DownScan Imaging™, SideScan, ActiveTarget, StructureScan® 3D and more. Some of these high frequency transducers are designed to look to the sides of the boat and some look to the bottom, and some combine all three directions into one transducer. These transducers and associated MFD’s (Multifunctional Display) were originally built for the needs of bass fishermen and other fresh water anglers, though salt water use has become very popular.
Most of these imaging sounders operate at frequencies much higher than traditional fish finders in the range of 400kHz to 1200kHz. As with traditional sounders, the higher the frequency, the shorter the range.
These imaging sounders are able to produce a realistic, extremely detailed image of the structure under and to the sides of the vessel. Some of these systems can display a real time, live image of what’s under and around the boat. The narrow beam of an Imaging sonar requires that the boat be moving, not too fast, to paint a picture of the water below.
There is a lot of technology packed into these transducers. One particular transducer, the Lowrance ActiveTarget transducer is comprised of three arrays, each having 140 individual transducer elements, for a total of 420 elements. The more elements you have in your array, the more detailed picture you will see.
You may notice that the graphic displays from imaging sounders are often monochrome, using tones or shades to paint a picture. This helps display the fine resolution, details and shadows produced by these transducers.
The good news about imaging transducers is that they are relatively inexpensive as an add-on to a compatible MFD display, usually costing less than $400. Imaging sonar can be useful in the ocean and bays to help you determine if you are in an area with bait or other fish nearby.
The bad news is these imaging sounders are not a substitute for an Omni sonar. Their optimum use is in shallower water, with limited range, marking fish-holding areas like rockpiles, standing timber, weedbeds, ditches and drop-offs. While some of the live imaging sonars will show fish under and nearby the boat in salt water, mounting these can be a challenge on a traditional sport fishing boat.
There are salt water anglers who have experimented with this technology, mounting the transducers in unconventional ways to get an edge up on the fish and the competition.
If your MFD has the capability of adding one of these imaging transducers, it can provide you that additional amount of information that will help you catch more fish. §
