Backscatter sonar is gathering momentum in recreational fishing and is definitely here to stay. Commonly called side scan or structure scan, It shows the user the seabed directly below or out to the side of the boat in much higher definition than a regular sounder. It can be used to spot areas of foul ground, areas of harder ground and mark good anchoring areas or schools of bait fish (and which side of the boat they're on) in deeper water.

Although commercially available for a number of years, Humminbird was the first company to shift this technology to the mainstream market with their “Side Imaging” Sonar Technology. Lowrance quickly copied this with “StructureScan Sonar” and found themselves in a legal spat with Humminbird over patents. Perhaps feeling left out of the legal rumble, Raymarine released it's Dragonfly earlier this year and promptly were sued by Lowrance for using the “Downscan Imaging Sonar” Patent. Garmin have just announced their take on backscatter sonar which they have called “SideVu and DownVu” Phallic Transducerwhich will be available at the start of next year. I hope they have kept their lawyers well fed.

The actual operation of Backscatter sonar is very similar to a regular sounder. The transducer makes a noise and then listens for a return echo which are then displayed on the screen. The main difference between them is the frequency which is used, and the beam widths of the transducer.

Regular depth sounders use frequencies between 50 and 200kHz, where recreational backscatter sonars use between 455 and 800 kHz. This gives a much clearer picture, but also has a much shorter range. In practical terms the range of most of the scanning sonars is about 50M, with the best results in the 5-25M range

The beamwidth of a regular sounder is normally circular and in the region of 12 degrees for 200kHz and 40 for 50kHz. Side scanning sonar uses a much wider beam to see far out each side of the boat but very short beam fore/aft to maximise the definition of the bottom, The dimensions of this beam are about 5 or 6 degrees fore and aft, and about 160 degrees sideways, when using both sides and bottom scanning. In order to get the very narrow beamwidth, the transducers have to be very long, sometimes close to a foot long.

Downscanning/DownVu sStructureScanonar can be read in a similar way to your normal sounder. The latest information is presented at the right hand side of the screen, and everything to the left of this point is history. Most of the information is very similar to the regular sounder, so if you have experience with normal sounders you will be able to read this quite easily. The flip side is that its really not that different to your sounder, with most of the information

Side looking sonar is presented in a different way which can confuse even the smartest of boaties. In the image above, the vessel is located at point A, at the top of the white line. Everything to the left is sensed from the port sensor, everything to the right is sensed by the starboard sensor. The catch is the darker area to either side of the white line. This is the water column, and the distance from the white line to the end of this darker area is the depth at that particular point. Both point B and point C are directly below the boat. I find it easier to think of it as two seperate sounders presented back to back.

In this case, the image shows the boat moving from a shallower, rockier area into a deeper sandier area, passing over 2 schools of bait fish on the way. If you look closely you will also see a third school of bait fish on the Starboard side at about the same area as the first school passes under the boat. This clearly shows the advantage of sidescan. Not only can we tell there are fish about, but we can say more accurately where these fish are hiding.