Understanding echo sounders and fish finders

Anglers often spend quite a lot of money on their fish finder echo sounder, but fail to understand the correct way to use it. This is part one of a series that will help you understand the way sonar works and how to better utilise it.

This information comes from Ron Calcutt's Lowrance book of sonar and GPS. This book is available from Lowrance suppliers at $14.95.

The words sonar and echo sounder cover a lot of technological territory, some of it simple and straight forward, some of it complex and powerful. At the simple end of the scale there are sonar units designed for the sole purpose of monitoring the depth of water beneath a boat. They show a digital depth reading on a small display, and that's the end of it. More complex versions of this basic unit have alarms that go off when the water shallows to a predetermined depth, or if the alarm is being used as an anchor watch, when the depth gets either shallower or deeper than a predetermined range. From these digital units we step to the flasher models, where a circular display is calibrated to indicate various depths. Bands of light flashing in a circular window running around the calibrations indicate the depth of the bottom and, for those smart enough to be able to interpret all the additional little flashes that show on the display, the presence of fish or bait can also be detected. Further up the ladder again are the paper graph recorders, where a rapidly rotating stylus draws a trace of the bottom, and fish, on a roll of sensitised paper scrolling across the screen from right to left. Although these are capable of rendering high detailed information, the convenience of electronic displays has seen the paper graph machines fade into the background in recent years.

Liquid Crystal Graph Sounders

Liquid Crystal Graph (LCG) Sounders are the sonar units most sport fishermen use these days, and they use computer technology for both their processing power and display to deliver remarkable performance to size ratios. The shallow profile of the display contents means they fit neatly onto the dash of even very small boats, where space is often at a premium. Because the internal workings of an LCG sounder produce little heat, they do not require ventilation, and can therefore be completely sealed and waterproofed. The displays, particularly the new ones, are the best available for day time viewing, even in direct sunlight, and the new cold cathode back lighting of the screen and keyboard is also exceptionally good to work with at night.

Cathode Ray Tube Sonar

The Cathode Ray Tube (CRT) sonar works on the same principle as your television screen. The tube style of display allows this type of sonar to offer quite brilliant colour graphics, but there are a number of drawbacks associated with the system. The most obvious one for smaller boat operators is that the sets have to be very bulky in order to accommodate the cathode ray tube, and the tubes also generate a significant amount of heat, which means that the cabinets can not be completely sealed and water proofed. Originally designed for larger vessels where the sonar is housed in a wheelhouse or enclosed bridge, these screens have the same problems with the daylight as a standard TV, but they are exceptionally good at night. Apart from using quite different display systems, there is little difference in the basic way the bulk of LCG and CRT sonar's operate.

The Basics

The basic requirement of any echo sounder is to display the depth of water beneath the boat. To do this, it fires an ultrasonic pulse towards the sea bottom  through a device called a transducer, which can also detect the return of the signal after it has bounced off the bottom. The echo sounder's control and processing unit knows the speed at which that pulse travels through the water, so by measuring the time it has taken to hit bottom and return, it can accurately determine the depth of water in which it is operating. This basic function can be carried out by a very simple and unsophisticated device, which then shows you the depth information on a digital display. In reality, the pulses being fired at the bottom return with a great deal more information than is extracted by a simple digital display. If the returns of a succession of pulses are recorded on a screen as a graph, a picture of the bottom shape is created as the boat passes over it. This is also a relatively simple task for a modern electronic device.

The Details

As we had to move from a basic digital display to a screen capable of showing a graphic image to get this far, so we have to add increasingly sophisticated software, greater processing power and better display quality as we ask the machine to extract more and more information from those ultrasonic pulses it fires at the bottom.

The software has to do a lot more than just receive the signal, and some very sophisticated sampling procedures take place in order to separate and interpret all the details that return as echoes from that ultrasonic pulse. You can't move up this scale of performance by upgrading just one part of the system. If you give the machine better software that can extract a lot of information from returning signals, then you also have to upgrade the processing power so the sonar can translate electrical signals into a graphic form that can then be displayed on a screen. Remember that we are not talking about any of this happening at a relaxed pace, either.

This is a world in which speed is measured in terms of processing millions of instructions every second. With the computing technology available to us today, the sky's the limit in terms of what can be achieved with an echo sounder. Screen technology is also advancing at a great rate, so that increasingly detailed and complex information can be displayed in a way that non-technical people can understand. This has led to a situation where the main game is often obscured by the development of all sorts of peripheral functions and features that are being added to sounders, then touted by marketing experts as "Break through technology" or "A whole new way of looking at the underwater world!"

In some cases, the gimmicks are promoted in order to distract the buyer's attention from the fact that the basic performance of the product is not all that hot. The fact is that to get the most out of sonar, you need to be able to recognise and understand the foundation principles of this technology. No matter what else a sonar unit will do for you, it will spend the greater part of its life looking at the bottom right under, or immediately adjacent to, your boat. In a narrow or wide screen it will show you a short history of where you have just been, and the fish that were in that particular column of water, but the fact remains that only that part of the picture emerging from the right hand side of your screen represents what is under your boat right now! If that doesn't sound very impressive, consider that most of the worlds commercial fishing vessels are working in the same two-dimensional view that you are, often with a huge financial investment riding on the ability of that sounder to provide accurate information on the nature of the bottom and the position and qualities of fish in the area. In many cases they can accurately identify different species of fish by the particular way they show up on a sounder screen. Their equipment is generally larger, more powerful, and capable of producing a wealth of detail specific to their particular fishing method, but they are still working in two dimensions with the same sorts of traces and pictures that display on the screen of a sports fishermen's LCG.

So what are these foundation principles that are fundamental to all echo sounder performance? Taking this very briefly, as all of this information is dealt with in much greater detail later in this book, the most basic LCG sounder should show you:

Bottom contours, The changing depth of water, The presence of fish in the area

The amount of detail it can show will be limited by both the size of the screen and the relatively coarse nature of the screen display. It is important to be able to show a standard screen display as well as Fish ID mode where fish are represented by icons of fish symbols. The ability to display fish returns as curves, dots or a block of dots when dealing with a school, makes it possible to determine whether fish are holding in the one place or moving about. This is also essential in areas where high levels of aeration in the water cause sounders explore this new territory, but only do it when the extended views are coupled with a set offering first rate and powerful 2D sonar as its main function.

It should already be obvious to you that an echo sounder can do many things, and that the specifications from model to model, brand to brand, will vary greatly. All of the screens will look quite different, and even the shape of the screens will vary. The next chapter will explain how you can learn a great deal about an echo sounder, just by looking at that screen.