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ContinuousWave: Small Boat Electrical
Sonar - What do we actually "see"?
|Author||Topic: Sonar - What do we actually "see"?|
posted 04-12-2006 09:29 PM ET (US)
I am wondering what is actually displayed on the sonar/fishfinder screens we have. In thinking about this, the typical signal cone from the transducer is 60 degrees on a Lowrance 200 kHz transducer. Even at a depth of 30 feet, the beam will spread to a circle with a diameter of nearly 35 feet at the bottom. When we see a fish symbol or bottom feature on the sounder screen, where exactly is it? How accurate is the depiction? A large fish at the bottom near the outside diameter of the "painted" area is actually about 35 feet from the transducer while the same fish directly below the sounder would be just 30 feet from the transducer. No doubt this would present totally different reflected signal strengths and timing so what do we actually see on the screen? Also, if the fish were at the outside diameter of the beam it could be anywhere around a 17.32 ft radius.
It seems like there must be a lot of software interpreting the reflections and making decisions about what to put on a single vertical line of the display as it moves in time. If this is the case, then it also would be a reasonable conclusion that the software algorithms from some sonar suppliers would be better than others. Is there a significant difference between manufacturers? Who is the best?
posted 04-12-2006 10:34 PM ET (US)
The fact that a fish at the edge of the cone is farther from the transducer than a fish at the same depth when directly under the boat is the reason fish show up as arches. A rising fish will look like part of an arch. A decending fish will give a flattish hump or even a line. The faster a fish moves relative to the boat, the narrower the arch. Generally, the fatter the arch, the bigger the fish.
Remember, that the image you see on the screen of a fishfinder is a "history". The instantaneous reading is just one column of pixels. The next column of pixels is the interval immediatly preceding, and so on. The intensity of the return is indicated by colors and thickness of the line.
I think Lowrance has some good tuorials on their website on how to interpret what you see on a fishfinder.
posted 04-12-2006 11:47 PM ET (US)
The first thing you should do is turn off the Fish ID feature and start looking at the actual image on the screen, versus fish icons. You're a lot smarter than the machine is. Then, start playing with the gain and sea clutter controls. You'll find when you start increasing gain, you can see the thermocline layer in a deep freshwater lake, and that's where you'll typically find schooling fish like stripers and white bass. You'll also find balls of bait around that thermocline. In saltwater, jellyfish and globs of sargassum weed show up to. You can differentiate them from fish targets by their location in the water column, and the strength of the signal they paint on the screen.
IMHO, Raytheon/Raymarine has the best signal processing on their fishfinders and radars. They are best able to differentiate individual targets versus just painting a big glob on the screen. Maybe some of their signal processing technology from their military tracking radar contracts bled down to the public sector. BillS
posted 04-13-2006 12:14 AM ET (US)
I didn't mean to infer that I was using the fish symbols. I display the arches but I am just unsure how to interpret the results. Here is another example:
Directly under the boat is a small boulder, at the fringe of the "cone" there is a large boulder. The display at any moment in time can only write one image in a vertical column so which boulder do we see? If we see the large boulder it really isn't under the boat, If we see the small boulder then what is the benefit of a wide angle sonar transducer. Don't get me wrong, knowing you are within 50' to 100'of a good "laker" is better than not knowing.
This summer I will be at a lake camp where I will be able to make passes over known objects at different distances. I'll make some observations and post the results.
posted 04-13-2006 11:23 AM ET (US)
You see the sum of all the echos.
This shows up going over a flat sand bottom. The sand at the
Your two boulders will both show up, but may look like one
posted 04-14-2006 08:36 AM ET (US)
Most SONAR presentations show a plot of the echo return as a function of time. Most presentations are arranged so that the display raster is swept from left to right, with the newest data coming on-screen at the right, and the oldest data going off-screen on the left. (The X-axis.)
The vertical axis of the presentation represents time. The upper bound of the display is typically normalized to the time the echo sounding pulse was generated, and the bottom of the display is normalized to approximately the time that echoes return from the bottom. (The Y-axis)
The amplitude of the return echo signal is displayed as a variation in the luminance of the displayed target. In monochrome displays, the intensity function is usually shown inverted, that is, the stronger the echo the lower the luminance of the representation. The no signal condition is represented by a high luminance pixel. This produces a white screen on which black targets are drawn.(The Z-axis)
In color display systems, the amplitude of the return echo is represented by variations in the color of the displayed target. Using a false color presentation like this allows small variations in intensity of the return echo to more easily be distinguished.
A number of automatic signal processing features have been developed to assist the operator in configuring the display parameters. Automatic variation of the display timing to only show portions of the echoes received and to display this small segment on the entire display provide "bottom lock." Variations in the gain of the receiver in coordination with the time improve sensitivity for distant targets.
See this other discussion for a hyperlink to details of some of the digital signal processing techniques being used to enhance SONAR displays:
posted 04-14-2006 08:39 AM ET (US)
The LOWRANCE company has provided an excellent on-line tutorial which explains the operation of SONAR devices. I found it very informative. See:
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