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ContinuousWave: Small Boat Electrical
SONAR Raster Displays
|Author||Topic: SONAR Raster Displays|
posted 02-21-2013 01:56 PM ET (US)
Most of us are familiar with modern SONAR raster display. The raster display is a representation of the echo signals received by a SONAR. The usual arrangement of the display is to represent time along the vertical axis. The time between transmission of a SONAR ping and the arrival of an echo of that signal is presented on the vertical scale. Time increases as one goes downward from the top. In this way echoes whose return took longer (and thus are farther away from the source) appear at a greater distance from the upper edge of the display. The horizontal axis is a representation of time. The newest echo signals appear on the extreme right, and older echo signals scroll off the screen on the extreme left. The display is a two-dimensional histogram of echo time intervals. The rate of scrolling right-to-left is usually fixed. The observer sees a display that shows the history of the echoes his boat has passed over. The greater the distance on the screen of an echo from the right side of the screen, the farther behind the boat's present position the echo occurred.
Even in the earliest SONAR displays for recreational use, a third parameter was added to the information by providing a visual representation of the strength of the echo. Early devices used a lamp whose brightness varied in proportion to the strength of the echo signal. These "flasher" displays gave the observer cues about the echo strength along with its time relationship. The flasher displays did not provide a histogram. They only showed the immediately occurring echoes.
Early raster displays of echo histograms were on cathode ray tube (CRT) displays. The brightness of the display could be modulated to represent signal strength, with a stronger echo producing a brighter representation on the CRT.
At some point, solid-state displays using liquid crystal technology replaced CRT displays. Some liquid crystal display (LCD) devices provided only on-off control of each pixel. This is a Black-White or 1-bit display technology, and it caused echo amplitude information to be lost. An echo was either shown or not shown, according to its amplitude compared to some threshold level.
More advanced LCD technology came along, allowing for displays to provide gray scales. The number of shades of gray in the gray scale varied. In some cases as few as eight different levels are provided. In other cases, as many as 256-level of gray are possible. The use of gray scale representation permitted echo amplitude to be returned to the raster presentation. The usual method was to decrease the brightness of the echo representation as echo strength increased. This is the reverse of the original method. Earlier the brighter the echo representation the stronger the echo signal it represented; in LCD technology the darker the echo representation the stronger the echo signal it represents.
At some point in the evolution of SONAR devices, a color display was added. The color display has added a further dimension to representation of SONAR echos: a false color mapping of echoes. As far as I know--which is really not very far, and which brings us to the actual focus of this article--the false color mapping of SONAR echoes is typically done according to signal strength. Manufacturers have developed various color palettes for representing SONAR echos. These palettes seem to tend to use colors in a brown tone for echoes that are typical of the sea bottom. The background color is usually set to white. SONAR echoes are represented with various colors, usually reserving oranges and reds for strong echoes. Using a brown tone for the sea floor representation may help novice users interpret the SONAR echo histograms, as they likely make an association with the sea floor and the color brown.
Exactly what signal characteristics produce which false-color mapping of the SONAR echoes onto the histogram display is not well known to me. I would be interested to hear of any information about the use of false-color representation of SONAR echoes in raster displays. If anyone has insight into this, please join the discussion.
posted 02-21-2013 02:17 PM ET (US)
I have done a cursory search (with GOOGLE) for information about SONAR color palettes. I found this description of one palette used:
A typical color scheme is:
Strong return = Yellow
These two sources seem to confirm that the general use of false-color representation is to convey information about echo strength.
posted 02-22-2013 11:01 AM ET (US)
There is a feature in my Lowrance HDS-7 that also shows a "Flasher" display along the right edge of my moving raster plot. I've found that the Flasher gives a more interpretable representation of echo signal strength than does the raster.
posted 02-22-2013 01:56 PM ET (US)
Dave--you probably grew up using the flasher style devices. I find them hard to interpret, having never used a flasher style device.
Another feature of the Lowrance HDS series is the ability to create SONAR log files. The files seem to actually record some form of raw data. When you play back a SONAR log you can apply display adjustments to the file data. For example, you can vary the SENSITIVITY and the COLORLINE controls. These controls seem to act like gain and clipping threshold controls, respectively. You can also select different color palettes to be used to display the raw data.
By playing back an actual SONAR log recording, I have found it very interesting to experiment with the SENSITIVITY, COLORLINE and color palettes, trying to produce a raster display that presents the most easily interpreted image. Here are a few examples:
In the images shown above, there is a double-bounce echo situation. The echoes are from a large wreck in about 100-feet of water. The echo off the upper part of the wreck at about 40-feet is so strong that it bounces for a second trip and reappears at 80-feet.
posted 02-24-2013 09:42 PM ET (US)
All a Flasher is is a signal strength meter, just like used for the past five or six millenia. The Flasher is the same rendering as the old CRT voice strength meters.
posted 02-25-2013 02:08 AM ET (US)
The Flasher presentation is really the same as the raster presentation without the history scrolling across right to left. I understand the basis of the presentation, but I just don't see the utility. I guess it is for thinking only in the present. I like to think in the past.
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