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As the fish is towed through the water it sends out an acoustic wave and listens for returned waves. How often
it sends out a acoustic wave depends on the Range setting. The figure below shows a top view of a series of
12 waves sent out from a passing fish (arrow). Since the Range and boat speed is known, the distances can
easily be figured.
In the figure below, we doubled the Range Switch setting from 25 m to 50 m. The boat speed stayed the same.
We are now covering twice the scan distance (left to right), which requires twice the time for each acoustic wave.
The distance between each acoustic wave is doubled, which results in less samples of the target and therefore
less resolution. The target will be displayed at one-half the vertical height.
In the figure below, we left the Range Switch setting at 50 m, but cut the boat speed in half. The result is that
we get the same resolution as the top figure while covering twice the scan distance.
Boat speed is important, slower boat speeds produce the highest resolution images.
Note: Ideal towing speed is 1 knot. Sonar View software offers manual and automatic boat speed correction to
compensate, but not with out loss of overall resolution, for faster than ideal boat speeds. Sonar View will display
properly proportioned images for boat speed of 1 to 5 knots with manual correction or 1/2 to 5 knots using the
automatic setting. Faster tow speeds will result in images that are vertically compressed. Slower tow speeds
will result in images that are vertically stretched.
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