StarFish 453OEM
System Manual
13
4.4.
Calculating The Distance To A Target
As mentioned earlier, the sonar display shows the recorded echoes over a period of time, and we have seen how
we can work out the depth below the sonar from this.
However, this also means that the range a target appears to be on
the display is not the distance it lies at from the sonar horizontally
across the seabed.
To understand why this is the case, consider the triangle shown in
the figure opposite.
As the sound waves travel from the sonar, they start to hit the
seabed, and each point of contact returns an echo along the
shortest path to the sonar.
To work out the actual distance at which a target lies, you need to use the mathematical formula (based on
Pythagoras’s theorem) …
2
2
Height
Range
Displayed
Range
Actual
4.5.
Acoustic Shadows
When sound from the sonar hits a submerged target with any
height above the seabed, an acoustic shadow will be cast away
from the sonar.
An experienced sonar operator can use the lengths of these
shadows, along with knowledge of the sonar altitude to get an
idea of the size and height of the object.
To help understand this, imagine you are in a darkened room, with a flashlight, standing above a ball. If you shine
the flashlight down on the ball, a small shadow is cast around it, while if you lie down level with the ball and shine
the light at it, a much longer shadow is produced stretching away from it.
This principle applies to side-scan sonar in a similar way; objects directly beneath the sonar will appear to have
very small shadows, while objects at greater distances will have longer shadows because they are being
illuminated (acoustically) on their side.
The shadows of targets can be elongated further if the towing altitude of the sonar is decreased or if in shallow
water.
Displayed
Range
Height
Actual Range
Acoustic Shadow
