Klein Associates Index



Basic Operation:

Approximate heights of targets can he found by calculation from measurements made on the sonar record. The geometry involved is that of similar triangles, as shown in the image below.

The ratio of the target height to the length of the shadow is equal to the ratio of the towfish height to the range to the end of the shadow, or:

Ht            Hf
___  =  _________
Ls         Rs  +  Ls

The only unknown factor is the target height, so we can rewrite this as:

          Ls  x  Hf
Ht  =  _______
         Rs + Ls
  • Ht   =  Target height
  • Ls   =  Acoustic shadow length
  • Hf   =  Towfish height above the bottom
  • Rs   =  Slant range to the target
  • Rs+Ls  =  Slant range to the end of the shadow

Using the record of the submarine USS 09 below. We can determine the height of the hull above the sea floor. In this case the towfish height is 10 meters, the length of the shadow is 10 meters, and the slant range to the end of the shadow is 57 meters. By calculation, then, the target height, the hull of the submarine is approximately 1.8 meters.above the bottom.

We always qualify the calculations as approximate for several reasons. Some of the sound will be diffracted around the top of the target, so the shadow will be a bit shorter than it would be in the ideal case. Secondly, it is often difficult to determine exactly what point to make the shadow length measurement from. The direct target return may be broad enough that it is not clear which point is producing the shadow. Making the measurement from the trailing edge of the direct return will result in a shorter computed height than a measurement from the leading edge. If clearances over targets are important, as for hydrographic purposes, it would he more conservative to measure shadow lengths from the leading edge of a target.

 


L-3 Corporate Web Sitenn