Aspect Angle: Your Point of View
The image of an object can be detailed when the object is viewed from one angle and meaningless when viewed from another angle. Figure 1 shows three views of the same cinder block. The top view shows the sides at approximately a 45° angle from the sound striking the block. One can see both near sides and the general rectangular shape of the block. In the middle view, the angle is less but the block is still recognizable. The bottom image shows the same block when turned so one side is perpendicular to the incoming sound pulses. The perpendicular wall returns a very bright return and masks the returns from the rest of the object. It is unrecognizable. When the block is viewed so no walls are perpendicular to the incoming sound, no one side takes precedence in brightness and the entire object returns sound more equally.
Figure 2 features a swimmer moving the block in a pool while the sonar remains in the same location.
In Figure 3, two salmon swim in circles illustrating the effect of aspect angle (the angle of the fish with respect to the sonar) versus target strength (visibility/brightness). A side aspect has a much higher target strength than a head (bad) or tail (worse) aspect, as evidenced by the fish image which practically disappears and reappears as the fish swim through the circle. (Courtesy of Louise Brennan, Marine Institute, Furnace, Newport, County Mayo, Ireland.)
 Figure 1: Three Views |
|
 Figure 3: Aspect angle versus target strength |
