While three people hold the tungsten ball underneath the ship, scientists inside monitor a computer screen to track where the ball is located within the four quadrants of the beam produced by the acoustic sonar's transducer. The goal is to collect at least 100 readings—shown as blue points in the inset image—which are recorded for each of the sonar's three transducers. Since the value, or target strength, returned by the tungsten ball is a known value, the scientists can then compare the values returned by the sonar and adjust accordingly. This insures that the number of fish that the sonar "sees" is as accurate as possible. The hand in this image is showing the location of the tungsten ball as viewed on the sonar's echogram.