Echo Sounder is a kind of acoustic measuring depth equipment, which can measure depth with the time between transmitted wave and echo on condition in which the speed of ultrasonic is fixed.
When we experiment on mine detection on the sea it is always found that the position of target in echo images is not fixed because of sway of ship and this reduces the veracity of detection.
When the radar seeker transmits several different frequency pulses, the echo amplitude of the target follows the Rayleigh distribution and that of the decoy is assumed fixed.