Aviation Electronics Technician 1 - Historic Naval Ships Association

The submarine reflects an echo of higher pitchthan that caused by the particles in the water, whichare not moving. When the echo from the oncomingsubmarine is higher in frequency than the echoes fromthe reverberations, the Doppler is high. The oppositeform of Doppler shift will occur when the submarineis heading away from the transducer. In this case, thepitch of the echo is lower than the pitch of thereverberations. (See figure 4-10.)The degree of Doppler indicates how rapidly thesubmarine is moving relative to the transducer. Forexample, a submarine moving directly toward thetransducer at 6 knots returns an echo of higherfrequency than one moving at only 2 knots. Also, asubmarine moving at 6 knots directly at the transducerreturns an echo of higher frequency than one movingonly slightly at the transducer. Refer to figure 4-11.This figure shows 12 submarines traveling at variousspeeds and courses with respect to a stationarytransducer supported by the helicopter. Notice howthe Doppler of each submarine is influenced by itsspeed and direction.Doppler also makes it possible to distinguish thedifference between a wake echo and a submarineecho. Relatively speaking, the submarine’s wake isstationary. Therefore, its wake returns an echo with afrequency different from that of the Doppler shiftedsubmarine echo.AIRBORNE SONAR SYSTEMLearning Objective: Recognize componentsand operating principles of an airbornesonar system.Figure 4-11-Varying degrees of Doppler effect due to differences in course and speed of submarines.4-9