Sonar systems are generally used underwater for range finding and detection. Active sonar emits an acoustic signal, or pulse of sound, into the water. The sound bounces off the target object and returns an echo to the sonar transducer. Unlike active sonar, passive sonar does not emit its own signal, which is an advantage for military vessels. But passive sonar cannot measure the range of an object unless it is used in conjunction with other passive listening devices. Multiple passive sonar devices must be used for triangulation of a sound source. No matter whether active sonar or passive sonar, the information included in the reflected signal can not be used without technical signal processing. To extract the useful information from the mixed signal, some steps are taken to transfer the raw acoustic data.
Active Sonar For active sonar, six steps are needed during the signal processing system.
Signal generation To generate a signal pulse typical analog implementations are oscillators and voltage controlled oscillators (VCO) which are followed by modulators. Amplitude modulation is used to weight the pulse envelopes and to translate the signal spectrum up to some suitable carrier frequency for transmission. First, in sonar system, the acoustic pressure field can be represented as s ( t , r → ) {\displaystyle s(t,{\vec {r}})} . The field function includes four variables: time t {\displaystyle t} and spatial coordinate r → = ( x , y , z ) {\displaystyle {\vec {r}}=(x,y,z)} . Thus, according to the Fourier transform, in frequency domain
s ( w , k → ) = ⨌ s ( t , r → ) ⋅ e − j ( w t − k → r → ) d x → d t , k → = ( k x , k y , k z ) , s ( t , r → ) = ⨌ s ( w , k → ) ⋅ e j ( w t − k → r → ) d k → d w , {\displaystyle {\begin{aligned}s(w,{\vec {k}})&=\iiiint s(t,{\vec {r}})\cdot e^{-j(wt-{\vec {k}}{\vec {r}})}d{\vec {x}}\,dt,\\{\vec {k}}&=(k_{x},k_{y},k_{z}),\\s(t,{\vec {r}})&=\iiiint s(w,{\vec {k}})\cdot e^{j(wt-{\vec {k}}{\vec {r}})}d{\vec {k}}\,dw,\end{aligned}}}
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