<P> The sound reflection characteristics of the target of an active sonar, such as a submarine, are known as its target strength . A complication is that echoes are also obtained from other objects in the sea such as whales, wakes, schools of fish and rocks . </P> <P> Passive sonar detects the target's radiated noise characteristics . The radiated spectrum comprises a continuous spectrum of noise with peaks at certain frequencies which can be used for classification . </P> <P> Active (powered) countermeasures may be launched by a submarine under attack to raise the noise level, provide a large false target, and obscure the signature of the submarine itself . </P> <P> Passive (i.e., non-powered) countermeasures include: </P>

Explain in detail how sonar is employed to locate the objects