THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

430 15. Underwater AcousticsFigure 15.9. Schematic of echo ranging process.Now let the sonar act as a detector, i.e., it is to give an indication on some kindof display that an echoing target is present. When the input signal-to-noise ratioexceeds a specific detection threshold DT, thus meeting preset probability criteria,a relay can be activated to indicate on a display that a target is present. Otherwise,when the input signal-to-noise ratio falls below the detection threshold DT, theindication will be that a target is absent. But when the target is just being detected,the signal-to-noise ratio equals the detection threshold DT, i.e.,SL − 2TL+ TS − (NL − DI) = DT (15.15)Equation (15.15) characterizes the active-sonar equation as an equality in termsof the detection threshold. In recognizing that only a portion of the noise powerlying above the DT masks the echo, we could rearrange (15.15) as follows:SL − 2TL + TS = NL − DI + DT (15.16)Equation (15.16) places the echo level effects on the left-hand side and thosepertaining to the noise-masking background level on the other side. Equation(15.16) constitutes the active-sonar equation for the monostatic case, one in whichthe source and the receiving hydrophone are coincident and the echo from the targettravels back to the source. In some sonar applications, a bistatic arrangement is