Abstract: A simplified way of interpolation implementation for real time processing is presented. For passive localization the time delay can he determined by the cross-correlation peak. It is more eff ficient today to estimate cross-correlation function by computing the inverse FFT of cross-spectral density. The original sampling rate is as low as possible in order to reduce the computation. So the sampling rate of the cross-correlation computed is lower than the requirement of time delay estimate and a interpolation procedure is needed to raise the rate. The interpolation by zero augmented spectrum is really concise, fast and accurate. The results of the computer simulation and real underwater signal processing are shown in the paper.