a system is presented for enhancing sonar signals wherein doppler echoes eived in the time domain from a sonar system are processed by digitizing , shifting to baseband , fourier transforming to the frequency domain , applying a frequency stretching algorithm designed to expand low doppler signals , retransforming the signal back to the time domain , reshifting from baseband to original center frequency , converting to analog and outputting the resultant enhanced doppler time domain signals to either earphones of a sonar operator for audio monitoring or for further processing for visual display .