Underwater high-resolution range-dimension imaging using comb spectrum signal

The range resolution of an imaging sonar is difficult to be improved due to the restriction of the instantaneous bandwidth of the sonar system. To increase the range resolution of the imaging sonar, an imaging method with the use of the comb spectrum signal, which is composed of a set of continuous wave (CW) pulses with different center frequencies, is proposed. After the round-trip propagation these CW components, although sharing the same propagation range, have different phase shifts due to the frequency offset between them. In other words, the phase differences occur at different ranges. Thus, the beamformer (which is traditionally applied to the angle dimension to filter out echoes reflected back from a certain angle) can be utilized to process these CW components in the range dimension. We call such beamformer the range-dimension beamformer. Similar to the adaptive beamformer applied to the angle dimension, the range-dimension adaptive beamformer is proposed to obtain a high range resolution. Furthermore, we give the scanning vector, the covariance matrix, and the expression of the range-dimension adaptive beamformer. Via numerical examples we show that the proposed method can increase the range resolution effectively under the restriction of a certain instantaneous system bandwidth.