The Dependence of Signal Coherence on Sea-Surface Roughness for High and Low Duty Cycle Sonars in a Shallow-Water Channel

It is anticipated that high duty cycle (HDC) sonars will typically maintain the same bandwidth as the pulsed active sonars (PASs) that they might replace. This will significantly increase their time-bandwidth product, but may not produce the increased gain anticipated, if there are coherence limitations of the acoustic channel. To compare performance of HDC with conventional PAS in the littorals, a set of experiments was conducted as part of the Target and Reverberation Experiment in spring 2013 (TREX13). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer range target echoes from an air hose. The Pearson product-moment correlation coefficient (Pearson's R) was used to confirm significance of the results. Measurements showed that for an 18-s HDC pulse, the mean (coherent) component of the specular arrival decreased by as much as 5 dB as root mean square (rms) surface roughness increased, whereas the 0.5-s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean levels were used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1-dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for coherence loss from the matched filter were successfully used to interpret the results.