Target time smearing with short transmissions and multipath propagation

Abstract

In active sonar the target echo level is often estimated with a propagation model that adds all multipath arrivals. If the (post-correlator) transmitted pulse is short compared to the multipath time spread then there is effectively an extra loss (which may be substantial) since only a few of the paths contribute to the target echo at any one instant. This well known "time-smearing" loss is treated in a self-consistent manner with previous calculations of reverberation [Harrison, J. Acoust. Soc. Am. 114, 2744-2756 (2003)] to estimate the target response and the signal-to-reverberation-ratio. Again isovelocity water, Lambert's law, and reflection loss proportional to angle are assumed. In this important short pulse regime the target response becomes independent of boundary reflection properties but proportional to transmitted pulse length. Thus the signal-to-reverberation-ratio becomes independent of pulse length. The effect on signal-to-ambient-noise is also investigated and the resulting formulas presented in a table.

Efficient modeling of range-dependent ray convergence effects in propagation and reverberation

“…Since angle is related to travel time, convergence can alternatively be viewed as a function of time, and this is important for the target echo because the angle contributions do not all arrive at the same time, leading to "target time smearing" and a corresponding weakening of the target response with respect to the reverberation (Harrison, 2011b). The impulse response of a point target, say, tends to have all the convergence effects concentrated near the first return with late arrivals tending to decay more or less monotonically with time (Harrison and Nielsen, 2007).…”