Page 17 - Spring2019

Figure 6. Top: left, image showing the conical wave front of the ballistic shock wave at various detach points along the trajectory of the su- personic bullet; right, source-sensor geometry and bullet trajectory. Bottom: left, source of small arms fire; right, “N”-shaped waveforms of ballistic shock waves for increasing miss distances. From Ferguson et al. (2007).
Detecting the muzzle blast signals and measuring the time difference of arrival (time delay) of the wave front at a pair of spatially separated sensors provides an estimate of the source direction. The addition of another sensor forms a wide aper- ture array configuration of three widely spaced collinear sen- sors. As discussed in Estimating the Instantaneous Range of a Contact for submarines, passive ranging by the wave front curvature involves measuring the time delays for the wave front to traverse two pairs of adjacent sensors to estimate the source range from the middle sensor and a source bearing with respect to the longitudinal axis of the three-element ar- ray. Also, by measuring the differences in both the times of arrival and the angles of arrival of the ballistic shock wave and the muzzle blast enables estimation of the range and bearing of the shooter (Lo and Ferguson, 2012). In the ab- sence of a muzzle blast wave (due to the rifle being fitted with a muzzle blast suppressor or the excessive transmission loss of a long-range shot), the source (Figure 6, bottom left) can still be localized using only time delay measurements of the shock wave at spatially distributed sensor nodes (Lo and Fer- guson, 2012). Finally, Ferguson et al. (2007) showed that the caliber of the bullet and its miss distance can be estimated using a wideband piezoelectric (quartz) dynamic pressure transducer by measuring the peak pressure amplitude and duration of the “N” wave (see Figure 6, bottom right).
High-Frequency Sonar
Tomographic Imaging Sonar
The deployment of sea mines threatens the freedom of the seas and maritime trade. Sea mines are referred to as asym- metric threats because the cost of a mine is disproportionately small compared with the value of the target. Also, area denial
is achieved with an investment many times less than the cost of mine-clearing operations.
Once a mine like object (MLO) is detected by a high-fre- quency (~100 kHz) mine-hunting sonar, the next step is to identify it. The safest way to do this is to image the mine at a suitable standoff distance (say 250 m away). The acoustic image is required to reveal the shape and detail (features) of the object, which means the formation of a high-resolution acoustic image, with each pixel representing an area on the object ~1 cm long by ~1 cm wide. The advent of high-fre- quency 1-3 composite sonar transducers with bandwidths comparable to their center frequencies (i.e., Q ≈ 1) means that the along-range resolution δr ≈ 1 cm. However, real ap- erture-receiving arrays have coarse cross-range resolutions of 1-10 m, which are range dependent and prevent high- resolution acoustic imaging. The solution is to synthesize a virtual aperture so that the cross-range resolution matches the along-range resolution (Ferguson and Wyber, 2009). The idea of a tomographic imaging sonar is to circumnavigate the mine at a safe standoff distance while simultaneously insonifying the underwater scene, which includes the object of interest. Rather than viewing the scene many times from only one angle (which would improve detection but do noth- ing for multiaspect target classification), the plan is to in- sonify the object only once at a given angle but to repeat the process for many different angles. This idea is analogous to spotlight synthetic aperture radar.
Tomographic sonar image formation is based on image re- construction from projections (Ferguson and Wyber, 2005). Populating the projection (or observation) space with mea- surement data requires insonifying the object to be imaged
Spring 2019 | Acoustics Today | 15