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Acoustic Systems for Defense
Figure 5. Left: source-sensor node geometry. Right: zoom acoustic locations of artillery fire. From Ferguson et al. (2002).
• When is the vehicle at the closest point of approach to the microphone?
• What is the range to the vehicle at the closest point of approach?
The solution to this particular problem, together with the general application of acoustic signal-processing techniques to extract tactical information using one, two, or three mi- crophones can be found elsewhere (Ferguson, 2016). The speed of the truck is 20 km/h and the tachometer reading is 2,350 rpm. It has 6 cylinders and its closest point of approach to the microphone is 35 m.
Locating the Point of Origin of Artillery and Mortar Fire
Historically, sound ranging, or the passive acoustic localization of artillery fire, had its genesis in World War I (1914-1918). It was the procedure for locating the point where an artillery piece was fired by using calculations based on the relative time of arrival of the sound impulse at several accurately positioned microphones. Gun ranging fell into disuse, and it was replaced by radar that detected the projectile once it was fired. How- ever, radars are active systems (which make them vulnerable to counter attack) and so the Army revisited sound ranging with a view to complement weapon-locating radar.
Figure 5, left, shows two acoustic nodes locating the point of origin of 206 rounds of 105 mm Howitzer fire by triangu- lation using angle-of-arrival measurements of the incident wave front at the nodes. Zooming in on the location of the firing point shows the scatter in the grid coordinates of the gun primaries due to the meteorological effects of wind and temperature variations on the propagation of sound in the atmosphere (Figure 5, right). Ferguson et al. (2002) showed the results of localizing indirect weapon fire using acoustic sensors in an extensive series of field experiments conducted during army field exercises over many years. This work in-
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formed the full-scale engineering development and produc- tion of the Unattended Transient Acoustic Measurement and Signal Intelligence (MASINT) System (UTAMS) by the US Army Research Laboratory for deployment by the US Army during Operation Iraqi Freedom (2003-2011). This system had an immediate impact, effectively shutting down rogue mortar fire by insurgents (UTAMS, 2018).
Locating a Hostile Sniper’s Firing Position
The firing of a sniper’s weapon is accompanied by two acous- tic transient events: a muzzle blast and a ballistic shock wave. The muzzle blast transient is generated by the discharge of the bullet from the firearm. The acoustic energy propagates at the speed of sound and expands as a spherical wave front centered on the point of fire.
The propagation of the muzzle blast is omnidirectional, so it can be heard from any direction including those directions pointing away from the direction of fire. If the listener is po- sitioned in a direction forward of the firer, then the ballistic shock wave is heard as a loud sharp crack (or sonic boom) due to the supersonic speed of travel of the projectile along its trajectory. Unlike the muzzle blast wave front, the shock wave expands as a conical surface with the trajectory and nose of the bullet defining the axis and apex, respectively, of the cone (see Figure 6, top left). Also, the point of origin of the shock wave is the detach point on the trajectory of the bullet (see Figure 6, top right). In other words, with respect to the position of the receiver, the detach point is the position on the trajectory of the bullet from where the shock wave emanates. The shock wave arrives before the muzzle blast so, instinctively, a listener looks in the direction of propagation of the shock wave front, which is away from the direction of the shooter. It is the direction of the muzzle blast that coin- cides with the direction of the sniper’s firing point.