Page 16 - Spring 2019
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Acuustic Systems fur Defense
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Figure 5. Left: source-sensor node geometry. Right: zoom acoustic locations ofartilleryfire. From Ferguson et ol. (2002).
. When is the vehicle at the closest point of approach to the formed the full-scale engineering development and produc-
microphone? tion of the Unattended Transient Acoustic Measurement and
. What is the range to the vehicle at the closest point of Signal Intelligence (MASINT) System (UTAMS) by the US
approach? Army Research Laboratory for deployment by the US Army
The solution to this particular problem, together with the durmg F)Femu_°n Iraq‘ Freedm-n_ (2003-201_1)' This system
general application of acoustic signal-processing techniques had an émigethaie unpacibifiifi/igleglfélunmg down rogue
to extract tactical information using one, two, or three mi- mortar re Y insurgents ( ’ )'
Cr°P:‘°rt‘_e:] can :1: _f°““1‘:“‘:1}s]ew}:::: (Fe:E“5°“’ 2016c)h:nTh_e Locating a Hostile Snipefs Firing Position
517" ° 1 3 tr“ ‘5 20 I 3“ 9 ‘“ °met°1' '93 3 ‘5 The firing of a sniper’s weapon is accompanied bytwo acous-
2350 mm‘ It has 6 Cylinders and M C1053“ Pom‘ °f awmach tic transient events: a muzzle blast and a ballistic shock wave.
t0 ‘he mi“°Ph°“e is 35 m‘ The muzzle blast transient is generated by the discharge of
Locating the Point of Origin ofArtillery and Mortar Fire the bullet from the firearm. The acoustic energy propagates
Historically, sound ranging. orthe passive acoustic localization at the speed of sound and expands as a spherical wave front
of artillery fire, had its genesis in World War I (1914-1918). centered on the point of fire.
h'_was ‘he Pmcedurh for locah-'h_g the Pohh when an hnhiery The propagation of the muzzle blast is omnidirectional, so it
Piece  fired by “sm_g Cakulahohs based on the relahhh "me can be heard from any direction including those directions
°f_a‘r"va1°fthe sound "hpulse hi §eve_m1 accumhhy Posmohed pointing away from the direction of fire. If the listener is po-
mlcmphohes‘ Gun mhgmg fen hh°_ dmlse‘ aha it was replaced sitioned in a direction forward of the firer, then the ballistic
by radar ‘hh dflecfed the Prolechie ohm h was fired‘ H°w' shock wave is heard as a loud sharp crack (or sonic boom)
ever’ radars are “live syhems (which make them vulnerable due to the supersonic speed of travel of the projectile along
to Coumer attack) and so the Army “wished sound ranging its trajectory Unlike the muzzle blast wave front the shock
with aviewto c°mP1emeh"weaP°h'1°Cahhg radar‘ wave expands as a conical surface with the trajectory and
Figure 5, left, shows two acoustic nodes locating the point nose of the bullet defining the axis and apex, respectively, of
of origin of 205 rounds of 105 mm Howitzer fire by triangu- the cone (see Figure 6, top left). Also, the point of origin of
lation using angle-of-arrival measurements of the incident the shock wave is the detach point on the trajectory of the
wave front at the nodes. Zooming in on the location of the bullet (see Figure 6, top right). In other words, with respect
firing point shows the scatter in the grid coordinates of the to the position of the receiver, the detach point is the position
gun primaries due to the meteorological efl’ects of wind and on the trajectory of the bullet from where the shock wave
temperature variations on the propagation of sound in the emanates. The shock wave arrives before the muzzle blast so,
atmosphere (Figure 5, right). Ferguson et al. (2002) showed instinctively, a listener looks in the direction of propagation
the results of localizing indirect weapon fire using acoustic of the shock wave front, which is away from the direction of
sensors in an extensive series of field experiments conducted the shooter. It is the direction of the muzzle blast that coin-
during army field exercises over many years. This work i.n- cides with the direction of the sniper’s firing point.
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