Page 14 - Spring 2019
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Anoustil: Systerna for Defense >fl/Ill Q‘
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Compiling the Air Picture While at Depth 
During World War II, aircraft equipped with centi.rneter- \ U "" :-===w=- “" "“‘ ‘
wavelength radars accounted for the bulk of Allied defeats of
of U-boats from 1943 to 1945. U-boats spent most of their 55’)
time surfaced, running on diesel engines and diving only ‘
when attacked or for rare daytime torpedo attacks (Lans-
ford and Tucker, 2012).
11-. 1937_ a series of at.5ea experiments using Australian Figure 3. Left: cnntributinns made by un acnustic saurce in the uir ta
submarines and maritime patrol aircraft demonstrated the the -“’"”'tfi"’i't “t “ ’“9iV9" i" the 5997- F’0’" Wick (1972): Right-' W)’
detection, classification, localization, and tracking of aircraft PmhSf0r.u.b0mm immce (gins. urmws)’ direct mfmmwl (“d W-

_ _ rows), crmcal ungle where the refracted my lies ulnng the sea surface
“sing “ t°Wed “r”‘Y dePi°Yed f’°"‘ “ 5“i’"”‘ri“e (F°’3“s°“ (black arruws),am1sea surface reflectian (blue arrows). see text fur
and Speechley, 1989). The results subsequently informed the fimim expiummmh
full-scale engineering development of the Automated Threat
Overflight Monitoring Sysiem (ATOMS) for the Us Navy aircraft and then extract tactical information on the speed
submarine force‘ ATOMS offers early warning/long-range (using the Doppler effect), altitude (using the rate of change
detedion of ihreai aircraft by submerged submarines via of the instantaneous frequenc ) and identification (from the

. Y >

‘Owed arrays (Fnedmam 2006)‘ source/rest frequency of the blade rate) of the aircraft (Fer-
in a Serruna] naoer’ Urick (1972) indicated the oossihie exis_ guson, 1996). For a submariner, the upside of an aircraft on
tence of un to four separate contributions to the underwater top is identification of the aircraft and its mission profile by
sound field created by the presence of an airborne acoustic Preeessirig the °“tP“t or a sirigie hYdr°Ph°he With°“t giV'
source_ Figure 3’ [2/7, depicts each of these contributions; ing away the position of the submarine; the downside is that
direct refraction, one or more bottom reflections, the eva- there is ‘"3 eariY Warning-
heseeht WaVes a-“ti s°“hd scattered tr°rri a r °“Eh sea s“rfaee- The long-range detection of a submarine relies on reception of
When the aircraft flies overhead, its radiated acoustic noise the radiated noise from the aircraft after undergoing one (or
travels via the direct refraction path where it is received by a rnore) reflections from the seafloor (Ferguson and 5neechiey_
hYrir°Ph°he (after trahsrriissieh aer°ss the air'sea ihterfaee)- 2009). The intensity of the noise from the aircraft received via
N°Wa the ratio etthe sPeed ‘it sehhd ih air t0 that ih Water is direct refraction is considerably stronger (by 20 to 30 dB) than
0-223 at a eritieai ahgie etiheidehee (0,) : 13°, the “Eek W‘ for propagation involving a seafloor reflection, so a towed ar-
"W5 i“ Figiire 3 (the iheideht ray ih air ahd the refracted ray ray is required to detect the Doppler-shifted propeller blade
a-i°hB the sea surtaee) rielsiet the Pr°PaBati°h Path eta ei'iti' rate (and its harmonics) and to measure its angle of arrival
cal ray. The transmission of aircraft noise across the air-sea (hearing)_ A suhrnarine with its towed array denioyed has
interface occurs when the angle of incidence is less than fir rnany rninutea warning ofan annroaehing aircraft_ in Figure
(Figiire 3- red ‘"'"’”’s)- For ahgies °fiheidehee greater thah 3, right. the green arrows depict a bottom bounce path that
(at, the radiated noise of the aircraft is reflected from the sea enables the 1ong_range detection ofan aircraft
surface, with no energy being transmitted across the air-sea Fjfimufin the Instantaneous Run 2 "fa Contact
interface (Figure 3’ blue arrows)‘ The towedsarray sonar from a subrrigarine is only able to mea-
The reception of noise from an aircraft via the direct path sure the bearing of a contact. To estimate its range, the subma-
relies on the aircraft being overhead. This transitory phe- rine must undertake a maneuver to get another fix on the con-
nomenon lasts for a couple of seconds; its intensity and du- tact and then triangulate its position. This process takes tens of
ration depend on the altitude of the aircraft and the depth minutes. Another approach is to use wide-aperture array sonar,
of the receiver. Submariners refer to the overhead transit as which exploits the principle of passive ranging by wave front
an aircraft “on top.” Urick (1972) estimated the altitude of cunmture to estimate the bearing and iange of the contact at
the aircraft by measuring the intensity and duration of the any instant, without having to perform a submarine maneuver
acoustic footprint of the aircraft. An alternative approach (see Figure 4, left). The radius of curvature of the wave front
is to measure the variation in time of the instantaneous equates to the iange. Measurement of the differences in the ar-
frequency corresponding to the propeller blade rate of the rival times (or time delays ‘rrzand 123) of the wave front at two
12 1 Acuulclcl Tbday 1 Spring 2019

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