Clear air turbulence (CAT) is the leading cause of in-flight injuries and in severe cases can result in fatalities. The purpose of this work is to design and develop an infrasonic array network for early warning of clear air turbulence. The infrasonic system consists of an infrasonic three-microphone array, compact windscreens, and data management system. Past experimental efforts to detect acoustic emissions from CAT have been limited. An array of three infrasonic microphones, operating in the field at NASA Langley Research Center, on several occasions received signals interpreted as infrasonic emissions from CAT. Following comparison with current lidar and other past methods, the principle of operation, the experimental methods, and experimental data are presented for case studies and confirmed by pilot reports. The power spectral density of the received signals was found to fit a power law having an exponent of −6 to −7, which is found to be characteristics of infrasonic emissions from CAT, in contrast to findings of the past.

1.
J.
Croft
, “
Truer pictures of turbulence
,”
Aerosp. Am.
34
38
(
2005
).
2.
Federal Aviation Administration Advisory Circular AC No. 00-30B, “
Atmospheric turbulence avoidance
,” September 9, 1997.
3.
M. J.
Lighthill
, “
On sound generated aerodynamically
,”
Proc. R. Soc. London, Ser. A
211
,
564
587
(
1952
).
4.
W. C.
Meecham
and
G. W.
Ford
, “
Acoustic radiation from isotropic turbulence
,”
J. Acoust. Soc. Am.
30
,
318
322
(
1958
).
5.
W. C.
Meecham
, “
On aerodynamic infrasound
,”
J. Atmos. Terr. Phys.
33
,
149
155
(
1971
).
6.
J. W.
Wescott
, “
Atmospheric background at high altitudes
,”
Proceedings of the Symposium on Atmospheric Propagation
, U. S. Army Sig. Missile Support Agency AD-408716 (1961), pp.
182
193
.
7.
E. S.
Posmentier
, “
1–16 Hz infrasound associated with clear air turbulence predictors
,”
J. Geophys. Res.
79
,
1755
1760
, doi: (
1974
).
8.
J. A.
McDonald
, “
Naturally occurring atmospheric acoustical signals
,”
J. Acoust. Soc. Am.
56
,
338
361
(
1971
).
9.
J. D.
Lawrence
,
M. P.
McCormick
,
S. H.
Melfi
, and
D. P.
Woodman
, “
Laser backscatter correlation with turbulence regions of the atmosphere
,”
Appl. Phys. Lett.
12
,
72
73
(
1968
).
10.
P.
Vrancken
,
M.
Wirth
,
D.
Rempel
,
G.
Ehret
,
A.
Dolfi-Bouteyre
,
L.
Lombard
,
T.
Gaudo
,
D.
Rees
,
H.
Barny
, and
P.
Rondeau
, “
Clear air turbulence detection and characterization in the DELICAT airborne lidar project
,”
Proceedings of the 25th Laser Radar Conference 2010 (ILRC 25)
, Curran Associates, Inc. (
2010
), pp.
301
304
.
11.
R.
Targ
,
M. J.
Kavaya
,
R. M.
Huffaker
, and
R. L.
Bowles
, “
Coherent lidar airborne windshear sensor: Performance evaluation
,”
Appl. Opt.
30
,
2013
2026
(
1991
).
12.
W. K.
Hocking
, “
A review of mesosphere-stratosphere-troposphere (MST) radar developments and studies, circa 1997-2008
,”
J. Atmos. Sol.-Terr. Phys.
73
,
848
882
(
2011
).
13.
A. J.
Bedard
and
T. M.
Georges
, “
Atmospheric infrasound
,”
Phys. Today
March,
32
37
(
2000
).
14.
A. J.
Zuckerwar
,
Q. A.
Shams
, and
K.
Knight
, “
Wind noise reduction in a non-porous subsurface windscreen
,”
J. Acoust. Soc. Am.
132
,
1905
(
2012
).
15.
A. J.
Zuckerwar
,
J. E.
Teter
, and
W. E.
Robbins
, “
Infrasonic pistonphone
,”
J. Acoust. Soc. Am.
115
,
2527
(
2004
).
16.
C. H.
Knapp
and
G. C.
Carter
, “
The generalized correlation method for estimation of time delay
,”
IEEE Trans. Acoust., Speech, Signal Process.
24
,
320
327
(
1976
).
17.
J. V.
Olson
and
C. A. L.
Szuberla
, “
Uncertainties associated with parameter estimation in atmospheric infrasound arrays
,”
J. Acoust. Soc. Am.
115
,
253
258
(
2004
).
18.
J. V.
Olson
and
C. A. L.
Szuberla
, “
The least squares estimation of the azimuth and velocity of plane waves
,”
Inframatics
6
,
8
12
(
2004
).
19.
G. K.
Subramanian
and
A.
Muschinski
, “
First observations of microbaroms with single absolute barometers
,”
J. Atmos. Oceanic Technol.
28
,
933
943
(
2011
).
20.
U.S. Standard Atmosphere, 1976
(
U.S. Government Printing Office
,
Washington, DC
,
1976
).
21.
B.
Plovsing
,
Nord 2000: Outdoor Sound Propagation Model. Part 2: Propagation in an Atmosphere with Refraction
(
Danish Electronics, Light, and Acoustics
,
Horsholm
,
2000
), Appendix A.
22.
V. E.
Ostashev
,
V.
Mellert
,
R.
Wandelt
, and
F.
Gerdes
, “
Propagation of sound in a turbulent medium. I. Plane waves
,”
J. Acoust. Soc. Am.
102
,
2561
2569
(
1997
).
23.
M. A.
Johnson
,
R.
Raspet
, and
M. T.
Bobak
, “
A turbulence model for sound propagation from an elevated source above level ground
,”
J. Acoust. Soc. Am.
81
,
638
646
(
1987
).
24.
G.
Golub
and
W.
Kahan
, “
Calculating the singular values and pseudo inverse of a matrix
,”
SIAM (Soc. Ind. Appl. Math.) J. Numer. Anal., Ser. B
2
,
205
224
(
1965
).
25.
G.
Peters
and
J. H.
Wilkinson
, “
The lease squares problem and pseudo-inverses
,”
Comput. J.
13
,
309
316
(
1970
).
26.
R.
Penrose
, “
On best approximate solution of linear matrix equations
,”
Proc. Cambridge Philos. Soc.
52
,
17
19
(
1955
).
27.
D. K.
Wilson
, “
Performance bounds for acoustic direction-of-arrival arrays operating in atmospheric turbulence
,”
J. Acoust. Soc. Am.
103
,
1306
1319
(
1998
).
28.
T. M.
Georges
and
W. H.
Beasley
, “
Refraction of infrasound by upper-atmospheric winds
,”
J. Acoust. Soc. Am.
61
,
28
34
(
1977
).
29.
A.
Le Pichon
,
M. A.
Garces
,
E.
Blanc
,
M.
Barthelemy
, and
D. P.
Drob
, “
Acoustic propagation and atmosphere characteristics derived from infrasonic waves generated by the Concorde
,”
J. Acoust. Soc. Am.
111
,
629
641
(
2002
).
You do not currently have access to this content.