Communications – electrical: acoustic wave systems and devices – Underwater system
Reexamination Certificate
1982-06-28
2001-11-13
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Underwater system
Reexamination Certificate
active
06317388
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to systems for the generation of acoustic signals and more particularly to the generation of acoustic signals at a distance from a generating platform by using thermoacoustic effects.
BACKGROUND OF THE INVENTION
A submarine is a ship that can operate both on the surface of the water and completely submerged. In order to avoid detection by radar, surface ships and air patrols, a submarine is usually submerged. Modern submarines have the capability of remaining submerged for long periods of time. In fact, a modern submarine can circumnavigate the earth while running submerged. Thus, modern submarines may complete large portions of their missions while being submerged. In order to communicate or check the course of their own submarines, Navies want to determine the current location of their own submarines, without revealing the submarine's present location. Navies also want to determine the location of foreign submarines without revealing the location of the Navy's fleet.
One method utilized by the prior art for achieving the foregoing involved the placing of active and passive buoys in the water. The active buoys projected pulses sound into the water. These sound pulses would travel through the water until they hit an object, at which time the sound pulses would be reflected by the object and possibly detected by the passive buoys as an echo. The speed of sound in water is known. Thus, the range and bearing to the unknown object may be determined by triangulating the echoes received by three passive buoys.
For an echo produced by a submarine to be detected by passive buoys, the echo must be of greater strength than the other interfering echoes. The interfering echoes are caused by the noise produced by the passive buoys or by the reverberations produced by the sound generated by the active buoys. Reverberations are all the echoes returned to an active sonar system from the ocean itself. This includes the suspended marine organisms in the ocean as well as the irregularities of the ocean's bottom. A sonar operator will usually hear reverberations as quavering rings and echoes which directly bounce off submarines as pings. Hence, the sonar operator usually was able to distinguish between reverberations and echoes that were directly reflected by submarines.
Unfortunately, echoes do not travel in straight paths in the upper ocean where the active buoys are located. Echoes travel in the upper ocean in curved paths. Thus, the echoes produced by the submarine were usually quite weak (poor target illumination) and it was difficult to locate the submarine. Even though echoes travel in curved paths, it was possible to locate the submarine by triangulation if conditions were just right, i.e. the buoys and submarine must be in the right location for the sound which was produced by the active buoys to be reflected by a submarine and detected by a passive buoy. If no echoes are reflected by a submarine and then detected by the passive buoys, the submarine is not illuminated and its location is unknown. Sometimes, due to the location of the buoys and the submarine, there was an ambiguity in the location of the submarine. According to the detection buoys, the submarine was in two places at the time (physical impossibility). Since the sonar usually did not know the present location of the deployed buoys and he was unable to redirect the sound generated by the active buoys, it was very difficult to resolve the above ambiguity. Other disadvantages of the foregoing systems were that: buoys had a range of approximately one mile and as the submarine traveled the submarine might travel away from the buoy necessitating the dropping of another buoy so that the transmission between the aircraft or surface ship and the buoy might be continued; the active buoys were heavy; if the buoys were carried by an aircraft, the buoys would require a large amount of space and add to the weight of the aircraft which would reduce the amount of other equipment the aircraft would carry and/or reduce the aircraft's range; and the buoys were expendable which meant that the location of a submarine was relatively expensive.
SUMMARY OF THE INVENTION
This invention overcomes the disadvantages of the prior art by creating a sonar system that utilizes a thermoacoustic bi-static source as an active element and passive sensors as passive elements. The active element may be aboard an airborne platform, spaceborne platform or a surface vessel so that its location may be easily changed; and its passive elements may be aboard the aforementioned vehicles or the passive elements may be floating on the surface of the water. Thus, the moveable thermoacoustic source is used to generate acoustic signals from a platform into the water and the passive elements are used to detect the echoes produced by the acoustic source.
The apparatus of this invention utilizes the direct conversion of EM or particle Kinetic energy into acoustic energy by using either a pulsed infrared wavelength laser or particle beam which is fired into the water from an aircraft or satellite. The physical mechanisms producing sound are of two kinds: (1) thermal expansion of the water from heat generated by medium attenuation of a pulse of laser light or impinging particles, or (2) explosive vaporization of a small volume of water when the heat deposited by the laser or particle beam is large enough to raise the local water temperature above boiling threshold. Infrared laser light is usually used because of its high attenuation coefficient in water, which causes high thermal densities. The level of sound produced by infrared lasers is sufficient for communications at expected ranges of communication buoys. Infrared lasers may be controlled (modulated) to the extent required for an underwater communications system. Typical data rates are ~1-10 bits per second.
Modulation schemes which may be employed are on-off keying (OOK), pulse duration modulation (PDM), pulse amplitude modulation (PAM), and frequency shift keying (FSK). The foregoing modulation schemes may be used for lasers and particle beams.
When the density of the heat energy deposited by laser beam absorption is less than that required to vaporize a local volume of water (~2500 joules/cm
3
) the acoustic pressure at radial distance R and polar angle &thgr; from the beam impact point at the water surface is given by the following expression:
P
(
R
,
t
,
θ
)
=
k
2
π
∫
-
∞
+
∞
ⅆ
ω
M
(
ω
)
ω
2
exp
[
-
j
(
ω
t
-
R
/
c
o
)
]
·
sin
θ
where: k=&bgr;I
o
/(4&pgr;Rc
o
C
p
)
C
o
=speed of sound
C
p
=specific heat of water
I
o
=laser power output
t=time
&bgr;=thermal expansion coefficient of water
Here M(&ohgr;) is the Fourier transform of the modulation, and I
o
the laser power output prior to modulation. The above expression assumes that the useful portion of the acoustic signal is transmitted at a frequency with wavelength smaller than either the beam spot size or absorption depth.
If the modulation is a gaussian pulse
M
(
t
)
=
M
o
2
π
σ
t
exp
[
-
t
2
/
σ
t
2
]
where &sgr;
t
{tilde over (=)}(one-half of the laser pulse width). The Fourier transform of P(R,&thgr;) is proportional to the function
F(&ohgr;)=&ohgr;
2
exp[−&ohgr;
2
&sgr;
t
2
].
The frequency (&ohgr;
p
) when the spectral energy is the acoustic pulse peak is
ω
p
=
1
3
σ
t
-
1
as can be found by setting the derivative of F(&ohgr;) equal to zero.
Thus, the duration of the laser pulse (2&ohgr;
t
) controls the spectral W
p
. The bandwidth of the signal can be controlled by firing the laser a number of times at a repetition interval less than or equal to the duration of an acoustic pulse produced by a single laser pulse, or by simply lengthening the pulse duration for a
Carey Charles A.
Jensen Richard A.
Woodsum Harvey C.
Gomes David W.
Lockheed Martin Corporation
Pihulic Daniel T.
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