Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Reexamination Certificate
1999-07-19
2001-01-30
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
C367S103000, C367S119000, C367S122000
Reexamination Certificate
active
06181643
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to interferometers, such as sonars, that are used to detect and locate objects, and more particularly to interferometers having a single projector array.
BACKGROUND OF THE INVENTION
Interferometers, such as sonars, can be used to detect and locate objects. An active sonar projects a beam-like sound wave signal into the water, and an object that is in the path of the signal reflects a portion of the signal back to the sonar. The signal reflected back to the sonar is detected by the sonar and utilized to locate the object in the water.
FIG. 1
diagrammatically illustrates a conventional active sonar system
10
that is being used to locate an object
12
positioned below the surface of a body of water
14
. The conventional sonar system
10
includes a projector
16
that projects a sound wave signal s(t) into the water
14
. Portions of the signal projected from the projector
16
are reflected from the object
12
to a lower receiver
18
and an upper receiver
20
. The lower receiver
18
and the upper receiver
20
are separated by a vertical separation distance “d”. A direction of return arrow
22
illustrates the direction of travel of the reflected portions of the signal (that is, the echoes) that are received by the upper receiver
20
. A vertical arrival angle &thgr; that partially defines the position of the object
12
is estimated by measuring the difference in the time of arrival of the echoes at the lower receiver
18
and the upper receiver
20
. The vertical arrival angle &thgr; is the angle of intersection between a line perpendicular to the direction of the return arrow
22
and the vertical plane of the receivers
18
and
20
. The vertical arrival angle &thgr; is solved from:
D=d
sin(&thgr;)/
c
Inputs for solving for the vertical arrival angle &thgr; from the above equation consist of:
replica of output from the projector
16
:
r
1
(
t
)=
s
(
t
)+
n
1
(
t
)
echo at the lower receiver
18
:
r
2
(
t
)=
As
(
t
−&tgr;)+
n
2
(
t
)
echo at the upper receiver
20
:
r
3
(
t
)=
As
(
t−&tgr;−D
)+
n
3
(
t
)
In the immediately preceding equations, n
1
(t) represents noise and is typically zero for a noise-free replica. In addition, n
2
(t) and n
3
(t) are uncorrelated zero mean white Gaussian noise waveforms. The signal s(t), which is projected, or transmitted, from the projector
16
, is a pulse of length T seconds. The term “A” is the amplitude of the signal s(t). The delay “&tgr;” is the delay between transmission of the signal s(t) and reception of the corresponding echo at the lower receiver
18
. The delay “D” is the delay between the reception of the echo at the lower receiver
18
and the upper receiver
20
. The term “c” is a constant representing the speed of the projected signal s(t) through the respective medium, which in accordance with the preferred embodiment of the present invention is the speed of sound in water.
The conventional sonar system
10
requires both of the receivers
18
and
20
in order to determine the vertical arrival angle &thgr;. It is typical for each of the receivers
18
and
20
to include an array of many receiver elements, such as forty-eight receiver elements, and for each of the receiver elements to require its own analog conditioning electronics and analog-to-digital converter. That is, each of the receivers
18
and 20 includes an array of tens of hydrophones, and each hydrophone requires a separate electronics assembly. Further, digital processing circuitry is required for each of the receiver elements. Additionally, it is typical for the projector
16
to include an array of many projector elements, such as sixteen projector elements and for each projector element to require its own analog conditioning electronics and digital-to-analog converter. That is, the projector
16
includes an array of transducers, and each transducer requires a separate electronics assembly. Further, digital processing circuitry is required to present the desired signal to each of the projector elements.
Conventional sonar systems
10
are often deployed on watercraft, such as unmanned undersea vehicles, and in some cases the resources associated with the watercraft, such as space, power, weight and processing capability, are limited. Thus, the inclusion of many receiver elements, and their associated electronics assemblies and digital processing, can be prohibitive. In addition, the cost of the electronics for all of the parallel processing channels can be substantial.
It is also known for an interferometer to have two projectors and one receiver. For example, see H. Messer, G. Singal, and L. Bialy,
On the Achievable DF Accuracy of Two Kinds of Active Interferometers,
IEEE Transactions on Aerospace and Electronic Systems, Vol. 32, No. 3, July 1996, pp. 1158-1164, which is incorporated herein by reference.
The use of a sonar system having two separate projector arrays, each having many projector elements and their associated electronics assemblies and digital processing, and one receiver array having many receiver elements and their associated electronics assemblies and digital processing could be prohibitive in situations where space, power, weight and processing resources are limited. And again, the cost of the electronics for all of the parallel processing channels can be substantial. In addition, when using two separate projector arrays with many projector elements, each projector array may interfere with the other projector array by at least partially physically blocking the signals of the other projector array.
SUMMARY OF THE INVENTION
The present invention solves the problems identified above and provides other advantages by providing an interferometer, or more particularly a sonar system, which preferably has a single projector array and a single receiver array. The projector array comprises a first subset of projector elements that function as a first pseudo projector and a second subset of projector elements that function as a second pseudo projector. One or more of the projector elements of the projector array belong in both the first and the second subsets, such that one or more of the projector elements function as part of both the first and second pseudo projectors. This allows the system of the present invention to perform similarly to conventional systems while requiring less resources, such as less space, power, weight and processing capability, than conventional systems. The present invention also provides a system that is improved with respect to hardware and software complexities, cost, and reliability in comparison to conventional systems.
More specifically, the interferometer or sonar system of the present invention includes at least one projector assembly that includes a plurality of projector elements. At least a first and a second of the projector elements are each operative to provide a first signal so that at least the first and second projector elements function together as a first pseudo projector that provides the first signal. At least a third of the projector elements and the second projector element are operative to provide a second signal so that at least the second and third projector elements function together as a second pseudo projector that provides the second signal. Thus, the second projector element functions as part of both the first pseudo projector and the second pseudo projector while the first and third projector elements function only as part of the first pseudo projector and the second pseudo projector, respectively. A separation distance is defined between the centers of the first and second pseudo projectors. The first and second pseudo projectors are operative to contemporaneously provide the first and second signals, such that the second projector element provides a third signal that is a sum of the first and second signals. The first and second signals are preferably orthogonal sound waves that are provided periodically for a duration that is shorter than the period
Alston & Bird LLP
Lobo Ian J.
The Boeing Company
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