Note: Descriptions are shown in the official language in which they were submitted.
CA 02770247 2012-03-02
HOMING SYSTEM AND METHOD FOR AN AUTONOMOUS
UNDERWATER VEHICLE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the first application filed for the present
invention.
MICROFICHE APPENDIX
[0002] Not Applicable.
TECHNICAL FIELD
[0003] The present invention relates generally to a homing systems for use
in a marine
environment, and particular to systems and methods for long range homing of
autonomous
underwater vehicles.
BACKGROUND
[0004] Autonomous underwater vehicles (AUV's) are used in a variety of
marine
environments to explore beneath the water's surface. For example, AUV's may be
used to
perform marine surveys and to explore geological features of a sea bed as well
as take various
measurements of the underwater environment. AUV's are typically operated by a
controller that
may be installed on a ship. The ship controlling the AUV's mission may be
referred to as a
controller ship. The controller may be stationed at the surface of the water
near to the area that
the AUV is exploring. The controller may also assist with entry and extraction
of the AUV into
the marine environment. The AUV may be lowered into the water by the
controller ship. When
the AUV has completed its mission, it will typically return to the controller
ship for recovery. The
AUV may be given an acoustic homing signal in order for the AUV to navigate to
the controller
ship for recovery.
[0005] Recently, with advances in AUV technology and in particular,
advances in fuel cells,
underwater vehicles may travel for tens or even hundreds of kilometres before
they must be re-
fuelled. Hence, AUV's may be, at times, great distances from a controller
ship. For example,
an AUV may travel 50 kilometres from a controller ship. The controller ship
may be immobile or
it may be disadvantageous for the controller ship to move depending on the
conditions of the
marine environment. For example, when exploring in the arctic, it may be
necessary to
introduce an AUV into the water through a hole in an ice sheet. With thick ice
covering, it may
be impracticable for a ship to follow the AUV. Moreover, with a thick ice
covering, the controller
ship may drift with the ice flow, so that the AUV must navigate to a location
for retrieval that is
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some distance from the point of launch. Hence, the homing system used to
extract the AUV
from the water must allow for transmission over reasonably large distances.
[0006] As is known in the art, low frequency acoustic signals suffer lower
attenuation than
high frequency acoustic signal, and so are favoured for use in long range
homing systems.
However, low frequency homing signals may coincide with the frequencies that
are generated
by other acoustic sources in the water (such as wave action, ships or wild-
life). The AUV may
become lost if it attempts to follow a false homing signal.
[0007] Techniques that overcome deficiencies in existing homing systems
remain highly
desirable.
SUMMARY
[0008] A submarine homing system comprises an acoustic emitter configured
to emit an
acoustic signal comprising at least two narrowband tones, each narrowband tone
having a
respective predetermined center frequency. The homing system further comprises
an acoustic
receiver configured to receive the acoustic signal from the acoustic emitter,
and to produce one
or more receiver signals. A processor is communicably coupled to the acoustic
receiver and is
configured to process the receiver signals to calculate the direction from
which the acoustic
signal was received by the acoustic receiver.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] Embodiments will now be described by way of example only with
reference to the
appended drawings wherein:
[0010] FIG. 1 is a block diagram schematically illustrating principal
elements of a
submarine homing system in accordance with a representative embodiment of the
present
invention; and
[0011] FIG. 2 is a chart schematically illustrating a representative tone
combination usable
in the submarine homing system of FIG. I.
[0012] It will be noted that throughout the appended drawings, like
features are identified by
like reference numerals.
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DETAILED DESCRIPTION
[0013] It will be appreciated that for simplicity and clarity of
illustration, where considered
appropriate, reference numerals may be repeated among the figures to indicate
corresponding
or analogous elements. In addition, numerous specific details are set forth in
order to provide a
thorough understanding of the example embodiments described herein. However,
it will be
understood by those of ordinary skill in the art that the example embodiments
described herein
may be practised without these specific details. In other instances, well-
known methods,
procedures and components have not been described in detail so as not to
obscure the
example embodiments described herein.
[0014] Turning to FIG. 1, there is shown a system 2 comprising an
acoustic emitter 4
configured to emit an acoustic signal 6 into a body of water 8, comprising at
least two narrow-
band tones, each narrow-band tone having a respective predetermined center
frequency; a
receiver 10 configured to receive acoustic signals and produce corresponding
receiver signals;
and a processor 12 configured to analyse the receiver signals to calculate at
least a direction
from which the acoustic signal 6 was received by the receiver 10. Each of
these elements may
be constructed of any suitable combination of hardware and software. Such
construction details
are considered to be well within the purview of persons of ordinary skill in
the art, and thus will
not be described in detail herein.
[0015] The body of water 8 may be referred to interchangeably as a
marine environment,
and may be a lake, river or ocean, or any other body of water in which an
acoustic homing
signal may be used.
[0016] The acoustic emitter 4 may be configured as any suitable
combination of hardware
and software configured to emit the acoustic signal 6 into the body of water,
and may have any
suitable form (e.g. sonobouy etc.). In some embodiments, the acoustic emitter
4 may comprise
a Super Subcomms Multi-Mode Pipe Projector (SSMMPP), which is known in the
art. The
SSMMPP is capable of generating a nearly omni-directional acoustic field
having an intensity of
190 dB at 1370 Hz. The acoustic emitter 4 may be configured to operate at or
near the water
surface 14, or may be lowered to a desired depth below the surface. It is
contemplated that the
acoustic emitter 4 will normally be associated with a controller ship or
vessel being used to
retrieve an Autonomous Underwater Vehicle (AUV), but this is not essential.
More generally,
the acoustic emitter 4 may be used in any application where it is desired to
provide an acoustic
beacon that can be reliably detected by the receiver at long range.
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[0017] The receiver 10 and processor 12 will normally be incorporated in an
Autonomous
Underwater Vehicle (AUV) 16, as shown in FIG. 1. However, this is not
essential. More
generally, the receiver and processor may be used in any application in which
it is desired to
detect an acoustic beacon at a long range from the emitter, and calculate a
direction back
toward the emitter.
[0018] As noted above, the acoustic signal 6 comprises a combination of at
least two low-
frequency narrowband tones. More generally, the emitter 4 may emit an acoustic
signal
comprising a combination of n (where n is an natural number, 1-12) low-
frequency narrowband
tones, which are selected from a set of m (where m is a natural number)
possible tones. The
number of possible tones, m, is primarily dependent on the range of
frequencies over which the
acoustic emitter 4 is designed to operate, as well as the desired spacing
between the center
frequencies of each of the possible tones. By way of example, if the acoustic
emitter 4 emits at
frequencies between 1000 Hz and 1800 Hz, and the spacing between the center
frequencies of
each tone is 50 Hz, m would be17.
[0019] FIG. 2 illustrates an example combination of n=4 tones 18-24 The
band-width of
each tone will typically be a function of the respective wave-function of each
tone. Any desired
wave-function may be used, including, without limitation, square-wave,
triangular wave, saw-
tooth, sinusoidal and combinations thereof. Preferably, the tones 18-24
forming a given
combination are selected such that each of the tones can be readily
distinguished from the other
tones at the receiver 10. Preferably, the frequency range or width of each
tone is minimized so
as to limit dispersive effects in the marine environment 8. In some
embodiments, each tone 18-
24 is composed of a pure-tone sinusoidal signal.
[0020] In some embodiments, each of the n tones 18-24 of a given
combination are emitted
sequentially in time, and in a predetermined order. In such embodiments, the
order in which the
tones are emitted is preferably selected such that the time series of tones in
any given
combination is unique, at least among a set of possible tone combinations that
can be emitted
by a given emitter 4. For example, the number of possible n=4 tone
combinations is (n-1)!=6,
meaning that 6 unique combinations (of n=4 tones each) can be constructed in
which no tone
combination is a mere rotation of any other tone combination in the set. This
enables the
processor 12 to use known signal processing techniques to detect the time
series of tones in the
noisy acoustic signal received by the receiver 10 and so identify the specific
tone combination
being sent by the emitter 4.
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[0021] In other embodiments, the n tones of a given combination may be
emitted
simultaneously. In such embodiments, the center frequency of each tone within
a given tone
combination is preferably selected to minimize interference with the other
tones of that tone
combination. This enables the processor 12 to use known signal processing
techniques to
identify the specific tone combination in the acoustic signal received by the
receiver 10. Thus,
for example, the processor 12 can distinguish the acoustic signal 6 from noise
arising from other
sources in the marine environment 8, including broadband noise from passing
ships.
[0022] In some embodiments, one or more tone combinations may encode
information. For
example, in some embodiments, a selected tone combination may be used as an
identifier
associated with either the emitter 4 or the receiver 10. With this
arrangement, the processor 12
can use known signal processing techniques to identify a desired one acoustic
signal 6 from
among two or more acoustic signals received by the receiver 10. Thus, for
example, the AUV
16 may operate in a marine environment 8 in which two or more emitters 4 are
being used.
When each emitter 4 is controlled to emit a respective acoustic signal having
a unique identifier
(tone combination), the processor 12 can identify and use the respective
acoustic signal 6 from
a selected one of the emitters 4. In some embodiments, a selected tone
combination may be
used as a command. With this arrangement, the acoustic signal can be used to
trigger desired
behaviours of a AUV associated with the receiver 10 and processor 12.
[0023] The acoustic receiver 10 generates a set of one or more receiver
signals indicative of
the local acoustic field in the vicinity of the receiver 10. This local
acoustic field will normally
include the acoustic signal emitted from the emitter 4 and noise from other
acoustic sources in
the marine environment. In some embodiments, the acoustic receiver 10
comprises a plurality
of acoustic transducers (such as, for example, acoustic hydrophones) arranged
such that the
receiver signals contain information that can be used to calculate a direction
from which a
received acoustic signal was received. One possible arrangement capable of
this operation
comprises seven acoustic transducers arranged in three orthogonal 3-element
arrays, wherein
each of the orthogonal arrays shares a common center transducer. With this
arrangement,
each 3-element array generates respective detector signals that are indicative
of the acoustic
field component in a respective orthogonal axis, so that the direction from
which a selected
acoustic signal was received can be calculated from the relative intensities
of the respective
detector signals obtained from each of the three arrays. For example, the
processor 12 may
process the respective detector signals from each array, as described above,
to identify the
desired acoustic signal and determine the intensity of that acoustic signal as
detected by each
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transducer array. The respective intensities of the desired acoustic signal
detected by the three
arrays can then be used to calculate the direction (e.g. horizontal and
vertical angles) from
which the desired acoustic signal was received. Multi-element transducer
arrays of the type
described are known in the art, and so will not be further described herein.
[0024] In an embodiment in which one or more tone combinations are used to
encode
information in the form of commands, the processor 12 may also operate to
compare the tone
combination of the received acoustic signal to a set of predetermined tone
combinations (for
example stored in a memory), to identify a specific command encoded in the
acoustic signal.
The identified command can then be passed to a controller unit of the AUV 16
for execution.
[0025] Although the above has been described with reference to certain
specific example
embodiments, various modifications thereof will be apparent to those skilled
in the art without
departing from the scope of the claims appended hereto.
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