Patent Description:
Sonar or sound navigation ranging is a technique for the detection of objects under water and for measuring the water's depth by emitting sound pulses and detecting or measuring their return after being reflected. Passive sonar consists of listening for the sound made by vessels, while active sonar includes emitting pulses of sound (e.g., by a using an acoustic transponder) and listening for echoes.

One application for sonar technology is deep ocean observation (e.g., seabed imaging and mapping). Performing observations near the surface are difficult because of interference from surface reflections. Thus, currently, deep ocean observation is performed by systems near the sea bed. One such solution is a tow vehicle equipped with sonar that is towed behind a ship or vessel. The tow vehicle transmits pulsed signals (e.g., pulsed acoustic signals) that are reflected back from the bottom and objects on the seafloor. The tow vehicle has sensitive receivers (e.g., hydrophones) that receive the returning sound. The signals are then processed to generate an image based on the strength of the returned sound over the area the tow vehicle was sending the sound. However, the long tow lines produce a lot of drag that lifts the tow platform away from the seabed, meaning that towed operations in deep water may only be conducted at slow tow speeds. Another solution is the use of battery-powered autonomous unmanned vehicles (AUVs), which can travel faster. However, the batteries on such devices have limited capacity and much energy is expended getting down to and back from the sea floor, which reduces the range of such AUVs. Furthermore, the proximity to the sea floor also limits the effective area of coverage. Therefore, the area coverage rate of existing deep ocean observation systems is small, especially when compared to the vastness of the ocean.

What is desired is a solutions that provides an improved capability for search and protection of the seabed and seabed infrastructure.

The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not constitute prior art. <CIT> describes a method and system for reducing unwanted noise components/interfering targets detected through an ambiguous beam-steer direction, such as the ambiguous 'back-lobe' of a sensor array. <CIT> describes a torpedo seeker head for providing guidance information in three orthogonal axes is provided according to the invention. The torpedo seeker head includes three directional hydrophones, with each directional hydrophone capable of receiving an acoustic signal and generating a signal output therefrom. The three directional hydrophones are aligned with their response axes oriented substantially orthogonally, with any two directional hydrophones and respective axes defining a plane. An omni-directional hydrophone is also provided which measures a phase of a received acoustic signal. The signal outputs from the any two directional hydrophones, in conjunction with the phase from the omni-directional hydrophone are used to calculate a look angle to a target in a plane defined by the any two hydrophones, with the measurement being substantially independent of frequency.

Aspects of example embodiments of the present disclosure are directed to a submerged sensing system capable of cancelling or substantially reducing surface noise using a triplet cardioid. In some embodiments, the submerged sensing system includes an active towed array of hydrophone triplets capable of forming a cardioid null in a desired direction. The submerged sensing system may predict the incoming angle of unwanted surface return and reject it by pointing the cardioid null in that direction. The triplet element may steer the cardioid null in the direction of surface pulse reflection by adjusting one or more complex weights based on knowledge of the geometry of the tow, water depth, tow depth, etc..

According to the invention, there is provided a sensor system including the features set out in claim <NUM>.

In some embodiments, the processor is configured to receive, without nullifying, other signals from directions different from the incidence direction of the incoming signal, the other signals being received by the triplet element at a same time as the incoming signal.

In some embodiments, the first phase is different from the second phase.

In some embodiments, the incoming signal is a surface reflection of an acoustic signal off of a seabed.

In some embodiments, one end of each of the hydrophones is positioned along a circumference of a circle.

In some embodiments, the first to third hydrophones are spaced at equal intervals.

In some embodiments, a radius of the circle is less than or equal to <NUM>/<NUM> of a wavelength of the incoming signal.

The processor is configured to determine an incidence direction of the incoming signal based on a depth of the triplet element, a water column depth, a timing of transmission of a signal being reflected back from a seabed and a water surface as the incoming signal.

In some embodiments, the processor is configured to determine a first incidence direction of the incoming signal at a first time, to determine a second incidence direction of the incoming signal at a second time, and to steer the cardioid null from the first incidence direction to the second incidence direction to reject the incoming signal over time.

In some embodiments, the processor is further configured to calculate an output response of the sensor system as a weighted summation of intensities of signals received at the first to third hydrophones, wherein weights of the weighted summation are based on a radius of the triplet element and an angular location of the cardioid null generated by the sensor system.

In some embodiments, the processor is further configured to calculate an output response of the sensor system as:
<MAT>
where θ represents an incidence angle of the incoming signal, Phasor represents a signal seen by each of the hydrophones, Xn and Yn represent coordinates of each of the hydrophones of the triplet element in a cross-plane orthogonal to the first direction, and <MAT> represents a null-rotating term for steering the null in a particular direction.

In some embodiments, the Phasor is expressed by:
<MAT>
where x and y represent coordinates of a corresponding one of the hydrophones and λ represents a speed of sound at a water depth of the triplet element.

In some embodiments, in the null-rotating term, <MAT>, α□ and cτ□ are based on an angular location of a null generated by the sensor system, and λ represents the speed of sound at a water depth of the triplet element.

According to some embodiments of the present disclosure, there is provided a sensor system including: a plurality of triplet elements coupled together along a first direction, a triplet element of the plurality of triplet elements including a first hydrophone, a second hydrophone, and a third hydrophone configured to receive an incoming signal at a first phase, a second phase, and a third phase, respectively, the first to third hydrophones extending along a first direction; and a processor configured to determine an incidence direction of the incoming signal, and to dynamically generate a cardioid null in the incidence direction to reject the incoming signal based on the incoming signal at the first to third phases.

In some embodiments, the sensor system is configured to be towed by a tow body at a particular depth below a water surface, the tow body being towed by a surface vehicle.

In some embodiments, the tow body includes a transmitter configured to generate a pulsed acoustic signal emitted from sides of the tow body, and wherein the incoming signal is a reflection of the pulsed acoustic signal from a seabed and a water surface as the incoming signal.

In some embodiments, a radius of the triplet element is less than or equal to <NUM>/<NUM> of a wavelength of the incoming signal.

In some embodiments, the processor is configured to determine an incidence direction of the incoming signal based on a depth of the triplet element, a water column depth, a timing of transmission of a signal being reflected back from a seabed and a water surface as the incoming signal.

These and other features of some example embodiments of the present disclosure will be appreciated and understood with reference to the specification, claims, and appended drawings, wherein:.

The detailed description set forth below in connection with the appended drawings is intended as a description of some example embodiments of a system and a method for mitigating the effects of compression errors provided in accordance with the present disclosure and is not intended to represent the only forms in which the present disclosure may be constructed or utilized. The description sets forth the features of the present disclosure in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the scope of the disclosure. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.

Aspects of example embodiments of the present disclosure are directed to a submerged sonar-based sensing system that is capable of rejecting (or substantially reducing the received intensity of) the surface-reflected signals (i.e., sound waves reflected from the water surface). The sonar-based sensing system may be a submerged, low-frequency, active-sonar system, which may be able to image the seabed and/or submerged objects. If not canceled, the surface reflection noise can interfere with the desired signal receive from the depth and can lower signal-to-noise ratio (SNR) and imaging resolution. According to some embodiments, the sonar-based system uses a triplet hydrophone that allows a dynamic cardioid null to be steered towards this interference, thus significantly improving image SNR.

<FIG> is a schematic diagram of a sonar-based sensing system <NUM> according to some embodiments of the present disclosure. <FIG> illustrates return signals from different points along the seabed <NUM>, which reflect off the water surface <NUM> and reach the sonar-based sensing system <NUM>, according to some examples. As used herein the term "seabed" refers to the sea floor or ocean floor.

According to some embodiments, the sonar-based sensing system (also referred to as a survey system) <NUM> includes surface vehicle (also referred to as a launch and recovery vehicle) <NUM>, such as a ship or boat, which could be manned or autonomously operated; a tow body (e.g., an active depth-keeping tow body) <NUM> coupled to (e.g., is towed by) the surface vehicle <NUM> via a towing cable <NUM> and including a transmitter <NUM> (also referred as a projector) for generating pulsed acoustic signals emitted from the sides of a tow body <NUM>; and a receiver <NUM> coupled to (e.g., towed by) the tow body <NUM>, and configured to receive return signals (e.g., reflected acoustic signal) of the transmitted signal. In some embodiments, the receiver <NUM> includes a triplet element of hydrophones configured to form a cardioid null in a desired direction to cancel or substantially reduce the intensity of return signals reflecting from the surface of the water.

According to some examples, the tow body <NUM> may be an active depth keeping device deployed at a depth of about <NUM> to about <NUM> (e.g., about <NUM>) and may be towed at a speed of about <NUM> knots to about <NUM> knots (e.g., about <NUM> knots). The receiver <NUM> may have a seabed swath of about <NUM> and the sonar-based sensing system <NUM> may be capable of covering more than <NUM><NUM> per day.

As illustrated in <FIG>, operating the sonar-based sensing system <NUM> near the surface of water may presents a challenge as the surface of the ocean (i.e., the water-air interface) may act as a near perfect mirror that can generate a phase scrambled version of the image from an adjacent piece of seabed in the same range instant. This may be due to the fact that, at any given time, for every direct path of return of the emitted signal (as, e.g., represented by paths <NUM> and <NUM> in <FIG>), there is a corresponding indirect path (as, e.g., represented by paths <NUM>' and <NUM>' in <FIG>) of the same length by which a return signal from a nearby point along the seabed <NUM> reflects off the water surface <NUM> and returns to the receiver <NUM> at about the same time. If not nullified, the indirect return signals may destroy coherence and make synthetic aperture sonar impossible or very difficult. However, according to some embodiments, the receiver <NUM> utilizes triplet elements capable of nullifying or substantially nullifying the effect of the surface-reflected return signal, thus improving signal-to-noise ration of the detected return signal and improving detection resolution.

<FIG> respectively illustrate a side view and a perspective view of the triplet element <NUM>, according to some embodiments of the present disclosure. <FIG> illustrates a side view of telemetry electronics <NUM> and the shell <NUM> constituting a hydrophone <NUM> of the triplet element <NUM>, according to some embodiments of the present disclosure.

According to some embodiments, the triplet element <NUM> includes three hydrophones <NUM> that are organized axially (e.g., positioned along the circumference of a circle). In some embodiments, the three hydrophones <NUM> are spaced at equal intervals (e.g., <NUM> degrees apart); however, embodiments of the present disclosure are not limited thereto, and the three hydrophones <NUM> may be spaced at unequal intervals. The three hydrophones <NUM> may be coupled together (e.g., fixedly coupled together) by a pair of couplers <NUM> at each end of the trio. In some examples, the three hydrophones (i.e., the first to third hydrophones <NUM>, <NUM>-<NUM>, and <NUM>-<NUM>) may be identical or substantially identical.

The hydrophone <NUM> may detect sounds under water by converting received acoustic energy into electrical energy. The hydrophone <NUM> includes digitization and telemetry electronics <NUM> encased in a shell <NUM>. The shells <NUM> of the hydrophones <NUM> may be cylindrical in shape, and may extend parallel to one another along the direction of tow. The shell <NUM>, which may be made of ceramic (e.g., a lead zirconium titanate (PZT) ceramic) or any other suitable piezo-restrictive material, may be configured as a pressure housing and, in some embodiments, as a transducer (e.g., a piezoelectric transducer) that generates an electric potential when experiencing a pressure change (e.g., resulting from sound waves under water). The digitization and telemetry electronics <NUM> is configured to convert the electrical potentials from the shell <NUM> into digital signals for further processing. The hydrophone <NUM> may be configured to match the acoustic impedance of water.

According to some embodiments, while each hydrophone <NUM> is omni-directional (i.e., is capable of receiving signal from different direction with equal or substantially the same sensitivity), the triplet element <NUM> is capable of exhibiting directionality (i.e., can exhibit varying levels of sensitivity based on angle of incidence). This is due to the fact that the relative position of the three hydrophones results in a relative time delay between when each of the hydrophones <NUM> receives the incoming signal, which translates to a relative phase shift of the signal being received. In some embodiments, a signal processor of the sonar-based sensing system <NUM> (also referred to as a processor, null steerer, or beam former) amplifies the signals received by the three hydrophones differently to achieve destructive interference (or a null) at a desired angle. This allows the receiver <NUM> to ignore or reject noise in a one particular direction (e.g., to reject/ignore a surface-reflected acoustic signal), while listening for (i.e., receive without nullifying/rejecting) incoming signal from other directions. In some embodiments, a triplet radius of less than or equal to λ/<NUM>, where λ represents the wavelength of the signal generated by the transmitter <NUM>, allows for the formation of a cardioid response (e.g., a cardioid null) at an arbitrary look angle. The sensitivity of the receiver <NUM> increases the closer the triplet radius is to λ/<NUM>. At radiuses greater than λ/<NUM>, other nulls may appear in the cardioid, and thus the receiver response may not be as desired. In some examples, the triplet radius may be about <NUM> to about <NUM>.

<FIG> illustrates an example of the cardioid response of the receiver <NUM>, according to some embodiments of the present disclosure. <FIG> illustrates the intensity of the output of the receiver <NUM> as a function of incidence angle of the incoming signal, according to some embodiments of the present disclosure. <FIG> illustrates the capability of the sonar-based sensing system <NUM> to create a cardioid null to cancel or substantially reduce surface noise, according to some embodiments of the present disclosure.

Referring to <FIG>, the logarithmic output response of the triplet element <NUM> may be expressed as a function of the incidence angle θ of the signal being received by the triplet element <NUM> by Equation (<NUM>):.

<MAT>
where Phasor represents the intensity of the signal received by each of the three hydrophones, Xn and Yn represent the cartesian coordinates of each of the three hydrophones <NUM> of the triplet element <NUM> in a cross-plane (e.g., X-Y plane) orthogonal to the extension direction of the hydrophones <NUM> (e.g., the Z axis), the incidence angle θ is the radial angle in the cross-plane represented by the cartesian coordinates, and <MAT> represents the null-rotating term that can steer the null in a desired direction. For ease of illustration, the cardioid response in <FIG> is superimposed with the position of the three hydrophones <NUM> in the cross-plane.

Here, intensity of the hydrophone-received signal Phasor, which is a function of position of the hydrophone and the incidence angle of the received signal (e.g., the returned acoustic signal <NUM> or <NUM>') may be expressed by Equation (<NUM>):
<MAT>
where λ represents the wavelength of the incoming signal (e.g., wavelength of the return signal <NUM> or <NUM>'). In some examples, λ may be defined as the speed of sound at the depth of the triplet element <NUM> divided by the frequency of the return acoustic signal (which may be the same as the frequency of the transmitted signal). The position coordinates of the first to third hydrophones <NUM>, <NUM>-<NUM>, and <NUM>-<NUM> that are represented as (X<NUM>, Y<NUM>), (X<NUM>, Y<NUM>), and (X<NUM>, Y<NUM>) can be expressed through Equations (<NUM>)-(<NUM>):
<MAT>
<MAT>
<MAT>
<MAT>
<MAT>
<MAT>
where Radius is the radius of the triplet element, δ represents the angular offset of the triplet element <NUM> (e.g., the angular offset of the first hydrophone <NUM>) expressed in radians. In Equations (<NUM>)-(<NUM>), it is assumed that the three hydrophones <NUM> to <NUM>-<NUM> are equally spaced at angular intervals of <NUM> degree or <MAT> radians.

In the null-rotating term <MAT>, the parameters α□ and cτ□ for each of the hydrophones may be expressed by the following equations:
<MAT>
<MAT>
<MAT>
<MAT>
<MAT>
<MAT>.

Where β represents the angular location of the null generated by the triplet element <NUM>. In the example of <FIG>, β is at about <NUM> degree or about <MAT> radians. In the example of <FIG>, the y-axis of the graph is in dBs.

Thus, the output response of the receiver <NUM> may be calculated as a complex weighted summation of intensities of signals received at the first to third hydrophones <NUM> to <NUM>-<NUM>, wherein weights of the weighted summation are based on the radius of the triplet element <NUM> and an angular location of the cardioid null generated by the receiver.

According to some embodiments, the sonar-based sensing system <NUM> (e.g., a signal processor of the sonar-based sensing system <NUM>) can calculate the angle of incidence of the water-surface-reflected signal <NUM>'/<NUM>' based on the depth of the triplet element <NUM> (which, e.g., may be the same as the depth of the tow body <NUM>), the water column depth (i.e., a height of the water column corresponding to the location of the triplet element <NUM> from seabed <NUM> to water surface <NUM>), and the timing of the signal transmission. Once the angle of incidence of the water-surface-reflected signal <NUM>'/<NUM>' is determined, the sonar-based sensing system <NUM> (e.g., the signal processor of the sonar-based sensing system <NUM>) sets the value β equal to the calculated incidence angle of the surface-reflected signal. As noted above, in some embodiments, the determination of the incidence angle of the surface-reflected signal by the sonar-based sensing system <NUM> and the resultant null steering is time dependent (e.g., is based on the amount of time passed since the latest transmission by the transmitter <NUM>). This is because as more time passes, the signal transmitted by the transmitter <NUM> can reach further distances of the seabed and the return signal is further delayed. Therefore, as illustrated in <FIG>, at a first time, the sonar-based sensing system <NUM> (e.g., the signal processor) is configured to determine a first incidence direction of the incoming signal (see e.g., the angle of the return path <NUM>' in <FIG>), and at a second time (e.g., a later time), the sonar-based sensing system <NUM> (e.g., the signal processor) is configured to determine a second incidence direction of the incoming signal (see e.g., the angle of the return path <NUM>' in <FIG>) and to steer the cardioid null from the first incidence direction to the second incidence direction to reject the incoming signal over time.

Thus, as described above and as shown in <FIG> and 5B, the triplet element <NUM> can steer the null in the direction of the surface-reflected signal to reduce (e.g., minimize the dominant source of interference, which is the surface-reflected signal. In some examples, the receiver <NUM> can attenuate the surface-reflected signal by more than <NUM> dB, thus effectively cancelling/rejecting surface noise, and can achieve a signal-to-noise ratio (SNR) of greater than <NUM> dB.

In some embodiments, the steer angle is seeded with a geometric approximation (e.g., estimated water column depth and tow depth), and then optimized with a maximum coherence cost search to correct for unknown acoustic path characteristics.

According to some embodiments, the receiver <NUM> include a plurality of triplet elements <NUM> organized in array form along the tow direction (e.g., along the Z-axis), which allows for improved directional signal reception. For example, the receiver <NUM> may include an array of hundreds to thousands of triplet elements <NUM>. In some embodiments, the receiver array <NUM> may be divided into a plurality of modules, each including a plurality of triplet elements <NUM>. In some examples, the receiver array <NUM> may include <NUM> modules, where each of the modules includes <NUM> triplet elements <NUM>.

<FIG> illustrates a module <NUM> of a receiver <NUM>, according to some embodiments of the present disclosure.

Referring to <FIG>, according to some embodiments, the module <NUM> of the receiver array <NUM> includes a plurality of triplet elements <NUM> that are coupled together along the length of the triplet element <NUM> (e.g., along the Z-axis) and may be attached together by a plurality of rods <NUM> and held together at each end of the module <NUM> by a bulkhead <NUM>. The different modules <NUM> may be physically and electrically coupled to one another and the tow body <NUM> via a cable <NUM>.

In some embodiments, the bulkhead <NUM> includes attachment mechanisms for attaching to the rods <NUM> and securing them together, and electronic circuitry <NUM> that combine (e.g., sum) together the signals received from corresponding one of the hydrophones <NUM> from each of the triplet elements <NUM> to generate three electrical outputs. In other words, the electronic circuitry <NUM> of the bulkhead <NUM> may sum together the signals from the first hydrophones <NUM> to generate a first electrical output, sum together the signals from the second hydrophones <NUM>-<NUM> to generate a second electrical output, and sum together the signals from the third hydrophones <NUM>-<NUM> to generate a third electrical output. In some embodiments, the electronic circuitry <NUM> converts the three electrical signals to optical signals that are multiplexed onto an optical fiber in the cable <NUM> using different wavelengths (e.g., different colors) of light. However, embodiments of the present disclosure are not limited thereto, and the electrical outputs of each module <NUM> may be transmitted electrically along the cable <NUM>. The cable <NUM> transmits the output signals from each module <NUM> to a signal processor <NUM> for further processing.

According to some embodiments, the signal processor (also referred to as a processor, null steerer, or beam former) <NUM> is configured to determine an incidence direction of the incoming signal, and to dynamically generate a cardioid null in the incidence direction to reject the incoming signal based on the output signals from each of the modules <NUM>. The signal processor <NUM> may be incorporated into or implemented in any suitable device. For example, the signal processor <NUM> may be incorporated into a bulkhead <NUM> of a module <NUM>, the tow body <NUM>, the surface vehicle <NUM>, or an external location that receives the outputs of the receiver <NUM>.

Grouping the triplet elements <NUM> of the receiver array <NUM> into fewer modules <NUM> reduces the number of electronic circuitry <NUM> used in the receiver, which can lower power usage and overall system cost. However, because the azimuth resolution of the module <NUM> may be about one half of the length of the module <NUM>, the number of triplet elements <NUM> contained in one module <NUM> may be limited by the desired resolution. In some examples, the length of each module <NUM> may be about <NUM>, and the entire receiver array <NUM> may be hundreds of meters long. In some examples, the minimum 3dB width of a resolution cell, which may be obtained, may not be less than one half of the length of the receiver element.

As described herein, the sonar-based sensor system according to some embodiments of the present disclosure is capable of generate and dynamically steer a cardioid null and thus cancel or substantially reduce surface noise. The surface noise cancelation improves SNR and imaging resolution of the sensor system. As a result, the sonar-based sensor system according to some examples is capable of obtaining an <NUM> to <NUM> improvement in coverage area with <NUM> times the forward advance as compared to solutions of the related art.

As described above, unlike solutions of the related art that either sacrifice spectral performance or power efficiency, or induce significant transmitter processing complexity, the communication system according to some embodiments achieves high capacity constellations generated in the receiver with improved combined spreading gain/spectral efficiency without relying on high-power, and low efficiency linear amplifiers.

Claim 1:
A sensor system (<NUM>) comprising:
a triplet element (<NUM>) comprising a first hydrophone (<NUM>), a second hydrophone (<NUM>-<NUM>), and a third hydrophone (<NUM>-<NUM>) configured to receive an incoming signal at a first phase, a second phase, and a third phase, respectively, the first to third hydrophones extending along a first direction; and
a processor (<NUM>) configured to:
determine an incidence direction of the incoming signal based on a depth of the triplet element, a water column depth, a timing of transmission of a signal being reflected back from a seabed and a water surface (<NUM>) as the incoming signal, and
dynamically generate a cardioid null in the incidence direction to reject the incoming signal based on the incoming signal at the first to third phases.