WO2008105932A2

WO2008105932A2 - System and method for forward looking sonar

Info

Publication number: WO2008105932A2
Application number: PCT/US2007/078577
Authority: WO
Inventors: Paul Kraeutner; Wen Xu
Original assignee: Teledyne Rd Instruments, Inc.
Priority date: 2006-09-15
Filing date: 2007-09-14
Publication date: 2008-09-04
Also published as: WO2008105932A3

Abstract

Embodiments of a system and method for a forward looking sonar, for example, configured for use in an underwater vehicle, are disclosed. One embodiment includes a method of imaging a region. The method includes transmitting at least one acoustic pulse from a transducer and receiving a portion of the pulse scattered from a region. The method further includes generating a multi-channel signal based on the received portion of the pulse scattered from the region, sampling the scattering signal to generate a plurality of complex samples, and forming a plurality of beams in a horizontal plane based on the complex samples. The plurality of complex samples may be processed to obtain image data comprising angles of arrival from scattering components in a vertical plane for each horizontal beam. An image may be generated based on the image data. Other embodiments may include apparatuses such as sonar systems.

Description

SYSTEM AND METHOD FOR FORWARD LOOKING SONAR

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of, and incorporates by reference in its entirety, U.S. Provisional Application No. 60/845,094, filed on September 15, 2006.

BACKGROUND OF THE INVENTION

Field of the Invention

[0002] The present invention relates to acoustic transducers and beamformers that form simultaneous multiple beams in multiple planes.

Description of the Related Technology

[0003] Unmanned Underwater Vehicles (UUVs) are becoming a popular tool in ocean observation, hydrographic mapping, offshore engineering, as well as military applications such as mine countermeasure, anti-submarine warfare, and environmental monitoring/assessment. This trend to increased use of such UUVs has generated new opportunities as well as challenges for acoustic sensor system design. Particularly for small UUVs, the integrated sonar systems must be low-power, compact-size, and in some applications, expendable. Most existing commercial systems do not meet these requirements, nor can they be readily adapted to meet the requirements.

[0004] Forward looking sonar (FLS) can provide capabilities of obstacle detection, high-resolution imaging, acoustic beacon homing, as well as line-of-sight terrain profiling, which are all essential to an autonomous system. Those applications in turn require that the FLS be capable of high resolution discrimination of acoustic echoes in both the horizontal and vertical directions. Achieving high resolution in both dimensions has generally only been possible with large complex systems that employ profuse numbers of sensors and that are not only expensive but that also consume prohibitive amounts of power to make them viable on small UUVs and in other applications.

SUMMARY OF CERTAIN INVENTIVE ASPECTS

[0005] The system, method, and devices of the invention each have several aspects, no single one of which is solely responsible for its desirable attributes. Without limiting the scope of this invention, its more prominent features will now be discussed briefly. After considering this discussion, and particularly after reading the section entitled "Detailed Description of Certain Embodiments" one will understand how the features of this invention provide advantages that include, for example, a compact sonar imaging products for obstacle avoidance, mapping, and vehicle guidance for a broad spectrum of unmanned underwater vehicles.

[0006] One embodiment includes a method of imaging a region. The method comprises transmitting at least one acoustic pulse and receiving a portion of the pulse scattered from a region via an acoustic transducer having N elements, and. The method further comprises generating a multi-channel signal based on the received portion of the pulse scattered from the region and sampling the scattering signal to generate a plurality of complex samples. The method further comprises simultaneously forming a plurality of beams in a horizontal plane based on the complex samples. The method further comprises processing the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane. The number of scattering components M is less than N and the horizontal plane and the vertical plane are substantially orthogonal. The method may optionally include generating an image based on the image data.

[0007] One embodiment includes a system for imaging a region. The system includes at least one transducer configured to transmit at least one acoustic pulse and at least one transducer configured to receive a portion of the pulse scattered from a region. The at least one receiving transducer has N elements. The system further includes a receiver configured to generate a multi-channel signal based on the received portion of the pulse scattered from the region, and sample the scattering signal to generate a plurality of complex samples. The system further includes a beamformer configured to simultaneously form a plurality of beams in a horizontal plane based on the complex samples. The system further includes a processor configured to process the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane for each horizontal beam. The number of scattering components M is less than N, and the horizontal plane and the vertical plane are substantially orthogonal. The processor may also be configured to generate an image based on the image data.

[0008] One embodiment includes an underwater vehicle comprising a vehicle frame, a sonar system, and a housing configured to secure the sonar system to the vehicle frame. The sonar system includes at least one transducer configured to transmit at least one acoustic pulse and at least one transducer configured to receive a portion of the pulse scattered from a region. The at least one receiving transducer has N elements. The system further includes a receiver configured to generate a multi-channel signal based on the received portion of the pulse scattered from the region, and sample the scattering signal to generate a plurality of complex samples. The system further includes a beamformer configured to simultaneously form a plurality of beams in a horizontal plane based on the complex samples. The system further includes a processor configured to process the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane for each horizontal beam. The number of scattering components M is less than N, and the horizontal plane and the vertical plane are substantially orthogonal. The processor may also be configured to generate an image based on the image data.

[0009] One embodiment includes a system for imaging a region. The system includes means for transmitting at least one acoustic pulse and means for receiving a portion of the pulse scattered from a region. The receiving means has N elements. The system further includes means for processing the received portion of the pulse. The means is configured to generate a multi-channel signal based on the received portion of the pulse scattered from the region, and sample the scattering signal to generate a plurality of complex samples. The system further includes means for simultaneously forming a plurality of beams in a horizontal plane based on the complex samples. The system further includes means for processing the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane for each horizontal beam. The number of components M is less than N, the horizontal plane and the vertical plane are substantially orthogonal. The processing means may be further configured to generate an image based on the image data.

BRIEF DESCRIPTION OF THE DRAWINGS [0010] Figure 1 illustrates a beam footprint versus transmit pulse footprint according to one embodiment.

[0011] Figure 2 illustrates one embodiment of a three-dimensional forward looking imaging sonar. [0012] Figure 3 is a top level schematic block diagram of a sonar system according to one embodiment.

[0013] Figure 4 is a schematic block diagram that further illustrates a portion of the sonar system of Figure 3 in further detail.

[0014] Figure 5 is a schematic block diagram that further illustrates another portion of the sonar system of Figure 3 in further detail.

[0015] Figure 6 is a flowchart that illustrates one example of a method of generating sonar images according to one embodiment.

[0016] Figures 7A-D illustrate lakebed imaging and profiling test results for one embodiment. Figure 7A is a 2D profile of relatively flat lakebed and surface buoy (obstacle) located at 27 m range. Figure 7B is 2D depth profile of upward sloping lakebed and surface multipath. Figure 7C is an upward sloping lakebed bathymetry ahead of sonar assembled from individual depth profiles associated with each beam. Figure 7D is a 2D backscatter image of upward sloping lakebed ahead of sonar assembled from backscatter in each beam.

DETAILED DESCRIPTION OF CERTAIN EMBODIMENTS [0017] The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways as defined and covered by the claims. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout.

[0018] Current imaging technologies for the unmanned underwater vehicles are inadequate. Accordingly, various embodiments provide features such as a compact sonar imaging products for obstacle avoidance, mapping, and vehicle guidance for a broad spectrum of unmanned underwater vehicles.

[0019] Embodiments include a system and method for imaging and mapping the region of various looking directions from a stationary or mobile platform to which the system is attached. One embodiment uses angle-of-arrival estimation in conjunction with beamforming under various transducer array configurations to either improve system resolution or reduce system complexity. Embodiments may include a novel electronics architecture configured to provide low power operation in a compact space efficient device. For example, one embodiment of the method and apparatus provides a low- power, compact-size solution for many three-dimensional imaging and mapping applications particularly with small mobile platforms such as unmanned underwater vehicles. For example, a Multi-Beam Echo Sounder (MBES) is a downward and side looking sonar that is configured to determine the range to the seafloor in a number of discrete beams spanning a swath underneath and to port and starboard of the vehicle's forward path. Embodiments may also include a system and method that is a low-power, compact-size solution for many three-dimensional imaging and mapping applications particularly with small mobile platforms such as Unmanned Underwater Vehicles.

[0020] One embodiment of the method and apparatus performs array-based imaging and mapping. The apparatus has a transmit transducer array projecting a signal into the field, a receive transducer array collecting scattered signals from the field of interest, and an electronics subsystem handling generation and processing of signal to/from the transducer arrays. The apparatus has a desirable electronics architecture of power and space efficient, featuring a time-division multiplexing multi-channel sonar receiver, a high-speed low-power Field Programmable Gate Array (FPGA)-based data acquisition system, and a digital signal processing chain consisting of low-power highspeed digital signal processor (DSP) and FPGA.

[0021] One embodiment of a method implements angle-of-arrival estimation to add vertical information in conjunction with beamforming in various ways for transducer arrays of various configurations. For 3D imaging, one embodiment of the method combines angle-of-arrival estimation in the vertical plane with beamforming in the horizontal plane. For moderately complex operation geometry, the method can significantly reduce the number of transducer array elements while achieving the same or better resolution performance compared to a conventional beamforming-based system. For multi-beam echo sounder, angle-of-arrival estimation and beamforming are used as complementary tools applied in the same angular dimension. Beamforming is employed to obtain first-look data results; angle-of-arrival processing is then applied in a subsequent step to achieve sub-beam width resolution. This two-pass approach will be particularly beneficial in improving system performance in the outer beams.

[0022] In the context of underwater acoustic imaging, both beamforming and angle-of-arrival estimation are terms that describe methods for measuring the directional origin of propagating acoustic waves. Both methods make use of an array of sensors to simultaneously measure a propagating wave front at several discrete locations. The sensor array is comprised of individual transducer elements, typically organized along a line with equal spacing between elements.

[0023] The primary difference between directional measurements obtained using a beamforming approach versus an angle-of-arrival estimation approach is in the assumptions that are made about the backscatter. In the case of beamforming, backscatter is assumed to arrive at the receive transducer array from all directions at all times and therefore arrivals from a direction of interest must be filtered out from all others. On the other hand, for angle-of-arrival estimation, backscatter is assumed to arrive at the array from only a finite number of discrete directions at any given instant in time. Both methods have advantages and disadvantages.

[0024] Beamforming is an approach used in most existing electronic-scanning acoustic imaging and mapping systems. Generally, however, the angular resolution is inversely proportional to the array aperture size. For example, in order to achieve a 1.5° beam in one plane, with reasonably low sidelobes (i.e. leakage from directions outside the main beam) approximately 88 elements and therefore 88 receive channels are required. Thus, the required number of elements for a two-dimensional beamforming-based system is significant.

[0025] Furthermore, in the case of seafloor backscatter, beams that are steered at increasingly shallow grazing angles produce larger and larger spatial footprints on the seafloor, as shown in Figure 1. These larger footprints make it difficult to extract other important information from the backscatter, such as the range to the seafloor in the center of the beam which is very important for seafloor mapping.

[0026] As illustrated in Figure 1, an angle-of-arrival estimation approach on the other hand, directly estimates the directional origin of the backscatter by tracking the instantaneous position of the transmit pulse on the seafloor. Since the pulse footprint is, in general, much smaller than can be practically achieved by beamforming, the advantage in terms of achievable resolution of the angle-of-arrival estimation approach is substantial.

[0027] With much fewer array elements, an angle-of-arrival estimation-based system works well in typical geometries of shallow water vertical profiling, where only a discrete set of simultaneous signal arrivals presents, including sea-floor vertical features, water column targets above the seafloor, surface backscatter, and multipath. [0028] One embodiment includes a feature of combining beamforming and angle-of-arrival estimation for use in two-dimensional or three-dimensional imaging or mapping. The first of such combinations is for three-dimensional forward looking imaging, as shown in Figure 2.

[0029] Figure 2 illustrates one embodiment of a three-dimensional forward looking imaging sonar 1042 as embodiment in a underwater vehicle 102. For three- dimensional forward looking imaging, directional information is required in two dimensions. It has been found that the multiple-angle-of-arrival estimation approach is well suited to sorting backscatter arrivals in the vertical plane. Therefore, angle-of-arrival estimation is also well suited to forward-looking sonar requirements such as seabed profiling and separation of multipath from direct path seabed and obstacle arrivals in shallow water. Thus, in one embodiment beamforming is applied in the horizontal plane (using a relatively large number of array elements in that dimension) and to apply angle- of-arrival processing in the vertical plane (using a small number of array elements in the vertical dimension). Implementations may repeat this procedure in multiple simultaneous horizontal look directions and therefore produces an angle-of-arrival result simultaneously in each horizontal look direction.

[0030] For example, as illustrated in Figure 2, a transmitted pulse 108 propagating in each beam 110 intersects the seafloor 111 at time t at a range R (t) and the elevation angle θl (t). At substantially the same time and range, the acoustic pulse also intersects an example target 112, e.g., a mine-like target in the water column at an elevation angle of θ M (t). Backscatter 1 14 from the seafloor and the mine-like target are subsequently received by the sonar array 104 mounted on the nose of the vehicle 102 and angle-of-arrival processing is applied. Angle-of-arrival processing produces an estimate of the backscatter angles-of-arrival θ l(t) - O M (t) along with their associated amplitudes. The number of concurrent arrivals that can be resolved by angle-of-arrival processing is a function of N, the number of rows in the array and is typically up to N-I . The procedure for producing a three-dimensional image then is to apply angle-of-arrival processing to each horizontal beam in order to provide N-I elevation angles and associated amplitudes at each range step in each beam. Then using the azimuthal direction of each forward looking beam together with range from the two-way travel time and the elevation angles and amplitudes from angle-of-arrival processing, the backscatter may be plotted in three- dimensional space using spherical coordinates (i.e. range, azimuth, elevation ) relative to the vehicle position.

[0031] The procedure is very similar to that employed for two-dimensional imaging where only range and azimuth are available and are used to produce the well known flat, two-dimensional sector-scan display. Imaging resolution and performance are governed by the same factors with the exception of vertical resolution and performance which is governed by the underlying assumptions and angle-of-arrival estimation performance. Range resolution is derived from the signal bandwidth (pulse length), azimuthal resolution is derived from the two-way horizontal beampattern, and elevation angle (i.e. depth) resolution is derived from angle-of-arrival estimation. With fewer array elements and receive channels, one embodiment of the method can achieve the same horizontal resolution along with improved vertical resolution when compared to a beamform ing-based system. In an embodiment including such a combination, beamforming and angle-of-arrival processing are used as complementary tools applied in different angular dimensions.

[0032] For multi-beam bottom mapping, directional information is only required in one plane (e.g. a multibeam echo sounder with downward looking narrow beams distributed athwart ship). In this case, beamforming may be employed to obtain first-look data results, in a similar fashion to other multibeam systems. However, it may also be beneficial to apply an angle-of-arrival estimation approach in a subsequent post processing pass to enhance resolution (i.e. sub-beamwidth resolution). This two-pass approach can be particularly beneficial in improving system performance in the outer beams as shown in Figure 1. In an embodiment including such a combination, angle-of- arrival and beamforming are used as complementary tools applied in the same angular dimension.

[0033] Figure 3 is a top level schematic block diagram of the sonar system 104 according to one embodiment. In one embodiment, the sonar system 104 comprises an obstacle avoidance sonar. In one embodiment, the sonar system 104 is organized Embodiments such as illustrated in Figure 3 may also include a versatile sonar architecture which focuses specifically on minimization of size and power. In the example system 104 of Figure 3, the sonar system 104 includes a number of stackable modules, which in one embodiment, are each a circuit card assemblies (CCAs). However, as discussed below, in other embodiments the functionality discussed herein with reference to these CCAs may be distributed in any suitable arrangement of circuits, processors, or other components.

[0034] For example, the various illustrative CCAs, logical blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In one embodiment, one or more of CCAs are implemented using, for example, the VHDL logic device programming language under an environment such as that provided by the Xilinx ISE Foundation.

[0035] A carrier CCA 302 provides interface connectors for external control and power from a source such as a submersible vehicle or a tether to another control source such as boat, buoy, or pier. The carrier CCA 302 also provides output data from the sonar system back to that control source. In addition, the carrier CCA 302 provides power distribution and a mechanical mounting structure for all other CCA's. The carrier CCA 302 also provides the control signals to a digital signal processor (DSP) CCA 304. In one embodiment, the physical package comprises a compact CCA stack mounted on the carrier CCA 302, which fits comfortably into an unmanned underwater vehicle payload section measuring 7.5" in diameter by 6" in length. The total power consumption in one embodiment is about 10 watts. Desirably, the CCA architecture provides signal processing flexibility and its inherent power and size efficiency. It is to be recognized that additional processing capabilities can be easily and incrementally added by incorporating additional DSP CCAs 304, each connected via, for example, high speed serial ports. The interfaces

[0036] The DSP CCA 304 generates transmit signals and provides those signals to a transmitter CCA 306. The transmitter 306 then drives one or more transmit arrays 308 to transmit sonar pulses. In one embodiment, the sonar pulses may have an operating frequency of about 100 kHz. In another embodiment, the sonar pulses may have an operating frequency of about 150 kHz. In other example embodiments, the sonar pulses may have an operating frequency of between about 50 kHz and 600 kHz

[0037] Returned portions of the sonar pulses are received by one or more receive arrays 312. Signals provided by the receive arrays 312 are amplified and partially processed by a receiver CCA 314. The receiver CCA 314 and the transmitter CCA 306 may be driving by pulse timing data provided by the DSP CCA 304. Partially processed return data is provided to a data acquisition CCA 316 that further processes the return signals and provides the data back to the DSP CCA 304 for detection and output processing. The processed return data is then output via the carrier CCA 302. Depending on the embodiment, the return data output via the carrier CCA 302 may include one or more of image data or partially processed return data. In one embodiment, the receive arrays 312 comprises a two-dimensional array transducer such as disclosed in U.S. Patent No. 5,808,967, which is hereby incorporated by reference in its entirety. In one embodiment, the transducer array 308 and receiver array 312 are a single transducer array that is switched between transmit and receive operation. One or both of the transducer arrays 308 and 312 may comprise a series of independent but closely spaced (e.g., within a half wavelength) horizontal line arrays, each capable of being steered in the horizontal plane.

[0038] In one example embodiment, the transmit arrays 308 comprises three transducer elements, a central transducer element and two additional elements on either side of the central element, each positioned at an angle with respect to the central transducer element. In one embodiment, the angle is between 5 and 25 degrees. In one embodiment, the offset angle of each of the additional transducer elements is 15 degrees.

[0039] Figure 4 is a schematic block diagram that further illustrates a portion of the sonar system 104 in further detail. In particular, the carrier CCA 302 includes power supplies 322, e.g., digital and analog power supplies, and data interfaces 324. In addition to suitable data interfaces 324 such as IEEE 1394, USB, RS-232, and/or Ethernet for receiving control signals and outputting processed data to the external control source, the data interfaces 324 include one or more data interfaces such as IEEE 1394, USB, RS- 232, and/or Ethernet for communicating with other CCAs of the sonar system 104. In one embodiment, one or more of the CCAs may include a Micro-line bus for inter-CCA or external communication. [0040] In one embodiment, the DSP CCA 304 includes a high-speed, low- power digital signal processor along with a complement of data interfaces 342 (e.g., RS232 and IEEE 1394) with which to communicate with the external systems and with other CCAs. The DSP CCA 304 is configured to generate transmit signals and for angle- of-arrival processing via a detection and output processing module 344. In one embodiment, a receive beamformer 346 simultaneously forms horizontal beams based on data received from a transmit control module 348 and the data acquisition CCA 316 via a receiver interface bus 350. Alternatively, or in addition, the receive beamformer 346 may generate such beams in sequence for a scan of an area. The receiver interface bus 350 may be any suitable data bus. The transmit control module 348 generates transmit waveforms for, and provides transmit control to, the transmitter CCA 306 via a transmitter interface bus 354. The receiver interface bus 350 and the transmitter interface bus 354 may be embodied as any suitable data or signal bus, including serial or parallel data interfaces. A timing generator circuit or module 352 provides a ping clock and a ping reset signal to the transmitter CCA 306 and the receiver CCA 314.

[0041] The transmitter CCA 306 amplifies the transmit waveform generated by the DSP CCA 304 and drives the transmit array 308 under the control of the DSP CCA 304. The transmitter CCA 306 includes a control module 362, a transmit buffer 364, and one or more transmit amplifiers 366 that provide sonar pulses to the transmit arrays 308. The control module 362 receives ping timing signals from the DSP CCA 304 and controls the transmit buffer 364 based on the timing signals. The transmit buffer 364 receives waveforms for transmission from the DSP CCA 304 and provides those signals to the transmit amplifiers 366.

[0042] Figure 5 is a schematic block diagram that further illustrates another portion of the sonar system 104 in further detail. In one embodiment, the receive array 312 comprises a 4x12 array providing 48 channels of data. In one embodiment, the receive array 312 comprises 96 horizontal by 6 vertical transducer elements. It is to be recognized that as compared to a system that does not include

[0043] The receiver CCA 314 may comprise a single CCA or a number of stacked CCAs 314, each configured to process a number of channels, e.g., 12 channels per CCA. For example, one example system 104 includes 12 receive channels per receiver CCA 314. Each receives channel interfaces with each individual transducer element. Each channel passes through a preamplifier 372 and an analog band pass filter (BPF) 374. In one embodiment, a group, e.g., 12, receive channels are multiplexed via a multiplexer 376 into a single low-power broadband time varying gain (TVG) amplifier 378. TVG amplifiers 378 for two such set of 12 channels are illustrated in the example of Figure 5. Additional TVG amplifiers 378 (not shown) may amplify the remaining 24 channels. The signals from each of the TVG amplifiers 378 are passed through analog low pass filters 379 and to the data acquisition CCA 316. In one embodiment, the outputs of, for example, two of the TVG amplifiers are multiplexed into a single analog output so that the TVG amplifier outputs of 48 multiplexed channels can be provided to the data acquisition CCA 316 via two multiplexed analog outputs as illustrated in Figure 5. Through the use of time-division multiplexing, the number of TVG's and thus the total receive system power consumption are significantly reduced.

[0044] The data acquisition CCA 316 receives the multiplexed signals from the receiver CCA 314. In one embodiment, the data acquisition CCA 316 is stacked on the DSP CCA 304. In various embodiments, the data acquisition CCA 316, which may comprise a single CCA 316, or a stack of two or more CCAs 316. An example of the data CCA 316 includes two analog-to-digital converter (ADC) channels 384 that receives signals from the TVGs 378 of the receiver CCA 314, two digital-to-analog (DAC) channels 382 that provide gain control for the TVGs 378 and a lM-gate FPGA. The onboard FPGA may be used to perform receive channel de-multiplexing, complex demodulation, and signal-band noise filtering (i.e. digital filtering), beamforming and decimation or any other processing including that discussed with reference to the DSP CCA 304 to provides signal processing flexibility along with power and size efficiency. The signals from the receiver CCA are sampled at the ADCs 384 and demultiplexed via a demultiplexer 386. Digital low pass filters 388 are then applied to each of the channels. The channels are then multiplexed via a digital multiplexer 390 and provided to the DSP CCA 304 for further processing.

[0045] In one embodiment, the data acquisition CCA 316 may include a data sampling clock that is configured to provide timing control to the DSP CCA 304. In one such embodiment, the data acquisition CCA 316 generates two interrupts that are provide to the DSP CCA 304, one when ping starts and one when ping stops. In turn, the DSP CCA 304 may then use the interrupt signals to synchronize the ping operation sequence.

[0046] It is to be recognized that the various blocks or modules described with reference to the various CCAs 302-316 include functions that may in various embodiments be reallocated depending on the capabilities of the hardware used to implement the CCAs. In one embodiment, a single board may be used. In other embodiments, the functionality may be redistributed. For example, the functions of the receive beamformer 346 may be performed by the data acquisition CCA 316.

[0047] Figure 6 is a flowchart that illustrates one example of a method 600 of generating sonar images according to one embodiment. In one embodiment, the method 600 is performed using the system illustrated in Figures 3-6. The method 600 beings at a block 602 in which the transmit transducer array 308, transmits one or more acoustic pulses. Next at a block 604, the receiver array 310 receives a portion of the pulse that is scattered from a region, e.g., a target region of a body of water. The receive array 310 may include N transducer elements. Moving to a block 606, the receiver CCA 314, and/or the data acquisition CCA 316, generate a multi-channel signal based on the received portion of the pulse scattered from the region. In one embodiment, the receiver CCA 314 may be configured to filter the received portion of the pulse scattered from the region using the analog BPF 374 and the to generate a plurality of channel signals, each signal corresponding to one of a plurality of frequency bands. Such an example of the receiver CCA 314 may be further configured to amplify each of the channel signals via the TVG 378 for a portion of a time period, the portion being less than all of the time period, i.e., to time division multiplexing of the TVG 378. Desirably, such multiplexing allows use of fewer amplifiers thereby resulting in a smaller more compact system 100.

[0048] Moving to a block 610, the data acquisition CCA 316 samples the scattering signal to generate a plurality of complex samples. In one embodiment, the samples are provided to the DSP CCA 304. Next a block 612, the RX beamformer 346 simultaneously forms a plurality of beams in a horizontal plane based on complex samples. In one embodiment, the beams are formed with offset phase centers using, for example, linear combinations of the complex samples. In one embodiment, the number of beams is less than or equal to the number of receiving transducer elements. In one embodiment, each of the beams defines a fan or elliptically shaped beam. In one embodiment, the beams are formed in the horizontal plane while calculated angle of arrival processing is performed in the horizontal plane. In one embodiment, the horizontal plane is substantially orthogonal to a plane defined by a surface (or the bottom ocean, river, or seabed floor) of a body of water in which the beam is transmitted. In one embodiment having elliptically shaped beams, the beams are wider in the vertical dimension than in the horizontal dimension.

[0049] Proceeding to a block 614, the DSP CCA 304, and in particular, the detection and output processing module 344, processes the plurality of complex samples to obtain image data comprising angles of arrival from M, M<N, scattering components in a vertical plane for each horizontal beam. For example, in one embodiment, angle-of- arrival processing is applied to each horizontal beam in order to provide N/2 elevation angles and associated amplitudes at each range step in each beam. In one embodiment, angle of arrival processing is performed using a calculated angle of arrival method such as disclosed in U.S. Patent No. 6,130,641, entitled "Imaging Methods and Apparatus Using Model-Based Array Signal Processing, which is hereby incorporated by reference in its entirety. Next at a block 616, the detection and output processing module 344 generates an image based on the image data in the vertical plane and data associated with the plurality of beams in the horizontal beam. Alternatively, the detection and output processing module 344 may output, via the data interfaces 342 and 324, the image data in the vertical plane and data associated with the plurality of beams in the horizontal beam to allow an external processor to generate the image.

[0050] In one embodiment, the sonar system 104 may be configured to scan by sequentially transmitting a pulse from each transducer of the transmit arrays 308. In one embodiment, the transmit arrays 308 includes three transducers, e.g., a transducer element at -15 degrees from a central transducer element, a third transducer element at +15 degrees from the central transducer element. In this example of the system 104, a pulse is transmitted sequentially each of the three transducers, e.g., the -15° transducer, the central transducer, and +15° transducer. The processing described with reference to the method 600 may be repeated for each ping transmitted from each of the transducer of the transmit arrays 308. Thus, a forward area, e.g., from a UUV, may be scanned to, for example, provide obstacle avoidance ahead of the UUV.

[0051] One such embodiment of the system was successfully operated in a lake and the results are shown in Figures 7A-7D. During the test, the sonar was mounted on the end of a pole fixed to the side of the dock. The sonar depth was approximately 3.5 m and various downward tilt angles and pointing directions were used. Figure 7A shows a profile of the lakebed and surface buoy target. The lakebed is relatively flat in this data set and the surface buoy is readily visible above the sonar. Figure 7B shows a profile taken with the sonar pointed back towards the lake shore at an angle of -75 degrees. In this profile, the lakebed rises to the surface and a multipath image reflected by the surface is also visible. Figure 7C shows a composite bathymetric map of the lakebed ahead of the sonar derived from the data of Figure 7B and the profiles of the other 17 beams that span the 45 degree field-of-view. Figure 7D is a backscatter image for the same portion of the lakebed derived from the data from the same 18 beams as were used for Figure 1C.

[0052] In view of the above, one will appreciate that the invention overcomes the problem of generating images such as for use in a forward looking sonar. For example, one embodiment includes a sonar system and method that provides a compact and efficient forward looking sonar system for use in devices such as submersible vehicles or submarines. Moreover, applications of such a sonar system may include wide field-of-view, 3D detection and localization of underwater targets such as for mine counter measures, forward-looking 3d obstacle avoidance imaging, and/or forward looking 3D mapping of the seabed for hydrographic reconnaissance.

[0053] It is to be recognized that depending on the embodiment, certain acts or events of any of the methods described herein can be performed in a different sequence, may be added, merged, or left out all together (e.g., not all described acts or events are necessary for the practice of the method). Moreover, in certain embodiments, acts or events may be performed concurrently, e.g., through multi-threaded processing, interrupt processing, or multiple processors, rather than sequentially.

[0054] Those of skill will recognize that the various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention.

[0055] The steps of a method or algorithm described in connection with the embodiments disclosed herein may be embodied directly in hardware, in a software module executed by a processor, or in a combination of the two. A software module may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may reside in an ASIC. The ASIC may reside in a user terminal. In the alternative, the processor and the storage medium may reside as discrete components in a user terminal.

[0056] While the above detailed description has shown, described, and pointed out novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, and changes in the form and details of the device or process illustrated may be made by those skilled in the art without departing from the spirit of the invention. As will be recognized, the present invention may be embodied within a form that does not provide all of the features and benefits set forth herein, as some features may be used or practiced separately from others. The scope of the invention is indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

WHAT IS CLAIMED IS:

1. A method of imaging a region, the method comprising: transmitting at least one acoustic pulse; receiving a portion of the pulse scattered from a region via an acoustic transducer having N elements; generating a multi-channel signal based on the received portion of the pulse scattered from the region; sampling the scattering signal to generate a plurality of complex samples; simultaneously forming a plurality of beams in a horizontal plane based on the complex samples; processing the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane, wherein M is less than N, and wherein the horizontal plane and the vertical plane are substantially orthogonal; and generating an image based on the image data.

2. The method of Claim 1 , wherein processing the plurality of complex samples comprises applying angle-of-arrival processing to each horizontal beam to provide N/2 elevation angles and associated amplitudes at each range step in each of the horizontal beams.

3. The method of Claim 1, wherein each of the beams define a fan shape.

4. The method of Claim 1, wherein each of the beams is an elliptically shaped beam in which each of the beams is wider in the vertical plane than in the horizontal plane.

5. The method of Claim 1, wherein the horizontal plane is substantially orthogonal to a plane defined by a surface of a body of water in which the beam is transmitted.

6. The method of Claim 1, wherein generating the multi-channel signal comprises: filtering the received portion of the pulse scattered from the region to generate a plurality of channel signals, each signal corresponding to one of a plurality of frequency bands; and amplifying each of the channel signals for a portion of a time period, the portion of the time period for each channel being less than all of the time period.

7. The method of Claim 1, wherein simultaneously forming a plurality of beams in a horizontal plane based on complex samples comprises forming beams with offset phase centers.

8. A system for imaging a region, the system comprising: at least one transmit transducer configured to transmit at least one acoustic pulse; at least one receive transducer configured to receive a portion of the pulse scattered from a region, wherein the receive transducer has N elements; a receiver configured to: generate a multi-channel signal based on the received portion of the pulse scattered from the region; sample the scattering signal to generate a plurality of complex samples; a beamformer configured to simultaneously form a plurality of beams in a horizontal plane based on the complex samples; and a processor configured to: process the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane for each horizontal beam, wherein M is less than N, and wherein the horizontal plane and the vertical plane are substantially orthogonal; and generate an image based on the image data.

9. The system of Claim 8, wherein the processor is configured to apply angle- of-arrival processing to each horizontal beam to provide N/2 elevation angles and associated amplitudes at each range step in each of the horizontal beams.

10. The system of Claim 8, wherein the at least transmit transducer comprises a central transducer and two transducers configured to transmit at respective angles offset from the central transducer by about 15 degrees.

1 1. The system of Claim 8, wherein each of the beams define a fan shape.

12. The system of Claim 8, wherein each of the beams is an elliptically shaped beam in which each of the beams is wider in the vertical plane than in the horizontal plane.

13. The system of Claim 8, wherein the horizontal plane is substantially orthogonal to a plane defined by a surface of a body of water in which the beam is transmitted.

14. The system of Claim 8, wherein the receiver is configured to: filter the received portion of the pulse scattered from the region to generate a plurality of channel signals, each signal corresponding to one of a plurality of frequency bands; and amplify each of the channel signals for a portion of a time period, the portion of the time period for each channel being less than all of the time period.

15. The system of Claim 8, wherein the beamformer is configured to form the beams with offset phase centers.

16. An underwater vehicle, the vehicle comprising: a vehicle frame; a sonar system comprising: at least one transmit transducer configured to transmit at least one acoustic pulse; at least one receive transducer configured to receive a portion of the pulse scattered from a region, wherein the receive transducer has N elements; a receiver configured to: generate a multi-channel signal based on the received portion of the pulse scattered from the region; sample the scattering signal to generate a plurality of complex samples; a beamformer configured to simultaneously form a plurality of beams in a horizontal plane based on the complex samples; and a processor configured to: process the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane, wherein M is less than N, and wherein the horizontal plane and the vertical plane are substantially orthogonal; and generate an image based on the image data; and a housing configured to secure the sonar system to the vehicle frame.

17. The vehicle of Claim 16, wherein the processor is configured to apply angle-of-arrival processing to each horizontal beam to provide N/2 elevation angles and associated amplitudes at each range step in each of the horizontal beams.

18. The vehicle of Claim 16, wherein each of the beams is an elliptically shaped beam in which each of the beams is wider in the vertical plane than in the horizontal plane.

19. The vehicle of Claim 16, wherein the horizontal plane is substantially orthogonal to a plane defined by a surface of a body of water in which the beam is transmitted.

20. The vehicle of Claim 16, wherein the receiver is configured to: filter the received portion of the pulse scattered from the region to generate a plurality of channel signals, each signal corresponding to one of a plurality of frequency bands; and amplify each of the channel signals for a portion of a time period, the portion of the time period for each channel being less than all of the time period.

21. The vehicle of Claim 16, wherein the beamformer is configured to form the beams with offset phase centers.

22. A system for imaging a region, the system comprising: means for transmitting at least one acoustic pulse; means for receiving a portion of the pulse scattered from a region, the receiving means having N elements; means for processing the received portion of the pulse, wherein the processing means is configured to: generate a multi-channel signal based on the received portion of the pulse scattered from the region; sample the scattering signal to generate a plurality of complex samples; means for simultaneously forming a plurality of beams in a horizontal plane based on the complex samples; and means for processing the plurality of complex samples to obtain image data comprising angles of arrival from M scattering components in a vertical plane for each horizontal beam, wherein M is less than N, and wherein the horizontal plane and the vertical plane are substantially orthogonal, wherein the processing means is further configured to generate an image based on the image data.

23. The system of Claim 22, wherein the processing means is configured to apply angle-of-arrival processing to each horizontal beam to provide N/2 elevation angles and associated amplitudes at each range step in each of the horizontal beams.

24. The system of Claim 22, wherein each of the beams is an elliptically shaped beam in which each of the beams is wider in the vertical plane than in the horizontal plane, wherein the horizontal plane is substantially orthogonal to a plane defined by a surface of a body of water in which the beam is transmitted.

25. The system of Claim 22, wherein the receiving means comprises: means for filtering the received portion of the pulse scattered from the region to generate a plurality of channel signals, each signal corresponding to one of a plurality of frequency bands; and means for amplifying each of the channel signals for a portion of a time period, the portion of the time period for each channel being less than all of the time period.