GB2456426A - Underwater Detector - Google Patents

Abstract

The underwater detector includes a wave transmitting and receiving module 13 for transmitting a first transmission signal and a second transmission signal, frequencies of which are swept in an azimuth direction, and receiving a first echo signal and a second echo signal of the transmission signals, a frequency calculating module 15 for calculating frequencies of the first and second echo signals based on the frequencies of the first and second transmission signals in a predetermined azimuth direction and a traveling speed information of a ship, a filter module 16a, 16b for filtering the first and second echo signals based on the frequencies calculated by the frequency calculating module, and outputting a first received signal and a second received signal, and a received signal synthesizing module 18 for generating a synthesized received signal reduced grating lobes based on the first and second received signals.

Description

2456426

UNDERWATER DETECTOR Technical Field

[0001] The present invention relates to an underwater detector for detecting underwater by transmission and reception of an ultrasonic signal.

Background

[0002] An underwater detector for detecting a target object existing underwater by transmitting and receiving an ultrasonic signal, particularly, a scanning sonar is provided with a transducer including a plurality of elements. The shape of the transducer is cylindricality or hemisphere, and the elements are generally placed uniformly by the surface of the transducer. The scanning sonar transmits the ultrasonic signal to all surroundings by driving each of the elements of the transducer. Beamforming is performed using a set of elements in predetermined rows arranged in azimuth directions (circumferential direction) of the transducer to receive echo signals coming from predetermined directions. The beamforming is performed by adding the echo signals received by each of the elements arrayed in the wave transmitting and receiving module 13 in a predetermined azimuth direction, the echo signals being weighted a gain of the signals and adjusted the phases of the signals. The selection of the set of elements used is changed in turn in order to receive the signals from all azimuth directions; thereby, the scanning sonar performs detection for all azimuth directions.

[0003] However, the scanning sonar as described above generates grating lobes owing to the constitution of the transducer; thus, receives the echo signal from a direction other than the detecting direction to result in displaying a virtual image.

[0004] In order to reduce the grating lobes, the method is effective to some extent where a pitch between the elements is made narrow, that is, the density of the elements is made high. However, if the pitch between the elements is narrowed and the number of the elements increases, increase in proportion thereto, also a driving circuit for driving those, a control

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circuit for controlling, a receiving circuit, a circuit for processing the received echo signal and the like, thus leading to enormous hardware and a significantly increased cost.

[0005] Therefore, JP2005-106705(A) discloses an underwater detector in which a DFM (Directional Frequency Modulation) method is used where transmission and reception are performed with the ultrasonic signal, a band of which is limited in the directions as a method for reducing the virtual image due to the grating lobes.

[0006] Hereinafter, with reference to Figs. 12 and 13, the method for reducing the virtual image with the DFM method is described.

[0007] Fig. 12 is a conceptual diagram representing the main lobe and the grating lobes of the received signal Rx 1 obtained with a group of elements arrayed in the bow direction of the transducer. The height of the received signal Rxl corresponds to a signal level. Further, dotted lines represent the received signal Rxl having the signal level before filtering, and a solid line represents the received signal Rxl having the signal level after filtering.

[0008] In (a) of Fig. 13 is a correlation chart representing a relationship between the azimuth direction and the frequency in a transmission signal Txl to be transmitted and an echo signal Exl which is a reflected signal of the transmission signal. Further, (b) of Fig. 13 is a correlation chart representing a relationship between the azimuth direction and the signal level in the received signal Rxl obtained with elements which are similar to those in Fig. 12.

[0009] Firstly, as shown in (a) of Fig. 13, transmitted is the transmission signal Txl, the frequency of which is increased linearly from the stern via the portside, bow and starboard to the stem. The frequency of the transmission signal Txl is swept from fts at the stern, and increased linearly until fte from this stern, via the portside, bow and starboard to the stern; thus, the frequencies for the respective azimuth directions are different.

[0010] Then, the echo signal Exl of the transmission signal Txl is received with the element group arrayed in the starboard direction. At this time, as shown in Fig. 12 and (b) of Fig. 13, the echo signal Exl from azimuth directions other than the main lobe azimuth

direction (i.e., detecting direction) is also received owing to the grating lobes. In other words, as shown by dotted lines in (b) of Fig. 13, the received signal Rxl is made to have the signal level in a direction other than the detecting direction. Therefore, if display is performed based on the received signal Rx 1 like this, the virtual image is displayed.

[0011] Thus, in order to decrease the signal level of the grating lobes, the received signal Rxl is filtered by a bandpass filter having a center frequency at a frequency fc of the echo signal Exl coming from the main lobe direction. Note that BW in (a) of Fig. 13 represents a bandwidth of the bandpass filter.

[0012] This makes it possible in which as the received signal Rxl shown by a solid line in (b) of Fig. 13, the received signal Rxl is obtained which has the small signal level of the grating lobes; in other words, the signal level of the echo signal Exl coming from a direction other than the detecting direction is small and, thus, the virtual image is prevented from being displayed.

[0013] Here, since the underwater detector is exclusively used while the ship travels, the echo signal is made to have a Doppler shift. The ship travels in the bow direction, the frequency of the echo signal coming from the stern is lowered, and the frequency of the echo signal coming from the bow is heightened. Further, the frequencies of the echo signals from the portside direction and the starboard direction are not changed.

[0014] A correlation is shown in (a) of Fig. 14 between the azimuth direction and the frequency of the echo signal made to have the Doppler shift as described above. Tx2 in (a) of Fig. 14 is a transmission signal, and Ex2 is an echo signal. As shown in (a) of Fig. 14, the echo signal Ex2 has a frequency variation amount smaller on the side of the starboard than on the side of the bow.

[0015] The echo signal Ex2 is received with the element group arrayed in the bow direction. Further, as similar to the underwater detector of related art, the received signal Rx2 is filtered by the bandpass filter having the center frequency at the frequency fc of the echo signal Ex2 coming from the azimuth direction of the main lobe in order to reduce the signal level of the grating lobes.

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[0016] However, on the starboard side, the frequency of the echo signal Ex2 is swept gently and the shift thereof is small, and thus is made to be inside the bandwidth BW of the bandpass filter having the center frequency at the frequency fcl in the bow direction. Therefore, the received signal Rx2 can not be filtered on the starboard side from the bow, and the signal level of the grating lobes can not be decreased as shown in (b) of Fig. 14. Specifically, the underwater detector of related art can not have prevented the virtual image from being displayed during traveling.

[0017] Further, the DFM method disclosed in JP2005-106705(A), which performs the frequency sweep in the azimuth directions, effectively suppresses the virtual image displayed due to the received grating lobes existing in the azimuth direction of the transmitted main lobe. However, the suppressing effect is low for the virtual image displayed due to the received grating lobes existing in a vertical direction of the transmitted main lobe. In other words, the effect is exerted on only the azimuth directions, and the virtual image due to the grating lobes is still generated in the vertical direction.

[0018] Hereinafter, the virtual image due to the grating lobes in terms of the vertical direction is described with reference to Figs. 15 and 16.

[0019] Fig. 15 is a schematic view showing the main lobe of the ultrasonic signal transmitted from the transducer and two kinds of the grating lobes (GL). Particularly, when a tilt angle 20° occurs, obvious grating lobes directly below in direction (first GL) and obliquely upward in direction (second GL) are generated as shown in Fig. 15.

[0020] Fig. 16 shows a schematic view of a detected image obtained with the main lobe and the two kinds of grating lobes. In Fig. 16, the center represents a position of the transducer, and the radial direction represents a distance or a depth. Further, a circumferential direction represents an azimuth, and in Fig. 16, an upward direction is the bow direction, a downward direction is the stem direction, left is the portside direction, and right is the starboard direction.

[0021] Here, as shown in Fig. 15, a path from the transducer to the water bottom is shorter in the first GL and the second GL than in the main lobe. Therefore, as shown in Fig. 16,

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the virtual image of the water bottom detected with the first GL and the second GL has been displayed inside the real image of the water bottom detected with the main lobe.

Summary

[0022] It would be desirable to provide an underwater detector which can prevent or reduce the virtual image attributable to the grating lobes.

[0023] According to an aspect of the invention, an underwater detector includes a wave transmitting and receiving module for transmitting a first transmission signal and a second transmission signal, frequencies of which are swept in an azimuth direction, and receiving a first echo signal and a second echo signal of the transmission signals, a frequency calculating module for calculating frequencies of the first and second echo signals based on the frequencies of the first and second transmission signals in a predetermined azimuth direction and a traveling speed information of a ship, a filter module for filtering the first and second echo signals based on the frequencies calculated by the frequency calculating module, and outputting a first received signal and a second received signal, and a received signal synthesizing module for generating a synthesized received signal which is reduced grating lobes based on the first and second received signals.

[0024] As described above, for example, even when the echo signal is made to have a Doppler shift upon detecting underwater using the DFM (Directional Frequency Modulation) method, two kinds of received signals can be obtained, which are different in azimuth directions of grating lobes which cannot be filtered. The two kinds of received signals are compared to compensate for each other the azimuth directions which could not been filtered. In other words, a synthesized received signal equivalent to received signals which can be filtered in all azimuth directions can be obtained. Therefore, the virtual image attributable to the grating lobes can be prevented from being displayed.

[0025] According to another aspect of the invention, an underwater detector detects underwater based on transmission and reception of ultrasonic signals from and to a plurality of elements. The under water detector includes a wave transmitting and receiving module

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for transmitting a First transmission signal, a frequency of which is swept in an azimuth direction, and a second transmission signal, a frequency of which is different from that of the first transmission signal and is swept in an azimuth direction, from a group of the elements arrayed in all azimuth directions, and receiving a first echo signal of the first transmission signal and a second echo signal of the second transmission signal with a group of the elements arrayed in a predetermined azimuth direction. The under water detector includes a frequency calculating module for calculating frequencies of the first and second echo signals coming from the predetermined azimuth direction, a first filter for filtering a signal received by the wave transmitting and receiving module based on the frequency of the first echo signal calculated by the frequency calculating module to obtain a first received signal, a second filter for filtering a signal received by the wave transmitting and receiving module based on the frequency of the second echo signal calculated by the frequency calculating module to obtain a second received signal, and a received signal synthesizing module for generating a synthesized received signal where a signal level of grating lobes is lowered based on the first and second received signals. Azimuth directions of grating lobes of the first echo signal filtered by the first filter and azimuth directions of grating lobes of the second echo signal filtered by the second filter are different from each other.

[0026] As described above, for example, even when the echo signal is made to have a Doppler shift upon detecting underwater using the DFM (Directional Frequency Modulation) method, two kinds of received signals can be obtained, which are different in azimuth directions of grating lobes which cannot be filtered. The two kinds of received signals are compared to compensate for each other the azimuth directions which could not been filtered. In other words, a synthesized received signal equivalent to received signals which can be filtered in all azimuth directions can be obtained. Therefore, the virtual image attributable to the grating lobes can be prevented from being displayed.

[0027] The wave transmitting and receiving module may transmit the first transmission signal, the frequency of which is up-chirped in an azimuth direction and the second transmission signal, the frequency of which is different from that of the first transmission

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signal and is down-chirped in an azimuth direction, from a group of elements arrayed in all azimuth directions.

[0028] The wave transmitting and receiving module may transmit the first transmission signal, the frequency of which is triangle-swept in an azimuth direction and the second transmission signal, the frequency of which is different from that of the first transmission signal and is triangle-swept in the azimuth direction at a phase different from that of the first transmission signal, from a group of elements arrayed in all azimuth directions.

[0029] The received signal synthesizing module may generate the synthesized received signal by comparing signal levels of the first received signal and the second received signal for respective azimuth directions, and selecting the received signal having a lower signal level.

[0030] The received signal synthesizing module may generate the synthesized received signal by comparing signal levels of the first received signal and the second received signal for respective azimuth directions, selecting the received signal having a lower signal level when a difference of the signal levels is lower than a predetermined value, and selecting the received signal having a higher signal level when the difference of the signal levels equals to or more than the predetermined value.

[0031] The underwater detector may further include a reception aperture forming module for forming a first reception aperture and a second reception aperture, at least an element of the second reception aperture being not included in the first reception aperture. The first echo signal may be received by a group of the elements forming the first reception aperture among the plurality of elements, and the second echo signal may be received by a group of the elements forming the second reception aperture among the plurality of elements.

[0032] The plurality of elements may be arrayed in a transducer including a cylinder part and a hemispheric part adjacent to and below the cylinder part. The reception aperture forming module may form the first reception aperture in the cylinder part and the second reception aperture in the hemispheric part.

[0033] The underwater detector may further include a transmission aperture forming

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module for forming a first transmission aperture and a second transmission aperture, at least an element of the second transmission aperture being not included in the first reception aperture. The wave transmitting and receiving module may transmit the first transmission signal from the group of elements forming the first transmission aperture, and transmit the second transmission signal from the group of elements forming the second transmission aperture.

[0034] According to another aspect of the invention, an underwater detector detects underwater based on transmission and reception of ultrasonic signals from and to a plurality of elements. The under water detector includes a drive signal generating module for generating a drive signal for driving each of the plurality of elements, a transmission aperture forming module for forming a transmission aperture with a predetermined group of the elements among the plurality of the elements, a wave transmitting and receiving module for transmitting transmission signals based on the drive signals from the group of elements forming the transmission aperture, and receiving echo signals of the transmission signals with the plurality of elements, a reception aperture forming module for forming a plurality of different reception apertures, a received signal generating module for synthesizing the echo signals received by the group of elements forming the reception aperture to generate a plurality of received signals respectively corresponding to the plurality of reception apertures, a received signal selecting module for selecting the received signal having a lower signal level among the plurality of received signals, and a display module for displaying based on the selected received signal.

[0035] As described above, the reception of the echo signals with different apertures suppresses grating lobes existing in a vertical direction of the transmitted main lobe.

[0036] According to another aspect of the invention, a ship includes any of the above underwater detectors.

Brief Description of the Drawings

[0037] The present disclosure is illustrated by way of example and not by way of

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limitation in the figures of the accompanying drawings, in which the like reference numerals indicate like elements and in which:

[0038] Fig. 1 is a schematic view showing a configuration of an underwater detector according to Embodiment I of the present invention;

[0039] Fig. 2 is a graph and charts showing signals transmitted and received in the underwater detector of Embodiment 1;

[0040] Fig. 3 is a chart showing a synthesized received signal generated in the underwater detector of Embodiment 1;

[0041] Fig. 4 is a graph and charts showing signals transmitted and received in an underwater detector according to Embodiment 2 of the invention;

[0042] Fig. 5 is a schematic view showing a configuration of an underwater detector according to Embodiment 3 of the invention;

[0043] Figs. 6A and 6B are schematic views showing apertures of the underwater detector of Embodiment 3;

[0044] Fig. 7 is a schematic view showing a configuration of an underwater detector according to Embodiment 4 of the invention;

[0045] Fig. 8 is a view showing a detected image in an underwater detector of related art;

[0046] Fig. 9 is a view showing a detected image obtained by a reception aperture formed in a cylindrical shape;

[0047] Fig. 10 is a view showing a detected image obtained by a reception aperture formed in a hemispherical shape;

[0048] Fig. 11 is a view showing a detected image by the underwater detector of Embodiment 4;

[0049] Fig. 12 is a schematic view showing a signal received (received signal) in the underwater detector of related art;

[0050] Fig. 13 is a graph and a chart showing signals transmitted and received in the underwater detector of related art;

[0051] Fig. 14 is a graph and a chart showing signals transmitted and received in the

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underwater detector of related art;

[0052] Fig. 15 is a schematic view showing a main lobe and two kinds of grating lobes transmitted from a transducer; and

[0053] Fig. 16 is a schematic view showing a detected image obtained by the main lobe and the grating lobes shown in Fig. 15.

Detailed Description

(Embodiment 1)

[0054] An underwater detector 1 in Embodiment 1 has a main characteristic in which two kinds of transmission signal, frequencies of which are swept in azimuth directions, are transmitted to obtain two kinds of received signals, which are compared with each other to generate a synthesized received signal with a low signal level of the grating lobes.

[0055] Hereinafter, the underwater detector 1 is described with reference to Fig. 1 to Fig. 3.

[0056] The underwater detector 1 includes, as shown in Fig. 1, a drive signal generating module 10, a transmission aperture forming module 11, a transmission and reception (TX/RX) switching module 12, a wave transmitting and receiving (TX/RX) module 13, a first BMF 14a, a second BMF 14b, a frequency calculating module 15, a first post-filter 16a, a second post-filter 16b, a first post-amplifier 17a, a second post-amplifier 17b, a received signal synthesizing module 18, and a display module 19.

[0057] The drive signal generating module 10 generates a drive signal for driving a plurality of elements provided to the wave transmitting and receiving module 13, and outputs the same via the transmission and reception switching module 12 to the wave transmitting and receiving module 13. Frequency and the like of the transmission signal to be transmitted are determined with the drive signal.

[0058] The transmission aperture forming module 11 forms a transmission aperture in the wave transmitting and receiving module 13. The wave transmitting and receiving module 13 is provided with a transducer arranged with the plurality of elements, and the

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transmission aperture forming module 11 forms the aperture by selecting the element to be driven.

[0059] The transmission and reception switching module 12 outputs the drive signal generated by the drive signal generating module 10 to the wave transmitting and receiving module 13. Further, the transmission and reception switching module 12 outputs an echo signal received by the wave transmitting and receiving module 13 described later to the first BMF 14a and the second BMF 14b.

[0060] The wave transmitting and receiving module 13 transmits the transmission signal in all azimuth directions from the aperture formed by the transmission aperture forming module 11 based on the drive signal generated by the drive signal generating module 10. There are two kinds of transmission signals to be transmitted, which are a first transmission signal Tal and a second transmission signal Ta2, frequencies of which respectively are linearly increased and decreased from the stern via the portside, the bow and the starboard side to the stern.

[0061] Then, the wave transmitting and receiving module 13 receives a first echo signal Eal as a reflected signal of the first transmission signal Tal and a second echo signal Ea2 as a reflected signal of the second transmission signal Ta2.

[0062] The first BMF 14a and the second BMF 14b perform beamforming (BMF), that is, form a reception beam to output the received signal. The beamforming is performed by adding the echo signals received by each of the elements arrayed in the wave transmitting and receiving module 13 in a predetermined azimuth direction, the echo signals being weighted a gain of the signals and adjusted the phases of the signals. The predetermined azimuth direction is a detection direction, and the detection is performed by turning sequentially in azimuth directions a group of the elements of a predetermined azimuth direction concerning beamforming to perform detection for all azimuth directions.

[0063] Here, when a ship equipped with the underwater detector 1 travels, the echo signal is made to have a Doppler shift, thus, the frequencies of the echo signals in the respective azimuth directions are different from those of the transmission signal. An equation for

calculating the frequency of the echo signal is shown below. Here, fE(0) represents a frequency of the echo signal at an azimuth 0, fr(0) represents a frequency of the transmission signal at the azimuth 0, C represents the sound speed, and v(0) represents a vessel speed at the azimuth 0. In this embodiment, the ship may be any type of water vessel which travels on water surface and travels underwater.

fE{9)=C + v{9) fT(0) ...(1)

f C-v(0) T

[0064] The frequency calculating module 15 calculates the frequency fe(0) of the echo signal coming from each azimuth direction based on Equation (1). The frequency calculating module 15 calculates the frequency fE(0) of the received echo signal using the frequency fT(0) of the transmission signal based on the drive signal outputted from the drive signal generating module 10 and the vessel speed v(0) as the input values.

[0065] The first post-filter 16a and the second post-filter 16b filter the received signals outputted from the first BMF 14a and the second BMF 14b, respectively. The first post-filter 16a is a bandpass filter, and filters the received signal outputted from the first BMF 14a using as the center frequency the frequency of the first echo signal Eal coming from a predetermined azimuth direction calculated by the frequency calculating module 15 to obtain a first received signal Ral. Similarly, the second post-filter 16b is also a bandpass filter, and filters the received signal outputted from the second BMF 14b using as the center frequency the frequency of the second echo signal Ea2 coming from a predetermined azimuth direction calculated by the frequency calculating module 15 to obtain a second received signal Ra2.

[0066] The first post-amplifier 17a amplifies the first received signal Ral filtered by the first post-filter 16a. Similarly, the second post-amplifier 17b amplifies the second received signal Ra2 filtered by the second post-filter 16b. The first post-amplifier 17a and the second post-amplifier 17b perform a so-called TVG (Time Variable Gain) control where the amplification factor increases with time elapsed. In other words, attenuation of the signal propagated underwater is corrected in accordance with the path length thereof. Therefore,

the signal level of the received signal is fixed with respect to the path length.

[0067] The received signal synthesizing module 18 compares the first received signal Ral amplified by the first post-amplifier 17a with the second received signal Ra2 amplified by the second post-amplifier 17b to generate a synthesized received signal Ral2. The received signal synthesizing module 18 also compares the signal levels of the first received signal Ral and the second received signal Ra2 for the respective azimuth directions, and selects the received signal having a lower signal level to generate the synthesized received signal Ral2. In other words, one of the first received signal Ral and the second received signal Ra2, which has the lower signal level, is generated as the synthesized received signal Ral2 for the respective azimuth directions.

[0068] Therefore making it possible to obtain the synthesized received signal Ral2 with the grating lobes having the signal level lowered since the signal level of the first received signal Ral in an azimuth direction, which had not been filtered by the first post-filter 16a, can be lowered based on the second received signal Ra2 filtered by the second post-filter 16b.

[0069] Then, an image based on the synthesized received signal Ral2 is displayed on the display module 19.

[0070] As described above, the underwater detector I can prevent the virtual image due to the grating lobes from being displayed. Further, as described later, a suppressing effect by filtering on the grating lobes can be improved; and thus the displayed virtual image can be made smaller than that of related art.

[0071] Note that, in Embodiment 1, the two transmission signals are transmitted sequentially during one transmission period, and received during a reception period thereafter. However, the two transmission signals may be alternately transmitted and received. In this case, a received signal obtained at a previous time may be stored in a memory and the like, and a received signal obtained at this time may then be compared with the received signal stored in the memory to generate the synthesized received signal.

[0072] Next, referring to Figs. 2 and 3, the signal which is transmitted and received, and

generated in the underwater detector I is described. Note that, the signal level of each received signal shown in Figs. 2 and 3 is equal to the signal level of the reception beam.

[0073] In (a) of Fig. 2 is a correlation chart representing a relationship between the frequency and the azimuth direction of the transmission signal to be transmitted and the echo signal as the reflected signal thereof.

[0074] As shown in (a) of Fig. 2, the first transmission signal Tal transmitted from the wave transmitting and receiving module 13 is a signal, a frequency of which linearly increases (linear sweep) from ftsl to ftel from the stern via the portside, the bow and the starboard to the stern. Whereas, the second transmission signal Ta2 is a signal, a frequency of which linearly deceases (linear sweep) from fts2 to fte2 from the stern via the portside, the bow and the starboard to the stern. In this way, the frequencies of the first transmission signal Tal and the first transmission signal Tal are swept symmetrically. The frequency ftsl is larger than fts2, and frequency bands of the first transmission signal Tal and the second transmission signal Ta2 do not overlap each other.

[0075] Then, the first echo signal Eal is made to have a Doppler shift and becomes a signal, the frequency of which is swept steeply on the portside and gently on the starboard side. Conversely, the second echo signal Ea2 becomes a signal, the frequency of which is swept gently on the portside and steeply on the starboard side. Note that, in this case, the ship equipped with the underwater detector 1 travels in the bow direction; thus, the velocity component is 0 in directions of the portside and the starboard.

[0076] In (b) of Fig. 2 is a correlation chart representing a relationship between the azimuth direction and the signal level in the first received signal Ral filtered by the first post-filter 16a. Further, (c) of Fig. 2 is a relationship between the azimuth direction and the signal level of the second received signal Ra2 filtered by the second post-filter 16b. Note that, the first received signal Ral and the second received signal Ra2 are signals obtained by way of the element group arrayed in the bow direction.

[0077] As shown in (b) of Fig. 2, the signal level of the first received signal Ral on the starboard side can not be decreased because the signal is not filtered. Due to the first

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received signal Ral on the starboard side is inside the bandwidth BW of the first post-filter 16a (bandpass filter) using a frequency fcl in the bow direction as the center frequency since the frequency of the first echo signal Eal is gently swept on the starboard side, and a shift of the frequency is smaller than that of the first transmission signal Tal. On the other hand, the frequency is swept steeply on the portside, and the frequency shift becomes larger than that of the first transmission signal Tal at a point farther apart from the bow direction. That is, a filtering effect is improved more than that of related art on the portside and the signal level due to the grating lobes becoming smaller.

[0078] The second received signal Ra2 is a signal, as shown in (c) of Fig. 2, where the relationship of the starboard and the portside in the first received signal Ral is reversed. In other words, because of a similar reason to for the first received signal Ral, the signal on the portside is not filtered, and the filtering effect of the signal on the starboard side is improved.

[0079] Fig. 3 is a correlation chart representing a relationship between the azimuth direction and the signal level in the synthesized received signal Ral2. As shown in Fig. 3, the signal level of the synthesized received signal Ral2 for each of the azimuth directions is equal to one of the first received signal Ral shown in (b) of Fig. 2 and the second received signal Ra2 shown (c) of Fig. 2 having the lower signal level. That is, the synthesized received signal Ral2 is a signal generated by synthesizing for each azimuth direction one of the first received signal Ral and the second received signal Ra2 having the lower signal level. Further, a filtered component of each received signal has the signal level lower than that of related art; thus, the synthesized received signal Ral2 is a signal having a higher suppressing effect of the grating lobes than that of related art.

[0080] The two kinds of transmission signals transmitted underwater may be different in the filtered azimuth direction by a difference between the frequency variation widths of the echo signals thereof for the azimuth directions, and thus the transmission signal is not necessary linearly swept. That is, the signals may be a signal up-chirped to a high order or logarithm and a signal down-chirped.

[0081] Further, it may be in which the received signal synthesizing module 18 compares the signal levels of the first received signal Ral with the second received signal Ra2 for each azimuth direction, and selects when the difference between the signal levels is smaller than a predetermined value; the received signal having the smaller signal level, and selects when the difference between the signal levels is equal to or more than the predetermined value, the received signal having the larger signal level, thereby to generate the synthesized received signal Ral2.

(Embodiment 2)

[0082] Hereinafter, Embodiment 2 according to the invention is described with reference to Fig. 4. An underwater detector of Embodiment 2 is different from the underwater detector I of Embodiment 1 in which the transmission signal is subjected to a triangle sweep. Note that a configuration of the underwater detector in Embodiment 2 is similar to the underwater detector 1 in Embodiment 1; thus, an explanation thereof is omitted and only a signal transmitted and received is described.

[0083] In Fig. 4, (a) is a correlation chart representing a relationship between the frequency and the azimuth direction of the transmission signal transmitted. As shown in (a) of Fig. 4, a third transmission signal Ta3 transmitted from the wave transmitting and receiving device is a so-called triangle swept signal; a frequency of which increases linearly from the stern via the portside to the bow, and decreases from the bow via the starboard to the stern in a symmetric relation to the portside. A fourth transmission signal Ta4 is a signal obtained by shifting the frequency sweep of the third transmission signal Ta3 in an azimuth direction. That is, the third transmission signal Ta3 is different from the fourth transmission signal Ta4 in a phase in the azimuth direction. Amounts of the shift (phase shift) of the transmission signals may only be different from each other in the azimuth direction with the same frequency, and is not limited to 90° (from the bow direction to the starboard direction) unlike Embodiment 2.

[0084] Here, it should be mentioned that the effect of a Doppler shift is not taken into consideration for the sake of simplicity of the explanation in Embodiment 2. That is, as

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shown in (a) of Fig. 4, the frequencies of the third transmission signal Ta3 and a third echo signal Ea3 as a reflected signal thereof are equal, and the frequencies of the fourth transmission signal Ta4 and a fourth echo signal Ea4 as a reflected signal thereof are equal.

[0085] In Fig. 4, (b) is a correlation chart representing a relationship between the azimuth direction and the signal level in a third received signal Ra3 filtered by the first filter as the similar to Embodiment 1. Further, (c) of Fig. 4 is a correlation chart representing a relationship between the azimuth direction and the signal level in a fourth received signal Ra4 filtered by the second filter. Note that the third received signal Ra3 and the fourth received signal Ra4 are signals obtained by way of the element group arrayed in a direction slightly close to the starboard side from the bow direction.

[0086] Based on the third received signal Ra3 and the fourth received signal Ra4 described above, the synthesized received signal is generated by the received signal synthesizing module similarly to Embodiment 1, thereby obtaining the signal having the signal level of the grating lobes decreased to prevent the virtual image from being displayed.

[0087] Further, when such a transmission signal from the frequency of which is triangle-swept in the azimuth direction is affected by a Doppler effect, the effect of the grating lobes can be reduced with a similar configuration. In case of taking the effect of a Doppler shift into consideration, the echo signal corresponding to the transmission signal, which is subjected to a triangle sweep and has a peak of frequency in the bow direction like the transmission signal Ta3 shown in (a) of Fig. 4, has the frequency sweeps steeply and the shift of the frequency is large in the azimuth direction on both the portside and the starboard side, unlike the echo signal corresponding to the transmission signal subjected to a linear sweep as Embodiment 1. With this taken into consideration, in a case in which two kinds of transmission signals to be transmitted are two kinds of signals as are phased symmetrically in the directions of the portside and the starboard from the signal having a peck of frequency in the bow direction, the synthesized received signal with the filtering effect enlarged can be generated.

(Embodiment 3)

[0088] An underwater detector 2 according to Embodiment 3 performs a transmission and reception with two kinds of apertures and two kinds of frequencies to obtain two kinds of received signals for an identical detected area. Then, a main characteristic of the detector is when two kinds of received signals are compared with each other to prevent the virtual image, due to the grating lobes, from being displayed. Hereinafter, the underwater detector 2 is described with reference to Figs. 5, 6A and 6B.

[0089] The underwater detector 2 includes, as shown in Fig. 5, a drive signal generating module 20, a first transmission aperture forming module 21a, a second transmission aperture forming module 21b, a transmission and reception (TX/RX) switching module 22, a wave transmitting and receiving (TX/RX) module 23, a first pre-filter 24a, a second pre-filter 24b, a first pre-amplifier 25a, a second pre-amplifier 25b, a first reception aperture forming module 26a, a second reception aperture forming module 26b, a first received signal generating module 27a, a second received signal generating module 27b, a received signal synthesizing module 28 and a display module 29.

[0090] Note that, the configuration of the first BMF 14a described in Embodiment 1 corresponds to the first pre-filter 24a, the first pre-amplifier 25a, the first reception aperture forming module 26a and the first received signal generating module 27a in Embodiment 3. Further, the configuration of the second BMF 14b described in Embodiment 1 corresponds to the second pre-filter 24b, the second pre-amplifier 25b, the second reception aperture forming module 26b and the second received signal generating module 27b in Embodiment 3. Further, in Embodiment 3, the post-filter and the post-amplifier in a stage subsequent to beamforming is omitted in description.

[0091] The drive signal generating module 20 generates a drive signal for driving a plurality of elements provided to the wave transmitting and receiving module 23. Frequency and the like of the ultrasonic signal to be transmitted are determined with the drive signal. The drive signal generating module 20 outputs two kinds of drive signals to the wave transmitting and receiving module 23, and the underwater detector 2 transmits the

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ultrasonic signals having the frequencies different from each other (frequency fl and frequency f2) during one transmission period.

[0092] The first transmission aperture forming module 21a and the second transmission aperture forming module 21b form a transmission aperture in the wave transmitting and receiving module 23. The wave transmitting and receiving module 23 is provided with a transducer having a plurality of elements arranged. The first transmission aperture forming module 21a and the second transmission aperture forming module 21b form the aperture by selecting the element to be driven. Here, the transducer is configured to have a cylinder part and a hemispheric part connected to a lower portion of the cylinder part as shown in Figs. 6A and 6B. The first transmission aperture forming module 21a forms an aperture in a cylindrical shape (cylinder part) as shown in Fig. 6A, and the second transmission aperture forming module 21b forms an aperture in a hemispherical shape (hemispheric part) as shown in Fig. 6B.

[0093] The transmission and reception switching module 22 outputs a signal outputted from the first transmission aperture forming module 21a and the second transmission aperture forming module 21b to the wave transmitting and receiving module 23. The signal outputted from the first transmission aperture forming module 21a and the second transmission aperture forming module 21b is sequentially outputted during one transmission period by being switched with a switch and the like. Further, the transmission and reception switching module 22 outputs the echo signal received by the wave transmitting and receiving module 23 described later to the first pre-filter 24a and the second pre-filter 24b.

[0094] The wave transmitting and receiving module 23 transmits the ultrasonic signal from the aperture formed by the first transmission aperture forming module 21a and the second transmission aperture forming module 21b based on the drive signal generated by the drive signal generating module 20. Specifically, the wave transmitting and receiving module 23 transmits the ultrasonic signal of the frequency f 1 from the aperture formed in the cylindrical shape and the ultrasonic signal of the frequency f2 from the aperture formed

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in the hemispherical shape based on two kinds of drive signals outputted from the drive signal generating module 20. Then, the wave transmitting and receiving module 23 receives the echo signals of the respective transmitted ultrasonic signals, and outputs the echo signals to the first pre-filter 24a and the second pre-filter 24b, respectively.

[0095] The first pre-filter 24a and the second pre-filter 24b output a desired frequency component in the received echo signals. The first pre-filter 24a outputs an echo signal having a predetermined frequency width including only the frequency f 1 of the echo signal having the frequency fl and the frequency f2 to the first pre-amplifier 25a. That is, the echo signal of the ultrasonic signal transmitted from the aperture formed in the cylindrical shape is extracted and outputted to the first pre-amplifier 25a. Whereas, the second pre-filter 24b outputs an echo signal having a predetermined frequency width including only the frequency f2 of the echo signal having the frequency fl and the frequency f2 to the second pre-amplifier 25b. That is, the echo signal of the ultrasonic signal transmitted from the aperture formed in the hemispherical shape is extracted and outputted to the second pre-amplifier 25b.

[0096] The first pre-amplifier 25a and the second pre-amplifier 25b amplify the echo signal filtered to the desired frequency component. The first pre-amplifier 25a and the second pre-amplifier 25b perform a so-called TVG (Time Variable Gain) control where again amplification factor increases as the time elapses to correct attenuation of the echo signal propagated underwater depending on the path length thereof. The signal subjected to the TVG control by the first pre-amplifier 25a is outputted to the first reception aperture forming module 26a, and the signal subjected to the TVG control by the second pre-amplifier 25b is outputted to the second reception aperture forming module 26b.

[0097] The first reception aperture forming module 26a forms a reception aperture using the element group as a set provided to the cylinder part, and the second reception aperture forming module 26b forms a reception aperture using the element group as a set provided to the hemispheric part.

[0098] The first received signal generating module 27a performs beamforming at the

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reception aperture in the cylinder part formed by the first reception aperture forming module 26a, and generates the signal to be received. Similarly, the second received signal generating module 27b performs beamforming at the reception aperture in the hemispheric part formed by the second reception aperture forming module 26b, and generates the signal to be received. Then, both generated signals to be received are outputted to the received signal synthesizing module 18. Further, information of the echo signal received by each of the elements is once stored in a memory and the like (not shown), and based on the stored information, the beamforming is performed.

[0099] Here, the signal levels of the echo signals obtained from the reception aperture of the cylinder part and the reception aperture of the hemispheric part are equal to each other for the main lobe, but different from each other for the grating lobes depending on absence or presence thereof. Specifically, the echo signal obtained from the aperture of the cylinder part is small in the level of the first GL and large in the level of the second GL shown in Fig. 15 or Fig. 16. Conversely, the echo signal obtained from the hemispheric part is large in the level of the first GL and small in the level of the second GL.

[0100] The received signal synthesizing module 18 compares the received signals with each other outputted from the first received signal generating module 27a and the second received signal generating module 27b, and selects the received signal having the smaller signal level. Then, the display module 29 displays the echo based on the selected received signal. As described above, the received signals obtained from the different apertures, particularly, the apertures formed in the cylinder part and the hemispheric part, are different from each other in positions where the virtual images are displayed due to the grating lobes. Therefore, selection and display of the received signal having the smaller signal level for every display position makes it possible to prevent the virtual image due to the grating lobes from being displayed.

[0101] The aperture for transmission and reception is not limited to being formed in the cylinder part and the hemispheric part, and may be formed in any manner so long as the positions of the main lobe are the same and the positions of the grating lobes are different.

Further, the shape of the transducer is not limited to the shape having the cylinder part and the hemispheric part, and may be a shape of only a cylinder part, only a hemispheric part, and a polyhedron.

[0102] Further, the aperture and the frequency are not limited to two kinds and may be two or more kinds. For example, it may be when three kinds of apertures are formed to obtain three kinds of received signals having different signal levels; echo is displayed based on the received signal having the smallest signal level.

(Embodiment 4)

[0103] An underwater detector 3 according to Embodiment 4 is different from the underwater detector 2 according to Embodiment 3 in mainly the transmission aperture and the transmission frequency. The underwater detector 2 of Embodiment 3 transmits two kinds of ultrasonic signals different from each other in the frequency and the transmission aperture. Whereas the underwater detector 3 of Embodiment 4 transmits one kind of ultrasonic signal in which the frequency and the transmission aperture are identical. Hereinafter, the underwater detector 3 is described with reference to Figs. 7 to 11.

[0104] The underwater detector 3 includes, as shown in Fig. 7, a drive signal generating module 30, a transmission aperture forming module 31, a transmission and reception (TX/RX) switching module 32, a wave transmitting and receiving (TX/RX) module 33, a pre-filter 34, a pre-amplifier 35, a first reception aperture forming module 36a, a second reception aperture forming module 36b, a first received signal generating module 37a, a second received signal generating module 37b, a received signal synthesizing module 38 and a display module 39.

[0105] The drive signal generating module 30 generates a drive signal for driving a plurality of elements provided to the wave transmitting and receiving module 33. Frequency and the like of the ultrasonic signal to be transmitted are determined with the drive signal. The underwater detector 3 transmits the ultrasonic signal of a predetermined frequency.

[0106] The transmission aperture forming module 31 forms the transmission aperture in

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the wave transmitting and receiving module 33. The wave transmitting and receiving module 33 is provided with a transducer having a plurality of elements arranged, all of which the transmission aperture forming module 31 drives and controls to form the aperture all over the transducer.

[0107] The transmission and reception switching module 32 outputs the signals outputted from the drive signal generating module 30 and the transmission aperture forming module 31 to the wave transmitting and receiving module 33. Further, the echo signal received by the wave transmitting and receiving module 33 described later is outputted to the pre-filter 34.

[0108] The wave transmitting and receiving module 33 transmits the ultrasonic signal of a predetermined frequency from the aperture all over the transducer formed by the transmission aperture forming module 31 based on the drive signal generated by the drive signal generating module 30. Then, the wave transmitting and receiving module 33 receives the echo signal of the transmitted ultrasonic signal and outputs the echo signal to the pre-filter 34.

[0109] The pre-filter 34 outputs the same frequency component as the transmission frequency of the received echo signal to the pre-amplifier 35. Then, the pre-amplifier 35 amplifies the echo signal filtered by the pre-filter 34. The pre-amplifier 35 performs a so-called a TVG (Time Variable Gain) control where a amplification factor increases as the time elapses to correct attenuation of the echo signal propagated underwater depending on the path length thereof. The signal subjected to the TVG control by the pre-amplifier 35 is outputted to the first reception aperture forming module 36a and the second reception aperture forming module 36b.

[0110] The first reception aperture forming module 36a forms a reception aperture using the element group as a set provided to the cylinder part, and the second reception aperture forming module 36b forms a reception aperture using the element group as a set provided to the hemispheric part.

[0111] The first received signal generating module 37a performs beamforming at the

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reception aperture in the cylinder part formed by the first reception aperture forming module 36a, and generates the signal to be received. Similarly, the second received signal generating module 37b performs beamforming at the reception aperture in the hemispheric part formed by the second reception aperture forming module 36b, and generates the signal to be received. Then, both generated signals to be received are outputted to the received signal synthesizing module 38.

[0112] Here, the signal levels of the echo signals obtained from the reception aperture of the cylinder part and the reception aperture of the hemispheric part are equal to each other for the main lobe, but different from each other for the grating lobes depending on absence or presence thereof. Specifically, the echo signal obtained from the aperture of the cylinder part is small in the level of the first GL and large in the level of the second GL shown in Fig. 15 or Fig. 16. Conversely, the echo signal obtained from the hemispheric part is large in the level of the first GL and small in the level of the second GL.

[0113] The received signal synthesizing module 38 compares the received signals with each other outputted from the first received signal generating module 37a and the second received signal generating module 37b, and selects the received signal having the smaller signal level. Then, the display module 29 displays the echo based on the selected received signal. As described above, the received signals obtained from the different apertures, particularly, the apertures formed in the cylinder part and the hemispheric part, are different from each other in positions where the virtual images are displayed due to the grating lobes. Therefore, selection and display of the received signal having the smaller signal level for every display position makes it possible to prevent the virtual image due to the grating lobes from being displayed.

[0114] Next, shown in Fig. 8 is a detected image obtained when forming the aperture for transmission and reception all over the transducer; in other words, the detected image displayed by the underwater detector of related art. Further, shown in Fig. 9 is a detected image obtained when forming the aperture for transmission all over the transducer and forming the aperture for reception in the cylinder part; in other words, an image displayed

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based on only the received signal generated by the first received signal generating module 37a of the underwater detector 3. Shown in Fig. 10 is a detected image obtained when forming the aperture for transmission all over the transducer and forming the aperture for reception in the hemispheric part; in other words, an image displayed based on only the received signal generated by the second received signal generating module 37b of the underwater detector 3. The detected image displayed by the underwater detector 3 according to Embodiment 4 of the invention is shown in Fig. 11.

[0115] In the detected image shown in Fig. 8, the virtual image of the water bottom due to the first GL and the second GL is displayed inside the echo signal of the water bottom displayed in the shape in the outermost ring. Further, in the detected image shown in Fig. 9, the virtual image due to the first GL is small and the virtual image due to the second GL is large in comparison with Fig. 8. Whereas in the detected image shown in Fig. 10, the virtual image due to the first GL is large and the virtual image due to the second GL is small in comparison with Fig. 8. In the detected image shown in Fig. 11, the echo signal having the small signal level is selected and displayed for every display position of Figs. 9 and 10, the virtual images due to the first GL and the second GL are small. It is found that the virtual image due to the grating lobes can be prevented from being displayed as described above.

[0116] The reason why in the detected images in Figs. 8 to 11 the virtual images due to the grating lobes are not displayed with a ring shape as is the schematic view in Fig. 16 is because of the effect of a Doppler shift generated by the ship traveling. Further, what is displayed adjacent to the center of each detected image is an oscillation line.

[0117] Further, the underwater detector according to Embodiment 4 of the invention may transmit and receive two kinds of ultrasonic signals whose frequencies are different from each other with the identical transmission aperture and the identical reception aperture. Even in a case where the transmission aperture and the reception aperture are the same, if the frequencies are different, a pitch between the elements is different equivalently; thus, the positions of the grating lobes occurring are different. Therefore, the comparison of the

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received signals obtained by transmitting and receiving with the different frequencies makes it possible to prevent the virtual image due to the grating lobes from being displayed.

[0118] Further, the received signal synthesizing module may select, when the signal level difference between the first received signal and the second received signal is equal to or more than a predetermined value, one of the first received signals and the second received signals having the smaller signal level, and select when the signal level difference is equal to or less than the predetermined value, one of the first received signals and the second received signal having the larger signal level.

[0119] Further, using the DFM methods of Embodiments 1 and 2 together can improve the suppressing effect of the virtual image due to the grating lobes.

[0120] In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.

[0121] Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," "has," "having," "includes," "including," "contains," "containing" or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only

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those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by "comprises ...a," "has ...a," "includes ...a," "contains ...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms "a" and "an" are defined as one or more unless explicitly stated otherwise herein. The terms "substantially," "essentially," "approximately," "about" or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%. The term "coupled" as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is "configured" in a certain way is configured in at least that way, but may also be configured in ways that are not listed.

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Claims (13)

What is claimed is:

1. An underwater detector, comprising:

a wave transmitting and receiving module for transmitting a first transmission signal and a second transmission signal, frequencies of which are swept in an azimuth direction, and receiving a first echo signal and a second echo signal of the transmission signals;

a frequency calculating module for calculating frequencies of the first and second echo signals based on the frequencies of the first and second transmission signals in a predetermined azimuth direction and a traveling speed information of a ship;

a filter module for filtering the first and second echo signals based on the frequencies calculated by the frequency calculating module, and outputting a first received signal and a second received signal; and a received signal synthesizing module for generating a synthesized received signal which is reduced grating lobes based on the first and second received signals.

2. An underwater detector for detecting underwater based on transmission and reception of ultrasonic signals from and to a plurality of elements, the under water detector comprising:

a wave transmitting and receiving module for transmitting a first transmission signal, a frequency of which is swept in an azimuth direction, and a second transmission signal, a frequency of which is different from that of the first transmission signal and is swept in an azimuth direction, from a group of the elements arrayed in all azimuth directions, and receiving a first echo signal of the first transmission signal and a second echo signal of the second transmission signal with a group of the elements arrayed in a predetermined azimuth direction;

a frequency calculating module for calculating frequencies of the first and second echo signals coming from the predetermined azimuth direction;

a first filter for filtering a signal received by the wave transmitting and receiving

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module based on the frequency of the first echo signal calculated by the frequency calculating module to obtain a first received signal;

a second filter for filtering a signal received by the wave transmitting and receiving module based on the frequency of the second echo signal calculated by the frequency calculating module to obtain a second received signal; and a received signal synthesizing module for generating a synthesized received signal where a signal level of grating lobes is lowered based on the first and second received signals;

azimuth directions of grating lobes of the first echo signal filtered by the first filter and azimuth directions of grating lobes of the second echo signal filtered by the second filter being different from each other.

3. The underwater detector of Claim 1 or 2, wherein the wave transmitting and receiving module transmits the first transmission signal, the frequency of which is up-chirped in an azimuth direction and the second transmission signal, the frequency of which is different from that of the first transmission signal and is down-chirped in an azimuth direction, from a group of elements arrayed in all azimuth directions.

4. The underwater detector of Claim 1 or 2, wherein the wave transmitting and receiving module transmits the first transmission signal, the frequency of which is triangle-swept in an azimuth direction and the second transmission signal, the frequency of which is different from that of the first transmission signal and is triangle-swept in the azimuth direction at a phase different from that of the first transmission signal, from a group of elements arrayed in all azimuth directions.

5. The underwater detector of Claims 1 to 4, wherein the received signal synthesizing module generates the synthesized received signal by comparing signal levels of the first received signal and the second received signal for respective azimuth directions,

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and selecting the received signal having a lower signal level.

6. The underwater detector of Claims 1 to 4, wherein the received signal synthesizing module generates the synthesized received signal by comparing signal levels of the first received signal and the second received signal for respective azimuth directions, selecting the received signal having a lower signal level when a difference of the signal levels is lower than a predetermined value, and selecting the received signal having a higher signal level when the difference of the signal levels equals to or more than the predetermined value.

7. The underwater detector of Claim 2, further comprising a reception aperture forming module for forming a first reception aperture and a second reception aperture, at least an element of the second reception aperture being not included in the first reception aperture;

wherein the first echo signal is received by a group of the elements forming the first reception aperture among the plurality of elements, and the second echo signal is received by a group of the elements forming the second reception aperture among the plurality of elements.

8. The underwater detector of Claim 7, wherein the plurality of elements are arrayed in a transducer including a cylinder part and a hemispheric part adjacent to and below the cylinder part; and wherein the reception aperture forming module forms the first reception aperture in the cylinder part and the second reception aperture in the hemispheric part.

9. The underwater detector of Claim 7 or 8, further comprising a transmission aperture forming module for forming a first transmission aperture and a second transmission aperture, at least an element of the second transmission aperture being not included in the

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first reception aperture; and wherein the wave transmitting and receiving module transmits the first transmission signal from the group of elements forming the first transmission aperture, and transmits the second transmission signal from the group of elements forming the second transmission aperture.

10. An underwater detector for detecting underwater based on transmission and reception of ultrasonic signals from and to a plurality of elements, the under water detector comprising:

a drive signal generating module for generating a drive signal for driving each of the plurality of elements;

a transmission aperture forming module for forming a transmission aperture with a predetermined group of the elements among the plurality of the elements;

a wave transmitting and receiving module for transmitting transmission signals based on the drive signals from the group of elements forming the transmission aperture, and receiving echo signals of the transmission signals with the plurality of elements;

a reception aperture forming module for forming a plurality of different reception apertures;

a received signal generating module for synthesizing the echo signals received by the group of elements forming the reception aperture to generate a plurality of received signals respectively corresponding to the plurality of reception apertures;

a received signal selecting module for selecting the received signal having a lower signal level among the plurality of received signals; and a display module for displaying based on the selected received signal.

11. A ship comprising any of the above underwater detectors of Claims 1 through 10.

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12. An underwater detector substantially as described herein with reference to and as illustrated in the accompanying drawings.

13. A method of underwater detection substantially as described herein with reference to the accompanying drawings.

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