Device for and a method of determining the angle of incidence of a received signal and a scanning sonar

A transducer is constructed of a plurality of transducer elements which are arranged in multiple layers and columns forming as a whole a generally cylindrical shape. A scanning sonar employing the transducer thus constructed can be switched between horizontal scan mode in which a vertically focused beam having a narrow horizontal beam angle (high horizontal directivity) is steered around the transducer using all the transducer elements and vertical scan mode in which a horizontally focused beam having a narrow vertical beam angle (high vertical directivity) is steered in a vertical plane directed in a specified scan azimuth to find out the angle of incidence of a received signal. The horizontally focused beam is formed by using the transducer elements of specific columns centered on the specified scan azimuth. A vertical scan signal obtained in the vertical scan mode is multiplied by a chirp signal. A correlator provided in a succeeding stage converts the amount of Doppler shift into time data and determines the angle of incidence of the received signal.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
 The present invention relates to a device for and a method of determining
 the angle of incidence of a received signal by performing a vertical
 scanning operation using a generally cylindrical transducer, as well as to
 a scanning sonar employing such device and/or method.
 Scanning sonars are widely used for detecting underwater objects. A
 scanning sonar employs a transducer having a generally cylindrical shape
 as shown in FIG. 3 to detect underwater objects in all surrounding areas.
 Ultrasonic waves are radiated in all directions around the transducer and
 a vertically focused receiving beam, which is focused within a specified
 angle in a vertical plane to produce specific vertical directivity, is
 formed by summing signals reflected by the underwater objects and received
 by every vertical array, or column, of transducer elements with
 appropriate time delays inserted into the individual reflected signals.
 The receiving beam thus formed is rotated, as if a single column of
 transducer elements is mechanically rotated around the cylindrical shape,
 by switching the transducer elements from one vertical array to another
 around the transducer to scan the objects in all azimuthal directions. It
 is possible to control the downward-looking angle, or the tilt angle, of
 the receiving beam by varying the time delays applied to the individual
 transducer elements, whereby the scanning sonar can search out objects
 within a specific vertical sector area in all directions around the
 transducer.
 In the conventional scanning sonar of the aforementioned construction, the
 receiving beam directed to a specific tilt angle is horizontally rotated
 around the transducer in each horizontal scanning cycle. Thus, if it is
 desired to detect vertically distributed objects, it is necessary to carry
 out successive horizontal scanning cycles while sequentially varying the
 tilt angle of the receiving beam in one scanning cycle after another.
 Although the prior art discloses a transducer constructed in a spherical
 shape which can steer a receiving beam not only horizontally but also
 vertically, the spherically shaped transducer has such problems that it
 necessitates complex combinations of transducer elements for beam forming
 and its target detecting ability in horizontal scan mode, which is the
 most often used mode of operation, is inferior to that of the cylindrical
 transducer.
 SUMMARY OF THE INVENTION
 In view of the aforementioned problems of the prior art, it is an object of
 the invention to provide a device for and a method of determining the
 angles of incidence of a received signal that make it possible to
 vertically scan underwater objects using a transducer constructed of a
 plurality of transducer elements arranged in a cylindrical shape as well
 as a scanning sonar employing such device and/or method.
 According to the invention, a device for determining the angle of incidence
 of an incident ultrasonic signal comprises an ultrasonic transducer
 constructed of a plurality of transducer elements which are arranged in
 multiple layers and columns forming as a whole a generally cylindrical
 shape, a horizontally focused beam former which sums signals received by
 the individual transducer elements of specific columns centered on a
 specified scan azimuth for each layer to generate horizontally focused
 beam signals which provide high horizontal directivity, a vertical scan
 signal former which forms a vertical scan signal by sequentially selecting
 and combining the horizontally focused beam signals from the uppermost
 layer to the lowermost layer, or from the lowermost layer to the uppermost
 layer, and an angle detector which detects the angle of incidence of the
 incident ultrasonic signal based on the amount of frequency shift of the
 vertical scan signal.
 According to the invention, a method of determining the angle of incidence
 of an incident ultrasonic signal by using an ultrasonic transducer
 constructed of a plurality of transducer elements which are arranged in
 multiple layers and columns forming as a whole a generally cylindrical
 shape, the method comprising the steps of summing signals received by the
 individual transducer elements of specific columns centered on a specified
 scan azimuth for each layer to generate horizontally focused beam signals
 which provide high horizontal directivity, generating a vertical scan
 signal by sequentially selecting and combining the horizontally focused
 beam signals from the uppermost layer to the lowermost layer, or from the
 lowermost layer to the uppermost layer, and detecting the angle of
 incidence of the incident ultrasonic signal based on the amount of
 frequency shift of the vertical scan signal.
 A scanning sonar of the invention comprises an ultrasonic transducer
 constructed of a plurality of transducer elements which are arranged in
 multiple layers and columns forming as a whole a generally cylindrical
 shape, a horizontally focused beam former which sums signals received by
 the individual transducer elements of specific columns centered on a
 specified scan azimuth for each layer to generate horizontally focused
 beam signals which provide high horizontal directivity, a vertical scan
 signal former which forms a vertical scan signal by sequentially selecting
 and combining the horizontally focused beam signals from the uppermost
 layer to the lowermost layer, or from the lowermost layer to the uppermost
 layer, and an angle detector which detects the angle of incidence of an
 incident ultrasonic signal based on the amount of frequency shift of the
 vertical scan signal, a vertically focused beam former which sums signals
 received by the individual transducer elements of each column to generate
 vertically focused beam signals which provide high vertical directivity, a
 horizontal scan signal former which forms a horizontal scan signal by
 sequentially selecting and combining the vertically focused beam signals
 derived from the individual columns around the transducer, and a signal
 generator which takes out a signal from a specified direction based on the
 amount of frequency shift of the horizontal scan signal, wherein the
 vertically focused beam former, the horizontal scan signal former and the
 signal generator are activated in horizontal scan mode, and the
 horizontally focused beam former, the vertical scan signal former and the
 angle detector are activated in vertical scan mode.
 In one form of the invention, the angle detector and the signal generator
 of the aforementioned scanning sonar share a common correlator.
 A principle applied to this invention is now briefly described with
 reference to FIGS. 1A and 1B. If a receiving transducer R is moving in a
 straight line with velocity v and a signal of frequency fs is incident on
 the receiving transducer R at the angle of incidence .alpha. as shown in
 FIG. 1A, the apparent frequency f of the signal as it is received by the
 receiving transducer R differs from the actual frequency fs of the
 incident signal due to the Doppler effect. This can be expressed by
EQU f=(1+(v/c)sin .alpha.)fs
 where c is the sound velocity. If the frequency fs of the incident signal
 is known, it is possible to calculate the angle of incidence .alpha. by
 substituting the Doppler-shifted frequency f into the above equation.
 While the receiving transducer R is physically moved in the example shown
 in FIG. 1A, a similar effect can be produced by using a phased array
 technique in which the incident signal is received by a transducer array
 whose individual elements are sequentially switched. FIG. 1B depicts an
 example of such phased array approach, in which a plurality of receiving
 transducer elements are arranged in a straight line at regular intervals
 d. Sequentially switching the transducer elements at a switching frequency
 s yields the same result as would be obtained by physically moving a
 single transducer element at a velocity v=d.multidot.s.
 Based on the above principle, this invention provides means for vertical
 scanning (steering of an ultrasonic sounding beam in a specific vertical
 plane) using an ultrasonic transducer constructed of a plurality of
 transducer elements which are arranged in multiple layers and columns
 forming as a whole a generally cylindrical shape. More specifically, a
 specific number of transducer elements centered on a specified scan
 azimuth that are arranged in a circular arc in each layer are used for
 vertical scan operation. Signals received by these transducer elements in
 each layer are combined to produce a horizontally focused receiving beam
 having a narrow horizontal beam angle (high horizontal directivity) and a
 wide vertical beam angle. By forming the horizontally focused beam in this
 fashion, signals incident from outside a narrow sector area centered on
 the specified scan azimuth are rejected. Horizontally focused beam signals
 derived from the individual layers in the above-stated manner are
 sequentially selected from the uppermost layer to the lowermost layer, or
 from the lowermost layer to the uppermost layer, and combined together to
 produce a vertical scan signal. All the horizontally focused beam signals
 are derived from the same incident signal as is the case with the examples
 of FIGS. 1A and 1B. Thus, successive switching of the horizontally focused
 beam from the uppermost layer to the lowermost layer, or from the
 lowermost layer to the uppermost layer, produces the same effect that
 would be obtained by physically moving a single layer of transducer
 elements downward or upward in steps of the height of each layer.
 Accordingly, the horizontally focused beam signals derived from the
 individual layers are Doppler-shifted, in which the amount of Doppler
 shift (frequency shift) is a function of the angle of incidence .alpha..
 It is therefore possible to calculate the angle of incidence .alpha. of
 the incident signal by detecting the amount of Doppler shift. The
 aforementioned generally cylindrical shape of the transducer includes not
 only an exactly cylindrical shape but also a truncated cone shape of which
 top and bottom surfaces have different diameters.
 A scanning sonar employing a transducer having a generally cylindrical
 shape usually forms a static vertically focused receiving beam which has a
 narrow vertical beam angle (high vertical directivity) and is fixed to a
 specified tilt angle. In prior art technology, the vertically focused
 receiving beam is steered around the transducer and an incident signal is
 converted into a chirp signal and, then, the angle of incidence of the
 incident signal is determined by finding out an azimuth at which the chirp
 signal is detected (Japanese Examined Patent Publication No. 63-7350). The
 scanning sonar as claimed in claim 3 can be switched between the
 horizontal scan mode and vertical scan mode. Given this capability, the
 scanning sonar of the invention can detect not only horizontally
 distributed objects using the ordinary horizontal scan mode but also
 vertically distributed objects using the vertical scan mode. This
 dual-mode capability can be realized by using a correlator formed of a
 matched filter, for example, which is used for detecting the chirp signal
 in the horizontal scan mode to detect the amount of frequency shift in the
 vertical scan mode as well.
 As briefly described above, this invention makes it possible to vertically
 steer a horizontally focused beam within a specific vertical plane using a
 transducer array configured in a generally cylindrical shape. This means
 that the vertical scan operation can be accomplished by using a scanning
 sonar having basically the same construction as the conventional scanning
 sonars. The scanning sonar of this invention may usually be operated in
 the conventional horizontal scan mode and switched to the vertical scan
 mode whenever it becomes necessary to do so. As most of constituent
 components are commonly used in both the horizontal and vertical scan
 modes, it is possible to simplify the construction of the scanning sonar
 and reduce its physical size and production costs.
 These and other objects, features and advantages of the invention will
 become more apparent upon reading the following detailed description in
 conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
 A scanning sonar according to a preferred embodiment of the invention is
 now described with reference to the accompanying drawings. FIG. 2 is a
 block diagram of the scanning sonar of the invention and FIG. 3 is a
 perspective view of a cylinder-shaped transducer 1 used in the scanning
 sonar. As shown in FIG. 3, the transducer 1 is constructed of 480
 transducer elements Aij (i=1 to 30, j=1 to 16) stacked in 16 layers of
 annular arrays, each layer including 30 transducer elements. The
 transducer elements of odd-numbered layers ODD are displaced from those of
 even-numbered layers EVN in a circumferential direction as much as the
 width of each transducer element so that the transducer elements are
 arranged as a whole in an ordinary brickwork structure with alternate
 joint arrangement. The transducer 1 thus constructed is installed in a
 ship's hull in such a way that the axis of the cylindrical shape of the
 transducer 1 is vertically positioned. The transducer 1 is connected to a
 transmitter block 2 and a receiver block 3. The transmitter block 2
 supplies electric signals, which are phased in a particular fashion, to
 the individual transducer elements to emit an ultrasonic beam into the
 water.
 The transducer elements of the transducer 1 are individually connected to a
 beam former 11 provided in the receiver block 3. The beam former 11 forms
 a receiving beam by shifting the phase of received signals entered from
 the individual transducer elements in a particular fashion and then
 summing these signals. During horizontal scan mode, in which areas all
 around the transducer 1 are scanned at a specific tilt angle, a vertically
 focused beam whose response is sharply focused to produce a narrow
 vertical beam angle is formed by summing signals entered from all
 transducer elements in one vertical array after another. During vertical
 scan mode, in which a vertical cross section of the underwater situation
 in a particular azimuth is vertically scanned, signals received by and
 entered from six adjacent transducer elements centered on the specified
 scan azimuth, including three transducer elements each on the left and
 right sides of the scan azimuth, in each element layer are summed to
 produce a horizontally focused beam whose response is sharply focused to
 produce a narrow horizontal beam angle.
 To create the aforementioned horizontally focused and vertically focused
 beams, it is necessary to sum the signals entered from the individual
 transducer elements with a particular phase relationship among them. The
 beam former 11 amplifies the received signals and shifts them in phase by
 multiplying them by specific carrier signals to achieve a desired phase
 relationship. The carrier signals are produced by a horizontal scan mode
 carrier signal generator 12H and a vertical scan mode carrier signal
 generator 12V and supplied to the beam former 11 through a selector 13.
 Controlled by a controller 30, the selector 13 supplies the carrier
 signals produced by the horizontal scan mode carrier signal generator 12H
 to the beam former 11 in the horizontal scan mode, while it supplies the
 carrier signals produced by the vertical scan mode carrier signal
 generator 12V to the beam former 11 in the vertical scan mode.
 The scanning sonar of this embodiment can scan areas all around the
 transducer 1 at a tilt angle set between 0.degree. and 45.degree. below
 the water surface, for example, in the horizontal scan mode. In the
 vertical scan mode, it can scan through a vertical scan coverage of
 0.degree. to 45.degree. below the water surface, for example, in a desired
 azimuth.
 FIGS. 4A and 4B are diagrams illustrating vertically focused beams formed
 by the beam former 11 in the horizontal scan mode. Shown in FIG. 4A is a
 receiving beam produced when the tilt angle is set to 0.degree. to scan a
 full-circle area in a horizontal plane. When the signals received by the
 transducer elements of all the layers are summed for each vertical array,
 or column, in the equally phased form, that is, without inserting any
 phase delays, the beam former 11 produces a vertically focused beam having
 a narrow vertical beam angle (high vertical directivity) and a wide
 horizontal beam angle. In producing the vertically focused beam, the beam
 former 11 separately sums the signals fed from the transducer elements of
 the odd-numbered layers ODD (j=1, 3, 5, 7, 9, 11, 13, 15) and the signals
 fed from the transducer elements of the even-numbered layers EVN (j=2, 4,
 6, 8, 10, 12, 14, 16) and supplies vertically focused beam signals derived
 from the odd-numbered layers ODD and vertically focused beam signals
 derived from the odd-numbered layers EVN to a multiplexer 14 for
 horizontal scan operation.
 On the other hand, FIG. 4B shows a downward-looking receiving beam produced
 when scanning a full-circle area around the transducer 1 at a tilt angle
 greater than 0.degree.. To produce the receiving beam having such
 downward-looking directivity by summing the signals received by the
 transducer elements of each column of the transducer 1, phase delays which
 are made progressively greater from the uppermost layer to the lowermost
 layer are inserted into the signals received by the transducer elements of
 the individual layers. To create the aforementioned phase relationship
 among the signals fed from the transducer elements of the odd-numbered
 layers ODD (j=1, 3, 5, 7, 9, 11, 13, 15) and the even-numbered layers EVN
 (j=2, 4, 6, 8, 10, 12, 14, 16), the horizontal scan mode carrier signal
 generator 12H generates seven carrier signals for delaying the phases of
 the signals fed from the transducer elements of the odd-numbered layers
 ODD and the transducer elements of the even-numbered layers EVN except
 those of the first and second layers (j=1, 2) from the top of the
 transducer 1. It is to be noted here that the phases of the signals fed
 from the transducer elements of the first and second layers (j=1, 2) are
 not delayed. The amounts of phase delays for the third to the sixteenth
 layers are determined with reference to the phase of the signals picked up
 by the transducer elements of the first or second layer. The beam former
 11 first multiplies the signals fed from the transducer elements of the
 odd-numbered and even-numbered layers (j=3 to 15, 4 to 16) excluding the
 first and second layers by the respective carrier signals. Then, the beam
 former 11 separately sums the signals fed from the transducer elements of
 the odd-numbered layers ODD (j=1, 3, 5, 7, 9, 11, 13, 15) and the signals
 fed from the transducer elements of the even-numbered layers EVN (j=2, 4,
 6, 8, 10, 12, 14, 16) to form 60 vertically focused beam signals. The tilt
 angle of the vertically focused beam can be varied between 0.degree. and
 45.degree. by properly adjusting the amounts of phase delays for the
 individual layers. It is possible, if necessary, to form an upward-looking
 receiving beam by inserting progressively greater phase delays into the
 signals received by the transducer elements of upper layers.
 In the horizontal scan mode, the transmitter block 2 supplies electric
 signals shifted in phase to produce the same phase relationship as
 established among the individual layers of the transducer elements in
 producing the aforementioned receiving beam to all the transducer elements
 simultaneously and thereby forms a full-circle disklike or umbrellalike
 transmission beam set to the same tilt angle as the receiving beam.
 In the vertical scan mode, the transducer elements of only specific columns
 centered on a specified azimuth are used. FIGS. 5A to 5F are diagrams
 illustrating horizontally focused receiving and transmission beams formed
 in the vertical scan mode. More specifically, six adjacent transducer
 elements centered on the specified scan azimuth, including three
 transducer elements (i=k-3 to k+2) each on the left and right sides of the
 scan azimuth, in each element layer are used. Signals fed from the six
 transducer elements in each layer are summed, inserting greater phase
 delays into the signals received by the transducer elements close to the
 scan azimuth, or the center of the six transducer elements, as shown in
 FIG. 5A, to produce a horizontally focused beam having a narrow horizontal
 beam angle (high horizontal directivity) and a wide vertical beam angle as
 shown in FIG. 5B. Since such horizontally focused beam is formed for each
 layer, the beam former 11 outputs 16 horizontally focused beam signals
 (j=1 to 16) to a multiplexer 15 for vertical scan operation.
 To create a phase relationship as depicted in FIG. SA among the signals
 received by the six transducer elements in each layer, the vertical scan
 mode carrier signal generator 12V generates two types of carrier signals
 to be supplied to four central transducer elements. Since the amounts of
 phase shifting for the transducer elements on the left and right sides of
 the scan azimuth should be symmetrical, carrier signals with symmetrical
 phase delays are applied to the four central transducer elements (i=k-2 to
 k+1) without delaying the phases of the signals fed from the leftmost and
 rightmost transducer elements (i=k-3, k+2). The beam former 11 produces
 the horizontally focused beam signals by multiplying the signals fed from
 the central transducer elements (i=k-2 to k+1) by these carrier signals
 and then summing the signals derived from the transducer elements (i=k-3
 to k+2). As it is made possible to point the horizontally focused beam in
 any azimuth by specifying a desired column k, the direction of the
 horizontally focused beam need not be varied, but is fixed in the vertical
 scan mode, unlike the tilt angle of the vertically focused beam that is
 sequentially varied in the horizontal scan mode.
 In the vertical scan mode, the transmitter block 2 forms a transmission
 beam having a vertically wide beam angle by simultaneously feeding signals
 of which phase relationship among the individual layers is controlled as
 depicted in FIG. 5C or 5E to the individual transducer elements. The
 transmission beam equally broadened upward and downward shown in FIG. 5D
 is formed when the signals depicted in FIG. 5C are supplied, whereas the
 downward-looking vertically wide transmission beam shown in FIG. 5F is
 created when the signals depicted in FIG. 5E are supplied.
 In the horizontal scan mode, the multiplexer 14 successively switches the
 vertically focused beam signals i fed from the transducer elements of the
 individual columns in the order of ODD1, EVN1, ODD2, EVN2, . . . ODD30,
 EVN30 at specified time intervals, where ODDi and EVNi represent
 odd-numbered and even-numbered columns, respectively. With this switching
 operation, the multiplexer 14 rotates, or steers, the vertically focused
 receiving beam around the transducer 1 and thereby produces a horizontal
 scan signal. To accomplish smooth steering of the receiving beam, the
 vertically focused beam signals derived from a currently selected
 odd-numbered layer ODD are entered into a multiplier 16, in which the
 vertically focused beam signals are multiplied by a triangular wave signal
 for the odd-numbered layer ODD shown in FIG. 6A, the vertically focused
 beam signals derived from a currently selected even-numbered layer EVN are
 entered into a multiplier 17, in which the vertically focused beam signals
 are multiplied by a triangular wave signal for the even-numbered layer EVN
 shown in FIG. 6A and, then, the vertically focused beam signals of both
 the odd-numbered and even-numbered layers are summed by an adder 19. The
 triangular wave signals are produced by an interpolation signal generator
 18 and output to the individual multipliers 16, 17. As a result of the
 aforementioned operation, the adder 19 outputs the horizontal scan signal
 which is identical in receiving characteristics to a signal obtained by
 physically rotating a single transducer element. When a signal of a fixed
 frequency arriving from a distant sound source is received by such
 transducer element, the frequency of the received signal is
 Doppler-shifted as a result of rotation of the transducer element and a
 resultant horizontal scan signal contains a chirp signal component, in
 which the amount of frequency shift varies according to the relationship
 between the sound source and the angular position of the rotating
 transducer element as shown in FIG. 6B. A chirp signal is a signal whose
 frequency is gradually varied as shown in FIG. 6B. More specifically, the
 frequency of the chirp signal varies as follows. When the transducer
 element begins to receive the incoming signal, the frequency of the
 received signal is higher than that of the incoming acoustic signal due to
 a Doppler shift because the transducer element approaches the sound source
 in this stage. As the pointing direction of the beam gradually approaches
 the propagating direction of the incoming signal, the amount of frequency
 shift becomes smaller. The frequency of the received signal gradually
 decreases when the receiving beam is rotated further and the transducer
 element moves away from the sound source.
 The waveform of the chirp signal is uniquely determined if the angular
 velocity of the rotating transducer element is constant and the
 transmitting frequency of the sound source is fixed. Thus, it is possible
 to detect the existence of a sound source (target) and determine its
 direction by ascertaining the waveform of the chirp signal.
 Referring again to FIG. 2, a correlator 25 detects the existence of the
 above-mentioned chirp signal and compresses the beam. A pair of read-only
 memories (ROMs) 26 which serve as reference waveform storage devices are
 connected to the correlator 25 by way of a selector 27. These ROMs 26
 include a ROM 26H for storing a chirp signal waveform for the horizontal
 scan mode and a ROM 26V for storing a chirp signal waveform for the
 vertical scan mode. Although the ROMs 26H and 26V are provided separately
 and switched by the selector 27 in the block diagram of FIG. 2, a single
 ROM may be used to store reference waveform data for both the horizontal
 and vertical scan modes provided that selective switching of the reference
 waveform data for the two modes is allowed through controlled access to
 relevant addresses.
 The correlator 25 takes in 96 samples of the horizontal scan signal or of a
 vertical scan signal, which will be described later, through a selector 24
 and outputs the result of correlation between the sampled input data and
 the reference waveform data. In the horizontal scan mode, the samples of
 the horizontal scan signal are sequentially entered into and shifted
 through a 96-stage shift register of the correlator 25 in synchronism with
 a clock which is identical to a vertically focused beam select clock
 supplied to the multiplexer 14. The 96-stage shift register also takes in
 96 samples of the reference waveform data. The sampled input data and
 reference waveform data are compared in each stage of the 96-stage shift
 register and a value obtained by summing comparison results is output by
 the correlator 25 as its output data. When an ultrasonic wave reflected by
 a target is received, the resultant horizontal scan signal contains a
 chirp signal corresponding to the magnitude of the reflected ultrasonic
 wave at a point in scan time corresponding to the azimuth of the target.
 When the input waveform containing the chirp signal is entered into the
 correlator 25, it exhibits an extremely high correlation with the
 reference waveform which is also a chirp signal and, therefore, the
 correlator 25 provides a high output signal level. Information on the
 current scan azimuth is available from the controller 30 which outputs the
 select clock to the multiplexer 14. Thus, scan azimuth data obtained from
 the controller 30 and the output data of the correlator 25 are output to a
 display 4, whereby a video image of the detected target is displayed on
 the display 4 in the correct direction. The horizontal scan signal is
 formed successively while the vertically focused beam is rotated after
 each transmission of the transmission beam by the transmitter block 2 and
 ultrasonic waves reflected by nearby targets to distant targets are
 sequentially detected. Then, images of the detected targets are
 sequentially displayed on the display 4.
 In the vertical scan mode, the horizontally focused beam signals produced
 by the beam former 11 are entered into the multiplexer 15. The multiplexer
 15 has two processing channels, of which one channel (odd-numbered layer
 channel) sequentially selects the horizontally focused beam signals
 derived from the odd-numbered layers starting from their uppermost layer
 (j=1) and outputs a time-sequentially combined horizontally focused beam
 signal, while the other channel (even-numbered layer channel) sequentially
 selects the horizontally focused beam signals derived from the
 even-numbered layers starting from their uppermost layer (j=2) and outputs
 a time-sequentially combined horizontally focused beam signal. The
 combined horizontally focused beam signal derived from the odd-numbered
 layers is entered into a function multiplier 20 while the combined
 horizontally focused beam signal derived from the even-numbered layers is
 entered into a function multiplier 21.
 FIG. 7 is a detailed circuit diagram showing the configuration of the
 multiplexer 15, the function multipliers 20, 21 and a function generator
 22. FIGS. 8A and 8B are a switching time chart of the multiplexer 15 and a
 diagram showing function waveforms generated by the function multipliers
 20, 21, respectively.
 The horizontally focused beam signals fed from the odd-numbered layers ODD
 (j=1, 3, 5, 7, 9, 11, 13, 15) are supplied to switches SW1, SW3, SW5, SW7,
 SW9, SW11, SW13 and SW15 of the multiplexer 15, respectively, while the
 horizontally focused beam signals fed from the even-numbered layers EVN
 (j=2, 4, 6, 8, 10, 12, 14, 16) are supplied to switches SW2, SW4, SW6,
 SW8, SW10, SW12, SW14 and SW16 of the multiplexer 15, respectively. Then,
 based on a select clock fed from the controller 30, the odd-numbered layer
 channel of the multiplexer 15 sequentially selects the horizontally
 focused beam signals entered from the odd-numbered layers and the
 even-numbered layer channel of the multiplexer 15 sequentially selects the
 horizontally focused beam signals entered from the even-numbered layers
 with switching timing shown in the time chart of FIG. 8A, in which the
 signals selected from the alternate odd-numbered and even-numbered layers
 overlap as much as one-half of the duration of each sampling gate pulse.
 Outputs of the odd-numbered and even-numbered layer channels of the
 multiplexer 15 are entered into the function multipliers 20, 21,
 respectively. In the vertical scan mode, each successive scanning cycle
 begins when the switch SW1 is turned on and ends when the switch SW16 is
 turned off. The vertical scan signal is produced by sampling the
 horizontally focused beam signals during each successive vertical scanning
 cycle using window functions. As previously mentioned, the receiving beam
 is continuously rotated around the transducer 1 to accomplish
 uninterrupted scanning in the horizontal scan mode. In the vertical scan
 mode, however, the continuity of the vertical scan signal is interrupted
 when the receiving beam, which is steered downward in each scanning cycle,
 is returned from the lower limit to the upper limit of the vertical scan
 coverage. The horizontally focused beam signals are multiplied by the
 aforementioned window functions every scanning cycle to prevent the
 occurrence of sidelobes.
 The function multipliers 20, 21 multiply the input horizontally focused
 beam signals by the window functions, triangular wave signals for smooth
 interpolation and by a chirp signal which enables the correlator 25 to
 detect ultrasonic wave signals reflected by underwater targets. To prevent
 the occurrence of mirror images by multiplication, the function
 multipliers 20, 21 are connected in a doubled-balanced configuration. The
 function multiplier 20 branches the vertically focused beam signal (-cos
 .theta..sub.o) derived from the odd-numbered layers and entered from the
 multiplexer 15 into two signal paths. The vertically focused beam signal
 fed into one signal path is directly entered into a multiplier 203 while
 the vertically focused beam signal fed into the other signal path is
 delayed by a delay device 201 as much as .pi./2 and a resultant signal
 (-sin .theta..sub.o) is entered into a multiplier 202. The function
 generator 22 produces a signal -f.sub.ITP sin .theta..sub.M by multiplying
 a waveform signal -f.sub.ITP, which is obtained by combining a window
 function waveform and a triangular wave signal, and supplies the signal
 -f.sub.ITP sin .theta..sub.M into the multiplier 203. The function
 generator 22 also supplies a signal -f.sub.ITP cos .theta..sub.M, which is
 obtained by advancing the signal -f.sub.ITP sin .theta..sub.M as much as
 .pi./2, into the multiplier 202. Subsequently, outputs of the multipliers
 202, 203 are added by an adder 23. Then, it is possible to obtain the
 following signal which only contains components expressed by a sum
 .theta..sub.o +.theta..sub.M, excluding components expressed by a
 difference .theta..sub.o -.theta..sub.M, as shown in FIG. 8B:
EQU f.sub.ITP (cos .theta..sub.M.multidot.sin .theta..sub.o +sin
 .theta..sub.M.multidot.cos
 .theta..sub.o)=f.sub.ITP.multidot.sin(.theta..sub.o +.theta..sub.M) (1)
 On the other hand, the function multiplier 21 also branches the vertically
 focused beam signal (cos .theta..sub.e) derived from the even-numbered
 layers and entered from the multiplexer 15 into two signal paths. The
 vertically focused beam signal fed into one signal path is directly
 entered into a multiplier 213 while the vertically focused beam signal fed
 into the other signal path is delayed by a delay device 211 as much as
 .pi./2 and a resultant signal (sin .theta..sub.e) is entered into a
 multiplier 212. The function generator 22 produces a signal -f.sub.ITP sin
 .theta..sub.M by multiplying a waveform signal -f.sub.ITP, which is
 obtained by combining a window function waveform and a triangular wave
 signal, and supplies the signal -f.sub.ITP sin .theta..sub.M into the
 multiplier 213. The function generator 22 also supplies a signal
 -f.sub.ITP cos .theta..sub.M, which is obtained by advancing the signal
 -f.sub.ITP sin .theta..sub.M as much as .pi./2, into the multiplier 212.
 Subsequently, outputs of the multipliers 212, 213 are added by the 23.
 Then, it is possible to obtain the following signal which only contains
 components expressed by a sum .theta..sub.e +.theta..sub.M, excluding
 components expressed by a difference .theta..sub.e -.theta..sub.M, as
 shown in FIG. 8B:
EQU -f.sub.ITP (-cos .theta..sub.M.multidot.sin .theta..sub.e -sin
 .theta..sub.M.multidot.cos
 .theta..sub.e)=f.sub.ITP.multidot.sin(.theta..sub.e +.theta..sub.M) (2)
 It is possible to obtain a smooth vertical scan signal by adding the
 aforementioned signals derived from the odd-numbered and even-numbered
 layers.
 Referring now to FIGS. 1A and 1B, the frequency of ultrasonic waves
 reflected by an underwater target is Doppler-shifted, in which the amount
 of Doppler shift is related to the angle of incidence .alpha. of the
 reflected ultrasonic waves. It is therefore possible to determine the
 angle of incidence .alpha. by detecting the amount of Doppler shift. In
 this embodiment, the vertical scan signal is multiplied by a chirp signal
 so that the correlator 25 can convert the amount of Doppler shift
 (frequency value) into time data.
 FIGS. 9A and 9B are diagrams showing the principle of a method of detecting
 the amount of Doppler shift by entering the vertical scan signal
 multiplied by the chirp signal into the correlator 25, in which the angle
 of incidence is 0.degree. in FIG. 9A while the angle of incidence is
 30.degree. in FIG. 9B. FIGS. 9A and 9B each depict waveform
 representations at the top and frequency representations at the bottom.
 Each of these Figures depicts a situation in which a signal waveform is
 transferred to the right and, therefore, the right-hand side of the
 vertical scan signal represents its portion which occurred earlier in
 time. The vertical scan signal multiplied by the chirp signal shown on the
 left side in each of FIGS. 9A and 9B is a signal whose frequency varies
 along the time axis.
 On the other hand, a chirp signal supplied to the correlator 25 as a
 reference waveform shown on the right side in each of FIGS. 9A and 9B is a
 signal whose frequency varies over a broader frequency range than the
 input vertical scan signal. The range of frequency change of the reference
 chirp signal corresponds to the range of Doppler shift of the vertical
 scan signal obtained when ultrasonic waves are received at the angles of
 incidence between 0.degree. and 45.degree. below the water surface. The
 vertical scan signal is Doppler-shifted in accordance with the angle of
 incidence of the received signal. A chirp signal is superimposed on the
 received signal as if it is biased with the Doppler frequency. The
 waveform of the vertical scan signal matches some portion of the waveform
 of the reference chirp signal entered into the correlator 25. The portion
 of the waveform of the reference chirp signal that matches the input
 vertical scan signal corresponds to the amount of Doppler shift, or to the
 angle of incidence .alpha. of the received ultrasonic waves.
 When the vertical scan signal, which is shifted through the correlator 25
 based on a clock, matches a certain portion of the waveform of the
 reference chirp signal, the output signal level of the correlator 25 is
 maximized. Information on up to which point the vertical scan signal has
 been shifted when it has matched the portion of the waveform of the
 reference chirp signal is available from the controller 30 which outputs
 the clock for controlling the shift rate. Thus, scan angle (tilt angle)
 data calculated from the information on shift position and the output data
 of the correlator 25 are output to the display 4, whereby a video image of
 the target, which is supposed to exist at the tilt angle corresponding to
 the shift position where the correlator 25 has produced the high output
 signal level, can be displayed on the display 4. Vertical scanning is
 repeatedly made after each transmission by the transmitter block 2 and
 ultrasonic waves reflected by nearby targets to distant targets are
 sequentially detected. Then, the display 4 presents images of the detected
 targets as shown in FIG. 10 in accordance with the output signal of the
 correlator 25.
 It is to be noted that the transducer is not limited to the construction
 including 16 layers and 30 columns shown in FIG. 3. Its vertical scan
 coverage is not necessarily limited to 0.degree. to 45.degree. below the
 water surface either. In one variation of the invention, the vertical scan
 coverage may include upward-looking angles.
 Although the transducer 1 of the foregoing embodiment has a cylindrical
 shape, it is possible to employ a truncated cone-shaped transducer of
 which circular top surface has a larger diameter than the circular bottom
 surface. The transducer of this varied form is suited for searching
 underwater objects in areas closer to the sea bottom, rather than areas
 closer to the water surface, because transmission and receiving beams can
 be steered to greater tilt angles with the truncated cone-shaped
 transducer.
 Detection of the amount of Doppler shift (frequency shift) is not limited
 to the aforementioned method, in which the vertical scan signal is
 multiplied by the chirp signal.
 Furthermore, although the correlator 25 is commonly used in both the
 horizontal scan mode and the vertical scan mode in the foregoing
 embodiment, there may be provided separate correlators for the horizontal
 scan mode and the vertical scan mode so that completely difference
 channels are used in the individual modes.