GB2430743A

GB2430743A - Underwater Sounding Apparatus

Info

Publication number: GB2430743A
Application number: GB0617108A
Authority: GB
Inventors: Koji Tokuda; Hiromune Matsumoto
Original assignee: Furuno Electric Co Ltd
Current assignee: Furuno Electric Co Ltd
Priority date: 2005-08-30
Filing date: 2006-08-30
Publication date: 2007-04-04
Anticipated expiration: 2026-08-30
Also published as: JP2007064768A; GB0617108D0; GB2430743B

Abstract

An underwater sounding apparatus transmits acoustic waves into a body of water, receives echoes reflected by underwater targets and displays the target echoes. A first embodiment of the invention includes a filtering processor (6) which performs filtering operations on received echo signals by using filter coefficients suited to a specified target speed range. A band limits generator produces a set of desired filter coefficients by shifting a frequency range suited to a specific target speed range according to the moving speed of the underwater sounding apparatus. The set of filter coefficients are also associated with the direction of motion of the underwater sounding apparatus. A second embodiment of the invention underwater sounding apparatus includes a filtering processor (6) which performs filtering operation on received echo signals by using bandpass filters, each having a stopband formed in part of a passband, wherein the stopband is a frequency band covering frequencies of echo signals received from targets at zero speed. The underwater sounding apparatus thus configured can display echoes received from targets like fish schools traveling at speeds within a specified speed range only by erasing echoes from the sea bottom and substantially zero-speed targets like a plankton layer. When displaying the received echoes, the amount of Doppler shifting can be shown in different colours.

Description

TITLE OF THE INVENTION

UNDERWATER SOUNDING APPARATUS

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to an underwater sounding apparatus which transmits ultrasonic (acoustic) waves into a body of water, receives echoes reflected by underwater targets and displays the target echoes.

2. Description of the Related Art

There exist conventionally known underwater sounding apparatuses, such as fish finders and scanning sonars which transmit acoustic waves underwater, receive echoes from underwater targets and display the target echoes. Japanese Patent Application Publication No. 2003-84060 describes an example of such an underwater sounding apparatus.

The aforementioned Patent Application Publication discloses a technique for creating filtering characteristics optimized for individual directions in which a receiving beam of the underwater sounding apparatus is oriented in accordance with speed and traveling direction (heading) of a moving body (i.e., a ship) to obtain echo signals with a high signal-to- noise ratio (SNR).

This prior art technique is described in detail with reference to FIG. 9 which is a block diagram generally showing the configuration of the underwater sounding apparatus of Japanese Patent Application Publication No. 2003-84060.

As illustrated in FIG. 9, the conventional underwater sounding apparatus includes a transducer unit 101, a transmitter block 102, a transmitreceive switching circuit 103, a fixed passband filter 104, an mixing amplifier 105, a fixed passband filter 106, an amplifier 107, an analogto-digital (A/D) converter 108, a receiving beamformer 109, an automatic passband controller 110, a controller 111, a ship speed/heading sensing unit 112 and a demodulator 113.

The circuits designated by the numerals 104 through 110 and 113 in FIG. 9 together constitute a receiver block of the underwater sounding apparatus.

The transducer unit 101 converts electric transmit signals fed from the transmitter block 102 into acoustic waves, transmits the acoustic waves into a body of water, and converts acoustic echoes returning from underwater targets into electric signals (echo signals) . The transmitter block 102 is an electric signal generator for generating the electric transmit signals and supplying the same to the transducer unit 101. The transducer unit 101 produces and emits the acoustic waves having a specified pulselength at a specified tilt angle beneath the sea surface. The transmit-receive switching circuit 103 alternately switches signal paths over successive transmit and receive cycles. Specifically, the transmit-receive switching circuit 103 allows the electric transmit signals fed from the transmitter block 102 to pass into the transducer unit 101 during each successive transmit cycle and allows the echo signals from the transducer unit 101 to pass into the fixed passband filter 104 while blocking any transmit signals from the transmitter block 102 during each successive receive cycle.

The fixed passband filter 104 is a first-stage fixed passband filter provided in the receiver block for selectively passing signals of frequencies falling within a desired frequency band (passband) among the echo signals received by the transducer unit 101. The mixing amplifier amplifies the filtered echo signals and converts the same into intermediate-frequency (IF) signals. The fixed passband filter 106 is a second-stage fixed passband filter for further narrowing the frequency band of the echo signals by removing unwanted sideband signals contained in the frequency-converted echo signals (IF signals) . The amplifier 107 amplifies the echo signals up to levels suited for inputting into the A/D converter 108 provided in a succeeding stage.

The A/D converter 108 converts the amplified echo signals into digital signals. The receiving beamformer 109 forms a receiving beam which is steered in a horizontal plane to successively pick up the echo signals received from all directions around the transducer unit 101.

The automatic passband controller 110 determines a center frequency and bandwidth of the echo signal received from each direction based on information on the speed and heading of the ship obtained from the ship speed/heading sensing unit 112. Using center frequencies and bandwidths thus determined for all horizontal directions, the automatic passband controller 110 produces bandwidth- limited echo signals with high SNR.

The controller 111 is a control device provided with a central processing unit (CPU) and a memory for overall system control including operation of the automatic passband controller 110. The ship speed/heading sensing unit 112 includes sensors for detecting the speed and heading of the ship. For example, the sensor for detecting the speed of the ship may be a global positioning system (GPS) receiver, a current measuring apparatus, a speed sensor (e.g., an accelerometer whose output is integrated over time to calculate the ship's speed) integrally fitted in the transducer unit 101, while the sensor for detecting the heading of the ship may be the GPS receiver, a gyrocompass or an electronic compass.

The demodulator 113 demodulates, or detects envelopes of, the bandwidthlimited echo signals output from the automatic passband controller 110 and outputs the detected envelopes to a display unit (not shown).

In the underwater sounding apparatus thus configured, the transmitter block 102 supplies the pulsed transmit signals of a specific frequency to the transducer unit 101 through the transmit-receive switching circuit 103. The transducer unit 101 converts the transmit signals into acoustic waves and transmits the same into the body of water in the form of an acoustic transmitting beam. The transmitted acoustic waves are reflected by underwater targets, such as the sea bottom and fish schools. Upon receiving echoes from such underwater targets, the transducer unit 101 converts the acoustic echoes into electric signals (echo signals) and delivers the echo signals to the receiver block.

Prior to bearnforming operation, the fixed passband filter 104 selectively passes the signals of the specified frequency band by removing unwanted noise components contained in the received echo signals. The filtered echo signals are amplified and converted into the IF signals by the mixing amplifier 105. The frequency-converted echo signals (IF signals) passed through the fixed passband filter 106 which removes the unwanted sideband signals contained in the echo signals. The echo signals are further amplified by the amplifier 107 and converted into the digital signals by the A/D converter 108. According to the prior art Patent Application Publication, the AID converter 108 need not necessarily be located at this stage of the receiver block.

The receiving beamformer 109 performs the beamforming operation to form the receiving beam which is steered in the horizontal plane to successively pick up the echo signals received by the transducer unit 101 from all directions therearound. By carrying out the beamforming operation, the receiving beamformer 109 produces a string of echo signals picked up by the rotating receiving beam according to a spiral scanning pattern and outputs the string of echo signals to the automatic passband controller 110.

Upon completion of the beamforming operation, the automatic passband controller 110 determines the center frequencies and bandwidths of the echo signals received from the individual directions around the transducer unit 101 based on the information on the speed and heading of the ship obtained from the ship speed/heading sensing unit 112 and produces the bandwidth-limited echo signals with high SNR using the center frequencies and bandwidths thus determined for all horizontal directions. The demodulator 113 detects the envelopes of the bandwidth-limited echo signals produces by the automatic passband controller 110 and outputs data on the detected echo signal envelopes to the unillustrated display unit provided in a succeeding stage. The display unit presents a picture of echoes representing information on such underwater targets as the sea bottom and fish schools based on the data on the detected echo signal envelopes.

The automatic passband controller 110 of the underwater sounding apparatus of Japanese Patent Application Publication No. 2003-84060 is now described in further detail with reference to FIG. 10 which is a block diagram of the automatic passband controller 110 showing a specific configuration thereof.

As depicted in FIG. 10, the automatic passband controller 110 includes a frequency detector 201, a Doppler-shifted frequency estimator 202, a bandwidth estimator 203, a pair of multiplexers 204, 205, a mixer 206 and a variable passband filter 207.

The frequency detector 201 determines the center frequency and frequency variance of the echo signal received from each of horizontal directions in horizontal scan mode (from varying directions of tilt in vertical scan mode) by an autocorrelation method or a Fourier transform method, for instance. Although the frequency variance, in its strict sense, differs from the bandwidth, the frequency variance represents the degree of the spread, or dispersion, of frequency components of the echo signal and, thus, may be regarded as generally proportional to the bandwidth.

The Doppler-shifted frequency estimator 202 is used when it is difficult for the frequency detector 201 to determine Doppler-shifted frequencies of the received echo signals. Specifically, the Doppler-shifted frequency estimator 202 calculates Doppler-shifted frequencies expected to be observed from the echo signals received from all the horizontal directions around the transducer unit 101 in the horizontal scan mode (from varying directions of tilt in the vertical scan mode) by using the ship speed information derived from the speed sensor of the ship speed/heading sensing unit 112. The information on the ship's heading is used for converting a vector representing the ship's speed over the ground in a direction referenced to true north into a vector representing the ship's speed through the water in a direction referenced to a forward direction of the transducer unit 101 (or the ship's heading) in a case where a Doppler sonar, a current measuring apparatus or a GPS receiver capable of measuring the ship's ground speed is used as the speed sensor of the ship speed/heading sensing unit 112. The bandwidth estimator 203 estimates the bandwidth of the echo signal received from each direction based on information on transmit pulselength.

The multiplexers 204, 205 determine whether to use information on the center frequency and bandwidth obtained by the frequency detector 201 from the received echo signals or estimated values of the center frequency and bandwidth obtained by the Doppler-shifted frequency estimator 202 and the bandwidth estimator 203 from the ship's speed and transmit pulselength.

When the multiplexer 204 selects the center frequency detected by the frequency detector 201 or estimated by the Doppler-shifted frequency estimator 202, the mixer 206 shifts the center frequency of the echo signal according to the selected center frequency to produce a baseband (DC) signal. The variable passband filter 207 is a filter whose passband is matched to the bandwidth selected by the multiplexer 205. The variable passband filter 207 limits bandwidth of the baseband signal input from the mixer 206 to remove frequency components other than true echo signals, thus improving SNR of the received echo signals.

In the underwater sounding apparatus of Japanese Patent Application Publication No. 2003-84060, the automatic passband controller 110 regulates the frequency band of the echo signal received from each direction around the transducer unit 101 in the above-described fashion.

This makes it possible to obtain echo signals with high SNR from all directions around the transducer unit 101 even when a frequency shift occurs in the received echo signals due to various causes, such as a change in ship position, motion of the ship or a water current.

According to the prior art technique of Japanese

Patent Application Publication No. 2003-84060 discussed above, however, the data output to the display unit for on- screen presentation directly represents levels of the received echo signals. An underwater sounding apparatus like a scanning sonar or a fish finder employing the prior art technique simply detects and displays levels of received echo signals without regard to information on speeds of underwater targets which reflect transmitted acoustic waves. Thus, the conventional underwater sounding apparatus has not been able to display only fish schools swimming through the water excluding the sea bottom, for example. Especially in a shallow-water area or in the presence of a layer of plankton in shallow water, for instance, fish echoes might be overlaid with, or buried in, bottom echoes, surface noise or a reflection from the plankton layer. A chronic problem of the conventional underwater sounding apparatus is that it is difficult, or totally impossible at the worst, to distinguish the fish echoes amidst such overlying disturbances.

Additionally, the prior art technique of Japanese

Patent Application Publication No. 2003-840G0 involves the use of narrowpassband filters which pass echo sgnais from only such targets that travel at speeds in a range generally equal to own ship's speed range in order to improve SNR of the received echo signals. In the conventional underwater sounding apparatus, Doppler-shifted frequencies of echo signals recei'.red from such fast-moving targets as bonitâs or tunas wiuch swim at much higher speeds than the ship do not fall within passbands of the filters. As a consequence, frequency components of the echo signals received from such fast-moving fish targets would be filtered out entirely or attenuated in part. This gives rise to a problem that the conventional underwater sounding apparatus might not be able to detect targets traveling at much higher speeds than the ship.

Accordingly, it is desirable to provide an underwater sounding apparatus capable of displaying a picture of echoes rece.ved from underwater targets on a display screen :n such a marner that the on-screen picture does not simply represent information on signa. levels of the received -Il- echoes but a combination of the signal level information and information on moving speeds of the underwater targets.

An underwater sounding apparatus may include a band limits generator which produces a set of desired filter coefficients by shifting a frequency range of a filter coefficient suited to a specified target speed range according to moving speed and direction of a moving body (e.g., a ship) on which the underwater sounding apparatus is installed, and a filtering processor which performs filtering operation on received echo signals by using the filter coefficients produced by the band.imits generator.

The underwater sounding apparatus thus structured can display echoes from targets traveling at speeds within a specified speed range only corresponding to the filter coefficients produced by the band limits generator.

In one example, each of the filter coefficients produced by the band limits generator constitutes a bandpass filter having a stophand formed in part of a passband, wherein the stopband is a freqi.iency band covering frequencies of echo signals received from targets at zero speed.

Thus the underwater sounding apparatus can display echoes received from targets like fish schools traveing at speeds w.th:n the specified speed range only by eras.ng echoes from the sea bottom and substantially zerospeed targets like a plankton layer.

The band limits generator may produce a set of filter coefficients adapted independently to individual directions by shifting a frequency range of a single filter coefficient suited to a specified target speed range for the individual directions according to moving speed and. direction of the moving body (e.g., a ship) on which the underwater sounding apparatus is installed or according to average frequencies determined by a complex autocorrelation method. This feature, rhen implemented in an apparatus like a scannng sonar for sounding all around the transducer unit by producing an umbrella-like transmitting beam at a specific tilt angle, makes it possible to properly perform the fi.tering operation in individual beam directions.

The band limits generator may update the filter coefficients on a realtime basis even during each. successive receive cycle in accordance with changes in moving speed or direction of the moving body or motion thereof, and the filtering processor may perform the filtering operation by using the updated filter coefficients.

This feature enables the underwater sounding apparatus to process the echo signals in real time.

The filtering processor may perform the filtering operation on the received echo signals by using a conventional filter coefficient having a specific passband as well as the filter coefficients produced by the band limits generator, and the underwater sounding apparatus may simultaneously present echoes obtained through the filtering operation performed by using different types filter coefficients. This feature enables a user to compare pictures of echoes obtained through the filtering operation performed by using the thfferent types filter coefficients. If the band limits generator produces filter coefficients adapted to suppress a bottom echo, for example, the user can observe a picture obtained with the conventional filter coefficient showing the bottom echo and a picture obtained with the filter coefficients produced by the band limits generator of the invention showing neither the bottom echo nor other zero-speed targets.

The underwater sounding apparatus may be provided with a motion compensating function which enables transmisson and reception of acoustic waves to and from intended underwater directions by coinpensazing for motion of the moving body (e.g., a ship). This feature enables transmission and. reception of acoustic signals with high -:4- precision.

The underwater sounding apparatus may present the received echo signals with different amounts of Doppler shifts in different colors according to freqi.iencies of the received echo signals. This feature enables a user to intuitively recognize noving speeds of individual targets (e.g., fish schools) from an echo image displayed on-screen.

It is possible to display a picture of echoes received from only fish schools traveling at speeds within a specific speed range on-screen by erasir.g echoes from the sea bottom and substantially zero-speed targets like a plankton layer. Thus, the underwater sounding apparatus may contribute to improving efficiency of fishing operation especially in search and tracking of fish schools regardless of weather conditions and conditions of a fishing site. Also, the underwater sounding apparatus may make it easier to discriminate fish echoes which have conventionally beex obscured by or buried in echoes of the sea bottom or a plankton layer, for example, by presenting a clearer echo image to the user, thus provid. ng an improved efficiency of fishing operation. Espec:al].y because it is possible to display only echoes of bottom fish conventionally bur.ed in the bottom echo on a display screen by erasing the bottom echo, the underwater sounding apparatus can track a school of bottom fish whose echoes have conventionally been buried in the bottom echo in real target lock operation, for example, which is one function of a conventional scanning sonar. Additionally, the scanning sonar can achieve to attenuate or completely suppress the bottom echo in vertical scan mode, which has been a pending objective to be achieved.

In one display mode, the underwater sounding apparatus simultaneously presents a picture of echoes obtained through a new filtering techniquc and a picture of echoes obtained through a conventional filtering technique. This permits the user to easily dlstlnguLsh fish echoes moving at specific speeds from echoes frort the sea bottom and substantially zero- speed targets like a plankton layer.

Since it is possible to selectively derect tarees moving at speeds within a specified speed range, the underwater sounding apparatus can be used for locating and monitoring a diver working in water and watching any approaching objects from a distance.

BRflF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram generally showing the configuration of an uridez-water sounding apparatus according to a first embodiment of the invention; FIG. 2 is a block diagram showing a specific configuration of a band limits generator of the underwater sounding apparatus of the first embodiment; FIG. 3 is a diagram showing a filtering characteristic of a low-pass filter having a passband suited to a specific target speed range; FIG. 4 is a diagram showing a filtering characteristic of a bandpass filter obtained by shifting the passband of the low-pass filter of FIG. 3; FIG. 5 is a diagram showing a filtering characteristic of a bandpass filter having a stopbarid in part of a passband produced by the band limits generator of the first embodiment; FIG. 6 is a block diagram showing a specific configuration of an automatic passband controller employed in an underwater sounding apparatus according to a second embodimer.t of he invention; FIG. 7 is a diagram showing examples of onscreen pictures with and without a bottom echo presented by ar underwater sounding apparatus according to a third embodiment of the invention; FIG. B is a diagram showing examples of on-screen pictures wLth and without an echo from a plankton layer presented by the underwater sou.nding apparatus of the third enthodirnent; FIG. 9 is a bock diagram generally showing the configuration of a conventional underwater sounding apparatus; and FG. 10 is a block diagram showing a specific configuration of an automatic passband controller employed in the conventional underwater sounding apparatus of FIG. 9.

FIRST EMBODIMENT

An underwater sounding apparatus accordng to a. first embodiment of the invention is now described with reference to FIGS. 1 to 5.

FIG. 1 is a block diagram generally showing the configuration of the underwater sounding apparatus according to the first embodiment. As shown n FIG. 1, the underwater sounding apparatus includes a front-end .ncerf ace 1, a transmitting beamformer 2, a rece:virg beamforming preprocessor 3, a receiving beamformer 4, a band limits generator 5, a filtering processor 6 and a display processor 7.

The front-end interface 1 is a transmit-receive switching circuit which alternately passes transmit signals fed from the transmitting beamformer 2 into a transducer unit (not shown) and echo signals received by the transducer unit into the receiving beamforming preprocessor 3 in successive transmit and receive cycles. The transmitting beamformer 2 produces time series data used for generating transmit signals for individual channels.

In the case of a scanning sonar, for example, the transmitting beamformer 2 produces data for generating acoustic waves having a specified pulselength to be transmitted underwater at a specified tilt angle. Although not illustrated, the underwater sounding apparatus (e.g., the scanning sonar) may be provided with a motion compensating function which enables transmission and reception of the acoustic waves to and from intended directions underwater by compensating for motion of a moving body (e.g., a ship), thus permitting transmission and reception of acoustic signals with high precision.

The receiving beamforming preprocessor 3 performs such operations as filtering operation for removing noise and gain correcting operation by applying time-varied gain (TVG), for example, prior to execution of beamforming operation and outputs preprocessed echo signal data to the receiving beamformer 4.

By using a phased array beamforrning technique, the receiving beamformer 4 forms a receiving beam which is steered in a horizontal plane to successively pick up the echo signals received from all directions around the transducer unit.

The band limits generator 5 stores a plurality of filter coefficients preprogrammed for specific target speed ranges in storage means like a memory. The specified target speed ranges are speed ranges predetermined for such targets that should be detected by the underwater sounding apparatus. Each of the filter coefficients preprogrammed for a specified target speed range provides filtering properties including a passband defining a range of frequencies which will pass through. If one of the filter coefficients preprogrammed for a particular target speed range is selected, the selected filter coefficient allows echoes from targets moving within the specified target speed range only to pass through when the moving body (own ship) is at zero speed. The band limits generator 5 acquires one of the filter coefficients corresponding to an intended target speed range from the storage means and produces an adapted set of filter coefficients which provide desired filtering characteristics suited to the intended target speed range in accordance with the moving speed of own ship.

In a case where the underwater sounding apparatus is a full-circle scanning sonar for sounding all around the transducer unit, f or example, the amount of Doppler shift varies from one beam direction to another when own ship is moving. For this reason, the band limits generator 5 shifts the range of frequencies of the passband defined by the filter coefficient acquired from the storage means for each beam direction according to moving speed and direction of the moving body (own ship), or based on average frequencies determined for individual directions by a complex autocorrelation method. Then, the band limits generator 5 can produce a set of filter coefficients adapted independently to individual beam directions. The adapted filter coefficients thus produced provide filtering characteristics including, for example, a stopband which attenuates or completely suppress echoes from targets at about zero speed as well as a passband which passes echoes from targets traveling at speeds within a specified speed range only.

The filtering processor 6 performs filtering operation on echo signals picked up by the receiving beam from all directions around the transducer unit by using the set of filter coefficients produced by the band limits generator 5 for the individual directions. The echo signals obtained through the filtering operation performed by the filtering processor 6 contain data on echoes from underwater targets traveling at speeds within the specified speed range only, with echoes from the sea bottom or a plankton layer drifting at substantially zero speed excluded.

The display processor 7 performs operation for displaying the echo data obtained through the filtering operation on a display screen. Specifically, the display processor 7 demodulates the echo signals picked up from all directions around the transducer unit, performs such operations as TVG gain adlustment and interpolation operations on the demodulated echo signals and outputs data for on-screen presentation of an echo image to a display unit (not shown) according to a currently selected presentation mode. The display unit presents the echo image representing information on the underwater targets according to the data delivered from the display processor 7. The underwater sounding apparatus offers a choice of multiple presentation modes including a mode in which the display unit shows echoes of different signal intensities in different ways and a mode in which the display unit shows echoes with different amounts of Doppler shifts in different colors according to frequencies of the received echo signals so that an user can intuitively recognize moving speeds of individual targets (e.g., fish schools) from the echo image displayed onscreen.

Now, the working of the underwater sounding apparatus of the presentembodiment of the invention is discussed.

In the underwater sounding apparatus configured as described above, the transmitting beamformer 2 supplies the pulsed transmit signals of a specific frequency to the unillustrated transducer unit through the frontend interface 1. The transducer unit converts the transmit signals into acoustic waves and transmits the same into the body of water in the form of an acoustic transmitting beam.

The transmitted acoustic waves are reflected by underwater targets, such as the sea bottom and fish schools. Upon receiving echoes from such underwater targets, the transducer unit converts the acoustic echoes into electric signals (echo signals) and delivers the echo signals to the receiving beamforming preprocessor 3 through the front-end interface 1.

As already mentioned, the receiving beamforming preprocessor 3 performs such operations as the filtering operation for removing noise and the gain correcting operation by applying TVG, for example, prior to execution of the beamforming operation.

The bearnforming operation is performed by the receiving beamformer 4. Specifically, the receiving beamformer 4 forms the receiving beam for successively picking up the echo signals by using the phased array beamforming technique.

Upon completion of the beamforming operation by the receiving beamformer 4, the band limits generator 5 produces a set of filter coefficients adapted independently to individual beam directions. The adapted set of filter coefficients thus produced provides such filtering characteristics that the receiving beam picks up echoes from targets traveling at speeds within a specified speed range only. The filter coefficients produced by the band limits generator 5 are updated on a real-time basis even during each successive receive cycle in accordance with changes in the ship's speed or heading or ship's motion.

The filtering processor 6 performs the filtering operation on the echo signals picked up by the receiving beam from the individual directions around the transducer unit by using the set of filter coefficients produced by the band limits generator 5.

Finally, the display processor 7 detects envelopes of the echo signals which have passed through the filtering processor 6. Subsequently, the display processor 7 performs such operations as the TVG gain adjustment and interpolation operations on the detected echo signal envelopes and outputs the resultant data to the display unit. The display unit presents the echo image representing information on the underwater targets according to the data delivered from the display processor 7.

Now, the band limits generator 5 constituting a characteristic part of the invention is described in detail with reference to FIG. 2 which is a block diagram showing a specific configuration of the band limits generator 5. The following discussion is based on the assumption that the underwater sounding apparatus transmits and receives the acoustic waves in continuous wave (CW) mode.

As illustrated in FIG. 2, the band limits generator 5 includes a frequency shifter 21, a frequency detector 22 and a filter coefficient calculator 23.

The frequency shifter 21 shifts the frequency of radial sampling data picked up along a radial direction (range direction) in each of the beam directions by as much as a frequency shift in accordance with an A/D sampling frequency set value so that a receiving frequency band is shifted to a baseband (DC).

The frequency detector 22 determines a center frequency of each of the echo signals received from all horizontal directions around the transducer unit in horizontal scan mode (from varying directions of tilt in vertical scan mode, and from varying directions of tilt in the individual horizontal directions in variable tilt angle scan mode) by using an autocorrelation method or a Fourier transform method, for instance. The underwater sounding apparatus of the present embodiment uses the complex autocorrelation method for carrying out frequency analysis in all directions in real time. Complex autocorrelation performed by the frequency detector 22 is a mathematical operation for calculating complex autocorrelation values along the radial direction from an autocorrelation start range with a specific time lag after transmission in each beam direction and then determining average frequencies and frequency variances for the individual beam directions.

The frequency detector 22 may be programmed to perform instead of the aforementioned mathematical operation, either selectively or not, an operation for calculating center frequencies corresponding to the ship 1s speed for all of the beam directions around the transducer unit (all the directions of tilt in the vertical scan mode) based on information on the ship's speed and true course acquired from external sources.

The filter coefficient calculator 23 performs operations (1) through (4) described below.

(1) First, the filter coefficient calculator 23 acquires one of the filter coefficients preprogrammed to provide filtering characteristics corresponding to an intended target speed range. As previously mentioned, a plurality of filter coefficients are stored in the storage means like a memory. The filter coefficient calculator 23 acquires one of the filter coefficients which provides a filtering characteristic suited to a userspecified target speed range, for example. FIG. 3 is a diagram showing an example of a filter coefficient acquired by the filter coefficient calculator 23. In the example shown in FIG. 3, the filter coefficient acquired by the filter coefficient calculator 23 defines a low-pass filter having a passband suited to a constant target speed range as illustrated.

(2) Next, the filter coefficient calculator 23 shifts a center frequency of the passband defined by the acquired filter coefficient based on the average frequencies determined by the frequency detector 22 for the individual beam directions around the transducer unit (for the varying directions of tilt in the vertical scan mode, and for the varying directions of tilt in the individual horizontal directions in the variable tilt angle scan mode) by using the complex autocorrelation method. In this way, the filter coefficient calculator 23 produces from one filter coefficient acquired from the storage means an adapted set of filter coefficients which provide varying filtering characteristics suited independently to the individual beam directions. Needless to say, the filter coefficient calculator 23 may be programmed to shift the center frequency of the passband defined by the acquired filter coefficient based on the ship speed and true course information acquired from external sources instead of the average frequencies determined by the frequency detector 22 for the individual beam directions by using the complex autocorrelatjon method - FIG. 4 is a diagram showing a filtering characteristic provided by an adapted filter coefficient obtained for one of the beam directions after operation (2) above performed by the filter coefficient calculator 23. The filter coefficient calculator 23 shifts the center frequency of the passband defined by the filter coefficient acquired in operation (1) above to obtain an adapted filter coefficient for producing a bandpass filter suited to each beam direction as shown in FIG. 4.

(3) The filter coefficient calculator 23 further shifts passband frequencies or cutoff frequencies of bandpass filters obtained from the filter coefficients produced by operation (2) above so that the underwater sounding apparatus can detect echoes from targets traveling at speeds in a specified target speed range. As a result, the filter coefficient calculator 23 produces a set of filter coefficients adapted independently to individual beam directions to create a passband covering a frequency range of echo signals received from the targets traveling within the specified speed range only. The filter coefficients obtained by operation (3) produce bandpass filters, each having a stopband formed in part of a passband as depicted in FIG. 5 created by positively caused frequency aliasing accomplished by shifting the passband or cutoff frequencies in a real-number region.

Referring to FIG. 5, each of the baridpass filters produced by operation (3) above has a stopband which cuts off frequencies of echoes from substantially zero-speed targets. The stopbarid of each bandpass filter produced by operation (3) is set to include Doppler-shifted frequencies caused solely by movement of own ship, so that the stopband excludes echo signals received from zero-speed targets, such as the sea bottom.

The passband of the bandpass filter shown in FIG. 5 is separated into two portions (left and right) by the stopband at the middle. The two separated portions (left and right) are hereinafter referred to as lower and upper passbands for the sake of simplicity. The lower passband of the bandpass filter shown at left in FIG. 5 covers a frequency range of echoes from targets moving within the specified speed range in directions generally the same as the moving direction of own ship, whereas the upper passband of the bandpass filter shown at right covers a frequency range of echoes from targets moving within the specified speed range in directions generally opposite to the moving direction of own ship. Thus, the bandpass filter shown in FIG. 5 can detect echo signals from the targets moving within the specified speed range in different directions. This kind of bandpass filter having a partial stopband within the passband can be configured by a combination of two bandpass filters or by a combination of a bandpass filter and a notch filter.

(4) Finally, the filter coefficients produced by operation (3) above are stored in a filter coefficient memory provided in the filtering processor 6. The foregoing discussion of the band limits generator S has dealt with a case where the underwater sounding apparatus transmits and receives the acoustic waves in the CW mode.

The above-described configuration of the first embodiment may be modified such that the band limits generator 5 generates complex matched filter coefficients along the radial direction (range direction) in each of the beam directions in a case where the underwater sounding apparatus transmits and receives the acoustic waves in frequency modulation (FM) mode. Accordingly, the frequency shifter 21 and the frequency detector 22 are deactivated when the underwater sounding apparatus is operated in the FM mode.

Also, although the foregoing discussion of operations (1) through (4) has described a case where the band limits generator 5 generates the bandpass filters each of which has a partial stopband within the passband, the invention is not limited to this configuration. For example, the above-described configuration of the embodiment may be modified such that the underwater sounding apparatus is provided with a bandpass filter having a stopband in part of a passband prepared in advance and the stopband and the passband of the bandpass filter are shifted in frequency for individual beam directions according to own ship speed to produce filter coefficients suited independently to the individual beam directions.

Filtering characteristics provided by the filter coefficients produced in the underwater sounding apparatus as discussed above are described below.

For the purpose of the following discussion, it is assumed that the underwater sounding apparatus is a scanning sonar which transmits acoustic waves in all directions around the transducer unit at a single frequency fO and own ship is moving straight ahead at 5 knots, producing Doppler shift fd in a forward direction. An echo signal received from any zero-speed target (whether it is the sea bottom or a fish school) located straight ahead has a Doppler-shifted frequency given by fO+fd. According to the present embodiment of the invention, the band limits generator 5 detects the Doppler-shifted frequency fO+fd in real time by the autocorrelation method or calculates the same based on the ship speed and true course information supplied from the external sources and, then, produces bandpass filters for the individual beam directions, each bandpass filter having a stopband containing the Doppler- shifted frequency fO fd to reject echoes from zero-speed targets combined with a passband to pass echoes from targets traveling at speeds within a specified target speed range only.

Subsequently, the filtering processor 6 performs the filtering operation on received echo signals by using the bandpass filters individually having the aforementioned stopbands for rejecting echoes from zero-speed targets.

This filtering operation makes it possible to prevent echoes from zerospeed targets, such as those from the sea bottom, a plankton layer or a non-moving fish school, from being displayed on-screen.

If there is a fish school ahead of own ship moving at speed a in the same direction as own ship, an echo from the fish school presents a Doppler shift expressed by fO f a. In this case, the echo received from the fish school has a Doppler-shifted frequency calculated as fO+fd+fa. If the Doppler-shifted frequency fO+fd+fa of the echo from the moving fish school is contained in the passband of the bandpass filter produced by the band limits generator 5, the echo from the fish school passes through the filtering processor 6, although echoes from targets at the Dopplershifted frequency fO+fd are suppressed by the stopband formed in part of the passband. Consequently, echoes from fish schools traveling at speeds within only the specified target speed range are displayed on-screen.

As thus far described, the underwater sounding apparatus according to the first embodiment of the invention produces bandpass filters, each having a passband to pass echoes from targets traveling at speeds within a specified target speed range only, and performs the filtering operation on echo signals by using the bandpass filters. As a result, the underwater sounding apparatus can remove echoes received from zero-speed targets, such as the sea bottom or a plankton layer, while displaying echoes from targets like fish schools traveling at speeds within the specified target speed range only. In particular, the underwater sounding apparatus of the invention can be set to present only relatively large- sized fishes like bonitos or tunas while erasing small-sized fishes like sardines by properly controlling passband and stopband settings of the bandpass filters. Therefore, the underwater sounding apparatus of the invention makes it possible to distinguish large-sized single fishes like bonitos or tunas from bait fish like sardines at a long distance, thus contributing to improving efficiency of fishing operation. This feature of the invention is especially advantageous for coastal and offshore bonito catchers and purse seiners which aim at catching the large-sized fishes.

SECOND EMBODIMENT

An underwater sounding apparatus according to a second embodiment of the invention is now described with reference to FIG. 6 which is a block diagram showing a specific configuration of an automatic passband controller 110 employed in the underwater sounding apparatus of the second embodiment.

The present invention can also be implemented by reconfiguring the automatic passband controller 110 of the conventional underwater sounding apparatus previously described with reference to FIG. 9.

Referring to FIG. 6, the automatic passband controller includes a frequency detector 31, a Doppler-shifted frequency estimator 32, a bandwidth calculator 33, a multiplexer 34, a filter coefficient generator 35, a filter coefficient memory 36 and a variable bandwidth filter 37.

The frequency detector 31 determines a center frequency of each of echo signals received from all horizontal directions around the transducer unit in horizontal scan mode (from varying directions of tilt in vertical scan mode, and from varying directions of tilt in the individual horizontal directions in variable tilt angle scan mode) by using the autocorrelation method or the Fourier transform method, for instance. The underwater sounding apparatus of the present embodiment uses the complex autocorrelation method for carrying out frequency analysis in all directions in real time.

The Doppler-shifted frequency estimator 32 is used when it is difficult for the frequency detector 31 to determine Doppler-shifted frequencies of the received echo signals. Specifically, the Doppler-shifted frequency estimator 32 calculates Doppler-shifted frequencies expected to be observed from the echo signals received from all the horizontal directions around the transducer unit in the horizontal scan mode (from varying directions of tilt in the vertical scan mode) by using ship speed information obtained from a speed sensor. Information on the ship's heading is used for converting a vector representing the ship's speed over the ground in a direction referenced to true north into a vector representing the ship's speed through the water in a direction referenced to a forward direction of the transducer unit (or the ship's heading) in a case where a Doppler sonar, a current measuring apparatus or a GPS receiver capable of measuring the ship's ground speed is used as the speed sensor.

The bandwidth calculator 33 calculates a bandwidth of echo signals which would be received from targets to be detected traveling at speeds within a user-specified target speed range, f or instance. The multiplexer 34 determines whether to use information on the center frequency obtained by the frequency detector 31 from the received echo signals or estimated values of the center frequency obtained by the Doppler-shifted frequency estimator 32 from the ship's speed.

The filter coefficient generator 35 generates a filter coefficient corresponding to the specified target speed range of targets to be detected based on the bandwidth of echo signals from the intended targets calculated by the bandwidth calculator 33. Subsequently, the filter coefficient generator 35 shifts a center frequency of a passband defined by the generated filter coefficient for individual beam directions around the transducer unit (for varying directions of tilt in the vertical scan mode, and for varying directions of tilt in the individual horizontal directions in the variable tilt angle scan mode) to thereby produce bandpass filters, each having a stopband formed in part of a passband. In this way, the filter coefficient generator 35 produces an adapted set of filter coefficients suited independently for receiving echoes from targets traveling at speeds within the specified speed range from the individual beam directions.

The filter coefficient memory 36 is a memory for storing the filter coefficients generated by the filter coefficient generator 35 for the individual horizontal directions. The variable bandwidth filter 37 is a filter whose passband and stopband are varied in accordance with the filter coefficients calculated by the filter coefficient generator 35 to pass or suppress signals of specific frequency bands.

It will be understood from the above discussion that it is possible to display echoes from targets traveling at speeds within the specified speed range only by making a simple design change to the automatic passband controller used in the conventional underwater sounding apparatus.

In the underwater sounding apparatus according to the second embodiment of the invention, the automatic passband controller 110 produces filter coefficients which provide filtering characteristics adapted to receiving echoes from targets traveling at speeds within the specified speed range for the individual beam directions around the transducer unit and performs filtering operation on echo signals by using the filter coefficients thus produced.

Consequently, like the underwater sounding apparatus of the first embodiment, the underwater sounding apparatus of the second embodiment can remove echoes received from zero- speed targets, such as the sea bottom or a plankton layer, while displaying echoes from targets like fish schools traveling at speeds within a desired target speed range only.

THIRD EMBODIMENT

An underwater sounding apparatus according to a third embodiment of the invention is now described.

The underwater sounding apparatus of the third embodiment performs filtering operation on received echo signals by using filters of the first or second embodiment adapted to receiving echoes from targets traveling at speeds within a specified speed range from individual beam directions as well as a conventional filter having a specific passband. Thus, the underwater sounding apparatus of this embodiment can simultaneously present echoes obtained through the filtering operation performed by using different types of filters. The aforementioned conventional filter having the specific passband is a bandpass filter having a fixed passband like the one generated by the prior art technique of Japanese Patent Application Publication No. 2003-84060 discussed earlier,

for example.

FIGS. 7 and 8 are examples of on-screen pictures given by the underwater sounding apparatus of the third embodiment. Specifically, FIG. 7 shows pictures obtained from an experiment conducted at a water depth of approximately 170 m, the picture at left showing an image with no bottom echo which has been suppressed by the filters of the invention, and the picture at right showing the bottom echo picked up through the conventional filter.

On the other hand, FIG. 8 shows pictures obtained from an experiment conducted at a water depth of over 600 m, the picture at left showing an echo from a midwater plankton layer located approximately from a surface of the sea to a water depth of lOOm, and the picture at right showing an image with no echo from the midwater plankton layer which has been suppressed by the filters of the invention. As previously discussed, the filters of the invention are bandpass filters, each having a stopband formed in part of a passband, for passing echoes from targets traveling at speeds within the specified target speed range while suppressing echoes from at least zero-speed targets.

As can be seen from FIGS. 7 and 8, the underwater sounding apparatus of the third embodiment can display a picture of echoes from only fish targets swimming at speeds within a specific speed range by erasing echoes from the sea bottom and substantially zero-speed targets like a plankton layer by means of the bandpass filters adapted to receiving echoes from targets traveling within the specified speed range only. The underwater sounding apparatus of the third embodiment can also present a picture of echoes obtained with the conventional filter whose passband is determined without regard to the target speed range besides the picture of the echoes from the fish targets only. As the underwater sounding apparatus simultaneously presents the two different pictures, the user can precisely recognize targets distributed underwater from a comparison of the two pictures and achieve an improved efficiency of fishing operation.

Claims

CLAIMS: 1. An underwater sounding apparatus which transmits acoustic waves

into a body of water, receives echoes reflected by underwater targets and displays the target echoes, said underwater sounding apparatus comprising a filtering processor which performs filtering operation on received echo signals by using a filter coefficient suited to a specified target speed range.
2. An underwater sounding apparatus which transmits acoustic waves into a body of water, receives echoes reflected by underwater targets and displays the target echoes, said underwater sounding apparatus comprising a filtering processor which performs filtering operation on received echo signals by using a bandpass filter having a stopband formed in part of a passband.
3. An underwater sounding apparatus according to claim 1 further comprising: a band limits generator which produces a set of desired filter coefficients by shifting a frequency range of a filter coefficient suited to a specified target speed range according to moving speed of a moving body on which said underwater sounding apparatus is installed and wherein the filtering processor performs filtering operation on received echo signals by using the filter coefficients produced by said band limits generator.
4. An underwater sounding apparatus according to claim 3 wherein the band limits generator produces a set of filter coefficients adapted independently to individual directions by shifting a frequency range of a single filter coefficient suited to a specified target speed range for the individual directions according to moving speed and direction of a moving body on which said underwater sounding apparatus is installed.
5. An underwater sounding apparatus according to claim 3 wherein the band limits generator produces a set of filter coefficients adapted independently to individual directions by shifting a frequency range of a single filter coefficient suited to a specified target speed range for the individual directions according to average frequencies determined by a complex autocorrelation method.
6. The underwater sounding apparatus according to claim 3, 4 or 5, wherein each of the filter coefficients produced by said band limits generator constitutes a bandpass filter having a stopband formed in part of a passband.
7. The underwater sounding apparatus according to claim 2 or 6, wherein the stopband is a frequency band covering frequencies of echo signals received from targets at zero speed.
8. The underwater sounding apparatus according to claim 2, 6 or 7, wherein the stopband of the bandpass filter can be varied within a specific range.
9. The underwater sounding apparatus according to one of claims 3 or 4, wherein said band limits generator updates the filter coefficients on a real-time basis even during each successive receive cycle in accordance with changes in moving speed and direction of the moving body and motion thereof, and said filtering processor performs the filtering operation by using the updated filter coefficients.
10. The underwater sounding apparatus according to one of claims 3 to 9, wherein said filtering processor performs the filtering operation on the received echo signals by using a filter coefficient having a specific passband as well as the filter coefficient suited to a specified target speed range, and said underwater sounding apparatus simultaneously presents echoes obtained through the filtering operation performed by using different types filter coefficients.
11. The underwater sounding apparatus according to one of claims 3 to 10, wherein said underwater sounding apparatus is provided with a motion compensating function which enables transmission and reception of the acoustic waves to and from intended underwater directions by compensating for motion of the moving body.
12. The underwater sounding apparatus according to one of claims 3 to 11, wherein said underwater sounding apparatus presents the received echo signals with different amounts of Doppler shifts in different colors according to frequencies of the received echo signals.
13. An apparatus substantially as herein described with reference to and as illustrated in the accompanying drawings.