Abstract:
This disclosure provides an underwater detection device, which includes a first generator for generating a first transmission signal, a second generator for generating a second transmission signal having a longer pulse width than that of the first transmission signal, an output unit for outputting the transmission signals generated by the first and second generators to a transducer for transmitting underwater ultrasonic waves based on the first and second transmission signals, respectively, receiving the reflection waves caused by the ultrasonic waves, respectively, and converting the reflection waves into a first reception signal and a second reception signal, respectively, an underwater detector for detecting at least one of a presence of a target object and a water bottom depth based on the first reception signal, and a bottom sediment discriminator for discriminating a water bottom sediment type based on the second reception signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-090436, which was filed on Apr. 14, 2011 the entire disclosure of which is hereby incorporated by reference. 
     1. Technical Field 
     The present invention relates to a fish finder, an underwater detection device, an underwater detecting method, and a computer readable media storing underwater detecting program, in which the underwater detection is performed by transmitting an ultrasonic wave underwater and based on a reflection wave caused by the ultrasonic wave. 
     2. Background of the Invention 
     Conventionally, as fish finders used for ships, a fish finder for discriminating a sea bottom sediment type (rocks, stones, sand, etc.) and displaying the sediment type has been known (see JP4348415B). The bottom sediment discrimination is performed by analyzing a sea bottom echo of a transmission pulse of an ultrasonic wave. For example, in a location where the sea bottom is hard like rocks or stones with a bumpy surface, a time width of the sea bottom echo is longer and, whereas, in a location where the sea bottom is soft like sand or mud with a relatively flat surface, the time width of the sea bottom echo is shorter. On a display screen, a similarity of the sea bottom echo with each bottom sediment type and the most similar bottom sediment type among the bottom sediment types are typically displayed. 
     However, because fish finders are generally for detecting a school-of-fish accurately, a transmission pulse width of the ultrasonic wave to be transmitted therefrom is relatively shorter so as to achieve a high resolution in distance. With the shorter transmission pulse width, an energy of the ultrasonic wave drops and a detectable distance range shrinks; thereby, the bottom sediment discrimination at a deep depth is difficult. 
     SUMMARY OF THE INVENTION 
     Thus, the present invention is made in view of the above situations, and provides a fish finder, an underwater detection device, an underwater detecting method, and a computer readable media storing underwater detecting program, which can both detect fish and/or a water bottom depth and discriminate a sea bottom sediment type accurately, with a single device. 
     According to one aspect of the invention, an underwater detection device is provided, which includes a first generator for generating a first transmission signal, a second generator for generating a second transmission signal having a longer pulse width than that of the first transmission signal, an output unit for outputting the transmission signals generated by the first and second generators to a transducer for transmitting underwater ultrasonic waves based on the first and second transmission signals, respectively, receiving the reflection waves caused by the ultrasonic waves, respectively, and converting the reflection waves into a first reception signal and a second reception signal, respectively, an underwater detector for detecting at least one of a presence of a target object and a water bottom depth based on the first reception signal, and a bottom sediment discriminator for discriminating a water bottom sediment type based on the second reception signal. 
     With this configuration, the detection of at least one of the target object (e.g., fish) and the water bottom depth and the discrimination of the water bottom sediment type are performed based on the ultrasonic waves dedicated for the respective purposes, which have the different pulse widths from each other. The ultrasonic wave for the detection of at least one of the target object and the water bottom depth is required to have a higher distance resolution, that is, a shorter pulse width, so as to detect at least one of a length and kind of the target object, and the water bottom depth. On the other hand, the ultrasonic wave for the water bottom sediment type discrimination is required to have a stronger energy (signal intensity) and a longer detection range and larger SN ratio, that is a longer pulse width. Thus, by using the pulse width of the ultrasonic wave for the water bottom sediment type discrimination longer than the pulse width of the ultrasonic wave for the detection of at least one of the target object and the water bottom depth, both the detection of at least one of the target object and the water bottom depth and the discrimination of the water bottom sediment type can be performed by a single device. 
     According to another aspect of the invention, an underwater detection device is provide, which includes a first generator for generating a first transmission signal of which a frequency changes, a second generator for generating a second transmission signal with a fixed frequency, an output unit for outputting the transmission signals generated by the first and second generators to a transducer for transmitting underwater ultrasonic waves based on the first and second transmission signals, respectively, receiving the reflection waves caused by the ultrasonic waves, respectively, and converting the reflection waves into a first reception signal and a second reception signal, respectively, a setting unit for causing the first generator to generate the first transmission signal through a frequency modulation, pulse compressing the first reception signal, and causing the second generator to generate the second transmission signal having a longer pulse width than the pulse compressed first reception signal, an underwater detector for detecting at least one of a presence of a target object and a water bottom depth based on the pulse compressed first reception signal, and a bottom sediment discriminator for discriminating a water bottom sediment type based on the second reception signal. 
     With this configuration, by generating the second transmission signal having the pulse width longer than that of the pulse compressed first reception signal, the detection of at least one of the target object and the water bottom depth based on the pulse compressed reception signal. The ultrasonic wave for the detection of at least one of the target object and the water bottom depth is required to have a long detection range, the SN ratio of the reception signal that is accurate for the detection, and a high distance resolution (high ability of distinguishing a group of the target objects). However, if the pulse width of the ultrasonic wave is short so as to achieve the high distance resolution, the energy of the ultrasonic wave drops, and as a result, the detection range is shortened and the SN ratio is reduced. On the other hand, if the pulse width of the ultrasonic wave is long, the target object of which a length is shorter than the pulse width, cannot be detected and the distance resolution degrades. Whereas, the ultrasonic wave for the water bottom sediment type discrimination is not required such a high distance resolution, and, because the water bottom is a huge (widespread) reflection body that causes the reflection wave to scatter (secondary reflection or reverb), if the frequency of the transmission signal is modulated to have a wide band, a noise included in the reflection wave from the water bottom increases. Therefore, by modulating only the frequency of the transmission signal for the detection of at least one of the target object and the water bottom depth (target object group detection) and pulse compressing the reception signal, similar effect to when the ultrasonic wave having the short pulse width is transmitted and received can be obtained without reducing the energy. As a result, the detection of at least one of the target object and the water bottom depth and the water bottom sediment type discrimination can be performed accurately. 
     The underwater detection device may further include a first filter for filtering the first reception signal, and a second filter for filtering the second reception signal, the second filter having a narrower band width than that of the first filter. 
     With this configuration, because a filter bandwidth can be narrowed as the pulse width of the transmission signal is longer, the second filter can have a narrower band width than that of the first filter, and as a result, the SN ratio of the second reception signal can further be increased than that of the first reception signal. 
     The output unit may output the first and second transmission signals alternately. 
     With this configuration, by transmitting the ultrasonic wave for the detection of at least one of the target object and the water bottom depth and the ultrasonic wave for the water bottom sediment type discrimination, both the detection of at least one of the target object and the water bottom depth and the discrimination of the water bottom sediment type can be performed by using a single transducer. 
     The underwater detection device may further include a display unit for displaying a detection result obtained from the underwater detector, and a discrimination result obtained from the bottom sediment discriminator. 
     The display unit may not be included in the fish finder, and a display device installed in a ship in advance may be used alternatively. 
     According to another aspect of the invention, a method of performing an underwater detection based on reflection waves caused by transmitting ultrasonic waves underwater is provided. The method includes generating a first transmission signal, generating a second transmission signal having a longer pulse width than that of the first transmission signal, outputting the generated transmission signals to a transducer for transmitting underwater the ultrasonic waves based on the first and second transmission signals, respectively, receiving the reflection waves, and converting the reflection waves into a first reception signal and a second reception signal, respectively, detecting at least one of a presence of a target object and a water bottom depth based on the first reception signal, and discriminating a water bottom sediment type based on the second reception signal. 
     With this configuration, the detection of at least one of the target object (e.g., fish) and the water bottom depth and the discrimination of the water bottom sediment type are performed based on the ultrasonic waves dedicated for the respective purposes, which have the different pulse widths from each other. The ultrasonic wave for the detection of at least one of the target object and the water bottom depth is required to have a higher distance resolution, that is, a shorter pulse width, so as to detect at least one of a length and kind of the target object, and the water bottom depth. On the other hand, the ultrasonic wave for the water bottom sediment type discrimination is required to have a stronger energy (signal intensity) and a longer detection range and larger SN ratio, that is a longer pulse width. Thus, by using the pulse width of the ultrasonic wave for the water bottom sediment type discrimination longer than the pulse width of the ultrasonic wave for the detection of at least one of the target object and the water bottom depth, both the detection of at least one of the target object and the water bottom depth and the discrimination of the water bottom sediment type can be performed by a single device. 
     According to another aspect of the invention, computer readable media configured to store in a non-transitory manner a computer executable program, which upon execution by a processor of a computer causes the computer to perform an underwater detection based on reflection waves caused by transmitting ultrasonic waves according to a method is provided. The program includes causing a computer to generate a first transmission signal, causing a computer to generate a second transmission signal having a longer pulse width than that of the first transmission signal, causing a computer to output the generated transmission signals to a transducer for transmitting underwater the ultrasonic waves based on the first and second transmission signals, respectively, receiving the reflection waves, and converting the reflection waves into a first reception signal and a second reception signal, respectively, causing a computer to detect at least one of a presence of a target object and a water bottom depth based on the first reception signal, and causing a computer to discriminate a water bottom sediment type based on the second reception signal. 
     With this configuration, the detection of at least one of the target object (e.g., fish) and the water bottom depth and the discrimination of the water bottom sediment type are performed based on the ultrasonic waves dedicated for the respective purposes, which have the different pulse widths from each other. The ultrasonic wave for the detection of at least one of the target object and the water bottom depth is required to have a higher distance resolution, that is, a shorter pulse width, so as to detect at least one of a length and kind of the target object, and the water bottom depth. On the other hand, the ultrasonic wave for the water bottom sediment type discrimination is required to have a stronger energy (signal intensity) and a longer detection range and larger SN ratio, that is a longer pulse width. Thus, by using the pulse width of the ultrasonic wave for the water bottom sediment type discrimination longer than the pulse width of the ultrasonic wave for the detection of at least one of the target object and the water bottom depth, both the detection of at least one of the target object and the water bottom depth and the discrimination of the water bottom sediment type can be performed by a single device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which the like reference numeral indicate like elements and in which: 
         FIG. 1  is a block diagram schematically showing a configuration of a fish finder according to an embodiment of the present invention; 
         FIG. 2  is a block diagram schematically showing a configuration of a transmission unit; 
         FIG. 3  a block diagram schematically showing a configuration of a signal processor; 
         FIG. 4  is charts showing a comparison of echo signal between when an ultrasonic wave is transmitted in a pulse width T 1  and when the ultrasonic wave is transmitted in a pulse width T 2 ; 
         FIG. 5  is a table showing experimental results of a signal-to-noise ratio (SN ratio) obtained when the ultrasonic wave having the pulse width T 1  is transmitted and when the ultrasonic wave having the pulse width T 2  is transmitted; and 
         FIG. 6  is a flowchart showing a procedure of processing that is performed by the fish finder. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, a suitable embodiment of a fish finder, an underwater detection device, an underwater detecting method, and a computer readable media storing underwater detecting program according to the present invention is described with reference to the appended drawings. The fish finder  1  transmits an ultrasonic wave pulse (hereinafter, may simply be referred to as “the ultrasonic wave”) from an oscillator toward a water bottom, and receives, by the oscillator, a reflection wave that is the ultrasonic wave reflected on, for example, a school-of-fish or the water bottom. The fish finder  1  performs calculation processing on the received reflection wave, and displays an image (echo data) of the school-of-fish or the water bottom on a display unit based on the processing result. Note that, the term “water bottom” as used herein refers to any water bottom or any water bottom surface at sea, lake, river, etc. 
       FIG. 1  is a block diagram schematically showing a configuration of the fish finder  1  in this embodiment. The fish finder  1  includes a controller  10  (setting unit), a storage  11 , a user interface  12 , a display unit  13 , a transmission unit  14 , a transmission-and-reception switch  15 , a transducer  16 , a receiver  17 , an A/D converter  18 , and a signal processor  19 . 
     The storage  11  is, for example, a nonvolatile memory, such as Read Only Memory (ROM), and stores a required program  11 A and various data. The user interface  12  receives various instruction inputs from an operator, for example, to set a water bottom detection range (water bottom depth). The display unit  13  displays echo data by having a vertical axis of a display screen thereof as a depth direction and a horizontal axis thereof as a time direction. The display unit  13  may not be included in the fish finder  1 , and a display device installed in a ship in advance may be used alternatively. 
     The controller  10  includes a microcomputer. The controller  10  reads out the program  11 A stored in the storage  11  and controls the fish finder  1  overall in response to the various instruction inputs by the operator through the user interface  12 . For example, the controller  10  sets a transmission cycle (described later) and the water bottom detection range of transmission signals for the transmission unit  14 , sets a sampling cycle for the A/D converter  18 , and passes on the instruction to the signal processor  19  to perform various processing. 
     The transmission unit  14  outputs to the transducer  16  two generated transmission signals alternately at the transmission cycle set through the controller  10 , via the transmission-and-reception switch  15  internally built with a trap circuit. The detailed configuration of the transmission unit  14  is further described later. 
     The transducer  16  is arranged at, for example, a bottom of the ship, and includes a plurality of ultrasonic wave oscillators. Based on the transmission signals from the transmission unit  14 , the transducer  16  transmits ultrasonic waves underwater. Further, the transducer  16  receives, by the ultrasonic wave oscillator, the reflection wave caused by the transmitted ultrasonic wave reflecting on a target object group (e.g., the school-of-fish) or the water bottom, generates an echo signal according to an intensity of the reflection wave, and outputs it to the receiver  17  via the transmission-and-reception switch  15 . The transducer  16  may not be included in the fish finder  1  and, a transducer installed in the ship in advance may be used alternatively. Hereinafter, the echo signal caused by reflecting on fish is referred to as the fish echo (first reception signal) and the echo signal caused by reflecting on the water bottom is referred to as the water bottom echo (second reception signal). 
     The receiver  17  amplifies the echo signal inputted from the transducer  16  and outputs it to the A/D converter  18 . The A/D converter  18  samples the echo signal at a predetermined timing, converts it into a digital signal, and outputs it to the signal processor  19 . The timing for the sampling is set by the controller  10  based on a distance resolution in the depth direction. 
     The signal processor  19  stores the digitalized echo signal in a memory (not illustrated) sequentially. The signal processor  19  filters the stored echo signal by using a reception filter to remove a noise element that is outside a predetermined frequency band therefrom, and performs school-of-fish detection processing, water bottom detection processing, and bottom sediment discrimination processing. The reception filter is provided for each of the two kinds of echoes. Information showing the result of the school-of-fish detection, the water bottom detection, and the bottom sediment discrimination performed by the signal processor  19  is displayed on the display unit  13 . 
     Next, the transmission unit  14  is described.  FIG. 2  is a block diagram schematically showing a configuration of the transmission unit  14 . The transmission unit  14  includes a school-of-fish detecting signal generator  141  (first generator), a bottom sediment discriminating signal generator  142  (second generator), and a transmission timing controller  143  (signal outputting module). 
     The school-of-fish detecting signal generator  141  generates a frequency modulation chirp signal (FM chirp signal) as one of the transmission signals which is for detecting the school-of-fish, based on the instruction through the controller  10 . The FM chirp signal (first transmission signal) is a signal of which a frequency suitable for the school-of-fish detection is swept in terms of time. An amplitude of the signal is constant but the frequency changes from a lowest frequency to a highest frequency within a time length corresponding to a wavelength of the signal. 
     The ultrasonic wave for school-of-fish detection (hereinafter, may be referred to as “the fish ultrasonic wave”) is required to have a high distance resolution (high ability of distinguishing the school-of-fish). If a pulse width of the ultrasonic wave is long, a target object such as fish, of which a length is shorter than the pulse width, cannot be detected and the distance resolution degrades. However, if the pulse width of the ultrasonic wave is short so as to achieve a high distance resolution, an energy (signal intensity) of the ultrasonic wave drops, a signal-to-noise ratio (SN ratio) is reduced, and fish cannot be detected accurately. Therefore, the fish ultrasonic wave based on the FM chirp signal generated by the school-of-fish detecting signal generator  141  is transmitted for a predetermined time period, and the fish echo obtained by reflecting on the target object (fish) is pulse compressed. Thereby, an effect similar to when the ultrasonic wave having the short pulse width and a strong energy is transmitted and received can be obtained and, thus, allows the transmission and reception of the ultrasonic wave suitable for school-of-fish detection. Note that, the fish ultrasonic wave can also be used as the ultrasonic wave for detecting a depth underwater. 
     The bottom sediment discriminating signal generator  142  generates a fixed frequency signal (second transmission signal) as the other one of the transmission signals which is for discriminating a water bottom sediment type, based on the instruction through the controller  10 . The bottom sediment discriminating signal generator  142  generates the transmission signal having a pulse width longer than at least the pulse width of the pulse compressed fish echo and determined according to the water bottom detection range. In the bottom sediment discrimination, a property of the water bottom echo is analyzed. For example, when the energy of the water bottom echo is strong, the water bottom echo is discriminated as a water bottom of which the water bottom sediment type is hard, such as a rock ground or a rock reef. When the energy of the water bottom echo is weak, the water bottom echo is discriminated as a water bottom of which the water bottom sediment type is soft, such as a sand ground. Therefore, unlike the fish ultrasonic wave, the ultrasonic wave for the bottom sediment discrimination (hereinafter, may be referred to as “the water bottom ultrasonic wave”) does not require the high distance resolution while requires a strong energy and a high SN ratio. For this reason, the bottom sediment discriminating signal generator  142  generates the transmission signal having the pulse width longer than at least the pulse width of the pulse compressed fish echo. This transmission signal is outputted to the transducer  16 , and then the transducer  16  transmits the ultrasonic wave having the pulse width of the generated transmission signal. 
     Note that, if the school-of-fish detection is performed through the school-of-fish detecting signal generator  141  generating the fixed frequency signal and without pulse compressing the fish echo, the bottom sediment discriminating signal generator  142  generates a transmission signal having a pulse width longer than that of the transmission signal generated by the school-of-fish detecting signal generator  141 . Thereby, the fish finder  1  transmits the water bottom ultrasonic wave having the pulse width longer than the fish ultrasonic wave. 
     The transmission timing controller  143  outputs the transmission signals generated by the school-of-fish detecting signal generator  141  and the bottom sediment discriminating signal generator  142  to the transducer  16  via the transmission-and-reception switch  15  at predetermined timings alternately. That is, the fish finder  1  transmits the fish ultrasonic wave and the water bottom ultrasonic wave from the transducer  16  alternately. Specifically, the fish finder  1  transmits the fish ultrasonic wave and receives the reflection wave thereof, and then transmits the water bottom ultrasonic wave and receives the reflection wave thereof. 
     Hereinafter, the processing of the signal processor  19  is explained in detail.  FIG. 3  is a block diagram schematically showing a configuration of the signal processor  19 . The signal processor  19  includes a bottom sediment discrimination processing module  191  (bottom sediment discriminating module), a pulse compressing module  192  (setting unit), a school-of-fish detecting module  193  (underwater detecting module), and a display processing module  194 . 
     The bottom sediment discriminating module  191  is inputted with the echo signal from the A/D converter  18 . The bottom sediment discriminating module  191  stores echo signal(s) for one measurement (one ping) in a memory (not illustrated) and performs processing described later every time the echo signal(s) for one ping is stored. The bottom sediment discriminating module  191  discriminates the water bottom sediment type based on the water bottom echo. The manner of discriminating the bottom sediment type is not particularly limited, and may be based on a matching level (similarity) of the water bottom echo with reference data. That is, the bottom sediment discriminating module  191  obtains the water bottom depth from the inputted echo signal, and, through using the echo signal within a predetermined time length range from the water bottom depth (i.e., the echo signal having an energy above a predetermined threshold) as the water bottom echo, matches a pattern of the water bottom echo with the reference data so as to calculate a similarity. As the manner of matching, the fuzzy inference or neural network may be used. The reference data includes plural kinds of data of mud, sand, stones and rocks, etc. Therefore, the strong energy and the high SN ratio are required to the water bottom echo. 
     Note that, a manner of obtaining the water bottom depth may be based on any of the following timings: a timing at which the echo signal has an energy above a predetermined value, a timing at which a differential value reaches the highest value, and a timing at which a correlation value of the echo signal with a reference signal having a waveform similar to the pulse width of the transmitted ultrasonic wave reaches a highest value. 
     The pulse compressing module  192  pulse compresses the fish echo. Specifically, the pulse compressing module  192  performs correlation processing on the fish echo and the FM chirp signal by a matched filter, extracts only a frequency band corresponding to the FM chirp signal from the fish echo, and removes other frequency band(s). In this manner, the fish echo is pulse compressed. Thereby, the resolution of the fish echo improves and fish can be detected accurately by the school-of-fish detecting module  193  described below. 
     The school-of-fish detecting module  193  uses the known method to detect the school-of-fish from the pulse-compressed fish echo. The school-of-fish detection includes discriminating a fish body length and a fish kind, and detecting an underwater depth to the school-of-fish. Because the resolution of the fish echo is improved through the pulse compression by the pulse compressing module  192 , the school-of-fish detecting module  193  can detect the school-of-fish highly accurately. The school-of-fish detecting module  193  outputs the processing result to the display processing module  194 . 
     Next, the echo signals obtained as a result of transmitting the ultrasonic waves having different pulse widths are explained.  FIG. 4  is charts showing a comparison of echo signal between when an ultrasonic wave having a pulse width T 1  is transmitted and when an ultrasonic wave having a pulse width T 2  is transmitted. Note that, in  FIG. 4 , the pulse width T 1 &lt;the pulse width T 2 . In  FIG. 4 , the charts on the left show the case where the ultrasonic wave having the pulse width T 1  is transmitted, and the two charts on the right show the case where the ultrasonic wave having the pulse width T 2  is transmitted, in which the two upper charts show the transmission pulse, and the two lower charts show the reception pulse. Note that, each chart in  FIG. 4  is devised for the convenience of explanation and does not show the actual echo signal. Further,  FIG. 4  shows the case where the echo signal is not pulse compressed. 
     When the ultrasonic wave having the shorter pulse width is transmitted, the distance resolution improves and the accuracy in distinguishing an individual fish (accuracy in distinguishing the fish body length and the fish kind) improves. Therefore, as shown in the two charts on the left in  FIG. 4 , when the ultrasonic wave having the pulse width T 1  is transmitted, the fish echo with higher accuracy can be acquired comparing to the case with the ultrasonic wave having the pulse width T 2 . Because the school-of-fish detection requires a high distance resolution (the horizontal axis in  FIG. 4 ) but does not require a high energy (the vertical axis in  FIG. 4 ), the ultrasonic wave having the shorter pulse width is suitable. 
     Whereas, when the ultrasonic wave having the longer pulse width is transmitted, the energy of the reflection wave, the detectable distance range, and the SN ratio increase. Therefore, as shown in the two charts on the right in the  FIG. 4 , when the ultrasonic wave having the pulse width T 2  is transmitted, the energy of the echo signal is stronger and the noise level is decreased (the SN ratio is increased). Therefore, the ultrasonic wave having the longer pulse width is suitable because it does not require a high distance resolution but requires a high energy. Further, in either case with the pulse width T 1  or the pulse width T 2 , it can be understood that a characteristic (shape) of the water bottom obtained from the water bottom echo is the same. 
     Next, the increase of the SN ratio by the transmission of the ultrasonic wave having the longer pulse width is explained.  FIG. 5  is a table showing experimental results of the SN ratio obtained when the ultrasonic wave having the pulse width T 1  is transmitted and when the ultrasonic wave having the pulse width T 2  is transmitted. Note that, the values from the table are rounded to one decimal place.  FIG. 5  shows the example of when the detection range is 100 [m], the ultrasonic wave having the pulse width T 1  of 1.0 [ms] is transmitted, and then the ultrasonic wave having the pulse width T 2  of 3.0 [ms] is transmitted, and as a result, the SN ratio is increased by approximately 4.8 [dB]. Moreover, a reception frequency band width is in proportion to an inverse number of the pulse width; thereby, in the example where a band width of the reception filter (first filter) for the ultrasonic wave having the pulse width T 1  of 1.0 [ms] is 1.5 [kHz], a band width of the other reception filter (second filter) for the ultrasonic wave having the pulse width T 2  of 3.0 [ms] becomes 0.5 [kHz]. Because the number of noise elements generally reduces when the band width of the filter is narrowed, in this example, the SN ratio is further increased by approximately 4.8 [dB]. In total, the SN ratio is increased by approximately 9.5 [dB]. Thus, by elongating the pulse width T 2 , the SN ratio increases comparing to the case using the pulse width T 1 . 
     Hereinafter, the operation of the fish finder  1  is explained.  FIG. 6  is a flowchart showing a procedure of processing that is performed by the fish finder  1 . The controller  10  reads out the program  11 A stored in the storage  11  and, thereby, processing shown in  FIG. 6  is performed. 
     The controller  10  transmits the fish ultrasonic wave from the transducer  16  and it receives the reflection wave (S 1 ). Specifically, the controller  10  generates the FM chirp signal by the transmission unit  14  and outputs it to the transducer  16 , and the transducer  16  transmits the fish ultrasonic wave. Further, after the transducer  16  receives the reflection wave, the controller  10  pulse compresses the fish echo outputted from the transducer  16 , by the signal processor  19 . Next, the controller  10  performs the school-of-fish detection based on the fish echo (S 2 ). 
     Sequentially, the controller  10  transmits the water bottom ultrasonic wave from the transducer  16  and receives the reflection wave (S 3 ). Here, the controller  10  sets the pulse width of the water bottom ultrasonic wave which is to be transmitted, so as to have the longer pulse width than the pulse compressed fish echo. After the transducer  16  receives the reflection wave, the controller  10  performs the water bottom sediment discrimination by the signal processor  19  based on the water bottom echo outputted from the transducer  16  (S 4 ). Then, the controller  10  finishes the processing. 
     As explained above, in this embodiment, the fish finder  1  receives the ultrasonic waves having the different pulse widths, and performs the school-of-fish detection and the water bottom sediment discrimination. Because required property differs between the school-of-fish detection and the water bottom sediment discrimination, by differing the pulse width of the reflection wave to be received, the school-of-fish detection and the water bottom sediment discrimination can be performed with a single device. 
     Note that, the specific configuration of the fish finder  1  may be modified as needed. The functions and effects described in this embodiment are merely illustrations of suitable functions and effects that are derived from the invention, and therefore, they are not limited to the description of the above embodiment. 
     The transmission unit  14  can generate transmission signals with a plurality of frequencies, and may transmit the transmission signals with two or more kinds of frequencies. For example, as the fish ultrasonic waves, the fish finder  1  may transmit an FM chirp signal of 40 kHz to 80 kHz and an FM chirp signal of 175 kHz to 225 kHz alternately, or alternatively, may transmit a fixed frequency signal of low frequency waves of 60 kHz and a fixed frequency signal of high frequency waves of 200 kHz. Further, as the water bottom ultrasonic wave, the fish finder  1  may transmit a fixed frequency signal of 60 kHz and a fixed frequency signal of 200 kHz alternately. 
     In the foregoing specification, specific embodiments of the present invention have been described. However, one of ordinary skill in the technique appreciates that various modifications and changes can be performed without departing from the scope of the present invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present invention. The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued.