SCANNING SONAR
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sonar such as a scanning sonar that is used for detecting fish in a wide area in the water.
2. Description of the Related Art
A scanning sonar is a type of a sonar that transmits an ultrasonic beam to a wide area in the water, receives an echo with scanning in a predetermined direction, and displays a image of a shoal of fish or the like on the basis of the echo received. In general, the scanning sonar has a horizontal mode for transmitting an umbrella-shaped ultrasonic beam into water at a predetermined tilt angle and receiving an echo with scanning overall horizontal azimuths anda vertical mode for transmitting an ultrasonic beam to a fan-shaped area on a vertical plane in a predetermined horizontal azimuth and receiving an echo with scanning for the fan-shaped area. The scanning sonar is described in. for example, JP-A-2003-202370, JP-A-57-196172, and JP-A-9-43350.
Fig. 16 is a diagram for explaining a principle of the scanning sonar. In the figure, reference numeral 50 denotes a scanning sonar mounted on a ship 51; 52, a transducer provided in the scanning sonar 50; 53 and 54, ultrasonic beams emitted from the transducer 52: and 55, a water surface. In the horizontal mode, the scanning sonar 50 transmits the umbrella-shaped ultrasonic beam 53 from the transducer 52 to all underwater azimuths at a predetermined tilt angle (a depression angle) at the same time. After transmitting the beam, the scanning sonar 50 performs scanning in a circumferential direction using the transducer 52, forms a reception beam for performing scanning in a spiral shape in an arrow C direction at high speed, and receives echoes reflected on a shoal of fish and the bottom of the water.In the vertical mode, the scanning sonar 50 transmits the ultrasonic beam 54 from the transducer 52 toa fan-shaped vertical section indicated by oblique lines in the figure. After transmitting the beam, the scanning sonar SO performs scanning in the vertical direction using the transducer 52, forms a reception beam for scanning a fan-shaped area in an arrow D direction, and receives echos reflected on a shoal of fish and the bottom of the water. Images of the shoal of fish and the bottom of the water are displayed in color on a display screen of the scanning sonar 50 according to signal intensity of the echo received by such scanning. Fig. 17 shows an example of a image displayed on the display screen of the scanning sonar 50. Reference numeral 60 denotes a display screen.A horizontal mode image H obtained by an echo received in the horizontal mode and a vertical mode image v obtained from an echo received in the vertical mode are displayed on the screen 60 side by side. In the horizontal mode image H, reference numeral 61 denotes a mark on a hull; 62a and 62b, images of shoals of fish obtained from the echo; and 63, a image of the bottom of the water obtained from the echo. The vertical mode image V consists of two images V1 and v2. Reference sign VI denotes a image obtained by vertical scanning in an azimuth VI in the horizontal mode image H: 64a, a image of a shoal of fish obtained from the echo; and 65a, a image of the bottom of the water obtained from the echo.Reference sign V2 denotes a image obtained by vertical scanning in an azimuth V2 in the horizontal mode image H; 64b, a image of a shoal of fish obtained from the echo; and 65b, a image of the bottom of water obtained from the echo. The images 64a and 64b of the shoals of fish in the vertical mode image V correspond to the images 62a and 62b of the shoals of fish in the horizontal mode image H, respectively.
In the conventional scanning sonar, as shown in Fig. 17, the images of the shoals of fish obtained from echo data are displayed by two-dimensional rendering. However, in such two-dimensional display, when a user tries to three-dimensionally image a shoal of fish in the water, a place of the shoal of fish, and the like, the user has to mentally combine the horizontal mode image H and the vertical mode image V. The user needs an experience to mentally combine the images. Therefore, it is difficult for a user who does not have much experience to easily image a three-dimensional image.
On the other hand, JP-A-9-43350 describes an ultrasonic sonar that makes it possible to three-dimensionally recognize spread and thickness of a shoal of fish. In the patent document, the ultrasonic sonar creates volume data in which an echo received from the sea is stored in three-dimensional orthogonal coordinates, subjects thevolume data to processing for changing data into an equivalent plane using a threshold value of an echo level, and superimposes respective equivalent planes obtained as a result of the processing in a translucent state with a light and shade concentration difference or a hue difference to display a image.
Although the ultrasonic sonar in JP-A-9-43350 can three-dimensionally grasp a shoal of fish, the ultrasonic sonar simply gives differences of concentrations and colors to a image of the shoal of fish two-dimensionally displayed to represent thickness in the vertical direction. It is difficult to intuitively recognize a state of the shoal of fish with such display. It is impossible to accurately grasp a state in the water comprehensively only with display of the shoal of fish.
SUMMARY OF THE INVENTION
In view of the problems, it is an object of the invention to provide a sonar with which it is possible to three-dimensionally recognize a state in the water easily and accurately.
In the invention, there is provided a sonar comprising a transmitter for transmitting an ultrasonic signal in a form of an umbrella-shaped transmission beam to a wide area in water from a transducer, a receiver for receiving echo signals by reception beams formed in different directions in the wide area, a signal generator for generating three-dimensional echo display data from the echo signals and an indicator for displaying the three-dimensional echo display data as a three-dimensional image on the screen thereof.
Since the underwater information obtained by scanning the wide area in the water is displayed as the three-dimensional image in this way, even a user who does not have much experience can three-dimensionally recognize the underwater information of a shoal of fish and the like easily. In addition, the user can quite obviously see a position and the like of the shoal of fish and the like in the scan area and accurately grasp an overall state in the water.
In a typical embodiment of the invention, there is provided a sonar comprising a transmitter for transmitting an ultrasonic signal in a form of an umbrella-shaped transmission beam to a wide area in water from a transducer, a receiver for receiving echo signals by reception beams formed in different directions in the wide area, a signal generator for generating three-dimensional echo display data from the echo signals and an indicator for displaying the three-dimensional echo display data as a virtual stereoscopic image similar to the umbrella-shaped transmission beam with the transducer as a top thereof on the screen of the indicator. Consequently, the underwater information is displayed as a conical stereoscopic image.
It is possible to apply the invention to A sonar comprising a first mode for receiving echo signals resulting from a transmitted ultrasonic signal by reception beams formed in different directions in a first wide area, a second mode for receiving echo signals resulting from a transmitted ultrasonic signal by reception beams formed in different directions in a second wide area, a signal generator for generating three-dimensional echo display data from the echo signals obtained by the first mode and the second mode and an indicator for displaying the three-dimensional echo display data as three-dimensional image on the screen thereof.Consequently, since pieces of underwater information in different scan areas are superimposed and stereoscopically displayed, it is possible to recognize a image of a shoal of fish and the like from many aspects and more accurately grasp the underwater information.
In a typical embodiment, the first mode is a horizontal mode for receiving an echo with scanning over horizontal all azimuths and the second mode is a vertical mode for receiving an echo by scanning a fan-shaped area of a vertical surface in a predetermined horizontal azimuth. However, the invention is not limited to this. As one of plural modes, for example, a slant mode for transmitting an ultrasonic beam to a semicircular slant surface area facing downward from a water surface to perform scanning is also conceivable.
In the invention, there is provided a sonar comprising a transmitter for transmitting an ultrasonic signal from a transducer to a wide area in water, a receiver for receiving echo signals by reception beams formed in different directions in the wide area, a signal generator for generating three-dimensional echo display data from the echo signals, a storing section for storing the three-dimensional echo display data and an indicator for displaying a history of the three-dimensional echo display data on the screen of the indicator on the basis of the three-dimensional echo display data stored in the storing section. Consequently, since plural sections of the shoal of fish are displayed by the number of histories simultaneously, it is possible to easily grasp a shape and a size of the shoal of fish.In addition, it is also possible to easily grasp a moving state and a moving place of the shoal of fish.
In a preferred embodiment of the invention, a three-dimensional image of the scan area is superimposed on the three - dimensional image of the echo signals and displayed on the screen. Consequently, since it is possible to stereoscopicallyrecognize a detection area by scanning clearly. it is possible to more accurately grasp the underwater information.
In a preferred embodiment of the invention, threshold setting means for setting a threshold value for a signal level of echo is provided and the history of the three-dimensional image is displayed only for echo having a signal level equal to or higher than the threshold value set. Consequently, since a image of the shoal of fish, a history of which is display, is a clear image with a image due to noise removed, it is possible to more easily grasp a state of the shoal of fish.
In a preferred embodiment of the invention, transparency setting means for setting transparency of an image that is stereoscopically displayed is provided. Consequently, as transparency is increased, a image in a superimposed portion is more clearly displayed, which makes it easier to recognize stereoscopic images of a shoal of fish, a scan area, and the like. If transparency is set lowered, it is possible to conceal a image behind the superimposed portion.
In a preferred embodiment of the invention, it is possible to arbitrarily change a viewpoint of an image that is stereoscopically displayed. Consequently, it is possible to accurately grasp a volume and the like of a shoal of fish by looking at the same image from various angles.
In a preferred embodiment of the invention, the sonar displays a three-dimensional image of a water bottom landform together with the three-dimensional image of the echo signals. Consequently, since the water bottom landform is displayed as a stereoscopic image, it is possible to stereoscopically recognize a state of the water bottom, a position of a shoal of fish, and the like. This makes it possible to more accurately grasp the state in the water.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
Fig. 1 is a block diagram of a scanning sonar according to the invention; Fig. 2 is a diagram showing a state of horizontal scanning and vertical scanning; Fig. 3 is a diagram showing an example of an underwater image displayed on a display unit; Fig. 4 is a diagram showing the underwater image from a different viewpoint; Figs. 5A to 5D are principle diagrams for explaining a history display mode; Fig. 6 is a diagram for explaining a change in a tilt angle in the history display mode; Fig. 7 is a diagram showing an example of display of an underwater image in the history display mode; Fig. 8 is a diagram for explaining a change in a tilt angle in the history display mode; Fig. 9 is a diagram showing another example of display of an underwater image in the history display mode; Fig. 10 is a diagram of an example of simplified history display;Fig. 11 is a flowchart showing procedures in displaying an underwater image; Fig. 12 is a diagram for explaining a polygon; Fig. 13 is a diagram showing another example of an underwater image displayed on the display unit; Fig. 14 is a conceptual diagram of a slant mode; Fig. 15 is an example of application of history display; Fig. 16 is a diagram for explaining a principle of a scanning sonar; and Fig. 17 is an example of a image displayed on a display screen of the scanning sonar.
DETAILED DESCRIPTION OF THE INVENTION
Fig. 1 is a block diagram of a scanning sonar 100 according to the invention. In the figure, reference numeral 10 is a transducer. The transducer 10 acoustically converts a transmission signal and transmits an ultrasonic beam into the water at the time of transmission and receives an echo returned after being reflected on a target in the water and converts the echo into an electric signal at the time of reception. For example, a cylindrical or spherical transducer is used as the transducer 10. A transmitter in the invention may be a device that performs both transmission and reception in the same manner as the transducer 10 or may be a device that performs only transmission. In the latter case, a receiver is provided separately from the transmitter. Reference numeral 11 denotes a scanning unit.The scanning unit 11 includes a transmission circuit that generates a transmission signal given to the transducer 10, a reception circuit that amplifies an echo received by the transducer 10 and extracts a signal of a predetermined frequency component with a filter, a transmission/reception switching unit that switches between a transmission operation and a reception operation, and a beam forming unit that forms a reception beam for receiving an echo with horizontal and vertical scanning.
Reference numeral 12 denotes an operation unit that includes keys and a dial for setting and adjusting various parameters such as a tilt angle and a horizontal azimuth angle. The operation unit 12 constitutes the threshold setting means and the transparency setting means in the invention. Reference numeral 13 denotes a control unit that controls operations of the scanning sonar 100. Reference numeral 14 denotes an image processing unit that generates underwater images of a shoal of fish and the like on the basis of signals of echoes received. The control unit 13 and the image processing unit 14 are constituted as a microprocessor (CPU) 15 of one chip. However. the control unit 13 and the image processing unit 14 may be provided independently. Reference numeral 16 denotes a Image RAM (VRAM) in which data of the underwater image generated by the image processing unit 14 is stored.Reference numeral 17 is a display unit that displays the underwater image in color on the basis of the data stored in the VRAM 16. The display unit 17 is constituted by a CRT, a liquid crystal display, and the like.
Reference numeral 18 denotes a memory consisting of a ROM, a RAM, or the like and 19 denotes a sea bottom map database. In the memory 18, areas in which parameters such as a history threshold Z, a number of times of histories N, and transparency a described later as well as a tilt angle 0 and a horizontal azimuth angler are provided. Although not shown in the figure, areas in which data of a received echo, data for three-dimensional rendering, and the like are temporarily stored are secured. The sea bottom map database 19 is a database in which stereoscopic display data of sea bottom landforms at respective points are accumulated in association with longitude and latitude. It is possible to make use of an electronic chart available on the market (e.g., an electronic chart issued by Geographical Survey Institute).Reference numeral 20 denotes a Global Positioning System (GPS) receiver for acquiring global coordinates. The GPS receiver 20 is mounted on a ship as a unit separate from the scanning sonar 100.
Fig. 2 is a diagram showing states of scanning in a horizontal mode (hereinafter referred co as "horizontal scanning") and scanning in a vertical mode (hereinafter referred to as "vertical scanning") in the scanning sonar 100. At the time of the horizontal scanning, the scanning sonar 100 transmits an umbrella-shaped ultrasonic beam 21 to all horizontal azimuths at the predetermined tilt angle and, then, forms a reception beam 22 with scanning in an arrow A direction to receive an echo. At the time of vertical scanning, the scanning sonar 100 transmits an ultrasonic beam 23 to a fan-shaped area of about 90[deg] on a vertical surface at the predetermined horizontal azimuth angle ȧnd, then, forms a reception beam 24 with scanning in an arrow B direction to receive an echo.The horizontal scanning and the vertical scanning may be alternately performed or may be simultaneously performed in parallel. Reference numeral 25 denotes a ship mounted with the scanning sonar 100; 26, a shoal of fish in the water; and 27, a sea bottom. Fig. 3 is a diagram showing an example of an underwater image displayed on the display unit 17 . Reference numeral 17a denotes a display screen of the display unit 17 . On the display screen 17a, a ship 31, a scan area 32 of an umbrella-shaped ultrasonic beam (hereinafter referred to as "umbrella-shaped beam") in the horizontal mode, a scan area 33 of a fan-shaped ultrasonic beam (hereinafter referred to as "fan-shaped beam") in the vertical mode, a fish shoal image 34, and a sea bottom landform image 35 are stereoscopically displayed as three-dimensional images.Although the scan area 32 of the umbrella-shaped beam is displayed as an image of a cone (a solid cone), the actual scan area 32 is a surface of a cone formed by a reception beam (strictly, since the reception beam has fixed thickness, the scan area 32 includes the surface of the cone and the vicinity of the surface). Reference numeral 36 denotes an isobath on the surface of the cone; 37, a crossing line of the scan area 32 of the umbrella-shaped beam and the scan area 33 of the fan-shaped beam; and 38, a crossing surface of the cone and the scan area 33 of the fan-shapedbeam. Reference numeral 39 denotes a data display space in which a present position (latitude and longitude) of the ship 31 acquired from the GPS receiver 20 and a water depth value in the position, and the like are displayed.A water depth is measured by a depth sounder mounted on a ship or the scanning sonar 100 having a sounding function. Note that display of a water depth value may be omitted.
The scan area 32 of the umbrella-shaped beam and the scan area 33 of the fan-shaped beam are displayed on the basis of the data such as the tilt angle 0 and the horizontal azimuth angle (,6 set through the operation unit 12 . The fish shoal image 34 is displayed on the crossing line 37 of the scan areas 32 and 33 on the basis of three-dimensional echo display data generated from data of an echo obtained by the horizontal scanning and data of an echo obtained by the vertical scanning. The sea bottom landform image 35 is displayed on the basis of sea bottom landform data in the present position acquired from the sea bottom map database 19. The isobath 36 is calculated from a range of data, the tile angle 8, and the like and displayed.
In Fig. 3, underwater information in the scan area 32 is displayed on the display screen 17a as a virtual solid (cone) image similar to an umbrella-shaped beam having a transducer (not shown) provided in the ship 31 as the top. Images formed by echoes reflected on floating objects matters in the water and the sea bottom are also included in the image of the underwater information other than the fish shoal image 34. However, in Fig. 3, for simplicity of explanation, these images are not shown. In Fig. 3, respective three-dimensional images of the ship 31, the scan area 32 of the umbrella-shaped beam, the scan area 33 of the fan-shaped beam, the fish shoal image 34, the sea bottom landform image 35, and the isobath 36 are superimposed on the identical display screen 17a and stereoscopically displayed.Therefore, the user can quite obviously grasp in which direction and depth and how much volume of shoal of fish are present viewed from the ship 31 according to a three-dimensional image. In particular, since the pieces of underwater information in the different scan areas 32 and 33 are superimposed and stereoscopicallydisplayed, it is possible to recognize the fish shoal image 34 from many aspects and more accurately grasp the underwater information. Since the scan areas 32 and 33 are stereoscopically displayed, it is possible to stereoscopically and clearly recognize a detection area. Moreover, since the sea bottom landform image 35 is displayed as a three-dimensional image, it is possible to stereoscopically recognize a state of the sea bottom, a position of a shoal of fish, and the like. This makes it possible to accurately grasp a state in the water.Note that, even when the echo image of the sea bottom is displayed, since the stereoscopic sea bottom landform image 35 overlaps the echo image, it is easy to see a sea bottom surface and distinguish fish swimming at the bottom like a flounder.
In this case, transparency of a image may be set using the operation unit 12. Transparency is a factor that represents, when images are superimposed, how clearly a image in the superimposed portion is seen. As transparency is increased, the image of the superimposed portion is more clearly displayed. As transparency is decreased, the image of the superimposed portion is less easily seen. For example, when transparency is high in Fig. 3, a sea bottom landform (not shown in Fig.
3) in the depth of the scan area 32 of the umbrella-shaped beam is clearly displayed. The fish shoal image 34, the isobath 36, and the like in the superimposed portion of the scan area 32 of the umbrella-shaped beam and the scan area 33 of the fan-shaped beam are also clearly displayed. This makes it easier to recognize stereoscopic images of a scan area, a shoal of fish, a sea bottom landform, and the like. when it is desired to conceal a image in the depth of the superimposed portion and emphasize only a image in the front, transparency only has to be set low.
Fig. 3 is an example of display in the case in which an underwater image stereoscopically displayed is viewed from a certain fixed direction. However, a viewpointfor viewing the underwater image may be arbitrarily changed. Fig. 4 is an example of display in that case and shows a stereoscopic image in the case in which the ship 31 in Fig. 3 is viewed from the right direction. Operation for changing a viewpoint is performed in the operation unit 12. It is possible to display an underwater image viewed from an arbitrary viewpoint by, for example, setting or switching a direction of a viewpoint. This makes it possible to more accurately grasp a volume of a shoal of fish and a state in the water. Figs. SA to 5D are principle diagrams for explaining a history display mode that is another embodiment of the invention .Fig. 5A shows an example in which one vertical mode image V1 and two horizontal mode images H1 and H2 are superimposed and stereoscopically displayed when an azimuth angle is fixed and a tilt angle is changed to perform history display. Since the horizontal mode images HI and H2 have a time difference, the time difference is a history. Fig. 5B shows an example in which one horizontal mode image HI and two vertical mode images V1 and V2 are superimposed and stereoscopically displayed when a tilt angle is fixed and an azimuth angle is changed to perform history display. Since the vertical mode images VI and V2 have a time difference, the time difference is a history. Figs. 5C and 5D show examples in which plural images of only one mode are used to perform history display. In Fig. 5C, history display is performed according to two horizontal mode images HI and H2.In Fig. 5D, history display is performed according to two vertical mode images VI and V2. Note that the history display mode is set by operation of the operation unit 12 (Fig. 1).
According to such history display, since it is possible to display plural sections of a shoal of fish by the number of histories simultaneously and stereoscopically grasp a state of the shoal of fish, a frame of one shoal of fish is seen. It is possible to easily estimate a size and the like of the shoal of fish. For example, as shown in Fig. 6, when a tilt angle is sequentially changed as 1, 2, and 3 on the basis of the principle in Fig. 5A and scanning is applied to the relatively large shoal of fish 26 at the respective tilt angles, data of echoes obtained from the shoal of fish 26 equivalent to the number of times of scanning are stored in the memory 18 . The number of times of scanning at this point is the number of histories.As a result, the three-dimensional fish shoal image 34 shown in Fig. 7 is stereoscopically displayed on the basis of the history data of the echoes stored in the memory 18. It is possible to easily grasp a shape of the shoal of fish and understand that a size of the shoal of fish is relatively large. In Fig. 7, as in Fig. 5A, since a horizontal mode image and a vertical mode image are superimposed and displayed, it is possible to recognize the shoal of fish from many aspects. This makes it possible to accurately grasp a shape and a size of the shoal of fish.
Note that, in Fig. 5, the images HI and H2 and the images VI and V2 are obtained by scanning at different time instants, respectively. If scanning is performed at different frequencies using the method described in JP-A-2003-202370, it is possible to obtain the images HI and H2 and the images VI and V2 simultaneously. Other than the cases shown in Fig. 5, it is also possible to perform history display by displaying the plural horizontal mode images HI and H2 and the plural vertical mode images VI and V2 simultaneously. Moreover, in the case of a sailing ship, it is also possible to perform history display utilizing the fact that a scan area changes as the ship advances.
When the history display mode is used, other than stereoscopicallygrasping a shape of a shoal of fish, for example, it is possible to grasp a moving state of the shoal of fish. In other words, as shown in Fig. 8, when a tilt angle is sequentially changed as 1, 2, and 93 and scanning is performed at the respective tilt angles, data of echoes obtained from the shoal of fish 26 moving in an X direction equivalent to the number of times of scanning are stored in the memory 18. The number of times of scanning at this point is the number of histories. As a result, histories of three-dimensional fish shoal images 34a, 34b, and 34c shown in Fig. 9 are displayed on the basis of the history data of the echoes stored in the memory 18.On the other hand, when the shoal of fish 26 does not move but stays in the same place, even in the history display mode, a image of the shoal of fish displayed on the display screen 17a is the three-dimensional fish shoal image 34 without a change as shown in Fig. 3.
In Fig. 9, the fish shoal images 34a, 34b, and 34c are superimposed and stereoscopically displayed on the identical display screen 17a together with the ship 31, the sea bottom landform image 35, and the scan areas 32 and 33. Thus, it is possible to quite obviously grasp, with a three-dimensional image, in which direction a shoal of fish is moving when the shoal of fish is viewed from the ship 31.
In the history display mode, as in the example described above, transparency of a image may be set using the operation unit 12 or a viewpoint for viewing a image may be arbitrarily changed. If a threshold value is set for a signal level of a received echo and history display is performed only for echoes having signal levels equal to or higher than the threshold value set, it is possible to remove a image due to noise and leave a clear his tory. The threshold value in this case is hereinafter referred to as "history threshold". Setting of the history threshold is performed in the operation unit 12.
It is also possible that, in the operation unit 12, history display is switched between a detail mode and a simple mode. For example, a stereoscopic image shown in Fig. 9 is displayed on the screen in the detail mode and simplified history display shown in Fig. 10 is performed in the simple mode. In Fig. 10, the scan area 33 of the fan-shaped beam, the sea bottom landform image 35. and the like in Fig. 9 are not shown.
Fig. 11 is a flowchart showing a procedure in displaying an underwater image. The procedure is executed by the control unit 13 of the microprocessor (CPU) 15. A procedure for image display will be hereinafter explained with reference to Fig.
11. First, in the operation unit 12, the history threshold Z, the number of times of histories N, and the transparency a are set (steps S1 to S3). As described above, the history threshold Z is a threshold value for distinguishing an echo signal level of a fish shoal image, ahistoryof which is displayed. The number of times of histories N is the number of times a fish shoal image is left as a history (e.g., in Fig. 9, the number of times of histories N is 3) . As described above, the transparency a is a factor indicating how clearly superimposed images are seen. These set values are stored in the predetermined areas of the memory 18 shown in Fig. 1.
Subsequently, horizontal scanning or vertical scanning by the scanning unit 11 is performed to obtain data of echoes equivalent to one scan (step S4) . In the case of the horizontal scanning, the data equivalent to one scan means data (a signal level) of echoes received while the reception beam 22 rotates once (360[deg]) in the A direction in Fig. 2. In the case of the vertical scanning, the data equivalent to one scan means data (a signal level) of echoes received while the reception beam 24 rotates about 90[deg] in the B direction in Fig. 2. The data of echoes obtained by one scan is temporarily stored in the predetermined areas of the memory 18.
Subsequently, the number of times of scanning and the number of times of histories N are compared to each other (step S5). As a result of the comparison, when the number of times of scanning does not exceed the number of times of histories N (NO in step S5), the control unit 13 proceeds to step S7. When the number of times of scanning exceeds the number of times of histories N (YES at step SS) . the control unit 13 proceeds to step S6. In the first time, since the number of times of scanning is 1, it is judged in step S5 that the number of times of scanning does not exceed the number of times of histories N. The control unit 13 proceeds to step S7 without executing step S6.
In step 57. the control unit 13 acquires data of a polygon on thebasis of the data of echoes obtained in step 54. Triangular polygons P shown in Fig. 12 are objects of acquisition of data. In Fig. 12, reference numeral 22 denotes a reception beam in the horizontal scanning and reference signs SP1, SP2, SP3, and the like denote sampling points of the reception beam 22. An arrow A indicates a scanning direction. The triangular polygon P is a unit triangle formed by connecting the three sampling points. In Fig. 12, only a part of the triangular polygons P is shown. A signal level of an echo is measured for each of the sampling points and data of the signal level is held in the memory 18. Therefore, each of the triangular polygons P owns data of signal levels at the three sampling points that are vertexes.The individual data obtained at each of the sampling points owns three-dimensional data (global coordinates) consisting of an X coordinate value and a Y coordinate value determined by latitude and longitude and a Z coordinate value determined by a tilt angle. Note that, in Fig. 12, the triangular polygons P formed by the horizontal scanning are shown. However, the same principle is applied to triangular polygons formed by the vertical scanning. In step S7, signal levels of echoes at three vertexes are read out from the memory 18 for one of the triangular polygons P formed in one scan.
Subsequently, the control unit 13 judges whether all the signal levels at the three points of the triangular polygon P acquired is equal to or larger than a threshold value (step S8). The threshold value is the history threshold Z set in advance in step S1 . In a place where a shoal of fish is present, since a signal level of an echo is large, it is highly likely that the triangular polygon P with all the signal levels at the three points equal to or higher than the threshold value is extracted. As a result of the judgment, when at least one of the signal levels at the three points of the triangular polygon P is less than the threshold value (NO in step S8), the control unit 13 neglects the triangular polygon P to return to step 57 and performs judgment for the next triangular polygon P.On the other hand, when all the signal levels at the three points of the triangular polygon P are equal to or higher than the threshold value (YES in step S8), the control unit 13 stores three-dimensional data (global coordinates) at the respective vertexes of the triangular polygon P in the memory 18 (step 59). The three-dimensional data constitute three-dimensional echo display data in the invention. Thereafter, the control unit 13 judges whether the processing has been completed for all the triangular polygons P (step S10) . When the processing has not been completed (NO in step S10), the control unit 13 returns to step S7 and applies the processing in steps S8 and S9 to the next triangular polygon P.
When it is judged in step S10 that the processing has been completed for all the triangular polygons P (YES in step S10), the control unit 13 shifts to step Sll and performs rendering of a sea bottom map. In step Sll, the control unit 13 acquires information (latitude and longitude) of a present position from the GPS receiver 20 and extracts data of a sea bottom landform corresponding to the present position from the sea bottom map database 19. The control unit 13 s tereoscopically displays the sea bottom landform image 35 shown in Fig. 3 on the display screen 17a of the display unit 17 on the basis of the data extracted.
Subsequently, the control unit 13 performs rendering of a fish shoal image using the three-dimensional data of the triangular polygons stored in step S9 (step S12). Triangular polygons used in this case are those, all signal levels at three points of which are equal to or higher than the threshold value, and which correspond to the shoal of fish. Thus, a three-dimensional image obtained from an aggregate of the triangular polygons directly represents a three-dimensional image of the shoal of fish. According to the processing in step S12, the fish shoal image 34 shown in Fig. 3 is stereoscopically displayed on the display screen 17a of the display unit 17. Subsequently, the control unit 13 renders a ship, a scan area, an isobath, and the like (step S13) .According to the processing in step S13, the ship 31, the scan area 32 of an umbrella-shaped beam, the scan area 33 of a fan-shaped beam, the isobath 36, and the like shown in Fig. 3 are stereoscopically displayed on the display screen 17a. Data values are also displayed in the data display space 39. Note that the order of steps S11 to S13 may be changed.
Next, the control unit 13 judges whether the rendering should be ended (step S14). when operation for ending the rendering is performed in the operation unit 12 (YES in step S14), the control unit 13 ends the processing. When operation for ending the rendering is not performed (NO in step S14). the control unit 13 returns to step S4 and acquires data equivalent to the next one scan. When the previous scanning is the horizontal scanning in step S4, the present scanning is the vertical scanning. Steps S5 to S14 are executed on the basis of data of echoes obtained by the vertical scanning. When the control unit 13 returns to step S4 from step S14 again, the horizontal scanning is performed this time. Steps S5 to S14 are executed on the basis of data of echoes obtained by the horizontal scanning.Thereafter, in the same manner, the horizontal scanning and the vertical scanning are performed alternately. However, it is not essential for the invention to perform the horizontal scanning and the vertical scanning alternately. The horizontal scanning and the vertical scanning may be performed simultaneously in parallel.
It is possible to read out stored data and display a history of fish shoal images as shown in Figs. 7 and 9 by storing data of echoes acquired in each time of scanning in the memory 18. While scanning is repeated several times, when the number of times of scanning exceeds the number of times of histories N in step S5 (YES in step S5) , the control unit 13 erases oldest data stored in the memory 18 (step S6) and updates a data history to a latest one. Consequently, a history of a latest fish shoal image is always displayed on the display screen 17a in Figs. 7 and 9.
In Fig. 3, the image of the scan area 32 of the umbrella-shaped beam in the horizontal mode, the image of the scan area 33 of the fan-shaped beam in the vertical mode, and the fish shoal image 34 in both the scan areas are displayed. However, a image of a scan area in one of the horizontal mode and the vertical mode and a fish shoal image in the area may be displayed. Fig. 13 is an example of the display in which a image of the scan area 32 in the horizontal mode and the fish shoal image 34 in the scan area 32 are stereoscopically displayed. Images of the ship 31, the sea bottom landform image 35, and the isobath 36 are also stereoscopically displayed together with the images described above. The images in Fig. 13 are equivalent to images obtained by excluding the scan area 33 in the vertical mode and a fish shoal image in the area from the images in Fig. 3.The fish shoal image 34 represents a three-dimensional image of a shoal of fish near the surface of a cone forming the scan area 32 in the horizontal mode.
Although the scanning sonar having both the horizontal mode and the vertical mode is explained as an example in the embodiment described above, it is also possible to apply the invention to a scanning sonar having only one of the modes. For example, in the case of a scanning sonar having only the horizontal mode, the image shown in Fig. 13 is displayed. It is also possible to apply the invention to a scanning sonar having three or more modes (e.g., the horizontal mode, the vertical mode. and a slant mode described later) . It is possible to apply the invention not only to the scanning sonar but also to sonars such as a PPI sonar and a fish finder.
As embodiments of the invention, various embodiments are conceivable other than the embodiment described above. For example, although the horizontal mode and the vertical mode are explained as scan modes in the embodiment described above, it is also possible to apply the invention when scanning is performed in the slant mode and the vertical mode. Fig. 14 is a conceptual diagram of the slant mode. In this mode, an ultrasonic beam is transmitted to an area of a semicircular slant surface 42 extending obliquely downward from a water surface 41 and scanning is performed over the area. Reference numeral 43 denotes a ship and 44 denotes a shoal of fish.
It is also possible that a three-dimensional display mode (a 3D mode) and a two-dimensional display mode (a 2D mode) can be switched in the operation unit 12 and, when the scanning sonar is set in the three-dimensional display mode, the stereoscopic image shown in Figs. 3. 4 , 7, 9, and the like is displayed and, when the scanning sonar is set in the two-dimensional display mode, a planar image shown in Fig. 17 is displayed.
Although the sea bottom map database 19 is used to display the sea bottom landform image 35 in the embodiment described above, a sea bottom landform may be stereoscopically displayed by applying processing to data of echoes obtained by scanning without using the sea bottom map database 19.
Although a threshold value is set for a signal level of an echo in the case of the history display shown in Figs. 7 and 9 in the embodiment described above, a threshold value may be set in the case of the usual display shown in Figs. 3 and 13.
It is also possible to apply the invention to a scanning sonar of an automatic tracking system that performs automatic tracking by changing a tilt angle according to movement of a shoal of fish. In this case, even when the shoal of fish moves irregularly, by adopting the history display of the invention, it is possible to accurately display a history of a fish shoal image with a function of the automatic tracking. A scanning sonar having such an automatic tracking function is described in, for example, JP-A-2003-315453.
It is also possible to display moving histories 34a to 34c of a shoal of fish together with moving histories 31a to 31c of a ship as shown in Fig. 15 by making practical use of the history display in the invention. Reference numerals 32a to 32c denote moving histories of an umbrella-shaped beam and 36a to 36c denote moving histories of an isobath. Although a scan area of a fan-shaped beam and a sea bottom landform image are not displayed in Fig. 15. the scan area and the sea bottom landform image may be displayed.
According to the invention, even a user who does not have much experience can three-dimensionally grasp a state in the water according to a stereoscopic image.