GB2444161A

GB2444161A - Echo image display apparatus

Info

Publication number: GB2444161A
Application number: GB0722828A
Authority: GB
Inventors: Sanae Nagai; Masaki Ogata; Yasushi Nishimori
Original assignee: Furuno Electric Co Ltd
Current assignee: Furuno Electric Co Ltd
Priority date: 2006-11-22
Filing date: 2007-11-21
Publication date: 2008-05-28
Anticipated expiration: 2027-11-21
Also published as: JP2008128900A; GB0722828D0; JP4781240B2; GB2444161B

Abstract

A scanning sonar measures echoes received from individual geographical points within a sounding area through lapse of time for each of plural layers defined by dividing a specific depth range and calculates mean values of echo levels detected from the individual points for each layer. The scanning sonar then determines the maximum values of the mean values of the echo levels at the individual points calculated for each of the layers and display means then displays image elements at the individual points based upon the determined maximum values. If three layers (1, 2, 3 fig. 4) are defined and a fish school (G1 fig. 4) located at a particular point (Z1 fig. 4) extends vertically from layer 1 to layer 3, the fish school (G1 fig. 4) existing at all depths in layer 2 and producing a maximum mean value of echo levels in layer 2, for example, the scanning sonar presents an echo image element at the point Z1 based on the mean value of the echo levels in layer 2. The apparatus reduces noise in the displayed image without obscuring the desired images (figs 11 to 13).

Description

ECHO IMAGE DISPLAY APPARATUS

The present invention relates to a display apparatus for displaying an echo image obtained by sounding underwater objects through transmission and reception of an ultrasonic (acoustic) wave.

A scanning sonar is an underwater echo-sounding apparatus which transmits an acoustic sounding beam (transmitting beam) underwater through a transducer unit, receives echoes from underwater objects, such as fish schools, by scanning a receiving beam in specified directions and displays an image of the underwater objects based on the received echoes.

Japanese Patent Application Publication Nos. 2006- 162480, 2003-315453 and 1998-90411 describe examples of conventionally known scanning sonars.

The scanning sonar of Japanese Patent Application Publication No. 2006-162480 is configured to generate three-dimensional echo presentation data from echo data obtained by scanning for underwater objects in horizontal scan mode and vertical scan mode and display a three-dimensional image of the underwater objects, such as fish schools and sea bottom features, based on the echo presentation data.

The scanning sonar of Japanese Patent Application Publication No. 2003-315453 is configured to accurately acquire a target and automatically track the target by controlling tilt angle of a horizontal scan sounding beam according to movement of the target based on the target depth detected by vertical scanning.

Japanese Patent Application Publication No. 1998-90411 proposes such an arrangement that scanning sonars are installed on both a primary ship (own ship) and a secondary ship and the scanning sonar installed on the primary ship displays past tracks of both the primary and secondary ships on a screen of a display unit together with target echoes, such as fish echoes, detected by the scanning sonars of both ships along the past tracks thereof, thus permitting easy recognition of horizontal locations, depths and sizes of fish schools and other targets.

In a scanning sonar configured to display an echo image of such targets as detected fish schools on-screen along a plotted track of a ship according to movement thereof as in the aforementioned arrangement of Japanese Patent Application Publication No. 1998-90411, the scanning sonar conventionally employs a peak hold technique in general.

The peak hold technique as used in the scanning sonar for echo image presentation is a signal processing technique in which, if the value of an echo level obtained in a current measurement cycle is larger than the value of an echo level obtained in a preceding measurement cycle, the scanning sonar updates the value of the echo level from the previously obtained value to the currently obtained value to constantly hold a peak value of the echo level and displays the echo image based on the peak value. According to this technique, it would be possible to clearly present echoes of such targets as fish schools which produce high echo levels. If the scanning sonar employing the peak hold technique detects an echo of a non-target object drifting underwater producing a high signal level, however, the echo of this underwater object might be enhanced on-screen and appear as noise which would obscure the target echoes.

Thus, the peak hold technique has a problem that this technique makes it difficult to discriminate true targets

against background noise.

One approach to suppressing the aforementioned kind of noise is an averaging technique in which the scanning sonar adds up the values of echo levels over successive measurement cycles for a specified number of times, divides a sum of the added values of the echo levels by the number of additions to obtain an average value of the echo levels, and displays an echo image based on the average value of the successive echo levels of each echo thus obtained.

Although the averaging technique serves to suppress noise, this approach has a problem that echoes of desired targets displayed on-screen tend to become feeble and unclear if the scanning sonar is set to present target echoes existing in a large depth range. If the scanning sonar is set to present target echoes existing in a depth range of 0 m to 1000 ni, for example, target echoes from all depths (e.g., approximately 50 m and 900 m) are equally averaged.

Accordingly, if a target is present at a depth of approximately 50 m but there is no target at all below this depth, an average value of echo levels of the target at about 50 m detected by the scanning sonar installed on a moving ship would be small, so that the echo of the target displayed on-screen would become unclear.

SUMMARY OF THE INVENTION

The present invention is intended to overcome the aforementioned problems of the prior art. Specifically, it is an object of the invention to provide an echo image display apparatus which can clearly present an image of target echoes while suppressing unwanted echoes, or noise.

According to the invention, an echo image display apparatus for displaying an echo image obtained by sounding underwater objects through transmission and reception of an acoustic wave includes measuring means for measuring echoes received from individual geographical points within a sounding area through lapse of time for each of plural layers defined by dividing a specific depth range, first calculating means for calculating mean values of echo levels detected from the individual points for each of the layers based on results of measurement by the measuring means, second calculating means for determining maximum values of the mean values of the echo levels at the individual points calculated for each of the layers by the first calculating means, and display means for displaying echo image elements at the individual points based on the maximum values of the mean values of the echo levels determined by the second calculating means. The display means displays the echo image while updating the echo image elements at the individual points in accordance with movement of own ship.

The echo image display apparatus thus configured can suppress unwanted speckles of noise by averaging operation and display an echo image based on the maximum values of the mean values of the echo levels detected from the individual layers. This configuration of the invention takes advantage of both the peak hold technique and the averaging technique, making it possible to clearly present an image of target echoes while suppressing unwanted noise.

In one feature of the invention, the echo image display apparatus is configured such that, when own ship is moving, the display means displays the echo image elements for the individual points contained in the sounding area while updating the echo image elements and continues to display the same echo image elements for the individual points which are no longer contained in the sounding area as displayed immediately before these points are left behind the sounding area. According to this feature of the invention, the echo image display apparatus displays current echo image elements for geographical points contained in the sounding area and previously obtained echo image elements for geographical points outside the sounding area.

In another feature of the invention, each of the layers is set to partly overlap the adjacent layers in a depth direction. The echo image display apparatus thus configured can display a clear echo image while preventing a reduction in echo levels when a target like a fish school extends vertically in two or more layers.

In another feature of the invention, the display means displays a grid indicating geodetic coordinates, overlaying the same on the echo image. Conventionally, when examining latitude/longitude of a particular point, an operator moves a cursor to that point on-screen and reads latitude/longitude coordinates displayed at cursor position.

In the echo image display apparatus of the present invention, however, the operator can directly read approximate latitude/longitude of a given point without moving the cursor on-screen.

In another feature of the invention, the display means displays a second echo image produced based on the echo levels detected by the measuring means in addition to the aforementioned first echo image produced based on the maximum values of the mean values of the echo levels.

While the first echo image is continually updated as the maximum mean values of the detected echo levels are calculated from one scan to the next, the mean values of the.echo levels do not vary so rapidly that changes in the first echo image displayed on-screen are rather slow, or moderate. Therefore, the first echo image might not be easy to interpret for users who are accustomed to conventional, ordinary echo images. Since the echo image display apparatus according to this feature of the invention presents the second echo image (ordinary image) in addition to the first echo image, the on-screen presentation is easier to interpret for such users.

In a further feature of the invention, the display means may display the first echo image behind current own ship position and the second echo image at the current own ship position. The echo image display apparatus thus configured displays the first echo image produced by an averaging/peak-hold technique of the invention in areas behind the current own ship position and the second echo image (ordinary image) at the current own ship position.

Alternatively, the display means may display the first echo image behind the current own ship position and the superimposed first and second echo images at the current own ship position. The echo image display apparatus thus configured presents a distinct echo of a fish school produced by the averaging/peak-hold technique of the invention overlaid on the second echo image (ordinary image), so that the user can easily recognize the fish school even when it is difficult to discriminate a true echo from the ordinary image alone.

Preferably, the echo image display apparatus is configured to superimpose the first and second echo images with preset transparency. If the transparency is properly set, it is possible to overlay the first echo image produced by the averaging/peak-hold technique of the invention with desired opacity on the second echo image (ordinary image), for example, allowing the user to easily interpret the superimposed images. This feature of the invention enables the user to discriminate fish schools by producing an on-screen presentation adapted to specific sea area or fish species, for instance.

The echo image display apparatus thus configured determines the maximum values of the mean values of the echo levels detected from the individual layers and displays the echo image based on the maximum values as described above. This configuration of the invention makes it possible to clearly present an image of target echoes while suppressing unwanted noise.

These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed description in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration diagram of a system including a scanning sonar according to a preferred embodiment of the present invention; FIG. 2 is a schematic diagram showing a general principle of underwater echo-sounding by the scanning sonar; FIG. 3 is an electrical block diagram of the scanning sonar; FIG. 4 is a diagram showing a working principle of the present invention; E'IG. 5 is a diagram showing an example of a mesh of latitude/longitude lines used for allocating detected echo level values to corresponding layers; FIG. 6 is a diagram showing how an echo image displayed on-screen is updated in accordance with movement of own ship; FIG. 7 shows an example of an echo image displayed on-screen according to the present invention; FIG. 8 shows an example of an echo image obtained by a conventional peak hold technique; FIG. 9 shows an example of an echo image obtained by a conventional averaging technique; FIG. 10 shows an example of an echo image obtained by an averaging/peak-hold technique of the present invention; FIG. 11 shows an example of an echo image actually displayed on-screen with simulated echoes according to the present invention; FIG. 12 shows an example of an echo image actually displayed on-screen with simulated echoes using the averaging technique; FIG. 13 shows an example of an echo image actually displayed on-screen with simulated echoes using the peak hold technique; FIG. 14 shows an example of on-screen presentation according to another embodiment of the present invention; FIG. 15 shows an example of onscreen presentation according to still another embodiment of the present invention; FIG. 16 is a flowchart showing a procedure of echo image presentation according to the present invention; FIG. 17 is a diagram showing an example of echo clipping; FIGS. 18A to 18C are diagrams showing relationships between the echo image displayed on-screen and grid line intervals; and FIG. 19 is a system configuration diagram of a system according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED

EODIMENTS OF THE INVENTION

FIG. 1 is a system configuration diagram of a system according to a preferred embodiment of the present invention, in which designated by the reference numeral 1 is a scanning sonar for sounding an underwater area and displaying an echo image, designated by the reference numeral 2 is a navigational aid (or a group of navigational aids) including a global positioning system (GPS) receiver and a gyroscope, for instance, designated by the reference numeral 3 is an echo sounder for measuring water depth, and designated by the reference numeral 4 is a current meter for measuring speed and direction of a water current.

FIG. 2 is a schematic diagram illustrating a general principle of underwater echo-sounding by the scanning sonar 1 which is installed on a ship 5 (own ship) . Referring to FIG. 2, designated by the reference numeral 6 is a transducer unit of the scanning sonar 1, designated by the reference numeral 7 is an acoustic beam (transmitting beam) emitted underwater from the transducer unit 6, designated by the reference numeral 8 is a receiving beam for receiving echoes reflected from underwater objects like fish schools, and designated by the reference numeral 9 is a water surface. The scanning sonar 1 simultaneously transmits acoustic waves in all directions around the transducer unit 6 to form the transmitting beam 7 having an omnidirectional umbrellalike beam pattern inclined obliquely downward by a specified tilt angle. The receiving beam 8 is a relatively narrow directional sounding beam which is formed by using a phased array technique and steered around the transducer unit 6 at high speed to scan a full-circle (3600) area in a spiral pattern.

The scanning sonar 1 receives return echoes by the receiving beam 8 and analyzes echo signals to obtain underwater information, such as distribution and movements of fish schools in a broad area. The navigational aid 2, the echo sounder 3 and the current meter 4 shown in FIG. 1 are together installed on the ship 5.

FIG. 3 is an electrical block diagram of the scanning sonar 1, in which designated by the reference numeral 10 is one of vibrating elements of the transducer unit 6, each of the vibrating elements 10 constituting part of a transmit/receive channel Ch (Chi, Ch2, Ch3) . Only the transmit/receive channel Chi is described below as all of the transmit/receive channels Ch are configured in the same way. As shown in FIG. 3, the transmit/receive channel Chi includes a transmit/receive circuit 11, a transmitter circuit 12 and a receiver circuit 14. The transmit/receive circuit 11 switches the transmit/receive channel Chi by alternately connecting the vibrating element 10 to the transmitter circuit 12 and the receiver circuit 14 over successive transmit and receive cycles. The transmitter circuit 12 delivers a pulse-width-modulated transmitting signal to the vibrating element 10 every transmit cycle whereas the receiver circuit 14 performs various processing operations, such as amplification and noise suppression, on a received echo signal. The transmit/receive channel Chi further includes an analog-to-digital (A/D) converter 15 for converting a processed echo signal output from the receiver circuit 14 from an analog form into a digital form as well as an interface 16 for establishing interconnection between upstream and downstream stages. The interfaces 16 of the individual transmit/receive channels Ch are together connected to another interface 17 for interconnection to upstream circuits.

Referring again to FIG. 3, the scanning sonar 1 includes a transmitting beamformer 18, an operating panel 19 and a central processing unit (CPU) 20. The transmitting beamformer 18 calculates phase delays and weight values of transmitting signals to be fed into the vibrating elements 10 of the individual transmit/receive channels Ch every transmit cycle to form the aforementioned umbrellalike transmitting beam 7. The operating panel 19 is provided with various keys and controls which allow an operator to set the tilt angle of the transmitting beam 7, select options on a display menu and enter later-described various settings. The CPU 20 serves as a control unit for performing overall control of the scanning sonar 1. The scanning sonar 1 further includes a receiving beamformer 21 and an image processor 22. The receiving beamformer 21 calculates phases and weights of the received echo signals output from the individual vibrating elements 10 and combines the received echo signals to produce a synthesized echo signal. The image processor 22 demodulates and samples the echo signal output from the receiving beamfornier 21 to produce echo data and performs later-described image processing operation on the echo data thus obtained.

The scanning sonar 1 further includes a display unit 23 and a memory 24. The display unit 23 is a display device like a liquid crystal display (LCD), for instance, which presents the underwater information on fish schools and other objects detected in a specified sounding area in the form of an echo image based on image data (echo data) generated by the image processor 22. The memory 24 constitutes a storage device for storing an image display program 25 which controls on-screen presentation of the echo image. The memory 24 has a data area 26 for storing echo level data to be mapped on a later-described mesh of parallels (latitude lines) and meridians (longitude lines) The memory 24 also has storage areas (not shown) for storing various programs and control parameters which are not directly concerned with the present invention.

The scanning sonar 1 thus structured constitutes one example of an echo image display apparatus of the present invention. Specifically, circuit blocks shown in FIG. 3 excluding the image processor 22 and the display unit 23 together constitute an example of measuring means mentioned in the appended claims. Also, the image processor 22 shown in FIG. 3 constitutes examples of first and second calculating means mentioned in the appended claims, while the display unit 23 constitutes an example of display means mentioned in the appended claims.

Although not illustrated in FIG. 3, the navigational aid 2, the echo sounder 3 and the current meter 4 shown in FIG. 1 are connected to the scanning sonar 1. Additionally, a motion sensor is connected to the scanning sonar 1 for detecting motion of the ship 5 and controlling the transmitting beam 7 and the receiving beam 8 so that the transmitting and receiving beams 7, 8 are always oriented in specified directions. The navigational aid 2, the echo sounder 3, the current meter 4 and the motion sensor may be of conventional types.

FIG. 4 is a diagram illustrating a working principle of the present invention. According to the invention, a depth range (sounding range) is divided into a plurality of layers. For example, a depth range of 0 m to 100 m is divided into four 25-rn regions, and layers 1, 2 and 3 are defined, layer 1 covering 0 rn to 50 rn, layer 2 covering 25 m to 75 m, and layer 3 covering 50 m to 100 m. As shown in FIG. 4, layer 2 overlaps parts of two adjacent layers 1 and 3 for reasons which will be explained later in this

Specification.

The scanning sonar 1 measures echo levels at individual geographical points within a selected sounding area through lapse of time and calculates mean values of the detected echo levels at the individual points for each layer. Referring to FIG. 4, designated by Gl, G2 and G3 are fish schools and designated by Zl, Z2 and Z3 are points in the sounding area where these fish schools Gl-G3 are present, respectively. The points Z1-Z3 are locations on the earth expressed in geodetic coordinates in terms of latitudes and longitudes. Withrespect to the fish school Gl located at the point Zl, for instance, the scanning sonar 1 measures levels of echo signals received from different portions of the fish school Gl at specified depth intervals and allocates values of the obtained echo levels to corresponding layers to which individual depths belong.

For example, an echo level value obtained from a 20-rn depth is allocated to layer 1, an echo level value obtained from a 30-rn depth is allocated to both layers 1 and 2, an echo level value obtained from a 60-rn depth is allocated to both layers 2 and 3, and an echo level value obtained from an 80-rn depth is allocated to layer 3. The scanning sonar 1 adds the newly obtained echo level value to the sum of echo level values at the point Zl previously accumulated in each successive scan (measurement cycle) and calculates a current mean value of the echo levels by dividing the updated sum of the echo level values by the number of additions thereof for each of layers 1, 2 and 3.

After calculating the mean values of the echo levels detected from the individual layers at each geographical point as described above, the scanning sonar 1 determines a maximum value of the mean values of the detected echo levels. In the example of FIG. 4, the fish school Gi extends vertically from layer 1 to layer 3. While layers 1 and 3 each include a depth range in which the fish school Gi is not present, the fish school Gi is present at all depths in layer 2 as illustrated. Therefore, when the echo level values detected at individual depths are allocated to corresponding layers 1, 2 and 3, the mean value of the echo levels allocated to layer 2 becomes maximum. Thus, the scanning sonar 1 extracts the mean value of the echo levels of layer 2 as a maximum value of the mean values of the echo levels at the point Zi and presents an echo image element at the point Zi according to this maximum value.

The scanning sonar 1 determines maximum values of mean echo levels of the fish schools G2 and G3 located respectively at the points Z2 and Z3 shown in FIG. 4 and presents echo image elements at the points Z2 and Z3 in a similar fashion. As will be understood from the above discussion, the fish school G2 produces a maximum mean value of the echo levels in layer 3 so that the scanning sonar 1 presents the echo image element at the point Z2 according to this maximum mean value. Also, the fish school G3 produces a maximum mean value of the echo levels in layer 1 so that the scanning sonar 1 presents the echo image element at the point Z3 according to this maximum mean value.

The scanning sonar 1 determines a maximum value of echo levels calculated for the individual layers at each geographical point and presents an echo image based on the maximum values thus determined. The above-described echo level averaging technique makes it possible to suppress unwanted echoes (noise) . Referring to FIG. 4, even if there is some underwater object other than a fish school at the point Zi in layer 1, drifting approximately 10 m deep, for example, and an unwanted echo (noise) from this object is detected, the scanning sonar 1 does not extract echo data having a small mean value of received echo levels from layer 1, so that the scanning sonar 1 does not present the unwanted echo (noise) from this kind of drifting object at the point Zi on-screen. Instead, the scanning sonar 1 presents an echo image element at each geographical point using the maximum mean value of the echo levels calculated for the individual layers so that a fish school at the relevant point, if any, can be shown clearly and distinctly.

For example, since the fish school Gi extends vertically through the entirety of layer 2 as illustrated in FIG. 4, the mean value of the echo levels of the fish school Gi detected in layer 2 at the point Zi is extracted as the maximum mean value which is large enough to produce a clear fish school image. If layer 2 is not set to overlap two adjacent layers 1 and 3 unlike the example of FIG. 4 but layer 3 shown in FIG. 4 is redesignated as "layer 2," the fish school Gi would extend through two (not three) layers.

In this case, the maximum mean value of the echo levels detected in the individual layers would decrease, causing the echo of the fish school Gi to become feeble. In contrast, the scanning sonar 1 of the present embodiment is configured such that layers 1, 2 and 3 overlap one another as depicted in FIG. 4 to overcome the aforementioned problem.

In determining a maximum value of echo levels at each geographical point, the scanning sonar 1 of the embodiment uses the earlier-mentioned mesh of latitude/longitude lines to aid in allocating measured echo level values to the corresponding layers. FIG. 5 shows an example of a mesh 40 of latitude/longitude (L/L) lines. This mesh 40 (hereinafter referred to as the L/L mesh 40) shows geographical locations on the earth expressed in latitude/longitude coordinates. The L/L mesh 40 is formed for each layer and data on the L/L mesh 40 is stored in the data area 26 of the memory 24 shown in FIG. 3.

Specifically, the data area 26 stores the result of additions (sum) of echo levels and the number of additions thereof for each layer, in which the echo levels are determined (measured) at specified depth intervals within a predefined depth range based on intensities of the received echo signals.

The L/L mesh 40 shown in FIG. 5 is a mesh for layer N, in which blank circles designated by "a" through "e" represent data on echo level values measured at a particular depth in layer N. Based on these data, the scanning sonar 1 calculates echo level values at intersection points P1, P2, P3 and so forth of the latitude/longitude lines and maps the calculated echo level values on the L/L mesh 40. For example, the intersection point P1 lies within a triangle iabc so that the scanning sonar 1 calculates an echo level value to be allocated to the intersection point P1 from the data on the echo level values marked "a", "b" and "c" by way of linear interpolation. Similarly, the intersection point P2 lies within a triangle acd so that the scanning sonar 1 calculates an echo level value to be allocated to the intersection point P2 from the data on the echo level values marked "a", "c" and "d" by way of linear interpolation. Also, the intersection point P3 lies within a triangle cde so that the scanning sonar 1 calculates an echo level value to be allocated to the intersection point P3 from the data on the echo level values marked "c", "d" and "e" by way of linear interpolation.

The scanning sonar 1 calculates echo level values to be allocated to the intersection points P1, P2, P3 and so forth of the latitude/longitude lines of the L/L mesh 40 for each layer in each successive measurement cycle and adds the newly calculated echo level value to the sum of the previously accumulated echo level values at each point in the aforementioned manner. Each time the newly calculated echo level value is added to the sum of the previously accumulated echo level values, the scanning sonar 1 calculates a current mean value of the echo levels to be allocated to each point by dividing the updated sum of the echo level values by the number of additions thereof for each layer. Further, thescanning sonar 1 compares the mean values of the echo levels detected from the individual layers at each point and extracts a maximum value of the mean values of the echo levels at each point. Subsequently, the scanning sonar 1 maps the maximum values of the mean values off the echo levels thus obtained for individual geographical points on a separately prepared imaging mesh.

The image processor 22 performs color grading operation to assign different colors to the varying maximum mean values mapped on the imaging mesh according to echo levels.

Finally, the display unit 23 presents a color-coded echo image on-screen.

As own ship moves, echo image elements displayed on-screen at individual points in the aforementioned manner are updated from one scan (measurement cycle) to the next, or each time the scanning sonar 1 calculates the maximum mean values of echo levels for the individual points. FIG. 6 is a diagram showing how the echo image is updated in accordance with movement of own ship.

Referring to FIG. 6, indicated by a solid line circle A is a sounding area of the scanning sonar 1 at the beginning of echo-sounding operation. Assuming that the value of echo level detected at a point P in layer N, for example, by a first scan (initial measurement cycle) is La, the mean value of the echo level at the point P is currently La/i = La. If own ship moves as shown by an arrow in FIG. 6, the sounding area shifts as shown by a dashed-line circle B, for example, before a second scan is made. Provided that the value of echo level detected at the point P by the second scan is Lb, the mean value of the echo levels at the point P becomes (La+Lb)/2 at this point in time. If own ship further advances, the sounding area further shifts as shown by a dashed-line circle C, for example, before a third scan is made. Provided that the value of echo level detected at the point P by the third scan is Lc, the mean value of the echo levels at the point P becomes (La-f-Lb+Lc)/3 at this point in time. Mean values of echo levels at individual intersection points of latitude/longitude lines are calculated in the aforementioned manner not only for layer N but also for other layers, so that the mean value of the echo levels detected from each layer at each point is successively updated from one scan (measurement cycle) to the next as own ship advances. Accordingly, the maximum value of the mean values of the echo levels detected at each geographical point is also updated successively in a similar way and, as a result, the echo image displayed on-screen based on the maximum mean values of the echo levels is updated from one scan to the next.

When own ship runs further forward up to a point where the aforementioned point P is left behind the sounding area of the scanning sonar 1 as shown by a dashed-line circle D in FIG. 6, the scanning sonar 1 will no longer receive any echo data from the point P. Then, the mean value of the echo levels in each layer at the point P, and thus the maximum mean value at the point P, is no longer updated and the echo image element presented for the point P remains unchanged from the maximum mean value obtained immediately before the point P is left behind the sounding area.

The scanning sonar 1 of the present embodiment displays an echo image element for every geographical point contained in the sounding area while updating the echo image element in accordance with movement of own ship, and when a particular geographical point is not longer contained in the sounding area as a result of own ship movement, the scanning sonar 1 remains to display the echo image element obtained from that geographical point immediately before the same geographical point is left behind the sounding area as mentioned above. Accordingly, the scanning sonar 1 displays current echo image elements for geographical points contained in the current sounding area and previously obtained echo image elements for geographical points outside the current sounding area. As the maximum mean values of the received echo levels calculated in each successive scan (measurement cycle) are stored in the memory 24, the scanning sonar 1 can display a past echo image on-screen. While the scanning sonar 1 continually updates the on-screen echo image by calculating the maximum mean values of the echo levels from one scan to the next in the above-described example, the invention is not limited to this arrangement. For example, the scanning sonar 1 may be so configured as to update the echo image every two or more number of scans only, and not in each successive scan.

FIG. 7 shows an example of an echo image 50 displayed on the display unit 23 of the scanning sonar 1. This echo image 50, which is obtained by an averaging/peak-hold technique of the present invention, moves on-screen as shown by an arrow in FIG. 7 according to the movement of own ship. As shown in FIG. 7, a past track 51 of own ship and a grid 52 of latitude/longitude lines are overlaid on the echo image 50. In plotting the past track 51, the scanning sonar 1 uses information on own ship position and speed fed from the navigational aid 2. Black speckles shown in FIG. 7 are echoes 53 of fish schools. The scanning sonar 1 of the present invention displays the echo image 50 clearly showing the echoes 53 with low noise, so that on-screen presentation of the scanning sonar 1 is easy to interpret even when the echo image 50 is shifted to follow the movement of own ship. Conventionally, when examining latitude/longitude of a particular point, an operator moves a cursor to that point on-screen and reads latitude/longitude coordinates displayed at cursor position.

Since the scanning sonar 1 of this embodiment displays the grid 52 of latitude/longitude lines as shown in FIG. 7, the operator can directly read approximate latitude/longitude of a given point without moving the cursor on-screen.

FIGS. 8 to 10 are diagrams schematically showing comparative examples of echo images obtained by the averaging/peak-hold technique of the present invention and conventional techniques. For the sake of simplicity, these Figures each show the echo image obtained by a single scan (measurement cycle) only. Specifically, FIG. 8 shows the echo image obtained by the earlier-discussed conventional peak hold technique. Although the echo image obtained by the peak hold technique clearly shows echoes 31 of fish schools, many speckles of noise 32 appearing on-screen tend to obscure the echoes 31 of the fish schools as previously mentioned. FIG. 9 shows the echo image obtained by the earlier-discussed conventional averaging technique. While the number of speckles of noise 32 appearing on the echo image obtained by the averaging technique greatly decreases as compared to the echo image of FIG. 8, the echoes 31 of the fish schools displayed on-screen tend to become feeble and unclear as illustrated. Shown in FIG. 10 is the echo image obtained by the averaging/peak-hold technique of the present invention. The averaging/peak-hold technique based on the earlier-mentioned working principle takes advantage of both the peak hold technique and the averaging technique to clearly present the echoes 31 of the fish schools while suppressing the speckles of noise 32.

FIGS. 11 to 13 are diagrams showing examples of echo images actually presented on the display unit 23 with simulated echoes using the averaging/peak-hold technique of the present invention and the conventional techniques.

Specifically, FIG. 11 shows the echo image obtained by the averaging/peak-hold technique of the invention, FIG. 12 shows the echo image obtained by the averaging technique, and FIG. 13 shows the echo image obtained by the peak hold technique. It can be seen that fish school echoes shown in FIG. 12 are feeble whereas many speckles of noise are present on the echo image of FIG. 13.

FIG. 14 is a diagram showing an example of on-screen presentation according to another embodiment of the present invention. In the example of FIG. 14, the scanning sonar 1 presents an echo image 50 (first echo image) obtained by the averaging/peak_hold technique of the invention in past sounding areas behind the current own ship position as well as an ordinary echo image 60 (second echo image) obtained based on measured echo levels in a sounding area around the current own ship position. Designated by 60a in FIG. 14 is an echo of a fish school. While the echo image 50 obtained by the averaging/peak-hold technique of the invention is continually updated as the maximum mean values of the detected echo levels are calculated from one scan to the next, mean values of the echo levels do not vary so rapidly that changes in the echo image 50 displayed on-screen are rather slow, or moderate. Therefore, the echo image 50 produced by using the averaging/peak-hold technique might not be easy to interpret for users who are accustomed to conventional, ordinary echo images. Since the scanning sonar 1 of this embodiment presents the echo image 50 with the ordinary echo image 60 overlaid thereon, the on-screen presentation is easier to interpret for such users. While the echo image 60 is an image obtained in horizontal scan mode for sounding all around the transducer unit 6 in the example of FIG. 14, the embodiment may be adapted to present an echo image in slant mode covering a semicircular sounding area in a slanted plane or an echo image produced by a plan position indicator (PPI) sonar employing a mechanically trained transducer unit. Also, while the ordinary echo image 60 is displayed around the own ship position in the example of FIG. 14, the embodiment may be so modified as to present an echo image obtained by the averaging/peak-hold technique around the own ship position as shown in FIG. 7 and the ordinary echo image 60 at another position.

FIG. 15 is a diagram showing an example of on-screen presentation according to still another embodiment of the present invention. In this example, the scanning sonar 1 presents the echo image 50 obtained by the averaging/peak-hold technique of the invention overlaid with the ordinary echo image 60 around the current own ship position. This form of presentation allows for easier recognition of fish schools. For example, the on-screen presentation of FIG. shows an echo 60a which is probably a fish school within the echo image 60. Even when it is uncertain whether this echo 60a is a true fish school, the scanning sonar 1 of this embodiment presents a distinct echo 53 overlaid on the echo 60a within the echo image 60, so that a user can easily recognize at a glance that this echo 60a represents a true fish school.

When the two echo images 50, 60 are overlaid as shown in FIG. 15, it is preferable to preset a transparency through the operating panel 19 (FIG. 3) for each echo image superimposed on-screen and overlay the echo images 50, 60 according to respective transparency settings. If the transparency is properly set for each of the echo images to be superimposed, it is possible to overlay the echo image obtained by the averaging/peak-hold technique of the invention with desired opacity on top of the ordinary echo image 60, for example, allowing the user to easily interpret the superimposed images. The transparency of each echo image may be determined based on sea area or species of fish, for instance. This approach enables the user to recognize the sea area or fish species with ease and accuracy since the individual echo images are displayed according to the transparency settings. The echo images may be overlaid in any order in this embodiment, so that the ordinary echo image 60 may be overlaid on top of the echo image 50 obtained by the averaging/peak-hold technique of the invention.

FIG. 16 is a flowchart showing a procedure of echo image presentation according to the present invention. In step Si, the Cpu 20 sets a plurality of layers in a specified depth range (or sounding range) . When setting the layers, the CPU 20 makes the data area 26 of the memory 24 usable for storing echo level data to be mapped on the L/L mesh 40 for each layer. Next, in step S2, the CPU 20 resets the sum of echo levels (Levelsum) and the number n of additions of the echo levels previously stored in the data area 26 for each layer to initial values (i.e., LevelSum = 0, n = 0) . Proceeding now to step S3, the CPU examines whether an echo clipping area is defined, where "clipping" is a process of defining (limiting) a display area in which the scanning sonar 1 presents detected echoes on the display unit 23 as a result of the image processing operation. FIG. 17 is a diagram showing an example of echo clipping. In the example of FIG. 17, designated by Ri is the echo clipping area (hatched area) in which the scanning sonar 1 presents the received echoes, designated by R2 is an area affected by a transmission line which will usually appear close to the bottom of own ship, designated by R3 is an area in which a sea bottom echo is detected, and designated by R4 is an area affected by propeller noise which will usually appear behind own ship. If the echo clipping area is set, the scanning sonar 1 displays no echoes in the aforementioned areas R2, R3 and R4. The echo clipping area is defined according to the depth range (or sounding range) and an angular extent (angle 8 shown in FIG. 17) entered through the operating panel 19. The Cpu 20 automatically clips (trims) the echo image within the echo clipping area thus defined according to water depth data received from the echo sounder 3, for instance. The echo clipping area may be set not only automatically but also manually. In the latter case, the water depth data is not required for setting the echo clipping area, so that the echo sounder 3 need not be connected to the scanning sonar 1.

If the echo clipping area is defined (Yes in step S3), the CPU 20 proceeds to step S4 in which the CPU 20 clips the echo image within the echo clipping area. If the echo clipping area is not defined (No in step S3), the CPU 20 skips step S4. In succeeding step S5, the Cpu 20 examines whether the scanning sonar 1 is set to perform level conversion or like operation, where level conversion is an operation for reducing the level of a sea bottom echo signal when the sea bottom is detected so that a bottom echo will not overlie other echoes. The scanning sonar 1 may be set to perform filtering operation for eliminating noise instead of the level conversion operation. The operator can select the level conversion or filtering operation through the operating panel 19. If the level conversion or like operation is selected (Yes in step S5), the Cpu 20 proceeds to step S6 to perform the selected operation. If neither the level conversion nor like operation is selected (No in step S5), the CPU 20 skips step S6. In succeeding step S7, the CPU 20 converts the echo data mapped in an XYZ-coordinate system into echo data mapped in a latitude, longitude and depth coordinate (L/L/D-coordinate) system. The L/L/D-coordinate system is a three-dimensional coordinate system expressed in L/L coordinates and a depth coordinate.

Proceeding now to step S8, the CPU 20 adds a newly obtained echo level value Li to the previously calculated sum LevelSum of accumulated echo levels to update the sum LevelSum for each geographical point in each layer as follows: LevelSum = LevelSum + Li In this step, the CPU 20 also increments the number n of additions of the echo levels as follows: n=n+l In succeeding step S9, the CPU 20 calculates the mean value (Average) of the echo levels for each geographical point by dividing the updated sum LevelSum of the echo level values by the number n of additions of the echo levels as follows: Average = LevelSum/n (if n # 0) Average = 0 (if n = 0) The CPU 20 performs averaging operation by executing steps S8 and S9 above.

Upon executing the averaging operation, the CPU 20 proceeds to step Sb to perform peak hold operation, in which the CPU 20 determines a maximum value of the mean values of the echo levels obtained for all layers at each geographical point by the aforementioned averaging operation. In succeeding step Sib, the CPU 20 checks out whether the ordinary echo image 60 should be displayed.

The scanning sonar 1 allows the operator to choose in advance whether to display the ordinary echo image 60 through the operating panel 19. If the ordinary echo image is to be displayed (Yes in step Sib), the CPU 20 proceeds to step S12 to perform conventionally known ordinary echo image processing operation. If it is not necessary to display the ordinary echo image 60 (No in step Sil), the CPU 20 skips step S12.

In succeeding step S13, the CPU 20 produces and displays an echo image like the one shown in FIG. 7 based on individual maximum values of the mean values of the echo levels obtained by the peak hold operation of step Sb. In a case where the ordinary (conventional) echo image processing operation was executed in step S12 above, the echo image obtained by the averaging/peak-hold technique of the present invention in step S13 is overlaid on the ordinary echo image 60 as shown in FIGS. b and 15, for example. While the CPU 20 executes step S13 after steps Sil and S12 as discussed above, the procedure of FIG. 16 may be modified such that the CPU 20 executes step S13 before step Sil. If the procedure is so modified, the ordinary echo image 60 is overlaid on the echo image obtained by the averaging/peak-hold technique of the present invention. Finally, the CPU 20 displays the grid 52 of latitude/longitude lines and other additional information in step S14 and completes the procedure.

It is recommended to use the azimuthal equidistant projection in producing the grid 52 displayed on-screen.

The azimuthal equidistant projection is advantageous in that distortion is small even in high-latitude areas, allowing the operator to recognize exact distances and directions and intuitively determine locations of fish schools with ease. The method of projection is not limited to the azimuthal equidistant projection, however. For example, it is possible to use the equal-area cylindrical projection in this invention. Even when the equal-area cylindrical projection is used, the scanning sonar 1 provides substantially the same presentation as would be obtained by the azimuthal equidistant projection, because the sounding area displayed on-screen is extremely small as compared to the entire earth even when a large display range is selected.

FIGS. 18A to 18C are diagrams showing relationships between the echo image displayed on-screen and the grid 52.

Specifically, FIG. 18A shows the relationship between the echo image and the grid 52 when the tilt angle is 00, and FIG. 18B shows the relationship when the tilt angle is 45 .

In the examples shown in FIGS. 18A and 183, the on-screen echo image is reduced in size but intervals of crossed parallel lines of the grid 52 are kept unchanged even when the tilt angle is increased. This means that the echo image is displayed in size in accordance with the intervals of the crossed parallel lines of the grid 52 in the example of FIG. 183. In contrast, FIG. 18C shows an example in which the echo image obtained atthe tilt angle of 45 is displayed in the same size as in FIG. 18A but the intervals of the crossed parallel lines of the grid 52 is increased.

This means that the grid 52 is presented in accordance with the size of the echo image in the example of FIG. 18C.

According to the invention, the echo image size may be varied while keeping the grid line intervals unchanged or the grid line intervals may be varied while keeping the echo image size unchanged as described above.

The aforementioned principle of the present invention can be implemented in various other ways than described in the foregoing discussion. For example, although the echo image is displayed on the display unit 23 of the scanning sonar 1 in the foregoing embodiment, the echo image may be displayed on a display screen of a personal computer connected on-line to the scanning sonar 1 as shown in FIG. 19. The invention is applicable not only to an on-line system like the one shown in FIG. 19 but also to an off-line system. For example, an off-line system may be so configured that a user can record data collected by the onboard scanning sonar 1 on such storage media as an optical disc and reproduce echo images on a display screen of a personal computer in a laboratory to analyze the collected data at a later time.

Also, locations of echoes detected by past scans may be automatically shifted in accordance with water current data supplied from the current meter 4. If the averaging operation and the peak hold operation are performed on the echo data shifted by the water current data, the user will be able to know the current locations of fish schools even when the fish schools are displaced by a water current.

Although the scanning sonar 1 clips the echo image displayed on-screen by setting a depth range and an angular extent (FIG. 17) in the foregoing embodiment, it is impossible to remove propeller noise by simply defining an angular extent when the ship 5 makes a large turn. To avoid this inconvenience, the scanning sonar 1 may be configured to clip the echo image to cut off an angular portion thereof in which the moving scanning sonar 1 leaves a wake, and thus propeller noise, behind.

Although a selected depth range (sounding range) is divided into a plurality of equal depth layers (FIG. 4) in the foregoing embodiment, the layers need not necessary be equal. Additionally, the layers may be set to overlap in any desired form. Furthermore, although the layers shown in FIG. 4 extend horizontally to cover all geographical points in the display area as indicated in FIG. 4, it is possible to set different layers in segmented areas according to the invention.

Moreover, although the system of FIG. 1 is provided with the echo sounder 3 dedicated to measuring the water depth, the scanning sonar 1 may be connected to an onboard fish finder instead of the echo sounder 3 to take in depth data.

Claims

1. An echo image display apparatus for displaying an echo image obtained by sounding underwater objects through transmission and reception of an acoustic wave, said echo image display apparatus comprising: measuring means for measuring echoes received from individual geographical points within a sounding area through lapse of time for each of plural layers defined by dividing a specific depth range; first calculating means for calculating mean values of echo levels detected from the individual points for each of the layers based on results of measurement by said measuring means; second calculating means for determining maximum values of the mean values of the echo levels at the individual points calculated for each of the layers by said first calculating means; and display means for displaying echo image elements at the individual points based on the maximum values of the mean values of the echo levels determined by said second calculating means while updating the echo image elements at the individual points in accordance with movement of own ship.

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2. The echo image display apparatus according to claim 1, wherein, when own ship is moving, said display means displays the echo image elements for the individual points contained in the sounding area while updating the echo image elements and continues to display the same echo image elements for the individual points which are no longer contained in the sounding area as displayed immediately before these points are left behind the sounding area.

3. The echo image display apparatus according to claim 1 or 2, wherein each of the layers is set to partly overlap the adjacent layers in a depth direction.

4. The echo image display apparatus according to any of the preceding claims, wherein said display means displays a grid indicating geodetic coordinates, overlaying the same on the echo image.

5. The echo image display apparatus according to any of the preceding claims, wherein said display means displays a second echo image produced based on the echo levels detected by said measuring means in addition to said first echo image produced based on the maximum values of the mean values of the echo levels.

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6. The echo image display apparatus according to claim 5, wherein said display means displays the first echo image behind current own ship position and the second echo image at the current own ship position.

7. The echo image display apparatus according to claim 5, wherein said display means displays the first echo image behind current own ship position and the superimposed first and second echo images at the current own ship position.

8. The echo image display apparatus according to claim 7, wherein the first and second echo images are superimposed with preset transparency.

9. A method of displaying an echo image obtained by sounding underwater objects through transmission and reception of an acoustic wave, the method comprising: measuring echoes received from individual geographical points within a sounding area through lapse of time for each of plural layers defined by dividing a specific depth range; calculating mean values of echo levels detected from the individual points for each of the layers based on results of measurement by said measuring means; -41 -determining maximum values of the mean values of the echo levels at the individual points calculated for each of the layers by said first calculating means; and displaying echo image elements at the individual points based on the maximum values of the mean values of the echo levels determined by said second calculating means while updating the echo image elements at the individual points in accordance with movement of own ship.

10. Echo image display apparatus substantially as described herein with reference to and as illustrated in the accompanying drawings.

11. A method of displaying an echo image substantially as described herein with reference to the accompanying drawings.

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