Underwater detection system for determining carrier frequencies of signals arriving from a wide area

• US 5,561,641 A
• Filed: 07/29/1994
• Issued: 10/01/1996
• Est. Priority Date: 02/06/1992
• Status: Expired due to Term

• Current Assignee: Furuno Electric Co, Ltd
• Original Assignee: Furuno Electric Co, Ltd
• Inventors: Kawai, Masato, Nishimori, Yasushi, Yoshioka, Takashi, Fukuoka, Itsuo, Hayashi, Tatsuo
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US08/256,989
• Time in Patent Office: 795 Days
• Field of Search: 367/87, 367/90, 367/99, 367/101, 367/110, 367/125, 367/103
• US Class Current: 367/90
• CPC Class Codes: G01S 15/58 Velocity or trajectory determination systems; Sense-of-movement determination systems velocity measurement in imaging systems G01S15/8979
• CPC Class Codes: G01S 15/96 for locating fish
• CPC Class Codes: G01S 3/80 using ultrasonic, sonic or infrasonic waves
• CPC Class Codes: G01S 3/805 using adjustment of real or effective orientation of directivity characteristics of a transducer or transducer system to give a desired condition of signal derived from that transducer or transducer system, e.g. to give a maximum or minimum signal
• CPC Class Codes: G01S 7/539 using analysis of echo signal for target characterisation; Target signature; Target cross-section
• CPC Class Codes: G01S 7/54 with receivers spaced apart
• CPC Class Codes: G01S 7/6263 in which different colours are used

Abstract

The present invention is directed to provide an underwater detection system which can determine frequency components contained in echoes arriving from all azimuthal directions at high speed and enhance target discriminating capabilities by presenting such frequency components.

The underwater detection system rotates a pair of ultrasonic receiving beams having a fixed phase angle difference, measures phase differences between two signals picked up by both ultrasonic receiving beams, determines carrier frequencies contained in target echoes from the phase differences, derives magnitudes and directions of moving velocities of targets from the carrier frequencies, and displays such target data in different colors. Furthermore, it provides such information as over-the-ground velocities of targets themselves upon compensating target velocity data with ship velocity data.

Claims

1. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam and a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beams produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage of one of said ultrasonic receiving means and said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/(2π

τ

);

a frequency/color conversion circuit for reading out specific color data depending on individual values of measured carrier frequencies f; and

a display unit for presenting the color data.

3. An underwater detection system according to claim 1 further comprising:

a variance calculating circuit for deriving variance data indicating the degree of variations in carrier frequency f fed from said carrier frequency measuring circuit; and

a frequency-variance/color conversion circuit for reading out specific color data depending on individual values of measured carrier frequencies f and variance data.

5. An underwater detection system according to claim 1 wherein the conversion circuit utilizes amplitude information included in input data as a reference, whereby only such input data that exceed a specified amplitude level are converted into color data.

6. An underwater detection system according to claim 1 further comprising:

storage means for storing individual carrier frequencies f fed from said carrier frequency measuring circuit;

a designating unit for specifying a desired area on said display unit;

data readout means for reading out data on carrier frequencies f from such memory locations of said storage means that correspond to the area specified by said designating unit;

a frequency histogram generator for generating a histogram upon determining the distribution of carrier frequencies f based on the data read out by said data readout means; and

display means for presenting the histogram.

7. An underwater detection system according to claim 1 wherein said display unit can simultaneously present at least two different pictures on a single screen.

30. An underwater detection system according to claim 1 wherein the color conversion circuit thereof is configured with a read-only memory (ROM) storing color data corresponding to individual values of input data.

37. An underwater detection system according claim 1 wherein said ultrasonic receiving means is configured with a pair of ultrasonic transducer elements which are rotated mechanically.

38. An underwater detection system according to claim 1 wherein said ultrasonic receiving means includes a plurality of ultrasonic transducer elements arranged in a circular array, selects a predefined number of adjacent transducer elements while shifting the selection in sequential steps by electrical switching, produces the first ultrasonic receiving beam by synthesizing the phases of signals received by each selection of transducer elements, and further produces the second ultrasonic receiving beam separated by the angular displacement θ

from the first ultrasonic receiving beam in a similar manner.

39. An underwater detection system according to claim 1 wherein said ultrasonic receiving means includes a plurality of ultrasonic transducer elements arranged in a linear array, selects a predefined number of adjacent transducer elements while shifting the selection in sequential steps by electrical switching, produces the first ultrasonic receiving beam by synthesizing the phases of signals received by each selection of transducer elements, and further produces the second ultrasonic receiving beam separated by the angular displacement θ

from the first ultrasonic receiving beam in a similar manner.

40. An underwater detection system according to claim 1 wherein said carrier frequency measuring circuit includes scanning means for steering the first and second ultrasonic receiving beams switchably from full-circle scanning to sector scanning operation and this switching function provides a higher sampling rate and consequent resolution enhancement.

41. An underwater detection system according to claim 1 wherein said carrier frequency measuring circuit further comprises:

second delay means which regards the first and second ultrasonic receiving beams as having a mutual angular displacement of 2π

-θ and

introduces a time delay τ

2 corresponding to the angular displacement 2π

-θ

into the signal received by one ultrasonic receiving beam which precedes the other ultrasonic receiving beam by τ

2;

second phase difference calculating means for determining the phase difference Δ

ψ

'"'"' between the signal output from said second delay means and the signal received by the other ultrasonic receiving beam; and

frequency calculating means for calculating carrier frequency f2=Δ

ψ

'"'"'/(2π

τ

2) from the phase difference Δ

ψ

'"'"' and outputting a true carrier frequency at which the carrier frequency f and the carrier frequency f2 coincide with each other.

42. An underwater detection system according to claim 1 wherein said carrier frequency measuring circuit further comprises:

second delay means which regards the first and second ultrasonic receiving beams as having a mutual angular displacement of 2π

+θ and

introduces a time delay τ

3 corresponding to the angular displacement 2+θ

into the signal received by one ultrasonic receiving beam which precedes the other ultrasonic receiving beam by τ

3;

second phase difference calculating means for determining the phase difference Δ

ψ

'"'"' between the signal outputted from said second delay means and the signal received by the other ultrasonic receiving beam; and

frequency calculating means for calculating carrier frequency f2=Δ

ψ

'"'"'/(2π

τ

3) from the phase difference Δ

ψ

'"'"' and outputting a true carrier frequency at which the carrier frequency f and the carrier frequency f2 coincide with each other.

43. An underwater detection system according to claim 1 wherein said carrier frequency measuring circuit further comprises:

second delay means which regards the first or second ultrasonic receiving beam in two successive scanning cycles as two ultrasonic receiving beams having a mutual angular displacement of 2π

for introducing a time delay τ

4 corresponding to the angular displacement 2π

into the signal received by the ultrasonic receiving beam which precedes the other ultrasonic receiving beam by τ

4;

second phase difference calculating means for determining the phase difference Δ

ψ

'"'"' between the signal output from said second delay means and the signal received by the other ultrasonic receiving beam; and

frequency calculating means for calculating carrier frequency f2=Δ

ψ

'"'"'/(2π

τ

4) from the phase difference Δ

ψ

'"'"' and outputting a true carrier frequency at which the carrier frequency f and the carrier frequency f2 coincide with each other.

51. An underwater detection system according to claim 43 wherein the frequency f1 attributed to the over-the-ground ship velocity is obtained by breaking down a Doppler frequency corresponding to the over-the-ground ship velocity acquired from an external navigational aid into each individual azimuthal component.

2. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam and a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beam produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage or one of said ultrasonic receiving means and said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/2π

τ

;

a variance calculating circuit for deriving variance data indicating the degree of variations in carrier frequency f fed from said carrier frequency measuring circuit;

a variance/color conversion circuit for reading out specific color data depending on individual values of variance data; and

a display unit for presenting the color data.

4. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam accompanied by a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beam produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage of one of said ultrasonic receiving means and said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/(2π

τ

);

a filter-amplitude/color conversion circuit for selecting from amplitude data derived from the signals detected by said detector means only such amplitude data that lie within a desired frequency range and then reading out specific color data depending on individual values of the selected amplitude data; and

a display unit for presenting the color data.

8. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam and a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beam produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage or said ultrasonic receiving means or said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/2π

τ

; and

an over-the-ground ship velocity compensation circuit for calculating carrier frequency fe representative of the over-the-ground velocity of a target itself by subtracting frequency f1 attributed to the over-the-ground ship velocity from each carrier frequency f fed from said carrier frequency measuring circuit.

10. An underwater detection system according to claim 8 wherein the frequency f1 attributed to the over-the-ground ship velocity is obtained by breaking down a Doppler frequency corresponding to the over-the-ground ship velocity acquired from such external equipment as a navigational aid into each individual azimuthal component.

11. An underwater detection system according to claim 8 wherein the frequency f2 attributed to the through-the-water ship velocity is obtained by breaking down a Doppler frequency corresponding to the through-the-water velocity acquired from such external equipment as a water current measuring apparatus into each individual azimuthal component.

12. An underwater detection system according to claim 8 wherein the frequency f1 attributed to the over-the-ground ship velocity is determined for each horizontal direction by storing individual carrier frequencies f fed from said carrier frequency measuring circuit into a frame memory, specifying a desired readout area of said frame memory, and sequentially reading out carrier frequencies f from the specified readout area in a predefined order of memory addresses.

13. An underwater detection system according to claim 8 wherein the frequency f1 attributed to the over-the-ground ship velocity is determined for each horizontal direction by storing individual carrier frequencies f fed from said carrier frequency measuring circuit into a frame memory, specifying a desired readout area of said frame memory, sequentially reading out carrier frequencies f from the specified readout area in a predefined order of memory addresses, estimating a cosine curve from the read data, and reading out again carrier frequencies f for individual horizontal directions with reference to the estimated cosine curve.

14. An underwater detection system according to claim 12 wherein each carrier frequency f stored in said frame memory is compensated with the frequency f1 attributed to the over-the-ground ship velocity.

15. An underwater detection system according to claim 12 wherein each carrier frequency f fed from said carrier frequency measuring circuit is compensated with the frequency f1 attributed to the over-the-ground ship velocity.

20. An underwater detection system according to claim 8 further comprising:

a frequency/color conversion circuit for reading out specific color data depending on the value of each carrier frequency fe representative of the over-the-ground velocity of the target itself or each carrier frequency fw representative of the through-the-water velocity of the target itself; and

a display unit for presenting the color data.

21. An underwater detection system according to claim 8 further comprising:

a frequency/color conversion circuit for reading out specific color data depending on the value of each compensated carrier frequency fe representative of the over-the-ground velocity of the target or each compensated carrier frequency fw representative of the through-the-water velocity of the target and also depending on whether such carrier frequency fe or fw, whichever applicable, is higher or lower than the transmitted frequency; and

a display unit for presenting the color data.

22. An underwater detection system according to claim 21 wherein the color data for a plurality of colors are classified into cool and warm color groups, for example, depending on whether each carrier frequency fe or fw, whichever applicable, is higher or lower than the transmitted frequency, and wherein specific colors are assigned in accordance with individual values of carrier frequencies fe or fw in both the cool and warm color groups.

23. An underwater detection system according to claim 8 further comprising variance calculating means for deriving variance data indicating the degree of variations in compensated carrier frequency fe representative of the over-the-ground velocity of each target or compensated carrier frequency fw representative of the through-the-water velocity of each target.

24. An underwater detection system according to claim 23 further comprising:

a variance/color conversion circuit for reading out specific color data depending on individual values of variance data; and

a display unit for presenting the color data.

25. An underwater detection system according to claim 23 further comprising a frequency-variance/color conversion circuit for reading out specific color data depending on individual values of compensated carrier frequencies fe or fw and variance data.

26. An underwater detection system according to claim 8 further comprising:

a filter-amplitude/color conversion circuit for selecting from amplitude data derived from the signals detected by said detector means only such amplitude data that lie within a desired frequency range and then reading out specific color data depending on individual values of the selected amplitude data; and

a display unit for presenting the color data.

27. An underwater detection system according to claim 20 wherein the conversion circuit thereof utilizes amplitude information included in input data as a reference, whereby only such input data that exceed a specified amplitude level are converted into color data.

28. An underwater detection system according to claim 20 further comprising:

storage means for storing compensated carrier frequencies fe representative of over-the-ground target velocities or compensated carrier frequencies fw representative of through-the-water target;

a designating unit for specifying a desired area on said display unit;

data readout means for reading out carrier frequency data from such memory locations of said storage means that correspond to the area specified by said designating unit;

a frequency histogram generator for generating a histogram upon determining the distribution of carrier frequencies based on the data read out by said data readout means; and

display means for presenting the histogram.

29. An underwater detection system according to claim 20 characterized in that said display unit can simultaneously present at least two different pictures on a single screen.

9. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam and a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beam produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage or said ultrasonic receiving means or said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/2π

τ

; and

a through-the-water ship velocity compensation circuit for calculating carrier frequency few representative of the through-the-water velocity of a target itself by subtracting frequency f2 attributed to the through-the water ship velocity from each carrier frequency f fed from said carrier frequency measuring circuit.

16. An underwater detection system according to claim 9 wherein the frequency f2 attributed to the through-the-water ship velocity is determined for each horizontal direction by storing individual carrier frequencies f fed from said carrier frequency measuring circuit into a frame memory, specifying a desired readout area of said frame memory, and sequentially reading out carrier frequencies f from the specified readout area in a predefined order of memory addresses.

17. An underwater detection system according to claim 9 wherein the frequency f2 attributed to the through-the-water ship velocity is determined for each horizontal direction by storing individual carrier frequencies f fed from said carrier frequency measuring circuit into a frame memory, specifying a desired readout area of said frame memory, sequentially reading out carrier frequencies f from the specified readout area in a predefined order of memory addresses, estimating a cosine curve from the read data, and reading out again carrier frequencies f for individual horizontal directions with reference to the estimated cosine curve.

18. An underwater detection system according to claim 16 wherein each frequency f stored in said frame memory is compensated with the frequency f2 attributed to the through-the-water ship velocity.

19. An underwater detection system according to claim 16 wherein each carrier frequency f fed from said carrier frequency measuring circuit is compensated with the frequency f2 attributed to the through-the-water ship velocity.

31. An underwater detection system comprising:

a carrier frequency measuring circuit including ultrasonic receiving means for receiving ultrasonic signals arriving from a wide searching area while rotating a first ultrasonic receiving beam accompanied by a second ultrasonic receiving beam, wherein the second ultrasonic receiving beam is delayed by a fixed angular displacement θ

relative to the first ultrasonic receiving beam and the first ultrasonic receiving beam precedes the second ultrasonic receiving beam by a time period τ

due to the angular displacement θ

;

detector means for detecting individual signals picked up by the two ultrasonic receiving beams produced by said ultrasonic receiving means;

delay means for delaying the signal picked up by the first ultrasonic receiving beam by the time period τ

at an output stage of said ultrasonic receiving means or said detector means;

phase difference calculating means for determining the phase difference Δ

ψ

between the two signals detected by said detector means; and

calculating means for determining the phase difference between the two signals detected by said detector means; and

calculating means for calculating carrier frequency f of the received ultrasonic signals from the phase difference determined by said phase difference calculating means in accordance with the equation f=Δ

ψ

/(2π

τ

);

measuring means for measuring a Doppler frequency shift contained in the carrier frequency determined by said carrier frequency measuring circuit; and

a velocity vector generating circuit for providing a velocity vector wherein, when a target at point P(x, y, z) in a rectangular coordinate system of which origin O is taken at the point of observation is moving at a velocity V₀ (X₀, Y₀, Z₀) relative to the origin O, said velocity vector generating circuit determines the velocity component V of said target at distance r in the direction of line OP based on the Doppler frequency shift determined by said measuring means, converts coordinates (x, y, z) of the point P and components (X₀, Y₀, Z₀) of the velocity V₀ into polar coordinate parameters (r, α

, β

) and (V₀, A₀, B₀), respectively, produces an equation v=f (r, α

, β

, V₀, A₀, B₀), further produces similar equations for other targets moving at the same velocity V₀ with said target, and estimates target velocity parameters V₀, A₀ and B₀ at the point P from these equations using the least squares method.

32. An underwater detection system according to claim 31 wherein the moving direction of a water current or fish school in each arbitrarily selected fractional area is displayed by way of a graphic symbol like an arrow mark based on the velocity vector.

33. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions on the wide area;

measuring means for measuring Doppler frequency shifts contained in the ultrasonic echo signals; and

a velocity vector generating circuit for providing a velocity vector wherein, when a target at point P(x, y, z) in a rectangular coordinate system of which origin O is taken at the point of observation is moving at a velocity V₀ (X₀, Y₀, Z₀) relative to the origin O, said velocity vector generating circuit;

determines the velocity component v of said target at distance r in the direction of line OP connecting the origin 0 and the P point based on the Doppler frequency shift determined by said measuring means,

produces an equation v=f(OP, V₀),

produces similar equations for other targets moving at the same velocity V₀ with said target, and

estimates the target velocity V₀ at the point P from these equations using the lease squares method.

34. An underwater detection system according to claim 33 wherein the point P and velocity V₀ are expressed in polar coordinates.

35. An underwater detection system according to claim 33 wherein the point P and velocity V₀ are expressed in rectangular coordinates.

36. An underwater detection system according to claim 33 wherein the moving direction and velocity level of a water current or fish school in each arbitrarily selected fractional area are displayed based on the velocity vector.

44. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

frequency measuring means for determining carrier frequencies of the received ultrasonic echo signals;

frequency conversion means for converting the carrier frequencies into Doppler frequency shifts;

variance calculating means for deriving variance data indicating the degree of variations in Doppler frequency shifts fed from said frequency conversion means;

variance/color conversion means for generating specific color data depending on individual values of variance data; and

a display unit for presenting the color data.

45. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

frequency measuring means for determining carrier frequencies of the received ultrasonic echo signals;

variance calculating means for deriving variance data indicating the degree of variations in carrier frequencies fed from said frequency measuring means;

variance/color conversion means for generating specific color data depending on individual values of variance data; and

a display unit for presenting the color data.

46. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area while rotating a first ultrasonic receiving beam and a second ultrasonic receiving beam;

frequency measuring means for determining carrier frequencies of the received ultrasonic echo signals

frequency conversion means for converting the carrier frequencies into Doppler frequency shifts;

detector means for detecting the received ultrasonic echo signals;

selection means for selecting from amplitude data derived from the detected signals fed from said detector means only such amplitude data that are associated with Doppler frequency shifts falling within a desired frequency range;

amplitude/color conversion means for generating specific color-coded signals depending on individual values of the amplitude data selected by said selection means; and

a display unit for presenting the color-coded signals.

47. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

frequency measuring means for determining carrier frequencies of the received ultrasonic echo signals;

detector means for detecting the received ultrasonic echo signals;

selection means for selecting from amplitude data derived from the detected signals fed from said detector means only such amplitude data that are associated with carrier frequencies falling within a desired frequency range;

amplitude/color conversion means for generating specific color-coded signals depending on individual values of the amplitude data selected by said selection means; and

a display unit for presenting the color-coded signals.

48. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

carrier frequency measuring means for determining carrier frequencies f of the received ultrasonic echo signals;

storage means for storing individual carrier frequencies f fed from said carrier frequency measuring means;

a display unit;

designating means for specifying a desired area on said display unit;

data readout means for reading out data on carrier frequencies f from such memory locations of said storage means that correspond to the area specified by said designating means;

a frequency histogram generator for generating a histogram upon determining the distribution of carrier frequencies f based on the data read out by said data readout means; and

display means for presenting the histogram on said display unit.

49. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

carrier frequency measuring means for determining carrier frequencies f of the received ultrasonic echo signals;

over-the-ground ship velocity compensation means for calculating carrier frequency fe representative of the over-the-ground velocity of a target itself by subtracting frequency f1 attributed to the over-the-ground ship velocity from each carrier frequency f fed from said carrier frequency measuring means.

50. An underwater detection system comprising:

transmitting means for transmitting an ultrasonic signal into a wide area;

receiving means for receiving ultrasonic echo signals arriving from all directions of the wide area;

carrier frequency measuring means for determining carrier frequencies f of the received ultrasonic echo signals;

through-the-water ship velocity compensation means for calculating carrier frequency fw representative of the through-the-water velocity of a target itself by subtracting frequency f2 attributed to the through-the-water ship velocity from each carrier frequency f fed from said carrier frequency measuring means.