Underwater detection system for determining carrier frequencies

Details Underwater detection system for determining carrier frequencies Underwater detection system for determining carrier frequencies

1994-07-29

1996-10-01

Lobo, Ian J.

Communications, electrical: acoustic wave systems and devices

Echo systems

Speed determination

367 99, 367110, 367125, G01S 1558, G01S 1596, G01S 752, G01S 762

Patent

active

055616410

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

This invention relates to an underwater detection system which can determine carrier frequencies (including angular frequencies) of ultrasonic signals arriving from a wide area of search and, more particularly, to an underwater detection system capable of presenting detected targets in different colors depending on their moving velocities in accordance with the measured carrier frequencies, for example.


BACKGROUND ART

Conventional sonars which present detected echoes in accordance with echo level information have such drawbacks as: saturation phenomenon; become impossible to distinguish; and intensity level.
Various improvements have so far been made to overcome these problems, including the use of a sonar receiver having a wide dynamic range, provision of an automatic gain control (AGC) and development of new signal processing techniques. Despite such efforts, if there is no level difference in received signals as stated in point 3 above, it is by no means possible to discriminate between them by using signal level information alone.
Should there exist a school of swimming fish, ultrasonic echoes returned from the fish school are shifted in frequency due to the so-called Doppler effect. If this shift in frequency (or in phase), known as the Doppler shift, can be detected without jeopardizing the sonar's ability to provide high-speed scanning (or real-time processing) and a wide searching area as well as its bearing and range resolutions, it would be possible to discriminate true targets among ultrasonic echoes from fixed targets such as the sea bottom. It would also be possible to recognize differences in moving velocities of fish schools, resulting in a significant improvement in target discriminating capabilities. For this reason, certain types of conventional sonars provide the ability to determine amounts of Doppler shifts (refer to Japanese Patent Publication No. 57-29975, for example).
Those conventional systems, however, have just been able to measure amounts of Doppler shifts of ultrasonic echoes arriving from only one particular direction and not provide high-speed measurement capabilities for Doppler shifts of ultrasonic echoes arriving from all directions. This is the reason why there has not been available yet a sonar capable of presenting moving velocities of underwater targets based on frequency information. It has long been desired therefore to develop a system that can provide real-time measurements of moving velocities of multiple fish schools throughout a wide searching area.


SUMMARY OF THE INVENTION

This invention is directed to solve the aforementioned problems of the conventional systems. Accordingly, it is an object of the invention to provide an underwater detection system which can derive frequency information from received echoes at high speed and present the frequency information in a manner which would enhance target discriminating capabilities.
An underwater detection system according to the present invention may include 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 .theta. relative to the first ultrasonic receiving beam; detector means for detecting individual signals picked up by the two ultrasonic receiving beams produced by the ultrasonic receiving means; delay means for delaying the signal picked up by the first ultrasonic receiving beam, which precedes the second ultrasonic receiving beam by a time period .tau. due to the angular displacement .theta., by the same time period .tau. at an output stage of the ultrasonic receiving means or the detector means; phase difference calculating means for determining the phase difference between the two signals detected by the detector means; and calculating means for calculating carrier frequency f of the received ultrason

REFERENCES:
patent: 3581192 (1971-05-01), Miura et al.
patent: 3711822 (1973-01-01), Muller
patent: 4282589 (1981-08-01), Evetts et al.
patent: 4686659 (1987-08-01), Yamamoto
patent: 5031157 (1991-07-01), Anderson et al.

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