Method and system for detecting objects at or below the water's surface

• US 5,646,907 A
• Filed: 08/09/1995
• Issued: 07/08/1997
• Est. Priority Date: 08/09/1995
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Maccabee, Bruce S.
• Primary Examiner(s): Hofsass, Jeffery
• Assistant Examiner(s): Lee, Benjamin C.
• Application Number:
  US08/514,464
• Time in Patent Office: 699 Days
• Field of Search: 367/93, 367/94, 367/909, 367/128, 367/7-10, 367/140, 367/141, 250/492.1, 73/801, 356/432, 356/432 T, 356/850, 340/555-557, 340/565-567
• US Class Current: 367/93
• CPC Class Codes: G01S 15/04 Systems determining presence of a target
• CPC Class Codes: G01S 15/86 Combinations of sonar systems with lidar systems; Combinations of sonar systems with systems not using wave reflection
• CPC Class Codes: G01S 17/04 Systems determining the presence of a target
• CPC Class Codes: G01S 17/86 Combinations of lidar systems with systems other than lidar, radar or sonar, e.g. with direction finders

Abstract

A method and system are provided for detecting objects at or below a surf of a body of water. A focused beam of amplitude varying electromagnetic radiation, e.g., a pulsed laser beam, is directed through the air towards the water'"'"'s surface. Acoustic energy under the water'"'"'s surface is monitored for any acoustic return that may be generated in the water as a result of the beam being incident on either an object at the water'"'"'s surface or the water'"'"'s surface above an object.

Claims

1. A method for detecting an object below a surface of a body of water, comprising the steps of:

radiating a beam of amplitude varying electromagnetic radiation through the air towards said surface, said electromagnetic radiation being of a wavelength absorbed by the water near said surface wherein said beam is converted from said electromagnetic radiation into acoustic energy; and

monitoring, from under said surface, said acoustic energy and an acoustic return generated in the water as a result of said acoustic energy being reflected from an object beneath said surface, wherein the presence of said acoustic return is indicative of the presence of said object and wherein differences between said acoustic energy and said acoustic return are used to characterize said object.

2. A method according to claim 1 wherein said beam of amplitude varying electromagnetic radiation comprises pulses of varying duration.

3. A method according to claim 1 wherein said beam of amplitude varying electromagnetic radiation comprises a time varying series of pulses.

4. A method according to claim 1 wherein said beam of amplitude varying electromagnetic radiation comprises a chirped series of pulses.

5. A method according to claim 1 wherein a wavelength of said electromagnetic radiation is selected from the group of wavelength bands consisting of a wavelength band from 2.5 to 3 microns and a wavelength band from 9 to 11 microns.

6. A method according to claim 1 wherein said beam of amplitude varying electromagnetic radiation comprises pulses of fixed duration.

7. A method according to claim 6 wherein said pulses are equi-spaced in time.

8. A method according to claim 1 further comprising the step of scanning said beam across said surface.

9. A method according to claim 8 wherein said step of scanning comprises the step of repeatedly performing said step of radiating for a plurality of spaced apart locations on said surface.

10. A method according to claim 9 wherein said plurality of spaced apart locations are equi-spaced.

11. A method for detecting an object below a surface of a body of water, comprising the steps of:

pulsing a laser beam to create a pulsed laser beam;

focusing said laser beam to create a focused pulsed laser beam;

directing said focused pulsed laser beam through the air towards said surface such that, if uninterrupted by an object, said focused pulsed laser beam is incident on said surface, said focused pulsed laser beam being of a wavelength absorbed by the water near said surface wherein said focused pulsed laser beam is converted into acoustic energy;

monitoring, from under said surface, said acoustic energy and an acoustic return generated in the water as a result of said acoustic energy being reflected from an object beneath said surface; and

analyzing said acoustic energy and characteristics of said acoustic return, said characteristics of said acoustic return including time of occurrence of said acoustic return, amplitude of said acoustic return, duration of said acoustic return, and frequency spectrum of said acoustic return, wherein the presence of said acoustic return is indicative of the presence of said object and differences between said acoustic energy and said characteristics of said acoustic return are used to characterize said object.

12. A method according to claim 11 wherein said pulsed laser beam comprises pulses of varying duration.

13. A method according to claim 11 wherein said pulsed laser beam comprises a time varying series of pulses.

14. A method according to claim 11 wherein said pulsed laser beam comprises a chirped series of pulses.

15. A method according to claim 11 wherein a wavelength of said pulsed laser beam is selected from the group of wavelength bands consisting of a wavelength band from 2.5 to 3 microns and a wavelength band from 9 to 11 microns.

16. A method according to claim 11 wherein said pulsed laser beam comprises pulses of fixed duration.

17. A method according to claim 16 wherein said pulses are equi-spaced in time.

18. A method according to claim 11 further comprising the step of scanning said focused pulsed laser beam across said surface.

19. A method according to claim 18 wherein said step of scanning comprises the step of repeatedly performing said step of directing for a plurality of spaced apart locations on said surface.

20. A method according to claim 19 wherein said plurality of spaced apart locations are equi-spaced.

21. A system for detecting an object below a surface of a body of water comprising:

a focused beam generator for directing a focused beam of amplitude varying electromagnetic radiation through the air towards said surface such that, if uninterrupted by an object, said focused beam is incident on said surface, said electromagnetic radiation being of a wavelength absorbed by the water near said surface wherein said beam is converted from said electromagnetic radiation into acoustic energy;

an underwater acoustic detection system for monitoring said acoustic energy and an acoustic return generated in the water as a result of said acoustic energy being reflected from an object beneath said surface; and

a processor coupled to said focused beam generator and said underwater acoustic detection system for analyzing said acoustic energy and characteristics of said acoustic return, said characteristics of said acoustic return including time of occurrence of said acoustic return, duration of said acoustic return, and frequency spectrum of said acoustic return, wherein the presence of said acoustic return is indicative of the presence of said object and differences between said acoustic energy and said characteristics of said acoustic return are used to characterize said object.

22. A system as in claim 21 further comprising means for scanning said focused beam across said surface.

23. A system as in claim 21 further comprising a floating platform for maintaining said focused beam generator above said surface.

24. A system as in claim 23 wherein said electromagnetic radiation is pulsed laser radiation.