Radar-acoustic hybrid detection system for rapid detection and classification of submerged stationary articles

• US 6,255,980 B1
• Filed: 11/12/1999
• Issued: 07/03/2001
• Est. Priority Date: 11/12/1999
• 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: Matthews, Anthony D.
• Primary Examiner(s): Sotomayor, John B.
• Application Number:
  US09/443,190
• Time in Patent Office: 599 Days
• Field of Search: 342/22, 342/25, 342/27, 342/28, 342/52, 342/58, 342/90, 342/189, 342/190, 342/191, 342/192, 342/193, 342/194, 342/195, 342/196, 342/197
• US Class Current: 342/22
• CPC Class Codes: G01S 13/862 Combination of radar systems with sonar systems
• CPC Class Codes: G01S 13/9027 Pattern recognition for feature extraction
• CPC Class Codes: G01V 11/00 Prospecting or detecting by methods combining techniques covered by two or more of main groups G01V1/00 - G01V9/00
• CPC Class Codes: G01V 3/17 operating with electromagnetic waves operating with millimetre waves G01V8/005

Abstract

A region of water is insonifed with a series of pulses. A synthetic apeture radar (SAR) is flown about the region and images the water surface above the sonified area. The radar has a carrier frequency (F_r) the acoustic source has a nominal carrier frequency of F_a. This acoustic frequency F_a is selected such that the acoustic wavelength is nearly, or exactly, half the wavelength of the radar carrier F_r where modified by the Sine of the incident of angle. Also practical are those acoustic wavelengths that are integer multiples of the wavelength that is half the radar wavelength. This arrangement will result in the Bragg condition in portions of the radar image of the water surface. The pulse repetition frequency (PRF) of the radar and the acoustic sources are equal (or can be such that the acoustic PRF is an integer multiple of the radar PRF), and locked in phase together by a radio frequency link between the SAR and the sonar transmitter. The sonar signal is repeatable and coherent with the radar signal. As a result, the SAR image can be processed, eliminating random surface wave motion, to detect and classify the structures that underlie the water surface.

Claims

1. A detection system arranged to detect an object under the surface of a liquid medium, comprising:

(a) at least one acoustic generator arranged for generating a detectable pattern of waves on the surface of said liquid medium, said at least one acoustic generator operating at a frequency range whereby said detectable pattern of waves has reinforcing characteristics at selected observation angles;

(b) at least one radar device arranged to scan said detectable pattern of waves on said surface of said liquid medium, said at least one radar device comprising means for coherently adding multiple scans of said detectable pattern of waves; and

,

(c) link means for coordinating transmittal of said at least one radar device and said at least one acoustic generator by phase-locking radar and acoustic transmissions.

2. The detection system of claim 1, wherein said at least one acoustic generator is tuned to a frequency range which is related by a specific relationship to a selected wavelength of said at least one radar device.

3. The detection system of claim 2, wherein said at least one radar scanner is tuned to achieve a predetermined relationship with frequencies generated by said at least one acoustic generator.

4. The detection system of claim 3, wherein said specific relationship is Bragg'"'"'s Law.

5. The detection system of claim 3, wherein said at least one radar device comprises means for scanning said detectable pattern of waves at multiple observation angles.

6. The detection system of claim 5, wherein said at least one radar device is mounted in an airplane and physically separated from said at least one acoustic generator.

7. The detection system of claim 3, wherein radar wavelength is twice the acoustic wavelength modified by the cosine of a selected observation angle.

8. The detection system of claim 7, wherein said observation angles are between 0 degrees to 90 degrees.

9. The detection system of claim 1, wherein said link means comprise a radio frequency link between said at least one radar device and said at least one acoustic generator.

10. The detection system of claim 9, wherein said at least one radar device is a synthetic aperture radar (SAR) system operating at 95 GHz, and said at least one acoustic generator operates at frequencies from approximately 672 kHz to 122 kHz.

11. A method of detecting an object under the surface of a liquid medium, comprising the steps of:

(a) generating acoustic signals in said liquid medium at a selected frequency range to form particular wave patterns on said surface of said liquid medium, whereby said particular wave patterns have reinforcing characteristics at predetermined angles of observation;

(b) radar scanning said particular wave patterns in a phase-locked relationship with generation of said acoustic signals to detect signals indicative of said particular wave patterns at selected observation angles; and

,

(c) coherently adding said signals at at least one selected observation angle to obtain an indication of said object.

12. The method of claim 11, wherein between substantially 40,000 and 50,000 scans of a selected portion of said liquid medium are carried out.

13. The method of claim 12, wherein said radar scanning is carried out at observation angles of between 30°

and 60°

.

14. The method of claim 13, wherein radar signals have twice the wavelength of acoustic signals as modified by the cosine of a predetermined angle of observation.

15. The method of claim 14, wherein said step of radar scanning takes place at 95 GHz with a 1 GHz bandwidth.

16. The method of claim 11, wherein said step of generating acoustic signals and said step of radar scanning are controlled to take place in synchronization with each other.