Method for atmospheric laser beam detection using remote sensing of off-axis scattering

• US 8,908,178 B1
• Filed: 03/05/2013
• Issued: 12/09/2014
• Est. Priority Date: 03/05/2013
• Status: Active Grant

• 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: Bendall, Charles S., Hanson, Frank E.
• Primary Examiner(s): Chowdhury, Tarifur
• Assistant Examiner(s): Akanbi, Isiaka
• Application Number:
  US13/785,060
• Time in Patent Office: 644 Days
• Field of Search: 356/434, 356/605, 356/614, 356/145, 356/622
• US Class Current: 356/343
• CPC Class Codes: G01C 1/00 Measuring angles
• CPC Class Codes: G01N 2021/1795 Atmospheric mapping of gases
• CPC Class Codes: G01N 21/53 within a flowing fluid, e.g. smoke
• CPC Class Codes: G01N 21/538 for determining atmospheric attenuation and visibility
• CPC Class Codes: G01S 3/784 using a mosaic of detectors
• CPC Class Codes: G01S 5/16 using electromagnetic waves other than radio waves
• CPC Class Codes: G01S 7/021 Auxiliary means for detecting or identifying radar signals or the like, e.g. radar jamming signals

Abstract

A method involves obtaining a first beam image on a focal plane of a first camera and a second beam image on a focal plane of a second camera from light scattered by ambient atmospheric aerosols in the path of a laser beam. First and second projected beam images are formed, representing the respective first and second beam images in the projected scenes of the respective first and second cameras. First and second ambiguity planes are then formed from the respective first and second projected beam images. An intersection of the first and second ambiguity planes is then determined, identifying the position of the laser beam. A source of the laser beam is then determined, along with a camera-source plane. A beam elevation angle with respect to this plane is then determined, as well as beam azimuth angles with respect to lines between the respective camera and the source.

Claims

1. A method comprising the steps of:

using a first camera to obtain a first beam image on a focal plane of the first camera and using a second camera to obtain a second beam image on a focal plane of the second camera from light scattered by ambient atmospheric aerosols in the path of at least part of a laser beam;

forming a first projected beam image that represents the first beam image in the projected scene of the first camera;

forming a second projected beam image that represents the second beam image in the projected scene of the second camera;

forming a first ambiguity plane from the first projected beam image;

forming a second ambiguity plane from the second projected beam image; and

determining an intersection of the first ambiguity plane and the second ambiguity plane, wherein at least a portion of the laser beam is located within the intersection.

2. The method of claim 1 wherein the step of forming the first ambiguity plane comprises the steps of:

determining a first vector representing the first projected beam image; and

determining a second vector representing the direction from the first camera to the beginning of the first vector, wherein the first ambiguity plane is defined by the first vector and the second vector.

3. The method of claim 2 wherein the step of forming the second ambiguity plane comprises the steps of:

determining a third vector representing the second projected beam image; and

determining a fourth vector representing the direction from the second camera to the beginning of the third vector, wherein the second ambiguity plane is defined by the third vector and the fourth vector.

4. The method of claim 1, wherein the step of determining the intersection of the first ambiguity plane and the second ambiguity plane comprises the steps of:

determining the orientation of the intersection; and

determining at least one of a point s₁ in the first ambiguity plane corresponding to the beginning of the laser beam within the first ambiguity plane and a point s₂ in the second ambiguity plane corresponding to the beginning of the laser beam within the second ambiguity plane.

5. The method of claim 4, wherein the step of determining an orientation of the intersection comprises:

determining a first normal vector normal to the first ambiguity plane;

determining a second normal vector normal to the second ambiguity plane; and

determining a unit beam vector formed by the cross product of the first normal vector and the second normal vector, wherein the intersection is parallel to the unit beam vector.

6. The method of claim 4, wherein the step of determining the position of at least one of a point s₁ and s₂ is performed according to s₁=C₁+v₁[(C₂−

C₁)·

n₂]/[v₁·

n₂] and s₂=C₂+v₂[(C₁−

C₂)·

n₁]/[v₂·

n₁], where C₁ is the position of the first camera, C₂ is the position of the second camera, v₁ is the first vector, v₂ is the second vector, n₁ is the first normal vector, and n₂ is the second normal vector.

7. The method of claim 6 further comprising the step of determining that one of s₁ and s₂ corresponds to the position S of a laser beam source based on at least one of a termination of the laser beam in an unobstructed scene and an enhanced brightness in one of the respective first projected beam image and the second projected beam image due to scattering from optical elements of the laser beam source.

8. The method of claim 7 further comprising the steps of:

determining a camera-source plane formed by positions S, C₁, and C₂;

forming a projection of the unit beam vector from S onto the camera-source plane;

determining a beam elevation angle of the beam unit vector relative to the camera-source plane;

determining a first beam azimuth angle φ

₁ corresponding the angle of the projection of the unit beam vector relative to the line from S to C₁; and

determining the second beam azimuth angle φ

₂ corresponding the angle of the projection of the unit beam vector relative to the line from S to C₂.

9. The method of claim 8 wherein the steps of determining the first and the second beam azimuth angles are based upon variation of radiance of the first beam image and the second beam image, wherein the beam angle of the laser beam is determined using the equation

10. The method of claim 9 further comprising the step of converting a measured signal in the beam image from each of the first and second cameras to a scaled radiance Y(θ

) that is proportional to radiance and the dependence of scaled radiance on a scattering angle θ

is obtained.

11. The method of claim 10, wherein the first and the second beam azimuth angles are determined by finding a solution to the equation

12. A system comprising:

a first camera and a second camera; and

a processor operatively connected to both the first and second cameras, the processor having computer-implementable instructions represented by computer-readable programming code stored therein, the processor configured to perform the steps of;

using a first camera to obtain a first beam image on a focal plane of the first camera and using a second camera to obtain a second beam image on a focal plane of the second camera from light scattered by ambient atmospheric aerosols in the path of at least part of a laser beam,

forming a first projected beam image that represents the first beam image in the projected scene of the first camera,

forming a second projected beam image that represents the second beam image in the projected scene of the second camera,

forming a first ambiguity plane from the first projected beam image,

forming a second ambiguity plane from the second projected beam image, and

determining an intersection of the first ambiguity plane and the second ambiguity plane, wherein at least a portion of the laser beam is located within the intersection.

13. The system of claim 12 wherein the step of forming the first ambiguity plane comprises the steps of determining a first vector representing the first projected beam image and determining a second vector representing the direction from the first camera to the beginning of the first vector, wherein the first ambiguity plane is defined by the first vector and the second vector, and the step of forming the second ambiguity plane comprises the steps of determining a third vector representing the second projected beam image and determining a fourth vector representing the direction from the second camera to the beginning of the third vector, wherein the second ambiguity plane is defined by the third vector and the fourth vector.

14. The system of claim 12, wherein the step of determining the intersection of the first ambiguity plane and the second ambiguity plane comprises the steps of:

determining the orientation of the intersection; and

determining at least one of a point s₁ in the first ambiguity plane corresponding to the beginning of the laser beam within the first ambiguity plane and a point s₂ in the second ambiguity plane corresponding to the beginning of the laser beam within the second ambiguity plane.

15. The system of claim 14, wherein the step of determining the orientation of the intersection comprises the steps of:

determining a first normal vector normal to the first ambiguity plane;

determining a second normal vector normal to the second ambiguity plane; and

determining a unit beam vector formed by the cross product of the first normal vector and the second normal vector, wherein the laser beam is parallel to the unit beam vector.

16. The system of claim 14, wherein the step of determining the position of at least one of a point s₁ and s₂ is performed according to s₁=C₁+v₁[(C₂−

C₁)·

n₂]/[v₁·

n₂] and s₂=C₂+v₂[(C₁−

C₂)·

n₁]/[v₂·

n₁], where C₁ is the position of the first camera, C₂ is the position of the second camera, v₁ is the first vector, v₂ is the second vector, n₁ is the first normal vector, and n₂ is the second normal vector.

17. The system of claim 16, wherein the processor is further configured to perform the step of determining that one of s₁ and s₂ corresponds to the position S of a laser beam source based on at least one of a termination of the laser beam in an unobstructed scene and an enhanced brightness in one of the respective first projected beam image and the second projected beam image due to scattering from optical elements of the laser beam.

18. The system of claim 17, wherein the processor is further configured to perform the steps of:

determining a camera-source plane formed by positions S, C₁, and C₂;

forming a projection of the unit beam vector from S onto the camera-source plane;

determining a beam elevation angle of the beam unit vector relative to the camera-source plane;

determining a first beam azimuth angle φ

₁ corresponding the angle of the projection of the unit beam vector relative to the line from S to C₁; and

determining the second beam azimuth angle φ

₂ corresponding the angle of the projection of the unit beam vector relative to the line from S to C₂, wherein the steps of determining the first and the second beam azimuth angles are based upon variation of radiance of the first beam image and the second beam image.

19. The system of claim 18, wherein the beam angle of the laser beam is determined using the equation

20. The system of claim 19, wherein the processor is further configured to perform the step of converting a measured signal in the beam image from each of the first and second cameras to a scaled radiance Y(θ

) that is proportional to radiance and the dependence of scaled radiance on a scattering angle θ

is obtained, wherein the first and the second beam azimuth angles are determined by finding a solution to the equation