SPECULAR ARRAY FOR RADIOMETRIC CALIBRATION AND METHOD

• US 20100032557A1
• Filed: 08/07/2009
• Published: 02/11/2010
• Est. Priority Date: 02/09/2007
• Status: Active Grant

• Current Assignee: Raytheon Company (Rtx Corporation)
• Original Assignee: Raytheon Company (Rtx Corporation)
• Inventors: SCHILLER, Stephen J.
• Granted Patent: US 8,158,929 B2
• Time in Patent Office: Days
• US Class Current: 250/252.100
• CPC Class Codes: G01J 1/02 Details
• CPC Class Codes: G01J 1/0242 Control or determination of height or angle information of sensors or receivers; Goniophotometry
• CPC Class Codes: G01J 1/04 Optical or mechanical part supplementary adjustable parts
• CPC Class Codes: G01J 1/0414 using plane or convex mirrors, parallel phase plates, or plane beam-splitters
• CPC Class Codes: G01J 1/08 Arrangements of light sources specially adapted for photometry standard sources, also using luminescent or radioactive material
• CPC Class Codes: G01J 1/4228 arrangements with two or more detectors, e.g. for sensitivity compensation
• CPC Class Codes: G01J 2001/4266 for measuring solar light
• CPC Class Codes: G01J 2005/0077 Imaging
• CPC Class Codes: G01J 5/02 Constructional details
• CPC Class Codes: G01J 5/0275 Control or determination of height or distance or angle information for sensors or receivers
• CPC Class Codes: G01J 5/08 Optical arrangements
• CPC Class Codes: G01J 5/0808 Convex mirrors
• CPC Class Codes: G01J 5/0846 having multiple detectors for performing different types of detection, e.g. using radiometry and reflectometry channels
• CPC Class Codes: G01J 5/53 Reference sources, e.g. standard lamps; Black bodies
• CPC Class Codes: G02B 5/1861 Reflection gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials G02B5/1809, G02B5/1828, G02B5/1833, G02B5/1838 and G02B5/1847 take precedence

Abstract

A specular array for radiometric calibration (SPARC) includes a plurality of spherical mirrors disposed upon a uniform background as at least one array of reflective points, at least two points reflecting a different intensity of directly incident sunlight. Each mirror has a radius of curvature and a diameter, the radius of curvature and the diameter providing a field of regard, the collective mirrors providing a collective minimum calibratability field of regard. Based upon the radius of curvature, the transmittance value of the sun to each mirror and from each mirror to a sensor being calibrated, the intensity of calibration light provided to the input aperture of a sensor to be calibrated within the collective minimum calibratability field of regard may be determined and used as a baseline for sensor calibration. An associated method of combined spatial and radiometric calibration is also provided.

Claims

1. A specular array for radiometric calibration, comprising:

a plurality of spherical mirrors disposed upon a uniform background as at least one array of reflective points, at least two points reflecting a different intensity of directly incident sunlight,

each mirror having a radius of curvature and a diameter, the radius of curvature and the diameter providing a field of regard,

the collective mirrors providing a collective minimum calibratability field of regard.

2. The specular array of claim 1, wherein the plurality of spherical mirrors are subgrouped;

a first subgroup of mirrors arranged systematically on a cross point grid array, the cross point grid array an order of magnitude larger than a predetermined ground sample pixel resolution,

at least one mirror disposed at a cross point of the grid array, the remaining plurality of mirrors individually offset at a predetermined location from the cross points of the grid array; and

a second subgroup of mirrors structured and arranged as an array of reflective points, at least two points reflecting a different intensity of directly incident sunlight.

3. A specular array for radiometric calibration, comprising:

a plurality of spherical mirrors disposed upon a uniform background, each mirror having a radius of curvature and a diameter;

a first subgroup of mirrors arranged systematically on a cross point grid array, the cross point grid array an order of magnitude larger than a predetermined ground sample pixel resolution, at least one mirror disposed at a cross point of the grid array, the remaining plurality mirrors individually offset at a predetermined location from the cross points of the grid array;

and a second subgroup of mirrors structured and arranged as an array of reflective points, at least two points reflecting a different intensity of directly incident sunlight;

wherein for each mirror, the radius of curvature and the diameter providing a field of regard, the collective mirrors providing a collective minimum calibratability field of regard.

4. The specular array of claim 3, wherein for the first subgroup, the mirrors are essentially identical.

5. The specular array of claim 3, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different radii of curvature.

6. The specular array of claim 3, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different diameters.

7. The specular array of claim 3, wherein for the second subgroup of mirrors, the mirrors are identical and subdivided into subsets of one or more collective mirrors as quasi point sources for each reflective point of the array.

8. The specular array of claim 3, wherein the first subgroup and the second subgroup are adjacent arrays.

9. The specular array of claim 3, wherein the first subgroup and the second subgroup are combined as the same array arranged systematically on the cross point grid array.

10. The specular array of claim 3, wherein at least a subgroup of spherical mirrors are structured and arranged to provide point source targets consisting of only a solar image provided by the directly incident sunlight.

11. The specular array of claim 3, further including at least one sky only mirror, structured and arranged to hide reflection of the sun while reflecting about the remaining sky illuminating the mirror surface, the sky only mirror providing a target that isolates the sky contribution from reflected intensity provided by the second subgroup of mirrors.

12. The specular array of claim 3, wherein the second subgroup of mirrors are identical mirrors disposed upon separate panels, each panel larger than the ground sample pixel resolution.

13. The specular array of claim 12, wherein the panels are about identical, the number of mirrors disposed upon at least one first panel different from the number of mirrors disposed upon at least one second panel

14. The specular array of claim 12, wherein at least one first panel has a first geometric shape and at least one second panel has a second geometric shape different from the first geometric shape.

15. The specular array of claim 3, wherein the collective minimum calibratability field of regard dictates the above ground placement of a sensor to be calibrated.

16. The specular array of claim 3, wherein the specular array is structured and arranged to orient the collective minimum calibratability field of regard toward a sensor to be calibrated.

17. The specular array of claim 3, further including an actuator structured and arranged to actuate the specular array and dynamically orient the collective minimum calibratability field of regard toward a sensor to be calibrated.

18. The specular array of claim 3, further including at least one information gatherer structured and arranged to gather atmospheric information.

19. The specular array of claim 18, wherein the information gatherer is a solar radiometer operable to determine transmittance of the atmosphere.

20. The specular array of claim 18, further comprising:

a sensor locator structured and arranged to determine the location of a sensor to be calibrated including a distance H between each mirror and the sensor; and

a processor operable to receive the radius of curvature and reflectance for each mirror in the specular array and a transmittance value for an atmospheric path from the sun to a mirror (T₁) and from a mirror to the sensor (T₂) to be calibrated,

the processor operable to determine the intensity (I_m) of the reflected directly incident sunlight provided by each mirror at an input aperture of the sensor to be calibrated.

21. A system for radiometric calibration, comprising:

a specular array structured and arranged to provide a collective minimum calibratability field of regard of direct reflective sunlight, the specular array comprising;

a plurality of spherical mirrors disposed upon a uniform background as at least one array of reflective points, at least two points reflecting a different intensity of directly incident sunlight,

each mirror having a radius of curvature, a diameter and a reflectance, the radius of curvature and the diameter providing a field of regard, the collective mirrors providing the collective minimum calibratability field of regard;

an atmosphere information gatherer operable to obtain at least one transmittance value of the atmosphere adjacent to the specular array;

a sensor locator operable to determine the location of a sensor to be calibrated including a distance H between each mirror and the sensor; and

a processor operable to receive the radius of curvature and reflectance for each mirror in the specular array and the transmittance value for an atmospheric path from the sun to a mirror (T₁) and from a mirror to the sensor (T₂) to be calibrated,

the processor operable to determine the intensity (I_m) of the reflected directly incident sunlight provided by each mirror at an input aperture of the sensor to be calibrated.

22. The system for radiometric calibration of claim 21, wherein the plurality of spherical mirrors are subgrouped;

a first subgroup of mirrors arranged systematically on a cross point grid array, the cross point grid array an order of magnitude larger than a predetermined ground sample pixel resolution,

at least one mirror disposed at a cross point of the grid array, the remaining plurality mirrors individually offset at a predetermined location from the cross points of the grid array; and

a second subgroup of mirrors structured and arranged as an array of reflective points, at least two points reflecting a different intensity of directly incident sunlight.

23. The system for radiometric calibration of claim 22, wherein for the first subgroup, the mirrors are essentially identical.

24. The system for radiometric calibration of claim 22, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different radii of curvature.

25. The system for radiometric calibration of claim 22, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different diameters.

26. The system for radiometric calibration of claim 22, wherein for the second subgroup of mirrors, the mirrors are identical and subdivided into subsets of one or more collective mirrors as quasi point sources for each reflective point of the array.

27. The system for radiometric calibration of claim 22, wherein the first subgroup and the second subgroup are adjacent arrays.

28. The system for radiometric calibration of claim 22, wherein the first subgroup and the second subgroup are a combined as the same array arranged systematically on the cross point grid array.

29. The system for radiometric calibration of claim 21, wherein the intensity I_m is determined by computing:

${I<br><br>\left(\mathrm{\lambda <br><br>}\right)}_m=\frac{r<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>E_o<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>R^2<br><br>T_1<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>T_2<br><br>\left(\mathrm{\lambda <br><br>}\right)}{4}<br><br>\left(1+\frac{\mathrm{fG}<br><br>\left(\mathrm{\lambda <br><br>}\right)}{1-G<br><br>\left(\mathrm{\lambda <br><br>}\right)}\right)<br><br><br><br>\mathrm{watts}<br><br>\text{/}<br><br>\mathrm{str},$

wherein r(λ

) is mirror specular reflectance, R is the mirror radius of curvature, E₀(λ

) is top-of-atmosphere solar irradiance, and G(λ

) is an estimation of sky contribution as a fraction of the reflected directly incident sunlight, providing a diffuse-to-global ratio.

30. The system for radiometric calibration of claim 29, wherein the processor is further operable to determine irradiance at the sensor (E_at-sensor) by computing:

$E_{\mathrm{at}<br><br>\text{-}<br><br>\mathrm{sensor}}<br><br>\left(\mathrm{\lambda <br><br>}\right)=\frac{r<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>E_o<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>R^2<br><br>T_1<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>T_2<br><br>\left(\mathrm{\lambda <br><br>}\right)}{4<br><br>H^2}<br><br>\left(1+\frac{\mathrm{fG}<br><br>\left(\mathrm{\lambda <br><br>}\right)}{1-G<br><br>\left(\mathrm{\lambda <br><br>}\right)}\right)<br><br><br><br>\mathrm{watt}<br><br>\text{/}<br><br>m^2<br><br><br><br>\mathrm{str}.$

31. A method for radiometric calibration, comprising:

providing a plurality of spherical mirrors disposed upon a uniform background, each mirror having a radius of curvature and a diameter, the mirrors arranged as;

a first subgroup of mirrors arranged systematically on a cross point grid array, the cross point grid array an order of magnitude larger than a predetermined ground sample pixel resolution, at least one mirror disposed at a cross point of the grid array, the remaining plurality mirrors individually offset at a predetermined location from the cross points of the grid array; and

a second subgroup of mirrors structured and arranged as an array of reflective points, at least two points reflecting a different intensity of directly incident sunlight;

wherein for each mirror, the radius of curvature and the diameter providing a field of regard, the collective mirrors providing a collective minimum calibratability field of regard;

placing a sensor to be calibrated within the collective minimum calibratability field of regard;

obtaining a transmittance value for an atmospheric path from the sun to a mirror (T₁) and from a mirror to the sensor (T₂) to be calibrated;

determining the intensity (I_m) of the reflected directly incident sunlight provided by each mirror to an input aperture of the sensor to be calibrated;

receiving and analyzing first reflected sunlight from the first subset of mirrors, the first reflected sunlight providing data revealing blurring and spatial effects of the sensor;

receiving and analyzing second reflected sunlight from the second subset of mirrors, the second reflected sunlight compared with the determined intensity to prove data revealing responsitivity of the sensor across about all spectral bands and dynamic ranges,

32. The method for radiometric calibration of claim 31, wherein for the first subgroup, the mirrors are essentially identical.

33. The method for radiometric calibration of claim 31, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different radius of curvature.

34. The method for radiometric calibration of claim 31, wherein for the second subgroup of mirrors, at least two mirrors of the subgroup have different diameters.

35. The method for radiometric calibration of claim 31, wherein for the second subgroup of mirrors, the mirrors are identical and subdivided into subsets of one or more collective mirrors as quasi point sources for each reflective point of the array.

36. The method for radiometric calibration of claim 31, wherein the first subgroup and the second subgroup are adjacent arrays.

37. The method for radiometric calibration of claim 31, wherein the first subgroup and the second subgroup are a combined as the same array arranged systematically on the cross point grid array.

38. The method for radiometric calibration of claim 31, wherein the intensity I_m is determined by computing:

${I<br><br>\left(\mathrm{\lambda <br><br>}\right)}_m=\frac{r<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>E_o<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>R^2<br><br>T_1<br><br>\left(\mathrm{\lambda <br><br>}\right)<br><br>T_2<br><br>\left(\mathrm{\lambda <br><br>}\right)}{4}<br><br>\left(1+\frac{\mathrm{fG}<br><br>\left(\mathrm{\lambda <br><br>}\right)}{1-G<br><br>\left(\mathrm{\lambda <br><br>}\right)}\right)<br><br><br><br>\mathrm{watt}<br><br>\text{/}<br><br>\mathrm{str},$

wherein r(λ

) is mirror specular reflectance, R is the mirror radius of curvature, E₀(λ

) is top-of-atmosphere solar irradiance, and G(λ

) is an estimation of sky contribution as a fraction of the reflected directly incident sunlight, providing a diffuse-to-global ratio.