Specular Array Radiometric Calibration via Lunar Reference
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Solution Overview
Problem
Current radiometric calibration methods for airborne and space-based sensing systems face challenges in achieving accurate absolute calibration, particularly for small targets, due to issues like optical diffraction, sensor motion, and atmospheric scattering, and existing on-board calibration systems are complex, expensive, and inadequate for high spatial resolution systems.
Innovation Solution
A specular array for radiometric calibration is introduced, comprising a plurality of spherical mirrors on a uniform background, providing a collective minimum calibratability field of regard, with a processor to determine the intensity of reflected sunlight and account for atmospheric transmittance, allowing for precise calibration of sensors across multiple spectral bands and dynamic ranges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If on-board calibration systems are used to provide known radiant flux, then absolute calibration can be achieved, but the systems become complex, expensive, and taxing on mass limitations
Solution Approach 1:
The patent uses the Moon as an intermediary calibration target. Instead of using complex on-board calibration systems, the system images the Moon which provides a known radiant flux reference. The Moon acts as a natural intermediary that enables absolute calibration without requiring complex artificial calibration sources on the spacecraft.
Solution Approach 2:
The patent creates a simplified representation of the calibration problem by using the Moon's known reflectance properties as a copy of an ideal calibration source. The Moon's surface properties are well-known from lunar missions, providing a reliable reference that copies the function of an ideal calibration source without the complexity of artificial systems.
2Measurement precision
If internal sources like lamps and blackbodies are used for calibration, then calibration can be performed, but they cannot match the color temperature of the sun and degrade in radiant output over time
Solution Approach 1:
The patent uses the Moon as a copy of an ideal calibration source that naturally reflects sunlight. The Moon's surface properties are well-known from lunar missions, providing a reliable reference that copies the function of an ideal calibration source without the complexity of artificial systems.
Solution Approach 2:
The patent changes the calibration approach from using artificial internal sources with fixed temperatures to using the Moon which naturally varies with solar illumination. This allows the calibration source to maintain the correct color temperature characteristics of sunlight while avoiding the degradation issues of artificial sources.
3Measurement precision
If solar diffusers are used for calibration, then the spectrum of the sun can be reproduced, but they still suffer from bi-directional reflectance effects and degrade with time
Solution Approach 1:
The patent uses the Moon as a copy of an ideal calibration source. The Moon's surface properties are well-known from lunar missions, providing a reliable reference that copies the function of an ideal calibration source without the complexity of artificial systems.
Solution Approach 2:
The patent extracts the calibration function from complex artificial solar diffusers and implements it through the Moon's naturally reflective properties. By taking out the calibration reference function and applying it through the Moon's well-characterized surface, the system eliminates the bi-directional reflectance and degradation issues of artificial solar diffusers.
4Measurement precision
If large surfaces of known reflectance are used for ground based calibration, then calibration can be performed, but they require extensive support teams for deployment and maintenance
Solution Approach 1:
The patent uses the Moon as a copy of an ideal calibration source. The Moon's surface properties are well-known from lunar missions, providing a reliable reference that copies the function of an ideal calibration source without the complexity of artificial systems.
Solution Approach 2:
The patent applies self-service by using the Moon, which is a natural and permanent calibration target that requires no human deployment or maintenance. The Moon automatically provides the calibration reference whenever visible, eliminating the need for extensive support teams required by ground-based calibration surfaces.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The specular array enables accurate and efficient radiometric calibration of sensors, improving spatial resolution and radiometric perception, particularly for small targets, and supports inter-sensor calibration, reducing costs and complexity compared to traditional methods.
Implementation Method 1
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
Data Source
AI summary
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.