Onboard Radiometer Calibration via NIST-Traceable LED Sources
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Solution Overview
Problem
Conventional onboard calibration sources for Earth observing radiometers, such as those on satellites, lack traceable standards for radiance calibration, especially at visible wavelengths, leading to instrument degradation and incorrect climate data records due to unmonitored biases and trends.
Innovation Solution
A calibration device comprising a housing with emitters configured to transmit narrow band wavelengths of light and detectors to measure radiance changes, combined with a honeycomb blackbody and integrating cavity, allowing for accurate calibration of radiometers in orbit by using NIST-traceable sources and monitoring changes in radiance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional onboard calibration sources (tungsten lamps, radiative black bodies) are used, then calibration can be performed onboard, but the sources lack traceable standards for radiance calibration especially at visible wavelengths
Solution Approach 1:
The patent introduces an intermediary calibration device that uses LEDs with known spectral characteristics as a mediator between the radiometer and traceable standards. The LEDs are calibrated on the ground with NIST-traceable standards and then used as a portable reference source onboard, bridging the gap between ground-based standards and space-based measurement.
Solution Approach 2:
The patent changes the fundamental parameter of the calibration source from thermal radiation (tungsten lamps, black bodies) to LED-based narrowband emission. This parameter change enables traceable calibration at visible wavelengths because LEDs can be manufactured with precise spectral characteristics that are traceable to national standards, unlike thermal sources which have inherently uncertain radiance at visible wavelengths.
2Reliability
If conventional calibration sources are used, then instrument calibration can be performed, but instrument degradation cannot be monitored due to lack of traceable standards
Solution Approach 1:
The patent implements a feedback mechanism where the radiometer repeatedly measures the known spectral radiance of the LED calibration source onboard. By comparing these measurements over time against the original ground calibration values, the system generates feedback information about instrument degradation, allowing scientists to monitor and correct for changes in detector sensitivity and optical component transmission.
3Ease of operation
If thermal radiation sources (incandescent lamps, black bodies) are used for onboard calibration, then calibration can be performed, but spectral resolution below 0.4 μm is insufficient
Solution Approach 1:
The patent replaces the mechanical/thermal radiation system (incandescent lamps, heated black bodies) with an electronic/optical system based on LEDs. This substitution eliminates the fundamental limitation of thermal sources in the visible spectrum, as LEDs generate light through electroluminescence rather than thermal radiation, enabling precise spectral characterization at wavelengths below 0.4 μm where thermal sources are ineffective.
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
Provides accurate and traceable calibration for radiometers, ensuring reliable climate data by accounting for instrument changes and maintaining spectral resolution across visible and near-infrared wavelengths, thereby correcting biases in climate records.
Implementation Method 1
a plurality of emitters disposed in the housing for transmitting light toward the imaging device. The emitters are configured to transmit narrow band wavelengths of light
Implementation Method 2
a plurality of detectors disposed in the housing for receiving a portion of the light transmitted toward the imaging device. The detectors are configured to measure a change in radiance of the light
Implementation Method 3
The calibration device may also include a honeycomb blackbody and an integrating cavity. The emitters are configured to radiate light into the integrating cavity
Data Source
AI summary
A calibration device for a radiometer includes an integrating cavity and a honeycomb blackbody mounted to the integrating cavity. A plurality of emitters are mounted to the exterior rim of the integrating cavity for transmitting narrow band wavelengths of light into and out of the integrating cavity. A controller, selectively, activates one or more emitters to radiate a single narrow band wavelength of light during an ON period and turn OFF during another period. A plurality of reference detectors are also mounted to the exterior rim of the integrating cavity for measuring the intensity of each narrow band radiation outputted from the integrating cavity. The reference detectors are effective in determining changes in intensity output by each of the narrow band emitters from the integrating cavity. The measured changes in intensity outputted by the emitters are used to calibrate the radiometer with the changes traced back to a NIST standard.