Abstract
We report on a remote sensor system architecture that has potential to improve absolute radiometric calibration accuracy in the solar bands by a factor of 10 when compared to conventional methods that rely on solar diffusers, calibrated bulb references, detector references, the sun or moon. We call the concept "NIST-in-Space" because it would bring the calibration accuracy of the National Institute of Standards & Technology (NIST) into space for the first time in the solar bands (250 nm – 3000 nm). NIST-in-Space should enable future space based remote sensors to evolve into climate class instruments that have the measurement accuracy needed to answer important questions related to climate change and land surface changes on shorter time scales.
A small aperture solar band (250 – 3000 nm) Fourier Transform Spectrometer (FTS) is the spectrally resolving sensor in this system. The FTS undergoes conventional calibration from a laser driven plasma white light source and black target. The plasma white light source is not required to have any calibration traceability. A second optical source comprised of an integrating sphere driven by a multitude of light emitting diodes (LED) becomes the primary reference. The LED sphere output radiance is short-term stabilized using monitor detectors as feedback. Long term stability and absolute radiometric accuracy is established by an uncooled, broadband, electrical substitution radiometer that views the LED sphere and provides a direct tie to the electrical Watt (SI). Successive views of both calibration sources by the FTS will allow the SI traceability to be transferred from the LED sphere to the white light source at a multitude of discrete wavelengths.
A spectrally resolved 0.1% radiometric calibration uncertainty is expected for this system. This is relative to 100% earth albedo radiance level. The system is robust against calibration degradation over decade long space missions.
System Concept for 0.1% Spectrally Resolved Radiometric Calibration in the Solar Bands
We report on a remote sensor system architecture that has potential to improve absolute radiometric calibration accuracy in the solar bands by a factor of 10 when compared to conventional methods that rely on solar diffusers, calibrated bulb references, detector references, the sun or moon. We call the concept "NIST-in-Space" because it would bring the calibration accuracy of the National Institute of Standards & Technology (NIST) into space for the first time in the solar bands (250 nm – 3000 nm). NIST-in-Space should enable future space based remote sensors to evolve into climate class instruments that have the measurement accuracy needed to answer important questions related to climate change and land surface changes on shorter time scales.
A small aperture solar band (250 – 3000 nm) Fourier Transform Spectrometer (FTS) is the spectrally resolving sensor in this system. The FTS undergoes conventional calibration from a laser driven plasma white light source and black target. The plasma white light source is not required to have any calibration traceability. A second optical source comprised of an integrating sphere driven by a multitude of light emitting diodes (LED) becomes the primary reference. The LED sphere output radiance is short-term stabilized using monitor detectors as feedback. Long term stability and absolute radiometric accuracy is established by an uncooled, broadband, electrical substitution radiometer that views the LED sphere and provides a direct tie to the electrical Watt (SI). Successive views of both calibration sources by the FTS will allow the SI traceability to be transferred from the LED sphere to the white light source at a multitude of discrete wavelengths.
A spectrally resolved 0.1% radiometric calibration uncertainty is expected for this system. This is relative to 100% earth albedo radiance level. The system is robust against calibration degradation over decade long space missions.