Abstract
The HyperSpectral Imager for Climate Science (HySICS), to be flown as the CLARREO Pathfinder payload in 2023, will acquire images of the Earth’s ground and atmosphere with unprecedented radiometric accuracies of <0.3% (k=1) achieved via on-orbit calibrations using the spectral solar irradiance. These high radiometric accuracies enable benchmarking of Earth radiances for climate studies and provide reference calibrations for other on-orbit Earth-viewing sensors.
The 2007 U.S. Academy of Sciences Decadal Survey for Earth Science recommended the Tier 1 mission CLARREO (Climate Absolute Radiance and Refractivity Observatory) to acquire high-accuracy, climate-benchmarking spatial/spectral radiances of the Earth's surface and to provide reference calibrations for other on-orbit assets. The more recent 2018 Decadal Survey similarly prioritized reference radiance inter-calibrations as one of its "Most Important Targeted Observables," providing on-orbit SI traceability for other programs such as the Global Space Based Inter-Calibration System (GSICS). To achieve these climate-benchmarking and inter-calibration capabilities, the space-borne imaging spectrometer for the CLARREO requires radiometric-accuracies that are nearly 10x better than any currently-flying spectrometer provides, necessitating innovative new on-orbit measurement techniques.
The HySICS is currently in development for the NASA CLARREO Pathfinder, a mission planned for launch to the International Space Station (ISS) in 2023 to demonstrate both the CLARREO-needed on-orbit radiometric accuracies and inter-calibrations of other space-based sensors. The instrument has a radiometric-uncertainty goal of 0.3% (k=1), which is much better than any current spaceflight reference detector or calibration light source is capable of providing. Instead of incorporating either of these traditional detector- or source-based calibration approaches into the instrument, the HySICS relies on on-orbit calibrations provided by direct views of the spectral solar irradiance, which is known on an absolute scale to ~0.2% from other space-based instruments. As opposed to using diffusors or other scattering surfaces that can degrade on orbit, the HySICS is designed for improved radiometric accuracies by regularly acquiring direct solar-irradiance measurements, avoiding concerns with on-orbit degradation that plague Earth-sensing optical instruments. A prototype HySICS demonstrated this solar-irradiance cross-calibration approach during two high-altitude balloon flights (Kopp et al., 2017).
We describe the radiometric-accuracy details of this Offner-based imaging spectrometer that contiguously covers 350 to 2300 nm with 6-nm spectral resolution and has an instantaneous nadir-looking field-of-view of 500 m and a swath width of 70 km from the ISS's orbit altitude.
Improved Radiometric Accuracies for Climate Science with HySICS
The HyperSpectral Imager for Climate Science (HySICS), to be flown as the CLARREO Pathfinder payload in 2023, will acquire images of the Earth’s ground and atmosphere with unprecedented radiometric accuracies of <0.3% (k=1) achieved via on-orbit calibrations using the spectral solar irradiance. These high radiometric accuracies enable benchmarking of Earth radiances for climate studies and provide reference calibrations for other on-orbit Earth-viewing sensors.
The 2007 U.S. Academy of Sciences Decadal Survey for Earth Science recommended the Tier 1 mission CLARREO (Climate Absolute Radiance and Refractivity Observatory) to acquire high-accuracy, climate-benchmarking spatial/spectral radiances of the Earth's surface and to provide reference calibrations for other on-orbit assets. The more recent 2018 Decadal Survey similarly prioritized reference radiance inter-calibrations as one of its "Most Important Targeted Observables," providing on-orbit SI traceability for other programs such as the Global Space Based Inter-Calibration System (GSICS). To achieve these climate-benchmarking and inter-calibration capabilities, the space-borne imaging spectrometer for the CLARREO requires radiometric-accuracies that are nearly 10x better than any currently-flying spectrometer provides, necessitating innovative new on-orbit measurement techniques.
The HySICS is currently in development for the NASA CLARREO Pathfinder, a mission planned for launch to the International Space Station (ISS) in 2023 to demonstrate both the CLARREO-needed on-orbit radiometric accuracies and inter-calibrations of other space-based sensors. The instrument has a radiometric-uncertainty goal of 0.3% (k=1), which is much better than any current spaceflight reference detector or calibration light source is capable of providing. Instead of incorporating either of these traditional detector- or source-based calibration approaches into the instrument, the HySICS relies on on-orbit calibrations provided by direct views of the spectral solar irradiance, which is known on an absolute scale to ~0.2% from other space-based instruments. As opposed to using diffusors or other scattering surfaces that can degrade on orbit, the HySICS is designed for improved radiometric accuracies by regularly acquiring direct solar-irradiance measurements, avoiding concerns with on-orbit degradation that plague Earth-sensing optical instruments. A prototype HySICS demonstrated this solar-irradiance cross-calibration approach during two high-altitude balloon flights (Kopp et al., 2017).
We describe the radiometric-accuracy details of this Offner-based imaging spectrometer that contiguously covers 350 to 2300 nm with 6-nm spectral resolution and has an instantaneous nadir-looking field-of-view of 500 m and a swath width of 70 km from the ISS's orbit altitude.