Monitoring perturbations in the long-term balance between Earth’s absorption of radiative energy from the Sun and its emission of infrared radiation to space is one of the primary objectives behind the Decadal Survey Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission. Through on-orbit traceability of measurements to SI standards, CLARREO will initiate benchmark climate data records that can be linked to future measurements and potentially be used to cross-calibrate other on-orbit instruments. We present results of studies that will aid in defining the requirements of an Earth-viewing spectrometer over the solar spectral domain for CLARREO, and an instrument concept for meeting those requirements.
To achieve the required level of accuracy in the shortwave requires an improvement on the order of a factor of ten over current state-of-the-art measurements of the Earth-scattered solar radiation. We are developing an Earth-viewing shortwave hyperspectral imager that will trace its calibration on-orbit to solar spectral irradiance, to be obtained by NOAA’s contribution to CLARREO as recommended in the Decadal Survey. Solar irradiance is known to better radiometric accuracy than any other calibration source available on-orbit. By cross-calibrating a hyperspectral imager with solar spectral irradiance, using techniques similar to those proven on the UARS and SORCE SOLSTICE instruments, the Earth-viewing imager will be calibrated, validated, and tracked on-orbit to the required accuracy and traceability levels.
A hyperspectral imager capable of cross-calibrating via direct solar irradiance measurements is currently being prototyped. Three precision attenuation methods allowing Sun and Earth-viewing are being developed and will be validated using a NIST-calibrated detector. This calibration technology will enable the creation of a climate data record that can be linked to future observations and thus establish a benchmark for detecting climate change.