Title: A Solar Irradiance Cross-Calibration Method Enabling Climate Studies Requiring 0.2% Radiometric Accuracies
Author: Paul Smith
Organization: Laboratory for Atmospheric and Space Physics
Co-Authors: Ginger Drake, Joey Espejo, Karl Heuerman, Greg Kopp

The 2007 NRC Decadal Surveyís CLARREO mission requires shortwave radiometric accuracy and SI-traceability to better than 0.2% for Earth climate studies on decadal time scales. These accuracies, being nearly ten times better than current on-orbit capabilities, will establish benchmark measurements of solar radiation scattered by the Earth, provide reference calibrations for other on-orbit instruments, and initiate a climate data record to be used for future policy decisions. The methods described here demonstrate a solution enabling such a high accuracy level via on-orbit end-to-end calibration of a hyperspectral imager using measurements of solar irradiances, which are known to better radiometric accuracies than any other calibration sources available on-orbit in the 350 - 2300 nm spectral region.

Cross-calibrating the hyperspectral imager with solar irradiances requires highly accurate attenuation methods, which are investigated in the work described here. The required attenuation between viewing solar and Earth radiances is approximately 5 orders of magnitude. Three methods are demonstrated to collectively provide this level of attenuation: decreasing optical entrance aperture size, shortening detector integration times, and inserting neutral density filters.
Apertures can achieve 3 orders of magnitude of attenuation by switching from a 20 mm aperture when observing the Earth to a 0.5 mm diameter aperture when observing the Sun. The aperture geometric areas are calibrated by NIST, and demonstrated to provide the expected attenuations with 0.1% uncertainties. Integration times from electronically shuttered focal plane arrays can provide attenuations of 2 orders of magnitude. The accuracy across this attenuation range is shown to be approximately 0.04%, measured by a NIST-calibrated silicon trap detector. Neutral density filters can achieve around 1 order of magnitude attenuation and can be calibrated on-orbit via lunar radiance observations, a technique that has been demonstrated in the lab to 0.06% uncertainty across the tested visible spectral range (450 - 700 nm).

The work described here verifies a solar cross-calibration approach to achieving the CLARREO mission requirements for shortwave radiometry, making it a viable option for long-term space-based climate studies.