Demonstrating the error budget for the Climate Absolute Radiance and Refractivity Observatory through solar irradiance measurements

Abstract

The Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission addresses the need to observe highaccuracy, long-term climate change trends and to use decadal change observations as a method to determine the accuracy of climate change. A CLARREO objective is to improve the accuracy of SI-traceable, absolute calibration at infrared and reflected solar wavelengths to reach on-orbit accuracies required to allow climate change observations to survive data gaps and observe climate change at the limit of natural variability. Such an effort will also demonstrate National Institute of Standards and Technology (NIST) approaches for use in future spaceborne instruments.The current work describes the results of laboratory and field measurements with the Solar, Lunar for Absolute Reflectance Imaging Spectroradiometer (SOLARIS) which is the calibration demonstration system (CDS) for the reflected solar portion of CLARREO. SOLARIS allows testing and evaluation of calibration approaches, alternate design and/or implementation approaches and components for the CLARREO mission. SOLARIS also provides a test-bed for detector technologies, non-linearity determination and uncertainties, and application of future technology developments and suggested spacecraft instrument design modifications. Results of laboratory calibration measurements are provided to demonstrate key assumptions about instrument behavior that are needed to achieve CLARREO's climate measurement requirements. Absolute radiometric response is determined using laser-based calibration sources and applied to direct solar views for comparison with accepted solar irradiance models to demonstrate accuracy values giving confidence in the error budget for the CLARREO reflectance retrieval.

Detector based calibration of a portable imaging spectrometer for CLARREO Pathfinder Mission

“…Past efforts as part of the CLARREO project performed component-level tests of the SOLARIS optics, detectors, grating, depolarizers and attenuators. 8,9 Past work has also determined the SOLARIS relative spectral response (RSR), detector-to-detector uniformity, noise, and temperature sensitivity. In addition, the measurements that related to the geometrical factor of instrument such as pixel pitch and orientation of array relative to fiducial were determined for a previous version of the detector package.…”

“…3 It can be considered as a portable version of NISTs Spectral Irradiance and Radiance Responsivity Calibrations Using Uniform Sources (SIRCUS). [4][5][6][7] GLAMR has already been used to characterize the radiometry of multiple sensors [8][9][10] but not at the level of uncertainty needed for the CPF Independent Calibration.…”

“…The portability of the SOLARIS instrument allows outdoor measurements of direct solar irradiance and solar-illuminated diffuser data. 9,11 The solar spectral shape and irradiance provide a reliable source for on-orbit characterization and calibration. The SOLARIS field measurements also provide test data sets that will be used to develop the processing methods and algorithms for on-orbit CPF measurements.…”

Development of a simulation environment to support intercalibration studies over the Algodones Dunes system

“…Note that SOLARIS does have a three-axis gimble to minimize view angle differences, but they will still exist operationally. 10 To quantify the impact of view geometry on the intercalibration accuracy, DIRSIG was used to simulate the Algodones Dunes at various geometries over a hemisphere (θ∶0 deg to 360 deg and ϕ∶0 deg to 80 deg). The different view geometries used to image the Algodones Dunes for Aqua-MODIS sensor are shown in Fig.…”

“…SOLARIS is the reflected solar instrument of the climate absolute radiance and refractivity observatory (CLARREO) mission. [9][10][11] The objective of the SOLARIS sensor is the development and inspection of calibration techniques, establish methods to obtain SI-traceability, and estimate reflectance from measurements of the sun and the scene. 9 The mission plan is to have the SOLARIS sensor placed on the ISS platform to investigate techniques and benefits in obtaining highly accurate measurements before placing high-budget satellites into service.…”