Abstract
The Absolute Radiance Interferometer (ARI) is an infrared spectrometer designed to serve as an on-orbit radiometric reference with the ultra-high accuracy (better than 0.1 K 3‑σ or k = 3 brightness temperature at scene brightness temperature) needed to optimize measurement of the long-term changes of Earth’s atmosphere and surface. If flown in an orbit that frequently crosses sun-synchronous orbits, ARI could be used to inter-calibrate the international fleet of infrared (IR) hyperspectral sounders to similar measurement accuracy, thereby establishing an observing system capable of achieving sampling biases on high-information-content spectral radiance products that are also < 0.1 K 3‑σ. It has been shown that such a climate observing system with <0.1 K 2‑σ overall accuracy would make it possible to realize times to detect subtle trends of temperature and water vapor distributions that closely match those of an ideal system, given the limit set by the natural variability of the atmosphere. This paper presents the ARI sensor's overall design, the new technologies developed to allow on-orbit verification and test of its accuracy, and the laboratory results that demonstrate its capability. In addition, we describe the techniques and uncertainty estimates for transferring ARI accuracy to operational sounders, providing economical global coverage. Societal challenges posed by climate change suggest that a Pathfinder ARI should be deployed as soon as possible.
Highlights
Since 1959, when the first instrument to view the Earth from space using omni-directional black, white, and gold hemispherical detectors was flown on Explorer VII by Verner Suomi and Robert Parent from the University of Wisconsin, it has been widely recognized that the global perspective of satellites is invaluable for exploring how the climate system of the Earth operates
The Absolute Radiance Interferometer (ARI) consists of a Calibrated Fourier Transform Spectrometer (CFTS), calibrated on-orbit in the traditional way using an ambient onboard blackbody reference and a space view, and an On-orbit Verification and Test System (OVTS) that provides end-to-end calibration verification with direct on-orbit traceability to absolute standards
Based on the radiometric uncertainty analysis completed via the perturbation of the complex calibration equation, a limit can be established for the radiometric uncertainty contribution associated with less significant contributors, such that the 0.1 K on-orbit requirement can still be met with sufficient margin
Summary
Since 1959, when the first instrument to view the Earth from space using omni-directional black, white, and gold hemispherical detectors was flown on Explorer VII by Verner Suomi and Robert Parent from the University of Wisconsin, it has been widely recognized that the global perspective of satellites is invaluable for exploring how the climate system of the Earth operates. The element of emphasizing absolute accuracy, proven on-orbit, to replace reliance on the overlap of subsequent measurement systems to establish long-term trends and (2) the importance of multiple, carefully chosen orbits to reduce spatial and temporal biases to acceptable levels [3] These proposals benefited from earlier hyperspectral sounder developments aimed at improving the temperature, water vapor, and trace gas profiling capability of future geosynchronous Earth orbit (GEO) and low Earth orbit (LEO) operational sensors for weather and atmospheric chemistry. The high accuracy of spectra from HIS soon attracted the attention of Tony Clough, author of the Fast Atmospheric Signature Code (FASCODE) line-by-line radiative transfer model, who quickly recognized their value for improving spectroscopy This led to the development of a highly accurate up-looking spectrometer, the University of Wisconsin Atmospheric Emitted Radiance Interferometer (AERI) under the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Program in the 1990s [9,10].
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