Calibration of the NIST Advanced Radiometer
In preparation for possible launch in 2013, NIST recently completed calibration of the NIST Advanced Radiometer (NISTAR). This instrument is designed to measure the absolute, spectrally integrated irradiance that is reflected and emitted from the sunlit face of Earth, as viewed from an orbit around the Earth-Sun L-1 point. From this vantage point, NISTAR, along with its companion imaging spectrometer, the Enhanced Polychromatic Imaging Camera (EPIC), will have a continuous “big picture” view of the illuminated Earth. Earth-orbiting satellites, in contrast, alternate between “daytime” and “nighttime” views.
NISTAR was designed and built between 1999 and 2001 by Ball Aerospace and Technology Corporation and the NIST Optical Technology Division, in conjunction with the Scripps Institute of Oceanography and NASA, as part of the Deep Space Climate Observatory (DSCOVR) mission. The instrument consists of four detectors: three electrical-substitution active cavity radiometers and a photodiode, plus several band-defining optical filters that can be used with any of the detectors.
NISTAR was calibrated in a thermal vacuum chamber in a clean-room environment at NIST, using a portable version of the NIST Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) facility. This calibration included system-level measurements of the relative spectral response of the NISTAR bands, using a wavelength-tunable laser and absolute responsivity measurements of each of the four NISTAR detectors at a wavelength of 532 nm. The standard uncertainty of the absolute responsivity calibration obtained using this technique was below 0.2 % (k = 1), limited by the signal-to-noise ratio of the NISTAR absolute detectors.
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