Characterization of a radiometric monitoring system for NASA code 618's SIRCUS-G

A tunable, intensity-stabilized, quasi-continuous wave (CW) laser system, patterned after the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) system at the National Institute of Standards and Technology (NIST) 1, has been installed and is being tested in the NASA Goddard Space Flight Center (GSFC) Code 618 Biospheric Sciences Laboratory’s Calibration Facility. This system is referred to as SIRCUS-G (SIRCUS-Goddard). The tunable output of the laser system is fiber-fed to a 76.2 cm diameter integrating sphere lined with Spectralon. The uniform radiance light emitted from the integrating sphere is used in system-level radiometric responsivity characterizations and wavelength calibrations of remote sensing instruments. The primary radiance reference standards in the responsivity characterizations are a three-element Si trap radiometer for the visible and near infrared and a radiometer employing an InGaAs detector. Both radiometers have been calibrated by NIST. These radiometers are located at the exit port of the Spectralon coated integrating sphere. In addition, a set of three radiometers are mounted on the 76.2 cm integrating sphere’s wall ports to monitor source radiance and to provide real-time sphere radiance data during the calibrations of remote sensing instruments. These monitor radiometers provide spectral coverage from 300nm to near 2500nm. This paper presents the results of our characterization of the performance of these monitor radiometers. Results are presented and discussed on monitor radiometer short- and long-term system stability, noise level, and total measurement uncertainty.