Nist Advanced Radiometer Calibrated And Delivered To Nasa - National Institute of Standards and Technology - Brief Article

Journal of Research of the National Institute of Standards and Technology, May-June, 2001

NIST staff recently completed a radiometric calibration of an advanced spaceflight instrument for NASA. The instrument, Scripps-NISTAR (NIST Advanced Radiometer), was developed to fly as part of the NASA Triana mission, which will view Earth from an L1 orbit (the Lagrange libration, or neutral gravity point between the Earth and the Sun)--about 1.5 mullion kiolometers from Earth. from there, an imaging camera and NISTAR will have a continuous, full, sunlit view of the Earth. NISTAR includes three active cavity electrical substitution radiometers for absolute irradiance measurements, and one silicon photodiode.

To properly simulate a sunlit Earth in the calibration laboratory, NIST researchers used NIST's SIRCUS (Spectral Irradiance and Radiance Responsivity Calibrations with Uniform Sources) facility. A tunable laser was fiber-optically coupled to an integrating sphere, which then served as a unform diffuse source of tunable monochromatic light, and the geometry was designed to provide the required 0.5[degrees] field of illumination. The NISTAR instrument's optical responsivity and other characteristics were measured using a wide variety of laser wavelengths in the range 488 nm to 850 nm (blue, green, red, near-infrared), during these tests, the NISTAR instrument was in a space-simulating thermal vacuum chamber, viewing the SIRCUS light through windows. This was the first time that the SIRCUS facility was used for an instrument in such a chamber. The results show that various adverse radiometric effects from using winddows and translating the fiber-optically coupled sphere are minimal and the resulting calibrarion uncertainties are below 1 %.

The NISTAR instrument was delivered to the NASA Goddard Space Flight Center and is ready for integration with the remaining spacecraft components for the Triana. To obtain additional information about the Triana mission, or to view the spacecraft's integration and testing pahse, visit http://trianaweb.gsfc.nasa.gov/