Calibration of a shortwave reference standard by transfer from a blackbody standard using a cryogenic active cavity radiometer

The Clouds and the Earth´s Radiant Energy System (CERES) instruments for the NASA Earth Observing System each contain three broadband scanning radiometers operating in bands at wavelengths from 0.3 to 5 μm, 8 to 12 μm and 0.3 to >150 μm. The primary radiance standard for CERES ground calibration is an absolute blackbody. This standard provides radiance calibration in the long wave spectral region over a blackbody temperature range of 200 K to 320 K. A cryogenic active cavity radiometer operating at liquid helium temperature is used to transfer the radiance scale of the blackbody to a secondary, shortwave calibration source. The longwave to shortwave calibration transfer makes use of the spectrally flat absorptance of the ACR across the entire spectral region. The thermal properties of the copper receiver cavity at 4 K permit a noise equivalent power performance adequate for calibration of the shortwave reference source in a series of narrow (10% wide) spectral bands which span the shortwave spectral region. The narrow spectral regions greatly reduce the errors in the relative spectral response knowledge of the shortwave source. Results are presented for a transfer that was made during validation tests of the ground calibration system in preparation for the CERES Functional Test Model instrument calibration