Analysis of CrIS FM1 TVAC Test Data in Preparation for On-orbit Radiometric Calibration

1. Introduction The Cross-track Infrared Sensor (CrIS) [1] is one of the key instruments for the National Polar-Orbiting Environmental Satellite System (NPOESS) [2]. As a Michelson interferometer, CrIS measures atmospheric emission spectrum in the infrared, which is to be used to produce the vertical temperature and moisture profiles of the Earth's atmosphere. The first Flight Module (FM1) of CrIS is to be launched on the NPOESS Preparatory Project (NPP) [3] satellite in 2011. As a part of prelaunch testing and characterization, the CrIS FM1 has undergone several phases of Thermal Vacuum tests; the last phase (TVAC3) of radiometric performance tests ended in December 2008. This paper presents the independent analyses of the TVAC3 test data of CrIS FM1 sensor by the NGAS team for NPP data product Calibration and Validation (Cal/Val). In addition to providing further verification of the instrument performance reported by the sensor manufacturer – ITT, our effort constitutes an important part of our preparation for the on-orbit Cal/Val of the CrIS FM1 data products. In particular, the residual radiometric calibration uncertainties, due in part to the limitations of TVAC test environment, led to our plan for on-orbit monitoring and further calibration. Sources of radiometric calibration uncertainties include: imperfect characterization of Internal Calibration Target (ICT) effective emissivity, detector nonlinearity, and the possible non-uniformity of the ICT surface brightness temperature. Our analyses of the TVAC data examine the sensitivity of the radiometric performance of CrIS FM1 to these parameters. In the remainder of this paper, we shall present the basic background information on the CrIS FM1 and its TVAC tests in Section 2. In Sections 3 and 4 we shall discuss the accuracy of the ICT radiance