Absolute Radiance Interferometer (ARI) for the CLARREO Pathfinder: On-Orbit Verification and Test (Part-2)

The infrared Absolute Radiance Interferometer (ARI) prototype instrument was developed for NASA's Climate Absolute Radiance and Reflectivity Observatory (CLARREO), and brought to a Technical Readiness Level of 6 (ready for a spaceflight mission) by our group, with funding from the NASA Earth Science and Technology Office (ESTO). An overview of the ARI instrument along with the status of the CLARREO Pathfinder mission on the International Space Station (ISS) is detailed in Part-1 with first author Henry Revercomb. The ARI consists of a Calibrated Fourier Transform Spectrometer (CFTS), calibrated on-orbit in the traditional way using an ambient blackbody and a space view, and an On-orbit Verification and Test System (OVTS) that provides end-to-end calibration verification. The OVTS consists of an On-Orbit Absolute Radiance Standard (OARS) and modules for measuring blackbody cavity emissivity and Instrument Line Shape using a Quantum Cascade Laser (QCL). The same 45° scene mirror that is rotated to expose the CFTS to the earth view and its calibration sources is used to select different OVTS views. The scene mirror is also used to direct the QCL beam into the OARS or Ambient Blackbody for emissivity measurements at 9.5 µm, or into an integrating sphere for the measurement of the ILS. The OARS is a variable temperature (-45°C to +40°C) independent high emissivity blackbody standard that can be viewed by the CFTS for periodic end-to-end checks of instrument radiometric accuracy and linearity. The underlying technologies in the OARS include an absolute temperature calibration scheme that uses the transient melt signatures of small quantities (<1g) of reference materials (gallium, water, and mercury) imbedded in the blackbody cavity to assign three well recognized absolute temperatures to the blackbody cavity temperature sensors. Absolute accuracies of better than 10 mK have been demonstrated using this novel approach. Measurements of the OARS emissivity from 5 to 50 µm, with an uncertainty of better than 0.0006, can be made using a carefully baffled “heated halo” placed in front of the OARS blackbody. Emissivity is calculated from the radiance measured by the CFTS while viewing the blackbody, combined with the knowledge of key temperatures and radiometric view factors. Laboratory performance results from vacuum testing of the ARI prototype will be presented that demonstrate the required performance for the CLARREO mission. In addition, the concept of operations for interleaving the OVTS into the routinely collected earth climate data will be outlined.