A New Class of Advanced Accuracy Satellite Instrumentation (AASI) for the CLARREO Mission: Interferometer Test-bed Tradestudies and Selection

NASA has selected CLARREO (Climate Absolute Radiance and Refractivity Observatory), a climate mission recommended by the 2007 Decadal Survey of the US National Research Council, as a potential new start in 2010. CLARREO will measure spectrally resolved radiance from the earth and atmospheric bending of GPS signals related to atmospheric structure (refractivity) as benchmark measurements of long-term climate change trends. CLARREO will provide more complete spectral and time-of-day coverage and will fly basic physical standards to eliminate the need to assume on-board reference stability. Therefore, the spectral radiances from this mission will also serve as benchmarks to propagate a highly accurate calibration to other space-borne IR instruments. Technology development and risk reduction for the CLARREO mission is being conducted at the Space Science and Engineering Center at the University of Wisconsin-Madison. The objective of this work is to develop and demonstrate the technology necessary to measure IR spectrally resolved radiances (3 - 50 micrometers) with ultra high accuracy (< 0.1 K 3-sigma brightness temperature at scene temperature) for the CLARREO benchmark climate mission. The ultimate benefit to society is irrefutable quantification of climate change and a solid basis for improving climate model forecasts. The proposed work (University of Wisconsin-Madison and Harvard University) was selected for the 2007 NASA Instrument Incubator Program (IIP) and will develop four primary technologies to assure SI traceability on-orbit and demonstrate the ultra high accuracy measurement capability required for CLARREO: (1) On-orbit Absolute Radiance Standard (OARS), a high emissivity blackbody source that uses multiple miniature phase-change cells to provide a revolutionary on-orbit standard with absolute temperature accuracy proven over a wide range of temperatures, (2) On-orbit Cavity Emissivity Modules (OCEMs), providing a source (quantum cascade laser, QCL, or “Heated Halo”) to measure any change in the cavity emissivity of the OARS, (3) On-orbit Spectral Response Module (OSRM), a source for spectral response measurements using a nearly monochromatic QCL source configured to uniformly fill the sensor field-of-view, and (4) Dual Absolute Radiance Interferometers (DARI), providing spectral coverage from 6 to 50 micrometers that can be inter-compared to dissect any unexpected systematic errors in overlapping spectral regions. Interferometer trade-studies for the UW DARI have been conducted and the design selection is complete. Instrument assembly, integration, testing and verification will begin in early 2010. The interferometer trade-study and design details will be presented herein.