Absolute Radiance Interferometer (ARI) for the CLARREO Pathfinder: Overview and Status (Part-1)

The NASA Climate Absolute Radiance and Reflectivity Observatory (CLARREO), recommended as a Tier 1 Mission by the first Earth science Decadal Survey of the National Research Council in 2007, plans to employ highly accurate, spectrally resolved infrared emission and reflected solar measurements to quantify global trends in the climate of the Earth. CLARREO implementation is being led by NASA LaRC and progress on the scientific justification and approach has been reported in BAMS (Wielicki, et al., 2013).

The planned stages for CLARREO implementation include (1) a CLARREO Pathfinder flight of IR and reflected solar instruments on the International Space Station (ISS) that is expected to enter Phase A in 2016 for a flight as early as 2019, and (2) the first full CLARREO mission that is expected to involve an international collaborative effort. The ISS Pathfinder will provide significant technical and cost risk reduction for the full mission. The technical readiness for these missions has been proven by NASA supported instrument developments, including that of the Absolute Radiance Interferometer (ARI), a prototype for the infrared portion of CLARREO. ARI was developed by our group teamed with the Anderson Group at Harvard University (supported by the NASA Earth Science Technology Office, ESTO).

The ARI instrument measures absolute spectrally resolved infrared radiance (3.7-50 µm) with ultra-high accuracy (< 0.1 K 3-sigma brightness temperature). Resolving spectral lines allows ARI to provide products for climate trending with much higher information content than those from current radiation budget measurements. While ARI accuracy requirements are demanding, the overall instrument is relatively simple and low-cost, because of the limited requirements on spatial sampling (25-100 km nadir-only footprints spaced at < 250 km) and on noise performance (decadal trending products are created by combining many samples). A key aspect of the ARI instrument is the On-orbit Verification and Test System (OVTS) for verifying its accuracy by reference to International Standards (SI). Laboratory results demonstrating agreement of calibrated ARI brightness temperatures to within 0.1 K 3-sigma of radiances from the OVTS will be shown. The new technologies developed for the OVTS are detailed in Part-2 with first author Fred Best.

The International Space Station (ISS) offers a good platform to demonstrate this new capability. The natural precession of the ISS orbit gives good time of day coverage for latitudes below 52 degrees and many opportunities for calibration transfer to high resolution sounding instruments (CrIS, IASI and AIRS) in sun-synchronous orbits. The value of these operational satellite systems for decadal climate trending can be greatly enhanced by implementing this on-orbit calibration transfer standard. Combining all of these observations would allow an ARI pathfinder mission on the ISS to demonstrate key CLARREO decadal trending capability with climate benchmark products covering all latitudes.