Climate Benchmark Quality IR Measurements for CLARREO: Update on CLARREO Pathfinder and the Absolute Radiance Interferometer

The NASA Climate Absolute Radiance and Reflectivity Observatory (CLARREO) will use highly accurate, spectrally resolved infrared emission and reflected solar measurements to quantify global trends in the climate of the Earth.  Near the end of 2015, NASA announced that it will conduct a CLARREO Pathfinder mission on the International Space Station (ISS), possibly including both an IR and a reflected solar instrument. Launch is planned for 2019. Other avenues for realizing the full CLARREO mission are also being pursed.

The technical readiness for this ISS mission 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 and ABB of Quebec, Canada (supported by the NASA Earth Science Technology Office).

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 involve large sample averages).

The key new aspect of the ARI instrument is the On-orbit Verification and Test System (OVTS) for proving its accuracy on-orbit by reference to International Standards (SI). The OVTS includes an On-orbit Absolute Radiance Standard (OARS), a high emissivity cavity blackbody that can be operated over a wide range of temperatures to verify ARI calibration. The OARS uses 3 small melt cells to establish its fundamental temperature scale to better than 10 mK and a broad-band heated-halo source for monitoring its cavity spectral emissivity.  A Quantum Cascade Laser (QCL) is also used by the OVTS to monitor the ARI spectral lineshape and the emissivity of its calibration blackbody relative to that of the OARS. These new technologies are now proven and ready flight.

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, allowing the start of a global benchmark record.