Establishing a High Quality Climate Benchmark of the Earth: Utilizing a New On-orbit IR Transfer Standard to Leverage Next Generation Operational Environmental Satellite Capabilities

The value of operational satellite systems for decadal climate trending can be greatly enhanced by implementing on-orbit transfer standards for testing and improving calibration accuracy. This concept is an important part of the Climate Absolute Radiance and Reflectivity Observatory (CLARREO) mission recommended as a Tier 1 mission in the 2007 Decadal Survey. While implementation as a NASA Decadal Survey mission is on hold, the technological development by NASA Earth Science Technology Office has proceeded very successfully. Here we report on the Absolute Radiance Interferometer (ARI), a flight prototype for the infrared portion of CLARREO. ARI has been developed by this group teamed with the Anderson Group at Harvard University, and is linked with the CLARREO program at NASA LaRC.

The ARI instrument was designed to measure absolute spectrally resolved infrared radiance over much of the Earth-emitted spectrum (3.7-50 microns at resolution < 1 cm-1) with ultra-high accuracy (<0.1 K 3-sigma brightness temperature). This is in contrast to current instruments dedicated to observing climate trends that, like the very first Suomi radiometers on Explorer 7, measure total integrated solar and IR radiances (with at most a broad window band). Resolving the spectrum allows ARI to provide benchmark products for climate trending with much higher information content than traditional measurements. While ARI requirements for accuracy and spectral properties 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 (climate 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) and testing on orbit. The OVTS includes an On-orbit Absolute Radiance Standard (OARS), which is a high emissivity cavity blackbody that can be operated over a wide range of temperatures to verify ARI calibration. The OARS uses multiple small phase change cells to establish its fundamental temperature scale to better than 5 mK absolute and a broad-band heated-halo source for monitoring its cavity spectral emissivity throughout the mission. A Quantum Cascade Laser (QCL) is also used by the OVTS to monitor the ARI instrument spectral lineshape and the emissivity of its calibration blackbody relative to that of the OARS. The ARI radiance measurements will also be tested for other systematic errors on orbit (non-linearity, polarization effects, and stray light). All of these new technologies have now been brought to the NASA Technical Readiness Level 6, meaning that they are ready for implementation in a flight program.

The International Space Station (ISS) offers an affordable opportunity to demonstrate this new capability. The natural precession of the ISS offers many orbit crossings for radiance intercomparisons with operational satellites in sun-synchronous orbits and gives good time of day coverage for latitudes below 52 degrees. Therefore, the ARI calibration that is verified on-orbit can be transferred to operational sounding instruments (CrIS at 1330 and IASI A & B at 0930) and to AIRS at 1330. Then, these sounders can be used to create climate benchmark products for all latitudes, especially filling in the important polar mission missed by the ISS orbit. The adequacy of limited sun-synchronous sampling for lower orbits can also be evaluated. In this way, an ARI mission on the ISS would leverage operational instruments to provide a demonstration of the new climate decadal trending capability offered by spectrally resolved radiances with high accuracy proven on-orbit. This might well offer a key new roll for the operational systems of the future.