The Cross-track Infrared Sounder (CrIS), a Fourier transform spectrometer with 1305 spectral channels, will produce high-resolution, three-dimensional temperature, pressure, and moisture profiles. These profiles will be used to enhance weather forecasting models, and they will facilitate both short- and long-term weather forecasting. Over longer timescales, they will help improve understanding of climate phenomena such as El Niño and La Niña.
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Installation of the CrIS instrument. Credit: Ball Aerospace
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The Cross-track Infrared Sounder (CrIS), provides soundings of the atmosphere with 1305 spectral channels, over 3 wavelength ranges: LWIR (9.14 - 15.38um); MWIR (5.71 - 8.26um); and SWIR (3.92 - 4.64 um). The CrIS instrument, a Fourier transform spectrometer, has an 8 cm clear aperture and utilizes plane mirror interferometer technology. CrIS scans a 2200km swath width (+/- 50 degrees), with 30 Earth-scene views. Each field consists of 9 fields of view, arrayed as 3x3 array of 14km diameter spots (nadir spatial resolution). Each scan (with an 8-second repeat interval) includes views of the internal calibration target (warm calibration point), and a deep space view (cold calibration point). The overall instrument data rate is <1.5Mbps. Only photovoltaic detectors are used in the CrIS instrument. The detectors are cooled to approximately 81K using a 4-stage passive cooler with no moving parts. They have a low-risk heritage design of over 50 space units.
The calibration of the interferometer is accomplished with both LASER wavelength calibration, and also with a Neon bulb spectral calibration. The internal calibration target (ICT) consists of a highly emissive, deep-cavity blackbody, utilizing a flight-proven, MOPITT-heritage design. Temperature knowledge of the ICT is better than 80mK. A passive vibration isolation system is included to allow instrument operation in a 50mG environment. The instrument optics are thermally decoupled from both the structure and the instrument electronics. The overall instrument design is modular, which allows for parallel assembly and rapid instrument integration.
The CrIS instrument consists of 6 modular assemblies: optical bench, scanning telescope, interferometer, PV focal plane arrays, 4-stage passive cooler, and electronics. The optical bench provides a stable structure for mounting all of the other assemblies. The scanning telescope scans the Earth views, the ICT, and deep space, and focuses the IR energy into the interferometer. The interferometer sequentially "breaks" the IR energy into the spectral bands, much like the "rainbow" from a DVD surface. The PV detectors sense the sequenced IR energy (from the interferometer), and provide an electrical signal corresponding to the incoming IR energy. The 4-stage cooler is used to cool the detectors, and hence reduce any spurious detector noise. The electronics assembly controls the instrument. It also conditions and formats the telescope scan and detector signals for output to the spacecraft.
