The NASA Earth Science Technology Office (ESTO) selected the Pyro-atmosphere Infrared Sounder (PIRS) airborne demonstration as part of its FireTech program in May of 2023. Upon completion, the PIRS will be flown in an aircraft to measure temperature and water vapor profiles, and total column carbon monoxide, above and in the vicinity of wildfires to support fire prediction and suppression efforts and scientific investigations of the meteorology of wildfires. The PIRS incorporates a wide field all refractive grating spectrometer operating in the MWIR region of the spectrum with 640 spectral channels from 4.08-5.13μm. The PIRS optics assembly was developed by BAE SYSTEMS for the CubeSat Infrared Atmospheric Sounder (CIRAS) project, as part of an earlier NASA and NOAA technology maturation program in support of reducing the size of infrared sounding instruments for space applications. Through the PIRS airborne flight development and demonstration, the full performance capabilities of the CIRAS instrument will have been demonstrated and build confidence in the performance expected from a future CIRAS spaceflight mission.
KEYWORDS: Equipment, Gas cells, Dark current, Fabry–Perot interferometers, Signal to noise ratio, Black bodies, Vacuum chambers, Thermography, Infrared radiation
Hyperspectral infrared sounders measure the profiles of temperature and water vapor in the atmosphere and the concentration of trace gas species. Instruments such as NASA Atmospheric Infrared Sounder (AIRS) on the Aqua spacecraft have proven their value for weather and climate research, atmospheric composition research, and high impact to the operational forecast at NWP centers worldwide. Reducing the size, weight and power of these instruments is key to enabling more rapid revisit when deployed in Low Earth Orbit (LEO), enabling new measurements such as 3D Atmospheric Motion Vector (AMV) winds, and reducing the cost of these instrument for future deployment in LEO, Geostationary Earth Orbit (GEO) and aircraft platforms. NASA and NOAA have sponsored technology maturation at JPL of the CubeSat Infrared Atmospheric Sounder (CIRAS) to demonstrate the use of wide field grating spectrometer optics and large format FPA technologies included in CIRAS for infrared sounding. These include a 2D format High Operating Temperature-Barrier Infrared Detector (HOT-BIRD), a silicon Immersion Grating, and Black Silicon for the CIRAS entrance slit and blackbody. Thermal Vacuum (TVac) performance testing of CIRAS has been completed achieving TRL 5 for a full scale brassboard of the instrument. Testing included spatial, spectral, and radiometric response of the instrument including measurements of the transmission in a gas cell. Results show excellent performance from the system with the exception of a high background flux from the Integrated Dewar Cryocooler Assembly (IDCA). The IDCA is not planned for flight use and projections of the performance in the flight configuration are discussed. Through this testing the instrument has reached TRL 5. Recently, the NASA Earth Science Technology Office (ESTO) awarded JPL a contract to fly the CIRAS in an aircraft, called the Pyro-atmosphere Infrared Sounder (PIRS), to measure the convective environment around wildfires.
The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua spacecraft measures the upwelling radiance of the atmosphere from 3.7 to 15.4 μμm. The AIRS radiometric calibration coefficients convert the counts measured from the instrument’s A/D converters (Level 1A) to SI traceable radiance units (Level 1B). The calibration equations are based on how the instrument operates and follow a simple second order relationship between counts and radiance. Terms are included to account for nonlinearity of the detectors, emissivity and temperature knowledge of the on-board calibrator (OBC) blackbody and radiometric offset due to coupling of the polarization of the scan mirror with the spectrometer. Radiometric coefficients have not been updated since launch and are used in the operational Version 5 available at the GES/DISC. A new set of coefficients, Version 7 (V7), were produced in 2018 but never released. This paper presents the coefficients for Version 8 (V8) with only a few changes from V7 relating to the additional time used in the training of the trend of the polarization coefficients. We then compare new coefficients, V8, with the latest operational version of the AIRS radiometric calibration coefficients Version 5 (V5) and the prior V7. The Version 8 coefficients utilize more of the pre-flight test data and show lower residuals to the tests than V5. V8 also removes a trend in the polarization seen in Module 5 and is expected to have more accurate nonlinearity than prior versions.
The exit of EOS Aqua from the A-train in early 2023 marks the end of 20 years of Atmospheric Infrared Sounder (AIRS) data from the 1:30 PM ascending node orbit. The AIRS 20-year data record shows impressive accuracy and stability. Trends in the radiometry relative to accepted stable geophysical references are at the -3 to +6 mK/yr level, likely caused by unaccounted for changes in the lower troposphere and increasing sensor aging effects. Previously unknown trends are seen in the distribution of clouds. The planned continuation of the AIRS data record with potentially 20 years of multiple Cross-track Infrared Sounder (CrIS) instruments may be used to confirm these trends. The overlap of three years of AIRS, SNPP-CrIS and JPSS1-CrIS shows radiometric agreement under cloud free ocean conditions at the 50 mK level. However, there are large day/night, land/ocean and cloud dependent differences between AIRS and CrIS data, which, even if explained by known footprint size differences, will complicate the climate change interpretation of trends from potentially 40 years of concatenated data.
AIRS has provided highly stable and accurate radiances since 2002, which has exceeded the expected instrument noise level. Despite the highly accurate data our analysis has shown warming as high as 5 Kelvin in the shorter wavelength portions of the AIRS spectrum for extremely cold scenes (less then 230 Kelvin), such as when AIRS views deep convective clouds. The warming only exists when AIRS is viewing a cold scene with neighboring warmer scenes. This work will demonstrate the spatial characteristics of these trends in relation to where the warm area is to the scene AIRS is viewing. Lastly, we will show that a linear regression correction approach that will account for the trend in the shorter wave portions of the AIRS spectrum.
The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002, with a 95% confidence design lifetime of 5 years. Since 2002 AIRS has performed exceptionally well. We now have 20 years of AIRS data. We show that AIRS data are an order of magnitude more accurate and more stable than the 100 mK absolute and 10 mK/yr required to measure climate trends. However, the real limitation on the usable absolute accuracy and stability are time varying residual cloud contamination at all wavelength, and surface contamination effects, particularly in the SW channels. The change in contamination and large interannual variabilities due to ENSO type effects complicate the interpretation of trends for climate. The 20 years of AIRS data contain at least two changes of potential climate significance: The change of the DCC count, but it is opposite in sign to the expected trend, and decrease in the count of SCT clear cases. Both indicate unexpected changes in the cloud structure.
Hyperspectral infrared measurements of Earth’s atmosphere from space have proven their value for weather forecasting, climate science and atmospheric composition. The CubeSat Infrared Atmospheric Sounder (CIRAS) instrument will demonstrate a fully functional infrared temperature, water vapor and carbon monoxide sounder in a CubeSat sized volume for at least an order of magnitude lower cost than legacy systems. Design for a CubeSat significantly reduces cost of access to space and enables flight in a constellation to reduce revisit time and enable new measurements including 3D winds. A technology demonstration of CIRAS is currently under development at JPL. The effort has completed integration and ambient testing of a high fidelity brassboard, complete with the flight configured optics assembly developed by Ball Aerospace with a JPL Immersion Grating and Black Silicon Entrance Slit. The brassboard includes a flight-configured High Operating Temperature Barrier Infrared Detector (HOT-BIRD) mounted in an Integrated Dewar Cryocooler Assembly (IDCA), enabling testing in the ambient environment. Ambient testing included radiometric testing of the system to characterize the instrument operability and NEdT. Spatial testing was performed to characterize the system line spread function (LSF) in two axes and report FWHM of the LSF. Spectral testing involved an air path test to characterize the spectral/spatial transformation matrix, and an etalon was used to measure the Spectral Response Functions (SRFs). Results of the testing show the CIRAS performs exceptionally well and meets the key performance required of the system. The end result of testing is the CIRAS instrument now meets TRL 4 with confidence in a brassboard configuration ready for thermal vacuum (TVac) testing necessary to achieve TRL 5 for the system.
AIRS has provided highly stable and accurate radiances since 2002. Stability at the 2 mK/yr has been demonstrated relative to the sea surface temperature (SST) in the 8-12 micron window channels. However, there is evidence of a larger trend in the shortwave channels: 8mK/yr when viewing clear ocean at about 300K, about 200 mK/yr when viewing 220K DCC at night in the tropical oceans, but a trend of less than 20 mK/yr for Dome C at 220K. Under these conditions, the 8-12 micron window channels show trends of less than 20 mK/yr. This study proposes that the trends in the shortwave channels are the result of a wavelength dependent contamination of the scan mirror and the on-board calibration (OBC) blackbody. Scattered light from nearby scenes while viewing DCCs causes the large apparent radiometric trend in the shortwave channels and can be removed via spatial linear regression. An experiment that assumes the OBC blackbody emissivity has slightly decreased is tested to explain the smaller trends. The emissivity experiment results show similar stability drifts at 1231 cm-1 but not to other window channels. The emissivity decrease may explain some of the stability drift, but other unknown physical mechanisms are likely as well.
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