H.H. Aumann (hha@airs1.jpl.nasa.gov), AIRS Project Scientist
AIRS, the Atmospheric Infrared Sounder on the EOS PM-1 platform, is a grating array spectrometer which covers the 3.7 15.4 µm region of the infrared spectrum. AIRS, in combination with the Advanced Microwave Sounding Unit (AMSU) and the Humidity Sounder Brazil (HSB) microwave sounders on EOS PM-1, will provide global temperature profile sounding capability with better than 1-K rms accuracy in 1-km vertical layers, and water vapor profiles with better than 10% rms accuracy under clear and partially-cloudy conditions in support of operational weather forecasting and climate research. The EOS PM-1 platform is scheduled for launch in December 2000. The spacecraft, built by TRW in Redondo Beach, CA, is the first in a series of common spacecraft for NASA, NOAA, and DoD polar-orbiting missions.
AIRS Hardware Status
The design of AIRS is essentially complete. The program technical focus is now on the AIRS engineering model (EM) and the flight model (FM) build and test. It is a schedule-driven environment focused on completing the EM by October 1997 and delivering the FM by October 1998.
The AIRS EM build is nearing completion. The spectrometer assembly, alignment, and warm wave front testing are complete. The wave front error, with the spectrometer at 300 K, shows excellent optical quality. The wave front error, using a 3.39 µm laser, tests the performance of the spectrometer from the entrance aperture through all optical surfaces, including the diffraction grating, to a retro-reflector mounted on the focal plane. Wave front error measurements with the spectrometer at 150 K and the focal plane at 58 K are the next steps.
AIRS uses a focal plane with 12 arrays, cooled to 55 K using an active cooler. The cooler and the focal plane have been successfully integrated in the engineering model vacuum dewar and tested. This was a major step. Early results are encouraging. Array M12 (14.67-15.4 µm) has a median noise equivalent temperature difference, NEDT, of 0.4 K with zero outages. Required for the FM is NEDT=0.35 K or better, with less than 2% outage. Outage is defined as any detector in an array that is a factor of 2 or more worse than the required array median NEDT. The build of the FM is paced by the IR focal plane build and test.
The AIRS Test and Calibration Facility (ATCF) build is complete and in the final checkout phase. The ATCF has spatial, spectral, and radiometric measurement capability. The ground support data acquisition station and processing software development are almost complete. The data acquisition hardware and processing software demonstrated their capability in support of the cold engineering model dewar testing. The AIRS Instrument Calibration plan, which gives an overview of the spectral, radiometric, and spatial calibration of AIRS, hardware tools (such as the ATCF), software tools, predicted calibration accuracies, and assumptions, is being circulated for final review. The EM testing will serve to validate these tools and assumptions. The plan for the calibration of the flight model includes two end-to-end tests: a spectral calibration test using a 6-m gas cell; and a radiometric test, where the AIRS (mounted in the ATCF) looks upward through a flat folding mirror into a vertical air column. Simultaneously, the same column is viewed with the Atmospheric Emitted Radiance Interferometer (AERI), an uplooking interferometer with spectral resolution and coverage equivalent to AIRS, which has been validated during intensive ground measurement campaigns. The tools available for AIRS calibration are adequate to meet the Functional Requirements Document (FRD) specified calibration accuracies.
AIRS Data Processing Status
The AIRS Level 1b and Level 2 algorithms have passed peer group reviews and can be downloaded from http://www-airs.jpl.nasa.gov/html/ATBD_home_ page/. The retrieval algorithm has four major stages: microwave retrieval, cloud clearing, first product generation using regression, and final product generation. The microwave retrieval uses the 57-GHz AMSU and 183-GHz HSB channels to create a first-guess solution independent of cloud cover. The cloud-clearing combines infrared and microwave channels by using 9 spots on the ground (3 x 3 AIRS footprints centered on an AMSU footprint) to solve for the fractional cloud cover, allowing for the possibility of multiple cloud formations, and cloud-clears using a TIROS-proven cloud clearing concept. The regression algorithm applies pre-calculated coefficients to the cloud-cleared radiances to quickly generate the first temperature and humidity profile solution. The final product is obtained by a physical retrieval. The retrieval accuracy, based on simulated data with up to 80% cloud cover, is 0.7 K rms in the lower troposphere. The moisture retrieval accuracy is better than 10% layer rms. Key to the physical retrieval algorithm is the fast forward algorithm. This algorithm accounts for the reflected thermal radiation and variable CO2 concentrations. The reflected thermal radiance is computed using 5 predictions to an accuracy of better than 0.1 K. Variations in the CO2 concentration (1% annual variability and at present a 0.5%/year upward trend) are handled as offsets to the existing fast transmittance calculations. Accuracies are better than the AIRS instrument noise with several easily computed predictors.
The AIRS Science Data Processing System (SDPS) is being developed in the rapid-prototyping mode. Prototype 6 is the current version of the SDPS. The functionality improvements of SDPS Prototype 6 over Prototype 5 include: use of a PCF (process control file) for opening Level 0 data files, Level 1b calibration smoothing across data granule boundaries and initial implementation of DC Restore correction, and integration of a 100-layer rapid transmission algorithm into the team algorithm. Prototype 7 will be the beta delivery to the DAAC, scheduled for June 1998. It will include the following additional functional improvements: full DAAC compatibility (toolkit for I/O and status messages, Earth Science Data Types [ESDTs], metadata), greatly reduced number of output files and use of Hierarchical Data Format (HDF) swath format, updated radiometric and spectral calibration, and re-integration of initial cloud clearing and initial retrieval code.
AIRS Data Product Validation Status
The AIRS Data Product Validation approach is described in the "AIRS Data Product Validation Plan," which was submitted to the EOS Project Science Office in August 1997. The plan can be found on the web at http://eospso.gsfc.nasa.gov/validation/valplans. html. The plan makes extensive use of already-established programs, such as the DOE ARM (Atmospheric Radiation Measurement) program in Oklahoma, and intensive campaigns using aircraft overflights of special test sites such as the CAMEX (Convection and Atmospheric Moisture Experiment) program, in addition to the more than 2000 radiosondes launched daily in support of global operational weather forecasting from balloons. Equipment typically available at special test sites are chilled mirror/frost-point hygrometers at 60-m and 30-m heights and at ground level, and hygrometers on tethered balloons and kite platforms below 1-km height and on aircraft overflights at up to 20-km altitude. Continuous observations of total precipitable water can be made by microwave radiometers, sun photometers, and the GPS (Global Positioning System). Profiles of water vapor and temperature can be made by Raman Lidar and AERI. The next CAMEX, CAMEX 3, is relevant to the AIRS program for the validation of forward and inverse algorithms in the tropical environment. CAMEX 3 is scheduled for August - September 1998 with flights from Florida. On board the NASA ER-2 will be the MODIS Airborne Simulator (MAS), the Microwave Imaging Radiometer (MIR, with 54, 118, and 183 GHz channels similar to AMSU-A and AMSU-B), and the IR Interferometer Sounder (with spectral coverage and resolution equivalent to AIRS), with the cross-track scanning capability. Lidar, an up-looking Fourier transform spectrometer (AERI), and a Differential Absorption Lidar (DIAL) system will be used to characterize the vertical temperature profile and water vapor profile in the lower troposphere with better than 1-km resolution and 5% accuracy.
For the latest information on AIRS and links to other information related to the EOS PM-1 program visit the AIRS homepage at http://www-airs.jpl.nasa.gov.