Steven E. Broberg

On-board calibration techniques and test results for the Atmospheric Infrared Sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS) is a space based instrument developed for measurement of global atmospheric properties; primarily water vapor and temperature. AIRS is one of several instruments on board NASAs Earth Observing System Aqua spacecraft. AIRS operates in the 3.7 - 15.4 micron region and has 2378 infrared channels and 4 Vis/NIR channels. AIRS spatial resolution is 13.5 km from the orbit of 705 km and it scans ±49.5 degrees. AIRS has a set of on-board calibrators including a single infrared blackbody source, a parylene spectral calibration source, a space view and a Vis/NIR photometric calibrator. The on-board calibration subsystems are described along with a description of special test procedures for using them and results from several tests performed to date. Results are exceptional indicating that the instrument is performing better than expected.

Version 5 product improvements from the atmospheric infrared sounder (AIRS)

The AIRS instrument was launched in May 2002 into a polar sun-synchronous orbit onboard the EOS Aqua Spacecraft. Since then we have released three versions of the AIRS data product to the scientific community. AIRS, in conjunction with the Advanced Microwave Sounding Unit (AMSU), produces temperature profiles with 1K/km accuracy on a global scale, as well as water vapor profiles and trace gas amounts. The first version of software, Version 2.0 was available to scientists shortly after launch with Version 3.0 released to the public in June 2003. Like all AIRS product releases, all products are accessible to the public in order to have the best user feedback on issues that appear in the data. Fortunately the products have had exceptional accuracy and stability. This paper presents the improvement between AIRS Version 4.0 and Version 5.0 products and shows examples of the new products available in Version 5.0.

AIRS/AMSU/HSB on EOS Aqua: first-year post-launch assessment

The Atmospheric Infrared Sounder (AIRS), Advanced Microwave Sounding Unit (AMSU), and Humidity Sounder from Brazil (HSB) are three instruments onboard the Earth Observing System (EOS) Aqua Spacecraft. Together, they form the Aqua Infrared and Microwave Sounding Suite (AIMSS). This paper discusses the science objectives and the status of the instruments and their data products one year after launch. All instruments went through a successful activation and calibration and have produced exceptional, calibrated, Level 1B data products. The Level 1B Product Generation Executables (PGEs) have been given to NOAA and the GSFC DAAC for production and distribution of data products. After nine months of operations, the HSB instrument experienced an electrical failure of the scanner. Despite the loss of HSB, early validation results have shown the AIRS and AMSU are producing very good temperature profiles.

Improving AIRS radiance spectra in high contrast scenes using MODIS

The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002. AIRS acquires hyperspectral infrared radiances in 2378 channels ranging in wavelength from 3.7-15.4 um with spectral resolution of better than 1200, and spatial resolution of 13.5 km with global daily coverage. The AIRS is designed to measure temperature and water vapor profiles for improvement in weather forecast accuracy and improved understanding of climate processes. As with most instruments, the AIRS Point Spread Functions (PSFs) are not the same for all detectors. When viewing a non-uniform scene, this causes a significant radiometric error in some channels that is scene dependent and cannot be removed without knowledge of the underlying scene. The magnitude of the error depends on the combination of non-uniformity of the AIRS spatial response for a given channel and the non-uniformity of the scene, but is typically only noticeable in about 1% of the scenes and about 10% of the channels. The current solution is to avoid those channels when performing geophysical retrievals. In this effort we use data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument to provide information on the scene uniformity that is used to correct the AIRS data. For the vast majority of channels and footprints the technique works extremely well when compared to a Principal Component (PC) reconstruction of the AIRS channels. In some cases where the scene has high inhomogeneity in an irregular pattern, and in some channels, the method can actually degrade the spectrum. Most of the degraded channels appear to be slightly affected by random noise introduced in the process, but those with larger degradation may be affected by alignment errors in the AIRS relative to MODIS or uncertainties in the PSF. Despite these errors, the methodology shows the ability to correct AIRS radiances in non-uniform scenes under some of the worst case conditions and improves the ability to match AIRS and MODIS radiances in non-uniform scenes.

Space View Issues for Hyperspectral Sounders

The expectation for climate quality measurements from hyperspectral sounders is absolute calibration accuracy at the 100 mK level and stability at the < 40 mK/decade level. The Atmospheric InfraRed Sounder (AIRS)1, Cross-track Infrared Sounder (CrIS), and Infrared Atmospheric Sounding Interferometer (IASI) hyperspectral sounders currently in orbit have been shown to agree well over most of their brightness temperature range. Some larger discrepancies are seen, however, at the coldest scene temperatures, such as those seen in Antarctic winter and deep convective clouds. A key limiting factor for the calibrated scene radiance accuracy for cold scenes is how well the effective radiance of the cold space view pertains to the scene views. The spaceview signal is composed of external sources and instrument thermal emission at about 270 K from the scan mirror, external baffles, etc. Any difference in any of these contributions between spaceviews and scene views will impact the absolute calibration accuracy, and the impact can be critical for cold scenes. Any change over time in these will show up as an apparent trend in calibrated radiances. We use AIRS data to investigate the validity of the spaceview assumption in view of the 100 mK accuracy and 40 mK/decade trend expectations. We show that the space views used for the cold calibration point for AIRS v5 Level-1B products meet these standards except under special circumstances and that AIRS v6 Level-1B products will meet them under all circumstances. This analysis also shows the value of having multiple distinct space views to give operational redundancy and analytic data, and that reaching climate quality requires continuing monitoring of aging instruments and adjustment of calibration.

Calibration status of the Atmospheric Infrared Sounder after eleven years in operation

The Atmospheric Infrared Sounder (AIRS) is a grating array infrared hyperspectral sounder with 2378 channels from 3.75 to 15.4 microns with spectral resolution 1200 to 1400 depending on the channel. AIRS was designed as an aid to weather prediction and for atmospheric process studies. It produces profiles of atmospheric temperature and water vapor. Because of its spectral coverage and spectral resolution it is sensitive to a number of trace atmospheric constituents including CO2, CO, SO2, O3, and CH4. The AIRS sensitivity, stability, and long life have led to its use in climate process studies and climate model validation, both of which place far more stringent requirements on calibration than weather forecasting does. This paper describes results from several special calibration sequences, originally developed for prelaunch testing, that have been used to monitor the AIRS calibration accuracy and instrument health on-orbit, including the scan mirror, space view response, and channel health. It also describes reanalyses of pre-launch calibration data used to determine calibration parameters. Finally, it shows comparisons of AIRS radiometry with two other hyperspectral infrared sounders presently in space—IASI and CrIS.

Level 1C spectra from the Atmospheric Infrared Sounder (AIRS)

The Atmospheric Infrared Sounder (AIRS), launched on the EOS Aqua spacecraft on May 4, 2002, is a grating spectrometer with 2378 channels in the range 3.7 to 15.4 microns. Spectra from grating spectrometers are capable of unsurpassed absolute radiometric accuracy, which makes them excellent potential sources for climate data records used in climate trending analyses. However, because each channel has its own detector and (partially) its own electronics, some individual channels can suffer from higher noise than other channels and, in extreme cases, can fail completely. Radiometric quality non-uniformity can complicate error and noise estimation for some products. In particular, crosscalibration with other instruments, frequency interpolation, and frequency shifting are made more difficult. AIRS level 1B data are calibrated spectral radiances from each channel. This paper describes results of creating level 1C spectra from level 1B data, where radiances from channels determined to be very noisy are replaced with values determined by taking advantage of channel-to-channel correlations. Several methods are evaluated and validated.

Evaluation of AIRS, MODIS, and HIRS 11 micron brightness temperature difference changes from 2002 through 2006

In an effort to validate the accuracy and stability of AIRS data at low scene temperatures (200-250 K range), we evaluated brightness temperatures at 11 microns with Aqua MODIS band 31 and HIRS/3 channel 8 for Antarctic granules between September 2002 and May 2006. We found excellent agreement with MODIS (at the 0.2 K level) over the full temperature range in data from early in the Aqua mission. However, in more recent data, starting in April 2005, we found a scene temperature dependence in MODIS-AIRS brightness temperature differences, with a discrepancy of 1- 1.5 K at 200 K. The comparison between AIRS and HIRS/3 (channel 8) on NOAA 16 for the same time period yields excellent agreement. The cause and time dependence of the disagreement with MODIS is under evaluation, but the change was coincident with a change in the MODIS production software from collection 4 to 5. AIRS and MODIS (Flight Model 1) are onboard the EOS Aqua spacecraft, launched into a 1:30 PM polar orbit on May 4, 2002. AIRS has 2378 infrared channels with high spectral resolution (1200) covering the 3.7 to 15.4 micron wavelength range, with a nominal spatial resolution of 13.5 km. MODIS has 36 relatively broad spectral bands with spatial resolution of 1 km for the LWIR bands. HIRS/3 is onboard NOAA-16 (L), launched into a 2:00 PM polar orbit on Sep. 21, 2000.

AIRS infrared polarization sensitivity and in-flight observations

The Atmospheric Infrared Sounder (AIRS) is a space-based instrument that measures the upwelling atmospheric spectrum in the infrared. AIRS is one of several instruments on the EOS-Aqua spacecraft launched on May 4, 2002. Typically, instrument polarization is not a concern in the infrared because the scene is usually not significantly polarized. A small amount of polarization is expected over ocean, which can be seen in the AIRS 3.7 mm window channels. The polarization is seen as a signal difference between two channels with the same center frequency but different polarizations. The observations are compared to a model that relies on measurements of instrument polarization made pre-flight. A first look at a comparison of the observations of sea surface polarization to expectations is presented.

Atmospheric sounding at JPL: current and future technologies

JPL is currently managing the instrument operations, calibration and data system for the Atmospheric Infrared Sounder (AIRS) on the EOS Aqua spacecraft. Aqua was launched on May 4, 2002 from Vandenberg Air Force Base. AIRS has 2378 infrared channels with high spectral resolution (1200) covering the 3.7 to 15.4 micron wavelength range. AIRS data are used to produce temperature and humidity profiles useful in predicting weather and monitoring climate. We discuss lessons learned on AIRS in the development and operations as well as plans for next generation systems including SIRAS, a wide field hyperspectral infrared imaging spectrometer which offers AIRS spectral performance at 24x the spatial resolution.