This paper evaluates the absolute accuracy and stability of the radiometric calibration of the Atmospheric Infrared Sounder (AIRS) by analyzing the difference between the brightness temperatures measured at 2616 cm−1 and those calculated at the top of the atmosphere (TOA), using the Real‐Time Global Sea Surface Temperature (RTGSST) for cloud‐free night tropical oceans between ±30° latitude. The TOA correction is based on radiative transfer. The analysis of the first 3 years of AIRS radiances verifies the absolute calibration at 2616 cm−1 to better than 200 mK, with better than 16 mK/yr stability. The AIRS radiometric calibration uses an internal full aperture wedge blackbody with the National Institute of Standards and Technology (NIST) traceable prelaunch calibration coefficients. The calibration coefficients have been unchanged since launch. The analysis uses very tight cloud filtering, which selects about 7000 cloud‐free tropical ocean spectra per day, about 0.5% of the data. The absolute accuracy and stability of the radiometry demonstrated at 2616 cm−1 are direct consequences of the implementation of AIRS as a thermally controlled, cooled grating‐array spectrometer and meticulous attention to details. Comparable radiometric performance is inferred from the AIRS design for all 2378 channels. AIRS performance sets the benchmark for what can be achieved with a state‐of‐the‐art hyperspectral radiometer from polar orbit and what is expected from future hyperspectral sounders. AIRS was launched into a 705 km altitude polar orbit on NASA's Earth Observation System (EOS) Aqua spacecraft on 4 May 2002. AIRS covers the 3.7–15.4 micron region of the thermal infrared spectrum with a spectral resolution of ν/Δν = 1200 and has returned 3.7 million spectra of the upwelling radiance each day since the start of routine data gathering in September 2002.

A selected subset of IRAS pointed observations has been examined to study infrared galaxies at 60 /¿m in the 50 to 100 mJy range. The observations cover a total area of 20 sq° and contain a total of 746 sources with signal to instrumental noise ratios SNR >5. Detailed observations of one of these fields at visual, near-infrared, radio and x-ray wavelengths indicate that these sources, the faintest extracted from the IRAS data, are dusty galaxies much like the bulk of infrared galaxies observed by IRAS. The area covered triples, in 20 different fields, the area of the previously most sensitive IRAS survey of sources in this flux density range. The number of sources is about seven times that previously extracted, and the source density is about twice that found in the previous study at this flux level. The observed 60 /¿m source counts are consistent with an evolutionary model of the number of infrared luminous galaxies that fits the observed radio source counts; the counts are inconsistent with a nonevolutionary model.

The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) is a new ground-based atmospheric Cherenkov telescope for gamma-ray astronomy. STACEE uses the large mirror area of a solar heliostat facility to achieve a low energy threshold. A prototype experiment which uses 32 heliostat mirrors with a total mirror area of ∼ 1200 m 2 has been constructed. This prototype, called STACEE-32, was used to search for high energy gamma-ray emission from the Crab Nebula and Pulsar. Observations taken between November 1998 and February 1999 yield a strong statistical excess of gamma-like events from the Crab, with a significance of +6.75σ in 43 hours of on-source observing time. No evidence for pulsed emission from the Crab Pulsar was found, and the upper limit on the pulsed fraction of the observed excess was < 5.5% at the 90% confidence level. A subset of the data was used to determine the integral flux of gamma rays from the Crab. We report an energy threshold of E th = 190 ± 60 GeV, and a measured integral flux of I(E > E th ) = (2.2 ± 0.6 ± 0.2) × 10 −10 photons cm −2 s −1 . The observed flux is in agreement with a continuation to lower energies of the power law spectrum seen at TeV energies.

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[1] Mid-tropospheric temperature soundings over tropical oceans by the Atmospheric Infrared Sounder, AIRS, using 4.3 micron CO 2 R-branch and P-branch channels independently measure about 260 K with one Kelvin semiannual variability. The difference between the soundings, which cancels seasonal variability, has increased over the past two years by 47 ± 9 mK/year. This trend is explained by the increase of 2.2 ± 0.4 ppmv/year and 0.6 ± 0.2 ppbv/year in the abundance of CO 2 and N 2 O, respectively, which results in a 35 mK/year trend. The ability to achieve closure at this level with only two years of AIRS data is very encouraging for measurements of other trends of atmospheric temperatures relevant to climate research.