The prediction of tropical cyclone rapid intensification is one of the most pressing unsolved problems in hurricane forecasting. The signatures of gravity waves launched by strong convective updrafts are often clearly seen in airglow and carbon dioxide thermal emission spectra under favorable atmospheric conditions. By continuously monitoring the Atlantic hurricane belt from the main development region to the vulnerable sections of the continental United States at high cadence, it will be possible to investigate the utility of storm-induced gravity wave observations for the diagnosis of impending storm intensification. Such a capability would also enable significant improvements in our ability to characterize the 3D transient behavior of upper-atmospheric gravity waves and point the way to future observing strategies that could mitigate the risk to human life caused by severe storms. This paper describes a new mission concept involving a midinfrared imager hosted aboard a geostationary satellite positioned at approximately 80°W longitude. The sensor’s 3-km pixel size ensures that the gravity wave horizontal structure is adequately resolved, while a 30-s refresh rate enables improved definition of the dynamic intensification process. In this way the transient development of gravity wave perturbations caused by both convective and cyclonic storms may be discerned in near–real time.

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An airborne hyperspectral imager operating in the midwave-infrared spectral range is described. The Mid-infrared Airborne Hyperspectral Imager (MAHI) features 3.3-nm spectral sampling over its 3.3-5.4 mu m wavelength range. MAHI operates in a roll-stabilized pushbroom configuration with 480 cross-track pixels, each with an instantaneous field-of-view (IFOV) of 0.94 mrad, to provide for a total FOV of 25.8 degrees. The sensor spectroradiometric performance is illustrated by case studies featuring the detection, identification, and quantification of a number of fugitive gaseous emissions from industrial sources.

A new airborne facility instrument for Earth science applications is introduced. The Mineral and Gas Identifier (MAGI) is a wide-swath (programmable up to ±42° off nadir) moderate spectral resolution thermal-infrared (TIR) imaging spectrometer that spans the 7.1- to 12.7-μm spectral window in 32 uniform and contiguous channels. Its spectral resolution enables improved discrimination of rock and mineral types, greatly expanded gas-detection capability, and generally more accurate land-surface temperature retrievals. The instrument design arose from trade studies between spectral resolution, spectral range, and instrument sensitivity and has now been validated by flight data acquired with the completed sensor. It offers a potential prototype for future space-based TIR instruments, which will require much higher spectral resolution than is currently available in order to address more detailed climate, anthropogenic, and solid Earth science questions.

A novel thermal-band imager is proposed for space-based Earth science measurement applications such as rock identification and volcano monitoring. The instrument, MAGI-L (Mineral and Gas Identifier - LEO), would also enable detection of gases from natural and anthropogenic sources. Its higher spectral resolution, compared to ASTER-type sensors, will improve discrimination of rock types, greatly expand the gas-detection capability, and result in more accurate land-surface temperatures. The optical design for MAGI-L will incorporate a novel compact Dyson spectrometer. Data from SEBASS have been used to examine the trade-offs between spectral resolution, spectral range, and instrument sensitivity for the proposed sensor.

Interpretation of surface radiation characteristics by overhead hyperspectral imagery often requires a compensation of the measured radiance data for the absorption and emission effects of the intervening atmosphere. We describe a procedure that accounts for these effects by direct use of the hyperspectral radiance data without recourse to ancillary meteorological data or atmospheric modeling. This in‐scene atmospheric compensation (ISAC) procedure is applicable to a broad class of problems. The present work concerns terrestrial surface characterization by remote sensing in the 8–13 μm atmospheric window band. A complete ISAC analysis is carried out in two steps. In the first step, unscaled atmospheric compensation spectra (transmittance and upwelling radiance) are extracted from the data. The step is carried out by application of a specially designed, line‐fitting procedure to a scatterplot constructed from the hyperspectral data. The compensation spectra are defined in terms of the slope and intercept parameters of the line. In the second step, these unscaled compensation spectra are scaled to quantitative compensation spectra. Here, this step is carried out using the strength of absorption in the 11.7‐μm water band. The compensation procedure is demonstrated by application to hyperspectral imagery obtained with the Aerospace Corporation's Spatially Enhanced Broadband Array Spectrograph System (SEBASS) hyperspectral imaging sensor at the Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) facility. Example applications demonstrate the ability of the method to remove atmospheric spectral structure from the observed data and reveal the spectral structure intrinsic to the underlying surface. This removal is demonstrated both for near‐blackbody surfaces composed of grass and for surfaces containing limestone gravel and Red Clay soil.

In May 1999, the airborne thermal infrared hyperspectral imaging system, Spatially Enhanced Broadband Array Spectrograph System (SEBASS), was flown over Mormon Mesa, NV, to provide the first test of such a system for geological mapping. Several types of carbonate deposits were identified using the 11.25-μm band. However, massive calcrete outcrops exhibited weak spectral contrast, which was confirmed by field and laboratory measurements. Because the weathered calcrete surface appeared relatively smooth in hand specimen, this weak spectral contrast was unexpected. Here we show that microscopic roughness not readily apparent to the eye has introduced both a cavity effect and volume scattering to reduce spectral contrast. The macroroughness of crevices and cobbles may also have a significant cavity effect. The diminished spectral contrast is important because it places higher signal-to-noise ratio (SNR) requirements for spectroscopic detection and identification. This effect should be factored into instrumentation planning and interpretations, especially interpretations without benefit of ground truth. SEBASS had the required high SNR and spectral resolution to allow us to demonstrate for the first time the ability of an airborne hyperspectral thermal infrared scanner to detect and identify spectrally subtle materials.

The alteration mineralogy of a porphyry-skarn system near Yerington, Nevada, was mapped using combined airborne hyperspectral HyMap and SEBASS data. The VNIR-SWIR HyMap data provided information about the abundance and levels of Tschermak substitution in white micas, as well as mapping the Mg-Fe chemistry of chlorite. The TIR SEBASS data provided unique information about the abundance and Fe-Al chemistry of garnet and Na-Ca chemistry of plagioclase feldspar. Both wavelength regions mapped calcite, dolomite, amphibole and epidote. The derived mineral maps were validated through spectral, mineralogical and chemical analyses of associated field samples. From these mineral maps, the geometry and physicochemistry of porphyry-skarn hydrothermal system becomes apparent, including the sites for Cu mineralization.

The Na-Ca chemistry of plagioclase felspar was measured and mapped remotely using airborne hyperspectral thermal infrared (TIR) SEBASS image data collected from a well-exposed porphyry system near Yerington, Nevada. This followed correction of the airborne SEBASS data for instrument and atmospheric effects and temperature-emissivity separation and was validated through associated field and laboratory studies. The later found a linear correlation between the relative intensities of the plagioclase /spl mu/FTIR emissivity spectral features at 9.6 and 10.0 /spl mu/m and the XRD dspacing of the felspar 111/130 hkl reflection, both of which are sensitive to the Na-Ca chemistry of plagioclase. The SEBASS derived felspar mineralogy maps clearly show variations in Na-Ca plagioclase felspar chemistry related to the nature of the host rock and any superimposed hydrothermal alteration.

We present intermediate-resolution (lambda/Delta lambda approximate to 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 mu m. The 9.7 mu m silicate feature is seen in absorption, and the 12.8 mu m [Ne II] fine structure line is seen in emission toward most of the observed nebulae. The strengths of both features vary enormously from nebula to nebula, suggesting large variations in the column densities of both Ne II and silicates toward these objects. Near-IR features attributed to polycyclic aromatic hydrocarbons (PAHs) are detected in six of the sources.Spherically symmetric dust shell models were calculated to obtain the best fits to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 mu m. The models are used to infer properties of the dust cocoon such as the distribution of density and temperature with radius, shell thickness, outer shell radius, and dust abundances. Our results are consistent with previous models that predict large dust cocoons with central cavities, sharp temperature gradients, and approximately constant density in the outer regions.

Thermal imagery from the spatially enhanced broadband array spectrograph system was analyzed for target detection purposes. The push-broom sensor was operated as part of the WESTERN RAINBOW experiment in October 1995. Data from 7.8- 13.4 microns were collected in 128 wavelength bands, with 128 pixels in the cross-track direction. The data set had nominal ground-resolution of better than one meter. Analysis techniques normally used in the reflective domain, with traditional imaging spectrometers, were used for the thermal data. Analysis was done in both the radiance and emissivity domains, following careful thermal calibration and atmospheric compensation. The techniques utilized were principal components, spectral angle mapper, and spectral matched filter. All wee successful, with the first two showing a success rate comparable to that found in similar experiments in the reflective domain. The principal components techniques was successful in discriminating man- made objects and disturbed earth from the desert background, much as expected. It was also successful in distinguishing between different categories of man-made objects. Of the latter two techniques, the spectral matched filter was more successful. This relatively greater success is attributed to the sensitivity of the spectral angle mapper to calibration errors, particular in the conversion from radiance to emissivity.

We describe a new form of prism spectrograph system based on aplanatic principles. The basic system is simple, comprising a prism with two spherical refracting surfaces, both operating near their aplanatic conjugates, and a spherical mirror operating near its center of curvature. This form provides a flat, accessible focal surface suitable for use with modem array detectors. Good image quality can be maintained over large wavelength intervals at fast focal ratios, making this form particularly useful for moderate resolution spectrography. Its simplicity, compactness, and tolerance of misalignment make it attractive for space and cryogenic instruments. We present three examples of operating instruments that have been constructed using this new form. (C) 1997 Society of Photo-Optical Instrumentation Engineers.

We describe the design and performance of an infrared imaging spectrograph that was first used as an airborne sensor in October, 1995. This instrument, called the spatially-enhanced broadband array spectrograph system (SEBASS), is intended to explore the utility of hyperspectral infrared sensors for remotely identifying solids, liquids, gases, and chemical vapors in the 2 to 14 micrometers 'chemical fingerprint' spectral region. The instrument, which is an extension of an existing non-imaging spectrograph uses two spherical-faced prisms to operate simultaneously in the atmospheric transmission windows found between 2.0 and 5.2 micrometers and between 7.8 and 13.4 micrometers (LWIR). ALthough the SEBASS instrument is designed primarily for use from an aircraft platform, it was used in March 1996 for a tower-based collection.

Spectroscopic studies in the 'fingerprint' region of the thermal IR from 3 to 14 microns of celestial dust components and the overall energy distribution of the sources are best served by moderate spectral resolution (R = lambda/Delta lambda approximately 30 to 200), high sensitivity observations. Spectral purity and the reproducibility of the spectral shape are critical as well, when using the spectral shape to assign temperatures to dust grains or to gas clouds based on the wavelength and shape of molecular bands. These sensor attributes are also important to the use of wavelengths and ratios of solid state features to derive compositions of dust grains in celestial sources. The advent of high quality linear arrays of blocked impurity band (BIB) detectors of Si:As permitted the development of a state-of-the-art, patented, cooled prism spectrograph. Developed at The Aerospace Corporation largely with in-house funds, the Broadband Array Spectrograph System (BASS) has been used for a variety of remote sensing applications, but especially for IR astronomical studies on the Kuiper Airborne Observatory and at the NASA Infrared Telescope Facility (IRTF). The attributes of the spectrograph, specifically having the pupil imaged onto the 2 linear 58 element detector arrays so that the effects of guiding errors are minimized, being able to maximally exploit the limited observing time by acquiring all 116 spectral channels simultaneously, and having all spectral channels imaged through the same aperture so that spectral mapping is readily and reliably accomplished, afford the scientist with a unique opportunity to conduct both surveys of examples of many different types of sources as well as in-depth studies of a given class of object by thoroughly sampling the class members. This duality was demonstrated with the BASS through a combination of KAO flights where spectral maps were obtained as part of in-depth studies of specific source regions (such as Orion and W3) and flights where up to 21 legs/flight were used to survey many different sources of different types.