Passive multi-spectral and hyper-spectral optical sensors offer great potential for remote-sensing in general and detection of low-concentration toxic species in particular. Correction for propagation through the intervening atmosphere (with molecular, aerosol and hydrosol constituents) is often the performance-limiting factor, a step which is unavoidably imprecise given uncertainties in the physical description of the atmospheric column. We propose to describe a general approach for mitigating the effects of these uncertainties using spectral sub-space projection, and to apply the technique to the difficult problem of trace-gas detection and quantification for environmental monitoring. While the approach may be applied to data collected by ground, aircraft or space-based sensors, we will illustrate it with hyperspectral off-nadir simulated imagery appropriate to a high-altitude aircraft. In addition to a description of the approach and simulated HSI data, detection/quantification performance using matched-filters for a number of common pollutants with and without sub-space projection will be presented. Use of matched-filters for detection has the additional benefit of quantitatively ranking the spectral weight of measurements for a given trace gas with the specified conditions and uncertainties. Such results bear directly on sensor design issues --spectral extent and resolution--and suggest that data volumes could be dramatically reduced by projection onto the target sub-space (s) prior to down-link.
Modern optical sensors can provide high quality multi/hyperspectral measurements of apparent radiance with high spectral and spatial resolution. A driving consideration if the full potential of this information stream be realized is mitigation of the effects of the --unavoidable -- uncertain elements which contribute to the radiance at aperture. These include atmospheric, geometric and shadowing effects, as well as variations in the optical properties of the target(s). Many techniques have recently been developed to address various aspects of this problem and much progress has been made. A general technique follows from calculation of a linear sub-space for the target vector, whose rank and extent reflects the uncertainty in the target radiance. With global coverage of thin cirrus exceeding eighty percent, it is natural to seek improved target detection performance for operational systems by anticipating the deleterious effects of thin clouds. We describe preliminary results for target detection within original and simulated hypercubes, with and without intervening thin clouds, using TSSP (Target Sub-Space Detection). The advantages of this technique for retrieval through realistic clouds -- non plane-parallel with spatially varying optical properties -- will also be discussed.
Modem optical sensors can provide high quality multi/hyperspectral data at high spatial resolution, permitting the application of diverse and sophisticated algorithms for remote sensing of the terrain and atmosphere. With global coverage of perceptible cloud exceeding seventy-five percent [Wylie & Menzel, 1999], it is important that the effects of intervening cloud be anticipated and minimized to realize the full potential of such systems. Cloud contamination also bears on the more general issue of "information content" in a HSI data stream. This paper will describe the application of the Vis-LWIR scene simulation tools CLDSIM / GENESSIS / MOSART for assessing spectral/spatial matched-filter algorithms for the detection and classification of features-of-interest against terrain, with and without thin clouds. Following a review of the methodology, the sensitivity of matched-filter SNR to cloud-cover, vs GSD, as captured in sequential subsets of the primary principal-components will be presented. The potential for mis-classification due to undetected thin-clouds will also be described.
MSTI-III is the third in the Miniature Sensor Iechno1o' Integration series of satellites originally
conceived of and developed by BMDO and now being conducted by the US Air Force1. As the name
implies, the MSTI . satellites are members of the class of satellites which have come to be known as
"smalisats" or, alternatively, as "lightsats". The satellite is tentatively planned for launch in early 1996
into a sun-synchronous orbit which will allow the satellite to revisit a given region at the same solar time,
that is, at the same solar illumination conditions.
Among the objectives ofthe MSTI-III mission is the characterization of the optical properties of clouds
in the mid-wavelength infrared (MWIR), short-wavelength-infrared (SWIR), and visible spectral regions as
a function of spectral band, latitude, season, cloud type, cloud altitude, and solar scattering geometry.
MSTI-III is planned to be operational for a minimum of one year to ensure the collection of a statistically
significant data base. MSTI-III will also image the EartWs limb and surface. Radiometric characterization
and calibration completed in Aug 1995 indicate that MSTI-III will provide high-spatial-resolution infrared
images with high signal-to-noise ratios, based upon the scene radiances predicted for the planned
measurement modes from existing models or estimated from empirical data.
The data-collection experiments planned for MSTI-III and the utility of the MSTI-III data base for the
validation and development of cloud, atmospheric, and surface radiance models are discussed.
The radiative properties of cirrus clouds are of wide interest because of the visually striking effects they produce, the significant global coverage of cirrus and thin cirrus,
and their intrinsic complexity. Within a limited class of crystal shape and size, hydrodynamic forces tend to orient the crystals with their long axis nearly horizontal, which can produce a narrow, intense specular reflection. First-principles scattering
calculations are especially difficult because the scatterers are diverse in shape, size and tilt distributions. However, a geometric optics approximation to the single-crystal BRDF combined with plausible distributions of flutter-angle can be used to both
simulate the appearance of the specular radiance feature, and extract microphysical cloud information from imagery of the specular point. This paper will review an empirical approach to radiative-transfer in the specular layer and integrate two different models for the specular BRDF into a cloud radiance simulation code. This will
then be used to illustrate the appearance of the subsun and Bottlinger's ring for various spectral bands. The extraction of microphysical information on the specular layer from vis - near-ir cloud imagery will then be discussed.
The surface temperature retrieved from thermal infrared satellite data in the presence (either known or unknown) of cirrus clouds can be in error. This error results from the absorption and scattering by the cirrus cloud of the surface and intervening atmospheric radiation. Using parameterizations of standard and subvisual cirrus cloud micrometeorology (e.g., particle size distributions, density) based on recent observations, the effects of cirrus clouds have been modelled to determine the range of temperature errors. MODTRAN is the primary tool used in this evaluation to establish the temperature difference over the 3 - 14 micrometers spectral region as a function of: optical depth, asymmetry parameter, scattering albedo, cloud temperature, cloud base and thickness, and ground temperature. The advantage to computing temperature differences as a function of scattering albedo and asymmetry parameter is to permit these results to be applied when a more accurate model of scattering by cirrus crystals has been developed. Temperature errors for realistic cirrus clouds may then be derived from the albedo and asymmetry parameter implied by this model.