Within the last few years, several commercial long-wave infrared (LWIR) hyperspectral imaging (HSI) systems have been developed for remote sensing of the ground from aircraft. While much less expensive and more practical to operate than sensors such as SEBASS and MAKO, which have been developed primarily for research and Government use, the commercial systems have poorer signal-to-noise and/or spectral resolution. We investigate the utility of three commercial systems—the Telops Hyper-Cam, SPECIM AisaOWL, and ITRES TASI-600—for quantitative retrieval of surface temperature and emissivity spectra. Atmospheric retrieval, correction and temperature-emissivity separation are performed on example data from these sensors using FLAASH-IR, a first-principles algorithm that incorporates radiation transport calculations and atmosphere models from MODTRAN. The results from the commercial sensors are noisy compared with SEBASS but otherwise appear to be reasonable. Applying a noise suppression algorithm to the radiance data yields better temperature retrievals and much cleaner emissivity spectra, with minimal loss of information, and should benefit scene classification applications.
Hyperspectral imaging (HSI) sensors have the ability to detect and identify objects within a scene based on the distinct attributes of their surface spectral signatures. Many targets of interest, such as vehicles, represent a complex arrangement of specular (non-Lambertian) materials with curved and flat surfaces oriented at varying view factors. This complexity, combined with possible changing atmospheric/illumination conditions and viewing geometries, can produce significant variations in the observed signatures from measurement to measurement, making detection and/or reacquisition challenging. This paper focuses on the characterization of visible-near infrared-short wave infrared (VNIR-SWIR) spectra for detection, identification and tracking of vehicles. Signature variations are predicted using a novel image simulation tool to calculate spectral images of complex 3D objects from a spectral material description such as the modified Beard-Maxwell BRDF model, a wireframe shape model, and a directional model of the illumination. We compare the simulations with recent VNIR-SWIR hyperspectral imagery of vehicles and panels collected at the Rochester Institute of Technology during an Autumn 2015 measurement campaign. Variations in both the simulated and measured spectra arise mainly from differences in the relative glint contribution. Implications of these variations on vehicle detection and identification are briefly discussed.
The MODTRAN6 radiative transfer (RT) code is a major advancement over earlier versions of the MODTRAN
atmospheric transmittance and radiance model. This version of the code incorporates modern software ar-
chitecture including an application programming interface, enhanced physics features including a line-by-line
algorithm, a supplementary physics toolkit, and new documentation. The application programming interface
has been developed for ease of integration into user applications. The MODTRAN code has been restructured
towards a modular, object-oriented architecture to simplify upgrades as well as facilitate integration with other
developers' codes. MODTRAN now includes a line-by-line algorithm for high resolution RT calculations as well
as coupling to optical scattering codes for easy implementation of custom aerosols and clouds.
Remotely sensed spectral imagery of the earth's surface can be used to fullest advantage when the influence of the atmosphere has been removed and the measurements are reduced to units of reflectance. Here, we provide a comprehensive summary of the latest version of the Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes atmospheric correction algorithm. We also report some new code improvements for speed and accuracy. These include the re-working of the original algorithm in C-language code parallelized with message passing interface and containing a new radiative transfer look-up table option, which replaces executions of the MODTRAN® model. With computation times now as low as ~10 s per image per computer processor, automated, real-time, on-board atmospheric correction of hyper- and multi-spectral imagery is within reach.
Land and ocean data product generation from visible-through-shortwave-infrared multispectral and hyperspectral
imagery requires atmospheric correction or compensation, that is, the removal of atmospheric absorption and scattering
effects that contaminate the measured spectra. We have recently developed a prototype software system for automated,
low-latency, high-accuracy atmospheric correction based on a C++-language version of the Spectral Sciences, Inc.
FLAASH™ code. In this system, pre-calculated look-up tables replace on-the-fly MODTRAN® radiative transfer
calculations, while the portable C++ code enables parallel processing on multicore/multiprocessor computer systems.
The initial software has been installed on the Sensor Web at NASA Goddard Space Flight Center, where it is currently
atmospherically correcting new data from the EO-1 Hyperion and ALI sensors. Computation time is around 10 s per
data cube per processor. Further development will be conducted to implement the new atmospheric correction software
on board the upcoming HyspIRI mission's Intelligent Payload Module, where it would generate data products in nearreal
time for Direct Broadcast to the ground. The rapid turn-around of data products made possible by this software
would benefit a broad range of applications in areas of emergency response, environmental monitoring and national
The QUick Image Display (QUID) model accurately computes and displays radiance images of aircraft and other
objects, generically called targets, at animation rates while the target undergoes unrestricted flight. Animation rates are
obtained without sacrificing radiometric accuracy by using two important innovations. First, QUID has been
implemented using the Open Scene Graph (OSG) library, an open-source, cross-platform 3-D graphics toolkit for the
development of high performance graphics applications in the fields of visual simulation, virtual reality, scientific
visualization and modeling. Written entirely in standard C++ and fully encapsulating OpenGL and its extensions, OSG
exploits modern graphics hardware to perform the computationally intensive calculations such as hidden surface
removal, 3-D transformations, and shadow casting. Second, a novel formulation for reflective/emissive terms enables
rapid and accurate calculation of per-vertex radiance. The bi-directional reflectance distribution function (BRDF) is a
decomposed into separable spectral and angular functions. The spectral terms can be pre-calculated for a user specified
band pass and for a set of target-observer ranges. The only BRDF calculations which must be performed during target
motion involves the observer-target-source angular functions. QUID supports a variety of target geometry files and is
capable of rendering scenes containing high level-of-detail targets with thousands of facets. QUID generates accurate
visible to LWIR radiance maps, in-band and spectral signatures. The newest features of QUID are illustrated with
radiance and apparent temperature images of threat missiles as viewed by an aircraft missile warning system.
Atmospheric Correction Algorithms (ACAs) are used in applications of remotely sensed Hyperspectral and Multispectral Imagery (HSI/MSI) to correct for atmospheric effects on measurements acquired by air and space-borne systems. The Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) algorithm is a forward-model based ACA created for HSI and MSI instruments which operate in the visible through shortwave infrared (Vis-SWIR) spectral regime. Designed as a general-purpose, physics-based code for inverting at-sensor radiance measurements into surface reflectance, FLAASH provides a collection of spectral analysis and atmospheric retrieval methods including: a per-pixel vertical water vapor column estimate, determination of aerosol optical depth, estimation of scattering for compensation of adjacency effects, detection/characterization of clouds, and smoothing of spectral structure resulting from an imperfect atmospheric correction. To further improve the accuracy of the atmospheric correction process, FLAASH will also detect and compensate for sensor-introduced artifacts such as optical smile and wavelength mis-calibration. FLAASH relies on the MODTRAN<sup>TM</sup> radiative transfer (RT) code as the physical basis behind its mathematical formulation, and has been developed in parallel with upgrades to MODTRAN in order to take advantage of the latest improvements in speed and accuracy. For example, the rapid, high fidelity multiple scattering (MS) option available in MODTRAN4 can greatly improve the accuracy of atmospheric retrievals over the 2-stream approximation. In this paper, advanced features available in FLAASH are described, including the principles and methods used to derive atmospheric parameters from HSI and MSI data. Results are presented from processing of Hyperion, AVIRIS, and LANDSAT data.
We describe a new visible-near infrared short-wavelength infrared (VNIR-SWIR) atmospheric correction method for multi- and hyperspectral imagery, dubbed QUAC (QUick Atmospheric Correction) that also enables retrieval of the wavelength-dependent optical depth of the aerosol or haze and molecular absorbers. It determines the atmospheric compensation parameters directly from the information contained within the scene using the observed pixel spectra. The approach is based on the empirical finding that the spectral standard deviation of a collection of diverse material spectra, such as the endmember spectra in a scene, is essentially spectrally flat. It allows the retrieval of reasonably accurate reflectance spectra even when the sensor does not have a proper radiometric or wavelength calibration, or when the solar illumination intensity is unknown. The computational speed of the atmospheric correction method is significantly faster than for the first-principles methods, making it potentially suitable for real-time applications. The aerosol optical depth retrieval method, unlike most prior methods, does not require the presence of dark pixels. QUAC is applied to atmospherically correction several AVIRIS data sets and a Landsat-7 data set, as well as to simulated HyMap data for a wide variety of atmospheric conditions. Comparisons to the physics-based Fast Line-of-sight Atmospheric Analysis of Spectral Hypercubes (FLAASH) code are also presented.