Christophe Miesch

Direct and inverse radiative transfer solutions for visible and near-infrared hyperspectral imagery

Two reciprocal direct and inverse radiative transfer models dealing with hyperspectral remote sensing in the visible-to-shortwave-infrared spectral domain are described in this paper. The first one, called COMANCHE, considers a flat and heterogeneous ground scene, with bidirectional reflectance effects, and computes spectral radiance hypercubes at the sensor level. Trapping and environment phenomena are take into account through specific optimized Monte Carlo modules. The reciprocal inverse algorithm, called COCHISE, considers a sensor-level hyperspectral image and retrieves the ground spectral reflectance distribution as well as the water vapor content. COCHISE removes the atmospheric and environment effects with the same modeling as COMANCHE, but consider however the Lambertian assumption for the ground reflectance. Both of the models are validated with existing radiative transfer codes (MODTRAN and AMARTIS, for instance), and also using experimental datasets from the Airborne Visible/Infrared Imaging Spectrometer. The comparisons show very good agreement regarding to the usual uncertainties involved insuche experiment. COCHISE is also applied on a additional dataset acquired by HyMap.

Monte Carlo approach for solving the radiative transfer equation over mountainous and heterogeneous areas

An algorithm based on the Monte Carlo method is developed to solve the radiative transfer equation in the reflective domain (0.4-4 microm) of the solar spectrum over rugged terrain. This algorithm takes into account relief, spatial heterogeneity, and ground bidirectional reflectance. The method permits the computation of irradiance components at ground level and radiance terms reaching an airborne or satelliteborne sensor. The Monte Carlo method consists of statistically simulating the paths of photons inside the Earth-atmosphere system to reproduce physical phenomena while introducing neither analytical modeling nor assumption. The potentialities of the code are then depicted over different types of landscape, including a seashore, a desert region, and a steep mountainous valley.

Radiative transfer solution for rugged and heterogeneous scene observations

A physical algorithm is developed to solve the radiative transfer problem in the solar reflective spectral domain. This new code, Advanced Modeling of the Atmospheric Radiative Transfer for Inhomogeneous Surfaces (AMARTIS), takes into account the relief, the spatial heterogeneity, and the bidirectional reflectances of ground surfaces. The resolution method consists of first identifying the irradiance and radiance components at ground and sensor levels and then modeling these components separately, the rationale being to find the optimal trade off between accuracy and computation times. The validity of the various assumptions introduced in the AMARTIS model are checked through comparisons with a reference Monte Carlo radiative transfer code for various ground scenes: flat ground with two surface types, a linear sand dune landscape, and an extreme mountainous configuration. The results show a divergence of less than 2% between the AMARTIS code and the Monte Carlo reference code for the total signals received at satellite level. In particular, it is demonstrated that the environmental and topographic effects are properly assessed by the AMARTIS model even for situations in which the effects become dominant.

Validation and robustness of an atmospheric correction algorithm for hyperspectral images

The Optics Department of ONERA has developed and implemented an inverse algorithm, COSHISE, to correct hyperspectral images of the atmosphere effects in the visible-NIR-SWIR domain (0,4-2,5 micrometers ). This algorithm automatically determine the integrated water-vapor content for each pixel, from the radiance at sensor level by using a LIRR-type (Linear Regression Ratio) technique. It then retrieves the spectral reflectance at ground level using atmospheric parameters computed with Modtran4, included the water-vapor spatial dependence as obtained in the first stop. The adjacency effects are taken into account using spectral kernels obtained by two Monte-Carlo codes. Results obtained with COCHISE code on real hyperspectral data are first compared to ground based reflectance measurements. AVIRIS images of Railroad Valley Playa, CA, and HyMap images of Hartheim, France, are use. The inverted reflectance agrees perfectly with the measurement at ground level for the AVIRIS data set, which validates COCHISE algorithm/ for the HyMap data set, the results are still good but cannot be considered as validating the code. The robustness of COCHISE code is evaluated. For this, spectral radiance images are modeled at the sensor level, with the direct algorithm COMANCHE, which is the reciprocal code of COCHISE. The COCHISE algorithm is then used to compute the reflectance at ground level from the simulated at-sensor radiance. A sensitivity analysis has been performed, as a function of errors on several atmospheric parameter and instruments defaults, by comparing the retrieved reflectance with the original one. COCHISE code shows a quite good robustness to errors on input parameter, except for aerosol type.

MATISSE: version 1.4 and future developments

This paper presents the MATISSE-v1.4 code whose main functionality is to compute spectral or integrated natural background radiance images. The spectral bandwidth extends from 765 to 3300 cm-1 (3 to 13 μm) with a 5 cm-1 resolution. Natural backgrounds include the atmosphere, low and high altitude clouds, sea and land. The most particular functionality of the code is to take into account atmospheric spatial variability quantities (temperatures, mixing ratio, etc) along each line of sight of the image. In addition to image generation capacity, the code computes atmospheric radiance and transmission along a line of sight with the same spectral characteristics as in imaging mode. In this case atmospheric refraction effects and radiation from high or low altitude clouds can be taken into account. A high spectral resolution mode is also available to propagate radiation from a high temperature medium in the same atmospheric state as that used for the image generation. Finally, an Application Programming Interface (API) is included to facilitate its use in conjunction with external codes. This paper describes the range of functionalities of MATISSE-v1.4 whose release is planned for April 2006. Future developments are also presented.

Phenomenological analysis of simulated signals observed over shaded areas in an urban scene

This paper analyzes the signal measured by optical remote sensors when acquiring data over a shaded part of an urban scene. The signal is much lower for this kind of target than for others because there is no direct downward irradiance. Here, a simple urban scene is considered with a shaded area. The signal observed by a high spatial resolution satellite sensor over an ordinary panchromatic band (500-700 nm) is computed thanks to a radiative transfer code [advanced modeling of the atmospheric radiative transfer for inhomogeneous surfaces (Amartis)] capable of dealing with ground topography and heterogeneity. The signal is analyzed, and it appears that environmental effects play a significant role. Moreover, because of the scattering that occurs at shorter wavelengths, it is also shown that a widening of the band to 440 nm sharpens the difference between signals coming from two different ground types (for whose the difference of reflectance is constant and equal to 0.1) by about 10%. This demonstrates that the band widening may be beneficial to observation in shadow, mainly because of scattering effects. A more realistic scene is also considered, in which each part is associated with realistic spectral properties. This simply shows the importance of the thematic in the choice of band, as it determines the effect of the widening.

Sensor radiance physical model for rugged heterogeneous surfaces in the 3–14 μm region.

We present a physical model describing the radiance acquired by an infrared sensor over a rugged heterogeneous surface. This model predicts the radiance seen over complex landscapes like urban areas and provides an accurate analysis of the signal, as each component is available at ground and sensor level. Plus, it allows data comparison from different instruments. Two representative cases (natural and urban) are analysed to show the composition and the construction of the sensor signal and to highlight the importance of having a 3D model, especially for rugged surfaces where environment weights in the overall spectral domain.

Irradiance calculation over mountainous areas in the reflective spectral domain: comparison with an accurate radiative transfer code

A new model has been developed to estimate irradiance at ground level over a rugged terrain in the reflective spectral domain in order to be used in an hyperspectral inversion code. Modtran4 allows to calculate atmospheric parameters over a flat scene which are then used to estimate the four components of irradiance over a mountainous area (direct, diffuse, reflected and coupling irradiance). This method have been compared with an accurate radiative transfer code called AMARTIS. Simulations are done at three wavelengths and for two solar configurations over a relief composed of two hills and flat terrain. Irradiances obtained with our model are in good agreement with this reference code except in shadow areas in the SWIR. Our model is also compared with a currently used model developed by Sandmeier whose results are worse than our models results. Current relative errors of our diffuse, reflected and coupling irradiance calculation model do not have much influence on total irradiance in most of the cases. This influence become significant for high beam incidence angles where Digital Elevation Model errors can be much more important.

Evaluation of the different irradiance components on a rugged terrain

An algorithm based on the Monte Carlo principle is developed to solve the radiative transfer problem in the reflective domain of the solar spectrum and is used to precisely evaluate ground irradiance on a rugged terrain. This method allows to simulate paths of photons inside the earth- atmosphere system without any assumption and calculate the different identified irradiance components and particularly those coming from environment. To establish the relative contribution of each of these terms, several typical relief and atmosphere configurations are considered. In a first step, two ground types simulations assuming lambertian reflectances are computed. Over vegetation-covered hills in the near IR, in the portion badly exposed to the direct solar beam, the environment irradiance contributes more than 20 percent of the total signal received at ground level. When severe slopes and higher reflectance values are considered, this contribution can exceed 60 percent in shadowed areas. These simulations demonstrate the necessity to take into account the high order terms when the region of interest presents important slopes and/or high reflectance ground. the case of non-lambertian reflectances is also dealt and it is shown that in the present configuration lambertian reflectances can be assumed to calculate the environment terms without significant errors on total irradiance, even in the shadow.

MATISSE--advanced earth modeling for imaging and scene simulation: first results

In this paper we present MATISSE 1.1 a new background scene generator, whose goal is to compute spectral or integrated radiance images of natural background, as well as the transmission of a hot gas signature. The spectral bandwidth for this version of the code is from 750 to 3300 cm-1 (3 to 13 μm) with a 5 cm-1 resolution. Gaseous absorption is computed by a Correlated K model. The spatial variability of atmospheric quantities (temperatures and mixing ratios, among others) is taken into account, using variable profiles along the line of sight. Natural backgrounds include the atmospheric background, low altitude clouds and the Earth ground. The radiation models used are designed for observation at low spatial resolution of clouds and soils, so a texture model was developed to increase the high spatial resolution rendering in the metric range. Intermediate outputs of the code deliver radiance and transmission restricted to a single line of sight, in which case atmospheric refraction effects are taken into account. Along this line of sight the transmission can also be computed using a line-by-line model, which is useful to propagate the radiation emitted by a hot gas source (fires, aircraft or missile plume). MATISSE 1.1 was released in June 2002, so this paper is devoted to a presentation of the first results obtained with the code and some validation tests.