Siegfried A. W. Gerstl

Multi-angle Imaging SpectroRadiometer (MISR) instrument description and experiment overview

The Multi-angle Imaging SpectroRadiometer (MISR) instrument is scheduled for launch aboard the first of the Earth Observing System (EOS) spacecraft, EOS-AM1. MISR will provide global, radiometrically calibrated, georectified, and spatially coregistered imagery at nine discrete viewing angles and four visible/near-infrared spectral bands. Algorithms specifically developed to capitalize on this measurement strategy will be used to retrieve geophysical products for studies of clouds, aerosols, and surface radiation. This paper provides an overview of the as-built instrument characteristics and the application of MISR to remote sensing of the Earth.

Nonlinear spectral mixing models for vegetative and soil surfaces

Abstract In this article we apply an analytical solution of the radiosity equation to compute vegetation indices, reflectance spectra, and the spectral bidirectional reflectance distribution function for simple canopy geometries. We show that nonlinear spectral mixing occurs due to multiple reflection and transmission from surfaces. We compare radiosity-derived spectra with single scattering or linear mixing models. We also develop a simple model to predict the reflectance spectrum of binary and ternary mineral mixtures of faceted surfaces. The two facet model is validated by measurements of the reflectance.

The radiosity method in optical remote sensing of structured 3-D surfaces

Abstract The radiosity method is a mathematical concept to describe the scattering of light between ideally diffuse (Lambertian) surfaces. The method presented takes reflections, transmission, and multiple scattering into account. Algorithms to find view factors and to solve the radiosity equations using the Gauss-Seidel iteration method are described. An example for a layered plant canopy model shows the relation between the radiosity method and radiative transfer. The application of the radiosity method to remote sensing problems of 3-D surfaces, e.g., calculation of a BRDF including internal shadowing effects is shown. Numerical results of radiosity calculations are compared with equivalent radiative transfer results. We conclude that the radiosity method is a valuable tool to model the transport of light in vegetative canopies as well as a tool to evaluate bidirectional reflectance characteristics of discrete leaf canopy structures, such as angular reflectance signatures.

MISR: A multiangle imaging spectroradiometer for geophysical and climatological research from Eos

The scientific objectives, instrument concept, and data plan for the multiangle imaging spectroradiometer (MISR), an experiment proposed for the Eos (Earth Observing System) mission, are described. MISR is a pushbroom imaging system designed to obtain continuous imagery of the sunlit Earth at four different view angles (25.8 degrees , 45.6 degrees , 60.0 degrees , and 72.5 degrees relative to the vertical at the Earths surface), in both the forward and aftward directions relative to nadir, using eight separate cameras. Observations will be acquired in four spectral bands, centered at 440, 550, 670, and 860 nm. Data analysis algorithms will be applied to MISR imagery to retrieve the optical, geometric, and radiative properties of complex, three-dimensional scenes, such as aerosol-laden atmospheres above a heterogeneously reflecting surface, nonstratified cloud systems, and vegetation canopies. The MISR investigation will address a number of scientific questions concerning the climatic and ecological consequences of many natural and anthropogenic processes, and will furnish the aerosol information necessary. >

3-D Scene Modeling of Semidesert Vegetation Cover and its Radiation Regime

To explore the potential of multiangle remote sensing for estimating biophysical or ecological parameters over a variety of landscapes, a modeling tool that is capable of handling three-dimensional (3-D) heterogeneous structures, deriving ecological parameters from the vegetation structure, and effectively working on different scene scales is very desirable. A 3-D scene modeling approach for these purposes is presented in this paper. This 3-D model fulfills its goal by taking advantage of radiosity theory and computer graphics techniques. It consists of two major modules: a modified extended L-systems (MELS) method to generate a 3-D realistic scene and a radiosity-graphics combined method (RGM) to calculate the radiation regime based on the 3-D structures rendered with MELS. The 3-D simulation tool is then evaluated using field measurements of both plant structure and spectra collected during the NASA Earth Observing Satellite Prototype Validation Exercise Jornada field campaign near Las Cruces, NM. The modeled scene reflectance is compared with measurements from three platforms (ground, tower, and satellite) at various scales (from the size of individual shrub component to satellite pixels of kilometers). The agreement with measured reflectances is excellent at all sampling scales tested. As an example of the models application, we use the model output to examine the validity of a linear mixture scheme over the Jornada semidesert scene. The result shows that the larger the sampling size (at least larger than the size of the shrub component), the better the hypothesis is satisfied because of the unique structure of the Jornada scene: dense plant clumps (shrub component) sparsely scattered on a predominantly bare soil background. A range of possible applications of this 3-D scene model is highlighted, and further work needed for 3-D modeling is also discussed.

Radiative transfer in three dimensional leaf canopies

Abstract The governing equation of transfer, cross sections, and a modified discrete ordinates method for numerical solution of the transfer equation for a three dimensional leaf canopy are formulated. Models for the total interaction and scattering cross sections are proposed, and preliminary transport calculations are reported. Convergence acceleration of the iteration on the scattering source using the coarse mesh rebalancing method and convergence criteria are discussed. The extent of flux distortions such as negative and oscillatory fluxes, ray effects, etc. is assessed, and their implications for optical radiometric remote sensing is discussed.

Radiation physics and modelling for off-nadir satellite sensing of non-Lambertian surfaces

Abstract The primary objective of this paper is to provide a deeper understanding of the physics of satellite remote-sensing when off-nadir observations are considered. Emphasis is placed on the analysis and modelling of atmospheric effects and the radiative transfer of non-Lambertian surface reflectance characteristics from ground-level to satellite locations. We evaluate the relative importance of spectral, spatial, angular, and temporal reflectance characteristics for satellite-sensed identification of vegetation types in the visible and near-infrared wavelength regions. Highest identification value is attributed to angular reflectance signatures. Using radiative transfer calculations to evaluate the atmospheric effects on angular reflectance distributions of vegetation surfaces, we identify atmosphere-invariant angular reflectance features such as the “hot spot” and the “persistent valley”. A new atmospheric correction formalism for complete angular reflectance distributions is described. A sample calculation demonstrates that a highly non-Lambertian measured surface reflectance distribution can be retrieved from simulated satellite data in the visible and near infrared to within about 20% accuracy for almost all view directions up to 60° off-nadir. Thus the high value of angular surface reflectance characteristics (the “angular signature”) for satellite-sensed feature identification is confirmed, which provides a scientific basis for future off-nadir satellite observations.

Coupled atmosphere/canopy model for remote sensing of plant reflectance features.

Solar radiative transfer through a coupled system of atmosphere and plant canopy is modeled as a multiple-scattering problem through a layered medium of random scatterers. The radiative transfer equation is solved by the discrete-ordinates finite-element method. Analytic expressions are derived that allow the calculation of scattering and absorption cross sections for any plant canopy layer from measurable biophysical parameters such as the leaf area index, leaf angle distribution, and individual leaf reflectance and transmittance data. An expression for a canopy scattering phase function is also given. Computational results are in good agreement with spectral reflectance measurements directly above a soybean canopy, and the concept of greenness- and brightness-transforms of Landsat MSS data is reconfirmed with our computed results. A sensitivity analysis with the coupled atmosphere/canopy model quantifies how satellite-sensed spectral radiances are affected by increased atmospheric aerosols, by varying leaf area index, by anisotropic leaf scattering, and by non-Lambertian soil boundary conditions. Possible extensions to a 2-D model are also discussed.

Discrete-ordinates finite-element method for atmospheric radiative transfer and remote sensing

Advantages and disadvantages of modern discrete-ordinates finite-element methods for the solution of radiative transfer problems in meteorology, climatology, and remote sensing applications are evaluated. After the common basis of the formulation of radiative transfer problems in the fields of neutron transport and atmospheric optics is established, the essential features of the discrete-ordinates finite-element method are described including the limitations of the method and their remedies. Numerical results are presented for 1-D and 2-D atmospheric radiative transfer problems where integral as well as angular dependent quantities are compared with published results from other calculations and with measured data. These comparisons provide a verification of the discrete-ordinates results for a wide spectrum of cases with varying degrees of absorption, scattering, and anisotropic phase functions. Accuracy and computational speed are also discussed. Since practically all discrete-ordinates codes offer a builtin adjoint capability, the general concept of the adjoint method is described and illustrated by sample problems. Our general conclusion is that the strengths of the discrete-ordinates finite-element method outweight its weaknesses. We demonstrate that existing general-purpose discrete-ordinates codes can provide a powerful tool to analyze radiative transfer problems through the atmosphere, especially when 2-D geometries must be considered.

Multiangle remote sensing: Past, present and future

Multiangle remote sensing has many new and important applications in the study of the earths land, ocean, and atmosphere. For land studies, multiangle remote sensing samples the bidirectional reflectance distribution function (BRDF) of land surfaces. The modeling and observation of land surface BRDFs has thus been an area of active research for the past decade. The International Forum on BRDF (IFB) was organized in December, 1998, in San Francisco to summarize recent progress in BRDF research, and to identify important future research topics and determine their priorities. This special issue of Remote Sensing Reviews presents a series of summary papers outlined at the IFB that focus on specific BRDF research areas. This paper provides an overview of the special issue by summarizing IFB discussions and individual papers. It also presents five primary courses of action for the BRDF community identified during the IFB. These include (1) identifying a set of key scientific questions to which multiangle remote sensing provides a qualitative and quantitative advances over more traditional approaches, as well as organizing case studies to show the value added by multiangle remote sensing; (2) exploring different inversion techniques, including data fusion and assimilation, to estimate land surface variables that are highly relevant to climate, environmental and ecological studies; (3) continuing the development of simpler BRDF models for analyzing satellite observations; (4) developing a benchmark validation database; and (5) strengthening graduate education program and outreach activities.