Multiple View Angles Research Articles

Operational bi-angle approach to retrieve the Earth surface albedo from AVHRR data in the visible band

Abstract The inference of surface spectral reflectance using visible observations is complicated mainly because of scattering effects. In the present paper we attempt a solution to the problem of retrieval of surface reflectance from satellite radiance measurements based on a solution of the radiative transfer equation. We have developed an operational method which relies on multiple view angle observations or multiple solar zenith angle observations of the surface to accomplish part of this task in a routine manner. This approach may be used for ERS-2 ATSR visible bands because there will be two view angle observations for the same area at essentially the same time. This approach can also be used with NOAA satellite AVHRR data by assuming that the distribution of the aerosol does not vary too rapidly as a function of time. Usually it is better to select the AVHRR data from around noon and at dusk in the same day. This approach requires that the visibility of the area should be more than 5 km for multiple...

Extension of off-nadir view angles for directional sensor systems

A knowledge-based system called VEG was expanded to infer nadir or any off-nadir reflectance(s) of a vegetation target given any combination of other directional reflectance(s) of the target. VEG determines the best techniques to use in an array of techniques, applies the techniques to the target data, and provides a rigorous estimate of the accuracy of the inference(s). The knowledge-based system, VEG, facilitates the use of diverse knowledge bases to be incorporated into the inference techniques. In this study, VEG used additional information to make more accurate view-angle extension techniques than the traditional techniques that only use spectral data from the unknown target. VEG used spectral data and a normalized difference technique to infer the percentage of ground cover of the unknown target. This estimate of percentage of ground cover of the unknown target along with information on the sun angle were then used to search a historical data base for targets that match the unknown target in these characteristics. This data captured the general shape of the reflectance distribution of the unknown target. This historical information was used to estimate the coefficients of the techniques for the conditions at hand and to test the accuracy of the techniques. The tests used in this study were difficult ones. For example, techniques were tested that make long angular extensions using one, two, or four input view angles to predict an unknown nadir value. Furthermore, a wide variety of unknown targets were tested. The errors (±proportional rms) obtained were on the order of 0.15. In addition techniques were tested that use seven or nine multiple view angles to predict the entire hemispherical reflectance distribution of an unknown target. The accuracy of these tests was relatively good considering the relatively dynamic and noisy nature of directional reflectance distributions. The accuracy of the techniques in this study depends on the smoothness of the historical reflectance distributions and the amount of historical data available that closely matches the unknown target.

Radiative transfer in shrub savanna sites in Niger: preliminary results from HAPEX-Sahel. 3. Optical dynamics and vegetation index sensitivity to biomass and plant cover

A shrub savannah landscape in Niger was optically characterized utilizing blue, green, red and near-infrared wavelengths. Selected vegetation indices were evaluated for their performance and sensitivity to describe the complex Sahelian soil/vegetation canopies. Bidirectional reflectance factors (BRF) of plants and soils were measured at several view angles, and used as input to various vegetation indices. Both soil and vegetation targets had strong anisotropic reflectance properties, rendering all vegetation index (VI) responses to be a direct function of sun and view geometry. Soil background influences were shown to alter the response of most vegetation indices. N-space greenness had the smallest dynamic range in VI response, but the n-space brightness index provided additional useful information. The global environmental monitoring index (GEMI) showed a large VI dynamic range for bare soils, which was undesirable for a vegetation index. The view angle response of the normalized difference vegetation index (NDVI), atmosphere resistant vegetation index (ARVI) and soil atmosphere resistant vegetation index (SARVI) were asymmetric about nadir for multiple view angles, and were, except for the SARVI, altered seriously by soil moisture and/or soil brightness effects. The soil adjusted vegetation index (SAVI) was least affected by surface soil moisture and was symmetric about nadir for grass vegetation covers. Overall the SAVI, SARVI and the n-space vegetation index performed best under all adverse conditions and were recommended to monitor vegetation growth in the sparsely vegetated Sahelian zone.

Learning class descriptions from a data base of spectral reflectance with multiple view angles

A program was developed to learn class descriptions from positive and negative training examples of spectral, directional reflectance data taken from natural surfaces such as bare soil, natural vegetation, or agricultural vegetation. The learning program combined a form of learning referred to as a learning by example with the generate and test paradigm to provide a robust learning environment that could handle error prone data. The learning program was tested by having it learn class descriptions of various categories of percent ground cover and plant height. These class descriptions were used to classify an array of targets. The class descriptions in this program comprised a series of different relationships between combinations of directional view angles, e.g., (30,50), (45,60), (10,180), etc. Where the values in parentheses are for zenith and relative azimuth view angles for a particular view. The program found the sequence of relationships that contained the most important information that distinguished the classes. The concept being learned was a sequence of relationships that optimized the discrimination of a class. >

Remote sensing of surface hemispherical reflectance (albedo) using pointable multispectral imaging spectroradiometers

Several techniques that use multiple off-nadir view angles taken in a single azimuth plane (called a string of data) to estimate hemispherical reflectance have been developed. The range of view angles that is required in order to obtain accurate inferences of hemispherical reflectance using string techniques was studied as applicable to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the High Resolution Imaging Spectroradiometer (HIRIS). Full and half strings out to 60°, 45°, 30°, and 15° were tested on ground data of a wide range of cover types and sun angles. Both a visible and near infrared band (AVHRR Bands 1 and 2) were tested using a knowledge-based system called VEG. Using string techniques that calculate a single coefficient from an internal spectral data base, the results showed small errors for the full string ± 60° (less than 4% error), HIRIS like data (less than 5% error), and MODIS like data (less than 7% error). Much larger errors occur in those cases where long extrapolations must be calculated (e.g., strings to only 15° and 30°). Systems that view fore and aft out to 60° would be desirable to minimize errors, assuming atmospheric corrections could be made at these angles. HIRIS (views 60° in forward and 30° in aft directions) is a good compromise by viewing out to 60° in only one direction and increases the error by approximately 2% or less. MODIS (full string ± 45°) still has a relatively small error of approximately 5% or less. Sensor systems capable of only viewing in the fore or aft directions (half strings) as opposed to both directions (full strings) greatly increase the error.

Rapid line scan NMR angiography.

This paper describes a new technique for NMR angiography based on rapid line scan projection imaging and presaturation of stationary spins. The resulting line scan angiograms are free from both misregistration artifacts common to subtraction methods and motion artifacts encountered in Fourier imaging. Moreover, line scan angiograms may be recorded within seconds and offer arbitrary fields of view as well as gradient zooming without problems due to aliasing. Three-dimensional information is obtained by rotation of the read gradient axis generating multiple view angles. Experimental line scan angiograms of the forearms of healthy volunteers have been recorded using a Bruker 2.35-T 40-cm magnet.

Extraction of spectral hemispherical reflectance (albedo) of surfaces from nadir and directional reflectance data

A radiative transfer model is used to investigate how the error of spectral hemispherical reflectance data obtained from nadir reflectance values varies with wavelength, solar zenith angle, leaf area index, and leaf orientation distribution. Several techniques employing multiple off-nadir view angles taken in azimuth planes are found to accurately infer spectral hemispherical reflectances, and to be well suited to sensor systems that scan in a known azimuth plane or view fore and aft in a known azimuth plane. The effects of errors in hemispherical reflectance on terrestrial energy budget and productivity calculations is also considered.

Atmospheric transmittance from spacecraft using multiple view angle imagery

A procedure for determining atmospheric transmittance using surface images acquired at several view angles from space is presented. Fourier filtering of the image data is used to mitigate the effects of atmospheric path radiance and instrument noise. Although it is essential for some contrast to be present in the images, knowledge of the atmospheric scattering phase function or the surface bidirectional reflectance function is not required. The method appears to provide absolute transmittances with an accuracy of better than 10 percent, which is useful in some applications.

Maximum-Likelihood Reconstruction for Single-Photon Emission Computed-Tomography

A mathematical model is formulated for a gamma camera used to observe single-photon emissions from multiple view angles. The model accounts for the statistics of radioactive decays, nonuniform attenuation, and a depth-dependent point-spread function. The maximum-likelihood method of statistics is used with the model to derive an algorithm for estimating the distribution of radioactivity.

Measurement of spatial distribution of refractive index in tissues by ultrasonic computer assisted tomography

<h2>Abstract</h2> Computerized tomography is used to calculate two independent images representing distributions of refractive index (<i>n</i>) and acoustic attenuation (α) within 1–3 mm thick cross-sections through excised organs, including canine hearts and human breasts. Values of <i>n</i> and α are calculated from profiles of propagation times and amplitudes, respectively, of digitized acoustic pulses obtained by rectilinear transmission scans of the tissue at multiple angles of view. Images of the local speed of ultrasound show high values in regions of muscle, breast parenchyma, medullary carcinoma and connective tissue and show low values in regions of fat. Images of acoustic attenuation show high values in regions of connective tissue and borders of scirrhus carcinoma and low values in regions of fat.

A Comparison of Two Photographic Techniques for Estimating Foliage Angle Distribution

There is an increasing interest in theoretical models which describe the interaction of solar radiation with vegetation canopies. Common to these models is a need to describe mathematically the geometric structure of the plant canopy. The amount of radiation reflected or absorbed by the canopy is primarily determined by the distribution of gaps in the foliage with respect to the radiation source. A measure of canopy geometry related to gap frequency at various view angles is the distribution of leaf angles. Two methods for measuring the distribution of leaf angles are discussed. The first method is to project orthogonally and photograph individual plants and relate the measured leaf angles in the projections to the canopy distribution of angles. The second method is a rapid in situ method based on ground level multiple view angle photography. A Fredholm integral equation relating foliage angles to the proportion of gap in the canopy as a function of view angle is then solved. Comparisons of the results using the two methods are made for a canopy of Western wheat grass (Agropyron smithii).