Showing papers in "IEEE Transactions on Geoscience and Remote Sensing in 1991"
TL;DR: The derived shade images are related to the observed variation in forest structure, i.e. the fraction of inferred shade in the pixel is related to different eucalyptus ages.
Abstract: Constrained-least-squares (CLS) and weighted-least-squares (WLS) mixing models for generating fraction images derived from remote sensing multispectral data are presented. An experiment considering three components within the pixels-eucalyptus, soil (understory), and shade-was performed. The generated fraction images for shade (shade image) derived from these two methods were compared by considering the performance and computer time. The derived shade images are related to the observed variation in forest structure, i.e. the fraction of inferred shade in the pixel is related to different eucalyptus ages. >
643Â citations
TL;DR: It is shown that estimates based on the correlation of the signal power spectra with an arbitrary weighting function are approximately Gaussian-distributed and a special case of a correlation-based estimator is a maximum-likelihood estimator that reaches the Cramer-Rao bound.
Abstract: Addresses the problem of Doppler frequency estimation in the presence of speckle and receiver noise. An ultimate accuracy bound for Doppler frequency estimation is derived from the Cramer-Rao inequality. It is shown that estimates based on the correlation of the signal power spectra with an arbitrary weighting function are approximately Gaussian-distributed. Their variance is derived in terms of the weighting function. It is shown that a special case of a correlation-based estimator is a maximum-likelihood estimator that reaches the Cramer-Rao bound. These general results are applied to the problem of Doppler centroid estimation from SAR (synthetic aperture radar) data. >
197Â citations
TL;DR: An algorithm to take advantage of this polarization diversity to suppress the speckle effect with much less resolution broadening than using spatial filtering is discussed and a vectorSpeckle filter which operates simultaneously in the polarization and spatial domains is tested.
Abstract: An algorithm to take advantage of this polarization diversity to suppress the speckle effect with much less resolution broadening than using spatial filtering is discussed. The coupling between polarization channels is minimized by using local intensity ratios. The degree of speckle reduction is similar to two-look or three-look processing. The same algorithm can also be used to process multifrequency polarimetric SAR. For three-frequency aircraft SAR data speckle reduction equivalent to six-look processing can be achieved. Further speckle reduction is possible by applying speckle filters in the spatial domain. In addition, a vector speckle filter which operates simultaneously in the polarization and spatial domains is tested. Experimental results with simulated polarimetric SAR as well as one-look and multilook parametric SAR data demonstrate the effectiveness of these speckle reductions, with minimum resolution broadening and coupling between polarimetric and frequency channels. Comparisons with other algorithms are also made. >
192Â citations
TL;DR: The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands to establish a model for a mixed coniferous forest stand.
Abstract: The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands. A mixed coniferous forest stand has been modeled at SAR frequencies (P-, L-, and C-bands). The extensive measurements of ground truth and canopy geometry parameters were performed in a 200 m-square hemlock-dominated plot inside a forest. Hemlock trees in the forest are modeled by characterizing tree trunks, branches, and needles (leaves) with randomly oriented, lossy dielectric cylinders whose area and orientation distributions are prescribed. The distorted Born approximation is used to compute the backscatter at P-, L-, and C-SAR frequencies. >
183Â citations
TL;DR: The combined effects of reflectance anisotropy and increased atmospheric path length on off-nadir observations are demonstrated to demonstrate a variation in vegetation indices as a function of view direction.
Abstract: The Advanced Solid-state Array Spectroradiometer (ASAS), an airborne, off-nadir-pointing imaging spectroradiometer used to acquire bidirectional radiance data for terrestrial targets, is described. As its platform aircraft flies over a target, the sensor can image the target through a sequence of at least seven fore-to-aft view directions ranging up to 45 degrees on either side of nadir. ASAS acquires data for 29 spectral bands in the visible and near-infrared portions of the spectrum (465 to 871 nm) with a resolution of 15 nm. The basic ASAS data product is a sequence of digital images acquired from multiple view directions and consisting of calibrated spectral radiance values. Examples of ASAS data from field experiments are presented. The data demonstrate the combined effects of reflectance anisotropy and increased atmospheric path length on off-nadir observations. One result of these effects is a variation in vegetation indices as a function of view direction. >
146Â citations
TL;DR: Results have shown that this median-filter-based ambiguity removal algorithm satisfies NSCAT mission requirements, and it therefore has been incorporated into the baseline geophysical data-processing system for NSCat.
Abstract: A description is given of the baseline NSCAT (the NASA scatterometer) ambiguity removal algorithm and the method used to select the set of optimum parameter values. An extensive simulation of the NSCAT instrument and ground data processor provides a means of testing the resulting tuned algorithm. This simulation generates the ambiguous wind-field vectors expected from the instrument as it orbits over a set of realistic mesoscale wind fields. The ambiguous wind field is then de-aliased using the median-filter-based ambiguity removal algorithm. Performance is measured by comparison of the selected wind fields with the true wind fields. Results have shown that this median-filter-based ambiguity removal algorithm satisfies NSCAT mission requirements, and it therefore has been incorporated into the baseline geophysical data-processing system for NSCAT. >
144Â citations
TL;DR: The image segmentation problem is solved by extracting kernel information from the input image to provide an initial interpretation of the image and by using a knowledge-based hierarchical classifier to discriminate between major land-cover types in the study area.
Abstract: A knowledge-based approach for Landsat image segmentation is proposed. The image segmentation problem is solved by extracting kernel information from the input image to provide an initial interpretation of the image and by using a knowledge-based hierarchical classifier to discriminate between major land-cover types in the study area. The proposed method is designed in such a way that a Landsat image can be segmented and interpreted without any prior image-dependent information. The general spectral land-cover knowledge is constructed from the training land-cover data, and the road information of an image is obtained through a road-detection program. >
123Â citations
TL;DR: A symmetrical form of the solution for an electrical source in a multibed well-logging environment is derived using local reflection and transmission operators of a single-bed boundary and a general recursive algorithm to derive generalized reflection and Transmission operators.
Abstract: A symmetrical form of the solution for an electrical source in a multibed well-logging environment is derived. The method uses local reflection and transmission operators of a single-bed boundary and a general recursive algorithm to derive generalized reflection and transmission operators. Using this method, the computation time scales linearly as N, where N is the number of beds in the environment. A computer program was developed to implement the solution. The program is robust and generates accurate results from 20 kHz to 25 MHz. >
116Â citations
TL;DR: A radar backscattering model for low-to-medium-density woodland in which each tree is treated as an individual backscatterer is described, and probabilities of radar beams intersecting tree crowns are introduced into the model.
Abstract: A radar backscattering model for low-to-medium-density woodland in which each tree is treated as an individual backscatterer is described. A forest stand viewed by radar is then an assemblage of scatterers standing on a background in a certain spatial pattern. The backscattering components arising from the various intersections of radar beams with the forest stand are calculated separately, and their Stokes matrices are then added together to achieve the Stokes matrix for total backscattering. Probabilities of radar beams intersecting tree crowns are introduced into the model. >
112Â citations
TL;DR: The author describes the development of the derivative ratio algorithm based on derivatives of a simple radiative transfer equation and indicates that some spectral features do survive the trip through the atmosphere and are recognizable using ratios of the spectral derivatives.
Abstract: The author describes the development of the derivative ratio algorithm based on derivatives of a simple radiative transfer equation. The limiting conditions of the algorithm are derived and demonstrated using examples of reflectance spectra of turbid water and an ash leaf. For these example targets, the algorithm indicates that some spectral features do survive the trip through the atmosphere and are recognizable using ratios of the spectral derivatives. The most detectable spectral features tended to be those that spanned bandwidths substantially larger than the minimum bandwidth tested (10 nm). >
108Â citations
TL;DR: A novel algorithm is proposed that works best with low contrast scenes, where the conventional look correlation technique fails, and needs no iterative processing of the SAR data and causes only low computational load.
Abstract: For high-precision synthetic aperture radar (SAR) processing, the determination of the Doppler centroid is indispensable. The Doppler frequency estimated from azimuth spectra, however, suffers from the fact that the data are sampled with the pulse repetition frequency (PRF) and an ambiguity about the correct PRF band remains. A novel algorithm to resolve this ambiguity is proposed. It uses the fact that the Doppler centroid depends linearly on the transmitted radar frequency for a given antenna squint angle. This dependence is not subject to PRF ambiguities. It can be measured by Fourier-transforming the SAR data in the range direction and estimating the Doppler centroid at each range frequency. The achievable accuracy is derived theoretically and verified with Seasat data of different scene content. The algorithm works best with low contrast scenes, where the conventional look correlation technique fails. It needs no iterative processing of the SAR data and causes only low computational load. >
TL;DR: Results of multivariate statistical modeling of water quality parameters versus simulated spectral band ratios for a water quality spectral bandset of the programmable multispectral imager (PMI), also known as the fluorescence line imagers (FLI), are presented.
Abstract: For a lake system in The Netherlands, Secchi disk transparency, seston dry weight, vertical attenuation coefficients, chlorophyll-a, and phaeopigments were determined. Inherent optical properties were estimated from measured apparent optical properties. Subsurface spectroradiometric measurements and airborne (non)-imaging spectrometric measurements were performed. This made it possible to model the performance of high-resolution multispectral scanning systems. Results of multivariate statistical modeling of water quality parameters versus simulated spectral band ratios for a water quality spectral bandset of the programmable multispectral imager (PMI), also known as the fluorescence line imager (FLI), are presented. >
TL;DR: A theoretical approach to cross-hole radar measurements is presented, and an approximate analytical form for the current distribution on an antenna in a borehole is derived, and the theoretical receiving signal of the borehole radar in the cross- hole measurement is calculated.
Abstract: A theoretical approach to cross-hole radar measurements is presented. An approximate analytical form for the current distribution on an antenna in a borehole is derived, and the theoretical receiving signal of the borehole radar in the cross-hole measurement is calculated. Theory is compared in the time-domain with the measured borehole radar signal obtained in a salt deposit. The radiation pattern of a borehole radar excited by a pulse is discussed. The accuracy of tomographic analysis, which is often applied to cross-hole radar measurements, is strongly governed by this radiation pattern. It is found that, in practice, most borehole radars have a pattern of a lambda /2 dipole antenna. However, the radiation pattern can be deformed by short pulse excitation, loading by the surrounding media, and coupling to the logging cable. >
TL;DR: The authors describe the evolution of the spaceborne imaging radar starting with the Seasat SAR, through the SIR-A, Sir-B, and S IR-C/X-SAR missions, to the Earth Observing System (Eos) SAR which is scheduled for launch as part of Eos in the late 1990s.
Abstract: The authors describe the evolution of the spaceborne imaging radar starting with the Seasat SAR, through the SIR-A, SIR-B, and SIR-C/X-SAR missions, to the Earth Observing System (Eos) SAR which is scheduled for launch as part of Eos in the late 1990s. A summary of the planned international missions, which may produce a permanent active microwave capability in space starting as early as 1991, is also presented, along with a description of the airborne systems which will be essential to the algorithm development and long-term calibration of the spaceborne data. A brief summary of the planetary missions utilizing SAR and a comparison of their imaging capabilities with those available on Earth are presented. >
TL;DR: The authors present two algorithms for performing shape matching on ice floe boundaries in SAR (synthetic aperture radar) images that use normalized correlation to match the psi -s curves, while the second uses dynamic programming to compute an elastic match that better accommodatesIce floe deformation.
Abstract: The authors present two algorithms for performing shape matching on ice floe boundaries in SAR (synthetic aperture radar) images. These algorithms quickly produce a set of ice motion and rotation vectors that can be used to guide a pixel value correlator. The algorithms match a shape descriptor known as the psi -s curve. The first algorithm uses normalized correlation to match the psi -s curves, while the second uses dynamic programming to compute an elastic match that better accommodates ice floe deformation. Some empirical data on the performance of the algorithms on Seasat SAR images are presented. >
TL;DR: A hybrid decision tree classifier design procedure that produces efficient and accurate classifiers for remote sensing problems is proposed and empirical tests suggest that the hybrid design produces higher accuracy with fewer features.
Abstract: In applying pattern recognition methods in remote sensing problems, an inherent limitation is that there is almost always only a small number of training samples with which to design the classifier. A hybrid decision tree classifier design procedure that produces efficient and accurate classifiers for this situation is proposed. In doing so, several key questions are addressed, among them the question of the feature extraction techniques to be used and the mathematical relationship between sample size, dimensionality, and risk value. Empirical tests comparing the hybrid design classifier with a conventional single layered one are presented. They suggest that the hybrid design produces higher accuracy with fewer features. The need for fewer features is an important advantage, because it reflects favorably on both the size of the training set needed and the amount of computation time that will be needed in analysis. >
TL;DR: This study deals with the first results obtained from the analysis of the first measurements carried out with the DUTSCAT multifrequency airborne scatterometer to establish a multifrequency dual-polarization radar signature database, and with it a multidimensional version of the current CMOD1 model.
Abstract: A series of airborne scatterometer measurements carried out with the DUTSCAT multifrequency airborne scatterometer are discussed. This study deals with the first results obtained from the analysis of these measurements. The objective of this activity is to establish a multifrequency dual-polarization radar signature database, and with it a multidimensional version of the current CMOD1 model. The main features of the data set are the following. The wind exponent of the upwind normalized radar cross section (NRCS) increases with frequency and incidence angle in the case of HH polarization. The upwind/downwind ratio is mainly negative at 20 degrees of incidence angle, always at C-, X-, and Ku1-bands. >
TL;DR: The system of equations developed in this study using three-stage least-squares and combinatorial screening accounted for 97% of the variability observed in average stand biomass per hectare and had an average bias of less than 1% with a standard error of approximately 3%.
Abstract: L-band HV multiple-incidence-angle aircraft synthetic aperture radar (SAR) data were analyzed in relation to average stand biomass, basal area, and tree height for 55 slash pine plantations located in northern Florida. This information was used to develop a system of equations to predict average stand biomass as a function of L-band (24.5-cm) radar backscatter. The system of equations developed in this study using three-stage least-squares and combinatorial screening accounted for 97% of the variability observed in average stand biomass per hectare. When applied to an independent data set, the biomass equations had an average bias of less than 1% with a standard error of approximately 3%. >
TL;DR: A study extracting and investigating the various components of these terrain influences using an imaging model set up for the geometric rectification of the SAR image and for a reconstruction of the imaging geometry.
Abstract: Synthetic aperture radar (SAR) images reveal radiometric image distortions that are caused by terrain undulations. The authors present the results of a study extracting and investigating the various components of these terrain influences. An imaging model, is set up for the geometric rectification of the SAR image and for a reconstruction of the imaging geometry. A prerequisite for the setup of this model is the use of a digital elevation model. Eight different geometric parameters are derived and investigated for their influence on grey-value variations in the geocoded SAR image. Image grey-value variations of three major land-use classes-forest, agricultural land, and urban/suburban areas-are examined. Empirical models of the SAR-backscatter variations are used to describe the relations between image grey values and various geometric parameters. >
TL;DR: It is shown that a DCT coding scheme reproduces faithfully the seismic waveform at approximately one-third of the original rate, revealing that sinusoidal transforms are well-suited for robust, low-rate seismic signal representation.
Abstract: The authors consider the development and evaluation of transform coding algorithms for the storage of seismic signals. Transform coding algorithms are developed using the discrete Fourier transform (DFT), the discrete cosine transform (DCT), the Walsh-Hadamard transform (WHT), and the Karhunen-Loeve transform (KLT). These are evaluated and compared to a linear predictive coding algorithm for data rates ranging from 150 to 550 bit/s. The results reveal that sinusoidal transforms are well-suited for robust, low-rate seismic signal representation. In particular, it is shown that a DCT coding scheme reproduces faithfully the seismic waveform at approximately one-third of the original rate. >
TL;DR: It is shown that statistics of logarithmically scaled (averaged) radar data can be described by simple analytical expressions using Riemann's zeta function and Eule's psi function, making possible the proper qualification of the impact of such transformation on the physical interpretation of remote sensing data and derived data.
Abstract: It is shown that statistics of logarithmically scaled (averaged) radar data can be described by simple analytical expressions using Riemann's zeta function and Eule's psi function. This achievement makes possible the proper qualification of the impact of such transformation on the physical interpretation of remote sensing data and derived data (i.e. field-averaged mean and standard deviation). The standard deviation for Rayleigh-fading objects is a constant at a logarithmic scale and depends on the number k only. Averaging at a logarithmic scale of radar data, which themselves result from the linear averaging over k independent samples or looks, introduces an offset dependent on the number k only. This offset does not depend on the mean level of the signal and therefore is the same for all Rayleigh-fading objects in the scene. >
TL;DR: The Michigan Microwave Canopy Scattering Model (MIMICS) is used to model scatterometer data that were obtained during the August 1987 EOS (Earth Observing System) synergism study and truck-based scatterometers were used to measure radar backscatter from a walnut orchard in Fresno County, California.
Abstract: The Michigan Microwave Canopy Scattering Model (MIMICS) is used to model scatterometer data that were obtained during the August 1987 EOS (Earth Observing System) synergism study. During this experiment, truck-based scatterometers were used to measure radar backscatter from a walnut orchard in Fresno County, California. Multipolarized L- and X-band data were recorded for orchard plots for which dielectric and evapotranspiration characteristics were monitored. MIMICS is used to model a multiangle data set in which a single orchard plot was observed at varying impedance angles and a series of diurnal measurements in which backscatter from this same plot was measured continuously over several 24-h periods. MIMICS accounts for variations in canopy backscatter driven by changes in canopy state that occur diurnally as well as on longer time scales. L-band backscatter is dependent not only on properties of the vegetation but also on properties of the underlying soil surface. The behavior of the X-band backscatter is dominated by properties of the tree crowns.
TL;DR: Results indicate that the K-distribution is very useful for describing the amplitude statistics of sea clutter (both the like- and cross-polarized channels) for low grazing angles.
Abstract: The important features of a coherent dual-polarized X-band radar designed to be of instrumentation quality for research use are described. The motivation for building the radar was to quantify the improvements attainable through the use of coherence and polarization in the detection of small floating targets in the presence of sea clutter. Results on the statistics of dual-polarized radar returns from the ocean surface (obtained with this radar at a site on the east coast of Canada) are presented. These results indicate that the K-distribution is very useful for describing the amplitude statistics of sea clutter (both the like- and cross-polarized channels) for low grazing angles. Analysis of the time-varying Doppler spectra of a small ice target and the neighboring sea clutter reveals differing Doppler characteristics, offering improved target detection. >
TL;DR: An algorithm for efficient SAR and ScanSAR processing is presented, and its validity is proved by image comparison and analysis.
Abstract: The multilook processing used in SAR (synthetic aperture radar) image formation to reduce the speckle noise is considered. Two multilook techniques are proposed for improving the radiometric resolution without altering the geometric resolution and the characteristics of the impulse response. These techniques are based on the formation of looks with different bandwidths. The final image is formed by giving each look a proper size and weighting, and by adding them incoherently. The looks with wide bandwidth contribute to an improvement of the overall geometric resolution, while the looks with narrow bandwidth improve the overall radiometric resolution. The equivalent number of looks is more than 2.3 times the number of independent looks and is superior to conventional multilook processing with overlapping. Finally, using the proposed techniques, an algorithm for efficient SAR and ScanSAR processing is presented, and its validity is proved by image comparison and analysis. >
TL;DR: A calibration procedure for imaging radar polarimeters was developed and applied to a set of images acquired by the NASA DC-8 multifrequency radar system, presenting the results of applying this technique to radar scenes of lava flows of varying roughness, temperate and tropical rain forests, and ocean water surfaces.
Abstract: The authors developed a calibration procedure for imaging radar polarimeters and applied it to a set of images acquired by the NASA DC-8 multifrequency radar system. The technique requires the use of ground reflectors of known cross-section for absolute calibration, that is, solution for sigma /sup 0/; however, the image data themselves can usually provide all information necessary for phase calibration and for antenna crosstalk correction. The accuracy of the approach, as measured by calculating the cross-section residuals of known targets in each calibrated scene, is on the order of +or-1-2 dB at the P- and C-band, but improves to +or-0.5 dB at the L-band. The authors present the results of applying this technique to radar scenes of lava flows of varying roughness, temperate and tropical rain forests, and ocean water surfaces. They also present several example applications which are feasible with calibrated data but which would be difficult to implement with uncalibrated data. >
TL;DR: The two dimensional transmission line matrix method was used to obtain images of buried pipes illuminated by electromagnetic pulses generated by a ground-penetrating radar for detection of dielectric or metal pipes buried in the ground.
Abstract: A subsurface radar for the detection of dielectric or metal pipes buried in the ground is investigated numerically. The two dimensional transmission line matrix method was used to obtain images of buried pipes illuminated by electromagnetic pulses generated by a ground-penetrating radar. The present study is only applicable for two-dimensional cases as it assumes infinitely long pipes and sources. However, the technique may be used for three-dimensional cases as long as the antenna is parallel to the underground pipes and is not near the ends or the bends of the pipes. For the cases shown, 480 grid points in the x direction and 220 in the gamma direction were used. The required memory to run the simulation software was 1.1 Mb. With 6000 iterations, the total CPU time for the case was 218 s on a MicroVax II workstation. >
TL;DR: The science requirements guiding the processing of MODIS data are reviewed, and the aspects of an operations concept for the production of data products from MODIS for use by the scientific community are discussed.
Abstract: The Moderate Resolution Imaging Spectrometer (MODIS) has been designated as a facility instrument on the first NASA polar orbiting platform as part of the Earth Observing System (Eos) and is scheduled for launch in the late 1990s. The near-global daily coverage of MODIS, combined with its continuous operation, broad spectral coverage, and relatively high spatial resolution, makes it central to the objectives of Eos. The development, implementation, production, and validation of the core MODIS data products define a set of functional, performance, and operational requirements on the data system that operate between the sensor measurements and the data products supplied to the user community. The science requirements guiding the processing of MODIS data are reviewed, and the aspects of an operations concept for the production of data products from MODIS for use by the scientific community are discussed. >
TL;DR: The measured and model results are combined and shown to be fully compatible and are expected to apply to any rubbled sea-ice surface when surface scattering dominates.
Abstract: During the March 1987 Labrador Ice Margin Experiment (LIMEX '87) two independent investigations were conducted to determine the C-band backscattering cross section of the marginal pack ice along the Newfoundland coast. In one experiment, data from a recently calibrated C-band airborne scatterometer were combined with C-band synthetic aperture radar (SAR) data to measure the normalized scattering cross section of the ice at incidence angles from 10 degrees to 74 degrees to within +or-2 dB. In the other experiment, detailed measurements of ice surface roughness and surface properties were made and the radar cross sections were predicted from a scattering model. In the present study, measured and model results are combined and shown to be fully compatible. By extension, the results are expected to apply to any rubbled sea-ice surface when surface scattering dominates. >
CNET1
TL;DR: The stability and accuracy of the rainfall profile retrieval in the presence of various types of errors are pointed out and illustrated by means of numerical simulations of data processing from a spaceborne rain radar operating basically at Ku band.
Abstract: A range-profiling algorithm for rainfall rate retrieval from a single-frequency downward-looking spaceborne radar is presented. The algorithm is based on a linear reformulation of the radar equation. The path-integrated attenuation given by the surface echo measurement is used as a constraint for normalizing the range-gated rain echoes. The expected performances are studied analytically and compared with those of the approach of W. Hitschfeld and J. Bordan (1954). The stability and accuracy of the rainfall profile retrieval in the presence of various types of errors are pointed out and illustrated by means of numerical simulations of data processing from a spaceborne rain radar operating basically at Ku band. >
TL;DR: Simulation results on several images are provided to indicate the effectiveness of the proposed 2-D adaptive block Kalman filtering method when used to remove the effects of speckle noise as well as those of the additive noise.
Abstract: A method for removing speckle from synthetic aperture radar (SAR) imagery by using 2-D adaptive block Kalman filtering is introduced. The image process is represented by an autoregressive model with a nonsymmetric half-plane (NSHP) region of support. New 2-D Kalman filtering equations are derived which taken into account not only the effect of speckles as multiplicative noise but also the effects of the additive receiver thermal noise and the blur. This method assumes local stationarity within a processing window, whereas the image can be assumed to be globally nonstationary. A recursive identification process using the stochastic Newton approach is also proposed which can be used on-line to estimate the filter parameters based upon the information within each new block of the image. Simulation results on several images are provided to indicate the effectiveness of the proposed method when used to remove the effects of speckle noise as well as those of the additive noise. >