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Geoscience and Remote Sensing, IEEE Transactions on

Issue 4 • Date July 1994

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Displaying Results 1 - 25 of 26
  • The wavenumber shift in SAR interferometry

    Page(s): 855 - 865
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1028 KB)  

    SAR surveys from separate passes show relative shifts of the ground wavenumber spectra that depend on the local slope and the off-nadir angle. The authors discuss the exploitation of this spectral shift for different applications: 1) generation of "low noise" interferograms benefiting phase unwrapping, 2) generation of quick-look interferograms, 3) decorrelation reduction by means of tunable SAR systems (TINSAR), 4) range resolution enhancement, and 5) the combination of SAR data gathered by different platforms (airborne and satellite) for a "long-time coherence" study.<> View full abstract»

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  • Estimating ocean coherence time using dual-baseline interferometric synthetic aperture radar

    Page(s): 846 - 854
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    A new technique for measuring the coherence time of the ocean surface at radar wavelengths has been developed and tested. This technique requires an interferometric synthetic aperture radar system with at least two unique baselines along the direction of platform motion. The coherence time of the surface may be presented as a high-resolution coherence time map. This technique was tested using the JPL AIRSAR along-track interferometer. Measurements of the ocean coherence at the L-band were made at high spatial resolution under a variety of conditions. A new operating technique that results in the required two baselines is described. Some parametric analysis is performed pertaining to the design of such a system. A representative interferometric data set acquired at the Strait of Messina in 1991 is presented to illustrate the technique. The interferometric phase data reveal and measure tidal currents flowing through the strait, while the coherence time image shows considerable structure not visible in the backscatter or phase image. Coherence times in and around the strait were measured to be of the order of 0.1 s at the L-band, which is in agreement with previous estimates and measurements View full abstract»

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  • The Los Alamos beacon receiver array

    Page(s): 954 - 958
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    Describes radio receivers that monitor transmissions from beacons on geosynchronous satellites. The receivers can detect ionospheric perturbations of a 300-3000 s period in the electron density integrated from beacon to receiver, for amplitudes as low as (1-2)×1013 m-2. Data are used in studies of atmospheric acoustic and acoustic-gravity waves View full abstract»

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  • Hyperspectral image classification and dimensionality reduction: an orthogonal subspace projection approach

    Page(s): 779 - 785
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    Most applications of hyperspectral imagery require processing techniques which achieve two fundamental goals: 1) detect and classify the constituent materials for each pixel in the scene; 2) reduce the data volume/dimensionality, without loss of critical information, so that it can be processed efficiently and assimilated by a human analyst. The authors describe a technique which simultaneously reduces the data dimensionality, suppresses undesired or interfering spectral signatures, and detects the presence of a spectral signature of interest. The basic concept is to project each pixel vector onto a subspace which is orthogonal to the undesired signatures. This operation is an optimal interference suppression process in the least squares sense. Once the interfering signatures have been nulled, projecting the residual onto the signature of interest maximizes the signal-to-noise ratio and results in a single component image that represents a classification for the signature of interest. The orthogonal subspace projection (OSP) operator can be extended to k-signatures of interest, thus reducing the dimensionality of k and classifying the hyperspectral image simultaneously. The approach is applicable to both spectrally pure as well as mixed pixels View full abstract»

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  • Turbulence-induced distortion of synthetic aperture radar images

    Page(s): 958 - 961
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    Streaking occurred on synthetic aperture radar (SAR) images made of Florida during the summer of 1992. There is strong evidence that the cause of these streaks is an atmospheric condition that occurs during warm, humid conditions typical of the southern Florida coast in the summer. This communication describes a mathematical model that explains the effects on SAR imagery due to this unique condition View full abstract»

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  • Special problems in the estimation of power-law spectra as applied to topographical modeling

    Page(s): 928 - 939
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    An increasing number of topographical studies find that natural surfaces possess power-law roughness spectra. Power-law spectra introduce unique difficulties in the spectral estimation process. The authors describe how an improper window choice allows leakage that yields a spectral estimate that is insensitive to the spectral slope. In addition, the commonly used Fourier-based spectral estimates have higher variances than other available estimators. Higher variance is particularly problematic when data records are short, as is often the case in remote sensing studies. The authors show that Capon's spectral estimator has less variance than Fourier-based estimators and measures the spectral slope more accurately. The authors also show how estimates of a 2D roughness spectrum can be obtained from estimates of the 1D spectrum for the isotropic power-law case View full abstract»

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  • Precision SAR processing using chirp scaling

    Page(s): 786 - 799
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    A space-variant interpolation is required to compensate for the migration of signal energy through range resolution cells when processing synthetic aperture radar (SAR) data, using either the classical range/Doppler (R/D) algorithm or related frequency domain techniques. In general, interpolation requires significant computation time, and leads to loss of image quality, especially in the complex image. The new chirp scaling algorithm avoids interpolation, yet performs range cell migration correction accurately. The algorithm requires only complex multiplies and Fourier transforms to implement, is inherently phase preserving, and is suitable for wide-swath, large-beamwidth, and large-squint applications. This paper describes the chirp scaling algorithm, summarizes simulation results, presents imagery processed with the algorithm, and reviews quantitative measures of its performance. Based on quantitative comparison, the chirp scaling algorithm provides image quality equal to or better than the precision range/Doppler processor. Over the range of parameters tested, image quality results approach the theoretical limit, as defined by the system bandwidth View full abstract»

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  • Use of copolar correlation coefficient for probing precipitation at nearly vertical incidence

    Page(s): 740 - 748
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    Presents observations of the copolar correlation coefficient between horizontally and vertically polarized echoes |ρ(0)|. These were made with ground-based and airborne weather radars at nearly vertical incidence. A sharp decrease of |ρ(0)| occurs at the bright band bottom, and is attributed to a varying mixture of hydrometeors with diverse shape, size, and thermodynamic phase. The largest contribution to decorrelation seems to come from wet aggregates; this is substantiated by consideration of two simple models. One consists of randomly oriented wet prolate spheroids, and the other considers an ensemble of distorted spheres. Prolates with axis ratios of 3 or distorted spheres with rms roughness equal to 15% of the diameter decrease the correlation to 0.8 at S band. At Ku band and for the size range encountered in the bright band, the decrease is a function of equivalent diameter because scattering is in the Mie regime. |ρ(0)| measurement at 13.8 GHz and from the aircraft are the first ever. Also, differential phase and differential reflectivity at a 10° off nadir are the first of its kind. These last two variables showed a distinct signature in the bright band. This is significant because it might lead to applications on airborne or spaceborne platforms View full abstract»

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  • Calibration of a polarimetric radar using a rotatable dihedral corner reflector

    Page(s): 837 - 845
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    Based on the existing mathematical formalisms of radar polarimetry, it is necessary to perform accurate and diversified polarimetric measurements in the real world to thoroughly investigate signature definition, identification, and classification of radar targets. For this study the Delft Atmospheric Research Radar (DARR) is used. This ground-based polarimetric FM-CW radar operates in the S-band. The purpose of the present paper is the polarimetric calibration of the DARR. Among the passive reflectors, a rotatable dihedral corner reflector is a suitable calibration object. It enables one to measure different scattering matrices with only one reflector. One alignment must be performed and the scattering matrices are measured at the same range. By measuring several scattering matrices, the accuracy of the calibration result can be estimated. A measurement campaign with a rotatable dihedral corner reflector was therefore performed. The experimental results and the calibration procedure are presented in this paper View full abstract»

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  • Accuracy of topographic maps derived from ERS-1 interferometric radar

    Page(s): 823 - 836
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    An interferometric radar technique for topographic mapping of surfaces promises a high-resolution approach to the generation of digital elevation models. The authors present analyses of data collected by the synthetic aperture radar instrument on-board the ERS-1 satellite on successive orbits. Use of a single satellite in a nearly repeating orbit is attractive for reducing cost and spaceborne hardware complexity; also it permits inference of changes in the surface from the correlation properties of the radar echoes. The data have been reduced to correlation maps and digital elevation models. The correlation maps show that temporal correlation decreases significantly with time, but not necessarily at a constant well-defined rate, likely depending on environmental factors. When correlation among passes remains high, however, it is possible to form digital elevation models. Analyses of noise expected in ERS-1 interferometric data collected over Alaska and the southwestern United States indicate that maps with relative errors less than 5 m rms are possible in some regions. However, orbit uncertainties imply that tie points are required in order to reduce absolute height errors to a similar magnitude. The authors find that about 6 tie points per 40×40 km scene with 5 m rms or better height accuracy are needed to keep systematic map height errors below 5 m rms. The performance of the ERS-1 radar system for topographic applications, though useful for a variety of regional and local discipline studies, may be improved with respect to temporal decorrelation errors and absolute height acuity by modifying the orbit repeat period and incorporating precise orbit determination techniques. The resulting implementation will meet many, but not all, objectives of a global mapping mission View full abstract»

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  • Dual dihedral angles for bidirectionally scattered electromagnetic waves

    Page(s): 940 - 942
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    A spherical coordinate system is typically used to quantify the directional characteristics of doubly hemispherical scattering of bidirectional reflectance distribution function (BRDF) entailing an incident angle or cone angle from surface normal, and a heading or azimuthal angle from the incident plane. An alternate angular coordinate system is described entailing two dihedral angles. One component is a dihedral angle from the incident plane, simply out-of-plane, and the second component is a dihedral angle from the specular beam to the detector direction projected onto the incident plane, or simply degrees off specular. The suggested system is believed easier to convey scattering characteristics, and to compute the scattering from complex surfaces such as mountains with exposed rocks, or buildings. This communication presents an example of an experimental configuration to measure a doubly hemispherical BRDF, a mathematical formulation to relate the angular coordinate systems, and a computational example View full abstract»

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  • A Nagao-Matsuyama approach to high-resolution satellite image classification

    Page(s): 749 - 758
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    A knowledge-based, hierarchical, unsupervised classification scheme for high-resolution multispectral satellite (HRMS) images is described. This scheme, which finds its conceptual bases in the work of Nagao and Matsuyama for structural analysis of aerial photographs, introduces a new filtering algorithm which is able to preserve fine linear structures of the image. An example of the application of this classification scheme to a Landsat Thematic Mapper multispectral image is presented View full abstract»

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  • Multisource classification of remotely sensed data: fusion of Landsat TM and SAR images

    Page(s): 768 - 778
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    Proposes a new method for statistical classification of multisource data. The method is suited for land-use classification based on the fusion of remotely sensed images of the same scene captured at different dates from multiple sources. It incorporates a priori information about the likelihood of changes between the acquisition of the different images to be fused. A framework for the fusion of remotely sensed data based on a Bayesian formulation is presented. First, a simple fusion model is given, and then the basic model is extended to take into account the temporal attribute if the different data sources are acquired at different dates. The performance of the model is evaluated by fusing Landsat TM images and ERS-1-SAR images for land-use classification. The fusion model gives significant improvements in the classification error rates compared to the conventional single-source classifiers View full abstract»

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  • Microwave (1-100 GHz) dielectric model of leaves

    Page(s): 947 - 949
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    A semiempirical formula for the complex dielectric permittivity of leaves from different plants is found from a comparison of published measurements covering the frequency range from 1 to 100 GHz. The explicit parameters are the dry-matter fraction md of the leaf and the permittivity εSW of saline water with a salinity of about 1 percent. The physical part of the formula is its basis on εSW while the empirical part is its linearity with md. The formula is applicable to fresh leaves; their m d values are in the range 0.1<md<0.5. A test indicates that besides the md variation and the spectral dependence the formula also describes the temperature variation correctly View full abstract»

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  • Modeling of geometric properties of loblolly pine tree and stand characteristics for use in radar backscatter studies

    Page(s): 800 - 822
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    An approach is presented to physically describe the tree and canopy components within loblolly pine forests in studies of microwave backscattering from forested canopies. The approach is based on a set of algorithms which describe the biomass characteristics of individual trees using measurements of tree diameter, height, and canopy depth. These algorithms predict bole, branch, and needle biomass; number, sizes, and orientations of primary and smaller branches; and number and density of needle clumps. Methods are discussed to aggregate the individual tree measurements into stand measurements (i.e., measurements per unit area). The results of the model clearly show how biomass distribution between tree components and canopy layers varies as a function of stand age for loblolly pines. The results also show the approach can be used to predict changes in the branch size and orientation within the canopy as a function of stand age, both on an individual tree basis as well as an entire stand basis View full abstract»

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  • Histogram-based morphological edge detector

    Page(s): 759 - 767
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    Presents a new edge detector for automatic extraction of oceanographic (mesoscale) features present in infrared (IR) images obtained from the Advanced Very High Resolution Radiometer (AVHRR). Conventional edge detectors are very sensitive to edge fine structure, which makes it difficult to distinguish the weak gradients that are useful in this application from noise. Mathematical morphology has been used in the past to develop efficient and statistically robust edge detectors. Image analysis techniques use the histogram for operations such as thresholding and edge extraction in a local neighborhood in the image. An efficient computational framework is discussed for extraction of mesoscale features present in IR images. The technique presented in the present article, called the Histogram-Based Morphological Edge detector (HMED), extracts all the weak gradients, yet retains the edge sharpness in the image. A new morphological operation defined in the domain of the histogram of an image is also presented. An interesting experimental result was found by applying the HMED technique to oceanographic data in which certain features are known to have edge gradients of varying strength View full abstract»

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  • An airborne 95 GHz dual-polarized radar for cloud studies

    Page(s): 731 - 739
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    A 95 GHz dual-polarization radar system was developed and flown on the University of Wyoming King Air research aircraft, from which it measured reflectivity, depolarization, and Doppler-derived velocity mean and standard deviation of a variety of clouds. This paper describes the radar and a data acquisition system that uses commercially available digitizers, signal processors, and signal generators. The authors also describe the tradeoffs between spatial resolution and ability to estimate reflectivity and velocity. This paper presents the first known airborne measurements of clouds made at 95 GHz; these are thought to be the most highly resolved millimeter-wave cloud images made to date. Depolarization, measured in terms of the linear depolarization ratio (LDR), was especially high in the melting band and in regions containing pristine ice crystals. These measurements demonstrate the advantages that high-spatial-resolution airborne millimeter-wave radars offer for the study of cloud microphysical properties View full abstract»

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  • Processing of multitemporal Landsat TM imagery to optimize extraction of forest cover change features

    Page(s): 918 - 927
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    Digital procedures to optimize the information content of multitemporal Landsat TM data sets for forest cover change detection are described. Imagery from three different years (1984, 1986, and 1990) were calibrated to exoatmospheric reflectance to minimize sensor calibration offsets and standardize data acquisition aspects. Geometric rectification was followed by atmospheric normalization and correction routines. The normalization consisted of a statistical regression over time based on spatially well-defined and spectrally stable landscape features spanning the entire reflectance range. Linear correlation coefficients for all bitemporal band pairs ranged from 0.9884 to 0.9998. The correction mechanism used a dark object subtraction technique incorporating published values of water reflectance. The association between digital data and forest cover was maximized and interpretability enhanced by converting band-specific reflectance values into vegetation indexes. Bitemporal vegetation index pairs for each time interval (two, four, and six years) were subjected to two change detection algorithms, standardized differencing and selective principal component analysis. Optimal feature selection was based on statistical divergence measures. Although limited to spectrally-radiometrically defined change classes, results show that the relationship between reflective TM data and forest canopy change is explicit enough to be of operational use in a forest cover change stratification phase prior to a more detailed assessment View full abstract»

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  • Inversion of induction tool measurements using the distorted Born iterative method and CG-FFHT

    Page(s): 878 - 884
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    The inversion of induction tool measurements using the distorted Born iterative method (DBIM) and the conjugate gradient-fast Fourier-Hankel transform (CG-FFHT) is described. The inverse problem is formulated in terms of an integral equation of scattering where the unknown to be sought is the conductivity in the rock formation, when the measurements along a borehole axis are performed. The nonlinear problem is linearized at each stage using the distorted Born approximation. The inhomogeneous medium Green's function in the distorted Born approximation is found by solving a volume integral equation using the CG-FFHT method, which allows a rapid solution to a large problem with reduced computational complexity and memory requirement. In this manner, the inverse problem is solved with a computational complexity proportional to NtlN log N where Ntl is the number of transmitter locations used in the data collection and N is the total number of pixels used to model the unknown formation. The memory requirement is of order NNtl View full abstract»

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  • Multidimensional analysis method for NOAA AVHRR images

    Page(s): 949 - 954
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    As a fundamental study of multiple image processing, the authors have developed a new technique for the analysis of multispectral remote sensing data. Though the basis of the authors' algorithm is similar to a histogram analysis, they have proposed a novel way of the representation of multidimensional image data, which facilitates a nonexpert to locate and assign different classes present in the data set. The authors' experimentation was done with NOAA satellite image data. The brightness of the received data from channels of different frequency bands are the different dimensions of the multispectral image. By using this method, the authors could successfully classify NOAA satellite data received from the northern part of Japan, and locate the plane areas as an exercise. The authors have quantitatively compared their result with official data. The authors' result was found to be only 1 percent in deviation with the official data View full abstract»

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  • Effect of multiple scattering on the estimation of rainfall rates using dual-wavelength radar techniques

    Page(s): 943 - 946
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    The effect of multiple scattering on the estimation of rainfall rates using incoherent backscattered power at two frequencies (16 and 34.8 GHz) is analyzed. Simulation of circularly polarized transmissions propagating through homogeneous rain is used to estimate the rainfall rates from echo power levels calculated with and without multiple scattering effects included. The results indicate that in homogeneous rain, the rainfall rate estimated using the multiple scattering calculation is always smaller than that using the conventional radar equation. The difference is significant for heavy rain cases. Since the multiple scattering process may exist in real rain situations, one must be careful in heavy rain cases when interpreting rainfall rates estimated using dual-wavelength algorithms View full abstract»

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  • An error and sensitivity analysis of the atmospheric- and soil-correcting variants of the NDVI for the MODIS-EOS

    Page(s): 897 - 905
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    Several soil- and atmospheric-correcting variants of the normalized difference vegetation index (NDVI) have been proposed to improve the accuracy in estimating biophysical plant parameters. In this study, a sensitivity analysis, utilizing simulated model data, was conducted on the NDVI and variants by analyzing the atmospheric- and soil-perturbed responses as a continuous function of leaf area index. Percent relative error and vegetation equivalent “noise” (VEN) were calculated for soil and atmospheric influences, separately and combined. The NDVI variants included the soil-adjusted vegetation index (SAVI), the atmospherically resistant vegetation index (ARVI), the soil-adjusted and atmospherically resistant vegetation index (SARVI), the modified SAVI (MSAVI), and modified SARVI (MSARVI). Soil and atmospheric error were of similar magnitudes, but varied with the vegetation index. All new variants outperformed the NDVI. The atmospherically resistant versions minimized atmospheric noise, but enhanced soil noise, while the soil adjusted variants minimized soil noise, but remained sensitive to the atmosphere. The SARVI, which had both a soil and atmosphere calibration term, performed the best with a relative error of 10 percent and VEN of ±0.33 LAI. By contrast, the NDM had a relative error of 20 percent and VEN of ±0.97 LAI View full abstract»

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  • Hidden Markov models applied to vegetation dynamics analysis using satellite remote sensing

    Page(s): 906 - 917
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    Details hidden Markov models (HMM) with respect to their ability to represent time series of remotely sensed data as well as to analyze vegetation dynamics at large scales. The present approach is shown to be a powerful way to classify and extract various dynamics parameters as well as to detect phenological anomalies. The methodology is applied and validated using the Advanced Very High Resolution Radiometer (AVHRR) normalized difference vegetation index (NDVI) time series. The model is then used to determine vegetation active cycle and the length of the growing season in the West African savanna View full abstract»

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  • Assessment of tilt capability for spaceborne global ocean color sensors

    Page(s): 866 - 877
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    The importance of tilt capability for Sun glint avoidance for future global ocean color missions was analyzed. The analyses focused on Sea-viewing Wide Field-of-view Sensor (SeaWiFS) mission, because its radiometric, orbital, and sensor characteristics are well defined. The analyses concentrated can two major questions: 1) does tilting to avoid Sun glint increase or decrease the total ocean coverage, and 2) at high latitudes far from the region of maximum Sun glint, should the sensor be tilted or untilted? For ocean coverage maximization, if the sensitivity of ocean color algorithms to Sun glint is of the same order as the error in the atmospheric correction algorithms, then a tilted sensor produces nearly 20 percent better coverage than an untilted one after 2 d in the absence of clouds, and 12 percent after 4 d including clouds. Thus, the tilt capability can improve the ocean coverage of future ocean color missions. At high latitudes differences in transmitted water-leaving radiance between tilted and untilted sensors were well within the algorithm errors. Furthermore, Sun glint radiances exceeding the algorithm errors occurred at high wind speeds as far as 70° from the solar declination, suggesting that sensors should remain in the tilted mode up to this limit View full abstract»

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  • A band-differenced angular signature technique for cirrus cloud detection

    Page(s): 890 - 896
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    A new approach to cloud detection is introduced that exploits the difference between two solar spectral reflectances as a function of view angle. The resulting band-differenced angular signature is sensitive to the contribution of Rayleigh scattering from above the tops of clouds and can be used to discriminate high clouds from lower level clouds and clear sky. The authors use model simulations to show that this technique could be applied to measurements from the Multiangle Imaging SpectroRadiometer, scheduled for launch on the first platform of the Earth Observing System. Results show the technique to work best over ocean and snow surfaces. Over such surfaces, the minimum detectable high cloud optical thickness (at 0.55 μm) would typically be 0.5 without the use of any a priori scene information, and lower if such information is available View full abstract»

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Aims & Scope

 

IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING (TGRS) is a monthly publication that focuses on the theory, concepts, and techniques of science and engineering as applied to sensing the land, oceans, atmosphere, and space; and the processing, interpretation, and dissemination of this information.

 

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Editor-in-Chief
Antonio J. Plaza
University of Extremadura