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Geoscience and Remote Sensing, IEEE Transactions on

Issue 1 • Date Jan 1995

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Displaying Results 1 - 25 of 25
  • Millimeter-wave radiometric observations of the troposphere: a comparison of measurements and calculations based on radiosonde and Raman lidar

    Page(s): 3 - 14
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    A comparison of clear-air brightness temperatures is performed between radiometric measurements and atmospheric radiative transfer calculations. The measurements were made using the NASA Goddard Space Flight Center's Millimeter-wave Imaging Radiometer (MIR) in a series of airborne and ground-based atmospheric experiments at six millimeter-wave frequencies: 89; 150; 183.3±1, ±3, ±7; and 220 GHz. With the inclusion of the 220 GHz channel, these measurements are the first passive observations of the atmosphere made simultaneously at the six frequencies. The MIR was operated concurrently with supporting meteorological instruments (radiosonde and Raman lidar) to construct a paired set of both spatially and temporally coincident calibrated brightness temperatures and atmospheric profile parameters. Calculated brightness temperatures based on the measured atmospheric profile parameters were obtained using a numerical radiative transfer model. Incremental water-vapor weighting functions were used to study the impact of radiosonde hygrometer errors on the radiative transfer calculations. The aircraft-based brightness temperature comparisons are generally within 3 K for the channels sensitive to the lower atmospheric levels (89, 150, 183.3±7, and 220 GHz), but show discrepancies of up to 11 K for the opaque channels (183.3±1 and ±3 GHz) caused primarily by radiosonde bias. The ground-based calculations are similarly found to be sensitive to hygrometer errors in the lower atmosphere. Ground-based comparisons between MIR observations and lidar-based calculations are typically within ±6 K View full abstract»

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  • A relaxation labeling technique for computing sea surface velocities from sea surface temperature

    Page(s): 216 - 220
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    The paper applies a relaxation labeling technique to computing sea surface advective velocities from sequential satellite sea surface temperature (SST) images. The technique first identifies prominent feature points in a pair of time-lapsed SST images, and then estimates sea surface current velocities by establishing feature point correspondences between the two images. In the context of relaxation labelling operation the authors have constructed a compatibility coefficient expressing flow field spatial smoothness, or coherency, suitable for fluid motion. New results are reported and are compared to those obtained using two other methods: the geostrophic currents from in situ measurements and the maximum cross-correlation (MCC) method. The three kinds of velocities are shown to be consistent to one another View full abstract»

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  • Disparity analysis: a wavelet transform approach

    Page(s): 67 - 76
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    Describes a new method for the computation of a disparity map between a couple of stereo images. The disparities are computed along the x and y axes, respectively at each point of the image. In order to compute the disparity field, first a set of ground control points is detected in both images. Next, a mapping of the disparities over the entire image is done using the kriging method. Finally, the stereo couple of images is registered using the disparity maps View full abstract»

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  • Studies of the cylinder-ground double bounce scattering mechanism in forest backscatter models

    Page(s): 229 - 231
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    An integration methodology is presented for computing fields backscattered by the cylinder-ground double bounce scattering mechanism, in which randomly tilted, finite length dielectric cylinders stand on a slightly rough lossy surface, Based on the Stratton-Chu integral technique, incorporating exact solutions of fields scattered by the cylinders, the method yields like- and cross-polarized responses and polarization phase differences. Numerical results are reported View full abstract»

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  • A comprehensive, automated approach to determining sea ice thickness from SAR data

    Page(s): 46 - 57
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    Documents an approach to sea ice classification through a combination of methods, both algorithmic and heuristic. The resulting system is a comprehensive technique, which uses dynamic local thresholding as a classification basis and then supplements that initial classification using heuristic geophysical knowledge organized in expert systems. The dynamic local thresholding method allows separation of the ice into thickness classes based on local intensity distributions. Because it utilizes the data within each image, it can adapt to varying ice thickness intensities to regional and seasonal changes and is not subject to limitations caused by using predefined parameters View full abstract»

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  • TOPEX/Poseidon Microwave Radiometer (TMR). II. Antenna pattern correction and brightness temperature algorithm

    Page(s): 138 - 146
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    For pt.I see ibid., vol.33, no.1, p.125-37 (1995). The calibrated antenna temperatures measured by the TOPEX Microwave Radiometer are used to derive radiometric brightness temperatures in the vicinity of the altimeter footprint. The basis for the procedure devised to do this-the antenna pattern correction and brightness temperature algorithm-is described in the paper, along with its associated uncertainties. The algorithm is based on knowledge of the antenna pattern, the ground-based measurements of which are presented along with their analyses. Using the results of these measurements, the authors perform an error analysis that yields the net uncertainties in the derived TMR footprint brightness temperatures. The net brightness temperature uncertainties range from 0.79 to 0.88 K for the three TMR frequencies, and include the radiometer calibration uncertainties which range from 0.54 to 0.57 K. the authors also derive an estimate of the uncertainty incurred by using brightness temperatures measured in the ~40 km TMR footprint to estimate path delay in the ~3 km altimeter footprint. The RMS difference in path delay averaged over the largest TMR footprint relative to that in the altimeter footprint is estimated to be about 0.3 cm. Finally, the authors discuss the error associated with using unequal beams at the three TMR frequencies to derive path delays, and describe an approach using along-track averaging of the algorithm brightness temperatures to reduce this error View full abstract»

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  • A comparison of backscattering models for rough surfaces

    Page(s): 195 - 200
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    The objective of the study is to examine the ease of applicability of three scattering models. This is done by considering the time taken to numerically evaluate these models and comparing their predictions as a function of surface roughness, frequency, incident angle and polarization with the moment method solution in two dimensions. In addition, the complexity of the analytic models in three dimensions and their analytic reduction to high and low frequency regions are also compared. The selected models are an integral equation model (IEM), a full wave model (FWM), and the phase perturbation model (PPM). It is noted that in three dimensions, the full-wave model requires an evaluation of a 10-fold integral, the phase perturbation model requires a 4- and 2-fold integral while the integral equation model is an algebraic equation in like polarization under single scattering conditions. In examining frequency dependence of IEM and PPM in two dimensions numerically, the same model expression is used for all frequency calculations, it is found that both the IEM and PPM agree with the moment method solution from low to high frequencies numerically View full abstract»

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  • Cloud fraction and cloud shadow property retrievals from coregistered TIMS and AVIRIS imagery: the use of cloud morphology for registration

    Page(s): 172 - 184
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    The Thermal Infrared Multispectral Scanner (TIMS) and the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) were operated simultaneously from the ER2 aircraft during a March 1990 test over the Rio Bravo region, Belize. Coregistration of the imagery obtained by these two instruments is necessary to utilize the data effectively. A technique for registering the TIMS imagery to AVIRIS imagery is presented. It takes advantage of the morphology of the fair weather cumulus (FWC) clouds present in the imagery for estimating inter-sensor distortions. It relies on an iterative process in which skew, nearest neighbor sampling, and cross-correlation (1D and 2D) are applied. Comparison between the AVIRIS three-band ratio (3BR) imagery and the coregistered TIMS imagery shows that TIMS is superior in detecting thin cloud and cloud edge pixels, especially over shadowed background. Although the seven scenes analyzed in the study were obtained within the same one-hour time period and over the same geographical region, the optimum temperature threshold for cloud detection, with respect to the background temperature, varies significantly from 2.1 to 3.3°C. These values agree with the AVIRIS 3BR cloud fraction equivalent temperature thresholds to within 0.5°C. When applying a cloud shadow mask from the AVIRIS near infrared imagery to the coregistered TIMS background imagery, a 1°C temperature differential is found between the shadowed and nonshadowed background. This significant radiative cooling by Fair Weather Cumulus cloud shadows could introduce errors in surface emissivity retrievals by other Earth Observing System (EOS) investigators View full abstract»

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  • A multivariate contrast enhancement technique for multispectral images

    Page(s): 212 - 216
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    Multispectral and true-color images are often enhanced using histogram-based methods, usually by adjustment of color components after transformation to a selected secondary color system. Enhancement aimed toward the preservation of certain important perceptual qualities generally calls for the secondary coordinate system to be perceptually based. However, independent modification of the secondary components seldom uses the full extent of the RGB gamut unless some color values are clipped at the RGB boundaries. Preserving perceptual attributes is sometimes less important than obtaining the greatest possible color contrast improvement. This is especially true for color composites derived from multispectral images, which have no significant basis in human perception. A new multivariate enhancement technique the authors have named “histogram explosion” is able to exploit nearly the full RGB extent without clipping. While not generally based upon a perceptual model, the method can preserve original hue values when parameters are chosen properly. Experimental results of histogram explosion are presented, along with an analysis of its computational complexity View full abstract»

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  • FOPAIR: a focused array imaging radar for ocean remote sensing

    Page(s): 115 - 124
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    FOPAIR, a FOcused Phased Array Imaging Radar, provides high-resolution X-band images of the ocean surface. The system is designed to provide high-speed imagery (up to 180 frames/s) for short range applications (50-400 m) from a fixed platform such as a pier or tower. FOPAIR employs a fast, sequentially sampled antenna array and uses a software-based beamforming technique to generate high resolution imagery without the need for multiple radar receivers or beamforming hardware typical of active phased arrays. A summary of the principles of operation and the design of the instrument is given, followed by examples of FOPAIR's imaging capability. To the authors' knowledge, these examples include the highest-resolution, highest-speed microwave images of the ocean surface produced to date. A brief comparison between FOPAIR and synthetic aperture radar techniques is also included View full abstract»

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  • TOPEX/Poseidon Microwave Radiometer (TMR). I. Instrument description and antenna temperature calibration

    Page(s): 125 - 137
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    The TOPEX/Poseidon microwave radiometer (TMR) is a three-frequency radiometer flown on the TOPEX/Poseidon (T/P) satellite in low Earth orbit. It operates at 18, 21, and 37 GHz in a nadir-only viewing direction which is co-aligned with the T/P radar altimeters. The TMR monitors and corrects for the propagation path delay of the altimeter radar signal due to water vapor and nonprecipitating liquid water in the atmosphere. The paper describes the TMR instrument and the radiometric instrument calibration required to derive antenna temperature (TA ) from the raw digital data. TA precision of 0.4 K is predicted on orbit in all expected thermal environments, TA accuracy of 0.5-0.6 K is expected following a post-launch field calibration campaign. These performance figures represent a significant improvement over those of the Seasat and Nimbus-G Scanning Multichannel Microwave Radiometer on which TMR is based. The improvements are the result of specific hardware design and calibration changes. Hardware changes include a redesigned feed horn, to reduce impedance mismatches, and the addition of radomes over the feed and sky horns, to reduce thermal variations. Calibration changes involve more extensive temperature cycling and data analysis during thermal/vacuum testing View full abstract»

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  • On the direct determination of reflectivity spectrum from synthetic aperture radar data

    Page(s): 224 - 228
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    A method for obtaining the Fourier transform of a synthetic aperture radar reflectivity map is described where the reflectivity map itself does not need to be produced from the radar data. The procedure essentially rearranges a sequence of numerical integrations in such a manner that allows one or more numerical integration to be replaced by a closed-form expression involving Fresnel integrals View full abstract»

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  • Behavior of the ocean radar cross-section at low incidence, observed in the vicinity of the Gulf Stream

    Page(s): 162 - 171
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    Observational results derived from the airborne C-band scatterometer RESSAC looking at small incidence angles (⩽21°) were acquired in the vicinity of the Gulf Stream during the Surface Wave Dynamics Experiment in February and March 1991. Although the instrument was primarily designed to study the directional spectra of long waves, it is also possible to process the data to examine the behaviour of the radar cross-section σ0 versus incidence θ and azimuth φ. Although the classical physical or empirical models of σ0(θ,φ) predict that the anisotropy of σ0 versus azimuth increases with wind speed for a given incidence angle θ, it is found that under some conditions, this behaviour does not occur. By contrast, the anisotropy shows strong minima on certain occasions. Given the high-resolution upper ocean current measurements from Airborne expendable Current Profilers deployed on 5 March 1991, the regions of anisotropic minima could be identified from the three sets of observations (March 4, 5, and 7) downwind of the core of the Gulf Stream with a maximum current of 2 m s-1, and downwind of the current speed maxima of an anticyclonic-rotating warm core ring juxtaposed along the North Wall. On three other days (February 27, March 2 and 6), no clear anisotropic minima of was found, because the upper ocean current effects were obscured by variability in surface wind field View full abstract»

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  • Determination of reflector angular position using directional phase-encoded wavefield

    Page(s): 15 - 25
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    Using a directional phase-encoded wavefield, experiments are performed to determine if reflector angular coordinates can be obtained from the encoded phase. Where the target sizes are small, experiments show that significant phase distortions are introduced in the echoes. A mathematical expression for the reflection-diffraction response of a rectangular plate is derived and confirms major phase distortions can arise due to edge diffractions. A scheme of phase reversal is then devised which eliminates the effects of this phase distortion, enables target direction to be determined, and distinguishes small reflectors from large ones. This scheme also tends to be less sensitive to measurement errors. The new results are yet another important support for the usefulness of the emerging concept on directional phase-encoded wavefield View full abstract»

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  • Localized Radon transform-based detection of ship wakes in SAR images

    Page(s): 35 - 45
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    Presents a Radon transform-based approach to the detection of ship wakes in synthetic aperture radar (SAR) images. The key element of this technique is a localization of the Radon transform, whereby the intensity integration is performed over short line segments rather than across the entire image. A linear feature detection algorithm, which utilizes this localized Radon transform, is then developed. In this algorithm, referred to as the feature space line detector algorithm, the transform space is subjected to processing which serves to isolate and locate the response of linear features and suppresses the response of false alarms. This algorithm is tested on both synthetic images corrupted by various levels of Weibull multiplicative noise and on actual SAR images of ship wakes. The results of this testing demonstrate the algorithm's robustness in the presence of noise, as well as its ability to detect and localize linear features that are significantly shorter than the image dimensions View full abstract»

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  • A technique for measurement of spaceborne SAR antenna patterns using distributed targets

    Page(s): 100 - 114
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    A method to measure the antenna elevation pattern of synthetic aperture radar (SAR) using distributed targets is introduced. The features of the method are (1) the antenna pattern model parameters are estimated as the solutions of maximum likelihood estimation; (2) To guarantee the uniformity, a screening process based on a chi-square similarity test is applied to the image data; and (3) Noise generated in the SAR receiver and data processor tends to broaden the estimated antenna pattern. To improve it, the estimated noise level is subtracted from the image data. The authors assume that the scattering coefficient over the evaluated images is unknown yet constant (although this could easily be extended to the case where the variation of γ or σ° with incidence angle is known), Three types of antenna pattern models are tested, Among them, the best result is given by a fourth order power model. This technique is applied to selected image data sets from the SIR-B mission, including several scenes analyzed previously by Moore from the Amazon rain forest and Illinois farmland. For the Amazon data (which give the best results), the authors found that the technique adopted in the paper gives residual errors on the antenna pattern fit of less than 0.08 dB for a given scene. Applying the antenna pattern estimated from one scene to others, residual errors of less than 0.3 dB are achieved View full abstract»

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  • Estimation of forest parameters through fuzzy classification of TM data

    Page(s): 77 - 84
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    Several studies have investigated the utility of Landsat 5 TM imagery to estimate forest parameters such as stand composition and density. Regression equations have generally been used to relate these parameters to the radiance responses of the TM channels. Such a method is not feasible in highly complex landscapes, where forest mixtures and terrain irregularities may obscure the existence of simple relationships. A fuzzy approach to the problem is presented based on a multi-step procedure. First, some typical forest plots with known features are spectrally identified. A maximum likelihood fuzzy classification with nonparametric priors is then applied to the study images, so as to derive fuzzy membership grades for all pixels with respect to the typical plots. Finally, these grades are used to compute the estimates of the forest parameters by a weighted average strategy. The method was tested on a complex, rugged area in Tuscany mainly covered by deciduous and coniferous forests. Two TM scenes and accurate ground references taken in spring and summer 1991 were utilized for the testing. The first results, statistically evaluated in comparison with those of a more usual multivariate regression procedure, are quite encouraging. The possible application of the fuzzy approach to other cases of environmental monitoring is finally discussed View full abstract»

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  • Scattering from ice crystals at 94 and 220 GHz millimeter wave frequencies

    Page(s): 93 - 99
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    Polarimetric scattering from cloud ice crystals modeled as hexagonal columns, hexagonal plates and stellar crystals are calculated at 94 and 220 GHz frequencies using the finite difference time domain (FDTD) method. Two orientation models are considered, In the first model the long axes of columns and broad surfaces of plates and stellar crystals are parallel to the horizontal plane and randomly oriented on this plane. The second model assumes uniform random orientation in three dimensions (3-D). The ratio of the backscattering cross sections at 220 and 94 GHz exhibits a steady decrease with increasing size up to about 1000 μm (2000 μm for columns) for horizontally aligned ice crystals at side incidence as well as for 3-D random orientation. This dual frequency ratio (DFR) may be useful in gauging the size of ice crystals, DFR may also be useful in discriminating between ice crystal types since it shows major differences between columns and planar crystals. The linear depolarization ratio (LDR), i.e. the ratio of the cross-polar to co-polar backscattering cross sections, at vertical incidence for the horizontal alignment model is much higher for columns (over 20 dB) compared to plates and stellar crystals. As a result LDR also has the potential for discriminating columns from planar crystals, Furthermore, the normalized Mueller matrices for horizontally aligned columns at vertical incidence are shown to be diagonal and a function of only LDR. This same result applies to the 3-D random orientation model for all three crystal types View full abstract»

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  • An expert system for land cover classification

    Page(s): 58 - 66
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    A framework to represent a broad class of problems in the analysis of remote sensing imagery is proposed, and an inference engine to tackle such problems is derived. A simple model for spectral knowledge representation is used along with a method for quantification of knowledge through an evidential approach. An automatic knowledge extraction technique is also proposed to gather knowledge from training samples. The techniques of knowledge extraction, representation and inferencing have been used to do a land cover analysis on two data sets, and the results are compared with contemporary digital techniques. It is found that the proposed approach has the advantages of avoiding commission errors, and can incorporate non-spectral and collateral knowledge, while its accuracy using only spectral knowledge is comparable with standard digital methods View full abstract»

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  • A composite discrete-continuous approach to model the microwave emission of vegetation

    Page(s): 201 - 211
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    A new approach to model the microwave emission of vegetation is described in the paper. The modeling is based on the combination of both the discrete approach to simulate single scattering albedo ω and the continuous approach to simulate vegetation scattering effects. This composite model COMPOS is designed first to account for absorption effects in a more accurate way than the continuous approach and secondly to remain relevant for inversion procedures. Sensitivity studies showed that the use of a priori information about the vegetation structure is relevant to simulate ω. So, a reduced number of input parameters can be used in the composite model. Simulations of single scattering albedo ω, of canopy opacity and of wheat emissivity have been compared with several sets of radiometric data. The comparisons show that the composite approach simulations are consistent with the microwave observations View full abstract»

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  • Removal of terrain effects from SAR satellite imagery of Arctic tundra

    Page(s): 185 - 194
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    Synthetic aperture radar (SAR) images of the Earth's terrestrial surface contain geometric and radiometric image effects which are caused by varying terrain elevation and slope. The radiometric effects tend to mask signal variations caused by other physical variables such as soil moisture and surface vegetation type, which are known to influence SAR backscatter signals. As a result, raw SAR images are of limited use in classifying surface vegetation type or quantifying the spatial distribution of soil moisture in regions of terrain relief, The authors present a technique for removing radiometric terrain effects from SAR images. Image correction was carried out in two steps. First, an existing modeling package was used in combination with digital elevation data in order to map the raw image pixels onto a geodetic coordinate system, thereby removing the geometric portion of the image distortion. Radiometric effects were then removed with the aid of a backscatter model which treats the reflected radiation as a combination of diffuse-Lambertian and specular components. Parameters in the backscatter model were determined by comparing two C-band SAR images of a test area in a region of Arctic tundra which were taken from ascending and descending orbit tracks of the ERS-1 satellite. The ascending and descending images displayed reductions in pixel value variance of 30% and 13%, respectively, after processing. Direct comparison of the two test area images reveals a dramatic improvement in image similarity after processing View full abstract»

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  • Compression of hyperspectral imagery using the 3-D DCT and hybrid DPCM/DCT

    Page(s): 26 - 34
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    Two systems are presented for compression of hyperspectral imagery which utilize trellis coded quantization (TCQ). Specifically, the first system uses TCQ to encode transform coefficients resulting from the application of an 8×8×8 discrete cosine transform (DCT). The second systems uses DPCM to spectrally decorrelate the data, while a 2D DCT coding scheme is used for spatial decorrelation. Side information and rate allocation strategies are discussed. Entropy-constrained code-books are designed using a modified version of the generalized Lloyd algorithm. These entropy constrained systems achieve compression ratios of greater than 70:1 with average PSNRs of the coded hyperspectral sequences exceeding 40.0 dB View full abstract»

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  • Dielectric constants of rubber and oil palm leaf samples at X-band

    Page(s): 221 - 223
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    The paper reports on the measured dielectric constants of leaves of two tropical crops, namely rubber and oil palm, as a function of moisture content at X-band. Using a microcomputer-based automated system consisting of a Wiltron scalar network analyzer and a slotted waveguide, the measurements are done based on the waveguide thin sheet technique. Theoretical values from the dual-dispersion model by Ulaby and El-Rayes [1987] and from the simple dielectric theory of Fung and Fung [1977] are compared with the experimental data. The model from Ulaby and El-Rayes is found to be able to give good estimates of the dielectric constants for the two types of leaf samples at X-band View full abstract»

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  • TOPEX/Poseidon microwave radiometer (TMR). III. Wet troposphere range correction algorithm and pre-launch error budget

    Page(s): 147 - 161
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    For pt.II see ibid., vol.33, no.1, p.138-46 (1995). The sole mission function of the TOPEX/Poseidon microwave radiometer (TMR) is to provide corrections for the altimeter range errors induced by the highly variable atmospheric water vapor content. The three TMR frequencies are shown to be near-optimum for measuring the vapor-induced path delay within an environment of variable cloud cover and variable sea surface flux background. After a review of the underlying physics relevant to the prediction of 5-40 GHz nadir-viewing microwave brightness temperatures, the authors describe the development of the statistical, two-step algorithm used for the TMR retrieval of path delay. Test simulations are presented which demonstrate the uniformity of algorithm performance over a range of cloud liquid and sea surface wind speed conditions. The results indicate that the inherent algorithm error (assuming noise free measurements and an exact physical model) is less than 0.4 cm of retrieved path delay for a global representation of atmospheric conditions. An algorithm error budget is developed which predicts an overall algorithm accuracy of 0.9 cm when modeling uncertainties are included. When combined with expected TMR antenna and brightness temperature accuracies, an overall measurement accuracy of 1.2 cm for the wet troposphere range correction is predicted View full abstract»

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  • Polarimetric measurements of sea surface brightness temperatures using an aircraft K-band radiometer

    Page(s): 85 - 92
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    Presents the first experimental evidence that the polarimetric brightness temperatures of sea surfaces are sensitive to ocean wind direction in the incidence angle range of 30 to 50°. The experimental data were collected by a K-band (19.35 GHz) polarimetric wind radiometer (WINDRAD) mounted on the NASA DC-8 aircraft. A set of aircraft radiometer flights was successfully completed in November 1993. The authors performed circle flights over National Data Buoy Center (NDBC) moored buoys deployed off the northern California coast, which provided ocean wind measurements. The results indicate that passive polarimetric radiometry has a strong potential for global ocean wind speed and direction measurements from space View full abstract»

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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