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Geoscience and Remote Sensing, IEEE Transactions on

Issue 1  Part 2 • Date Jan. 2010

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Displaying Results 1 - 25 of 32
  • [Front cover]

    Page(s): C1
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  • IEEE Transactions on Geoscience and Remote Sensing publication information

    Page(s): C2
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  • Table of contents

    Page(s): 297 - 298
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  • A Physically Based Screen for Precipitation Over Complex Surfaces Using Passive Microwave Observations

    Page(s): 299 - 313
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (2285 KB) |  | HTML iconHTML  

    Physically based passive microwave precipitation retrievals are difficult to develop over land because high nonuniform land emissivity values are difficult to distinguish from those of clouds. This paper uses an empirical approach to determine the covariance of emissivity at different microwave window channels and relies on this covariance to estimate the portion of the observed brightness temperatures that may be attributable to rainfall. One year (2006) of global cloud-free surface emissivity values were retrieved using data sets from multiple instruments on NASA's Aqua satellite. Correlations between the emissivities at different channels were developed for use in an empirical model within an optimal estimation retrieval. The optimal estimation retrieves surface temperature, total column water vapor, cloud water, and the emissivity at the 10.7-GHz horizontally polarized channel. From this retrieval and the covariance of emissivities, the 89.0-GHz brightness temperature at both polarizations can be estimated. Significant differences between the observed and retrieved high-resolution brightness temperatures are used to screen for precipitation, and results are compared to ground-based radar data for several study regions representing a variety of land surface types in the U.S. The Heidke Skill Score is used to determine the robustness of this methodology and, in all cases, demonstrates at least some increase in skill relative to random chance. View full abstract»

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  • Fire Detection by Microwave Radiometric Sensors: Modeling a Scenario in the Presence of Obstacles

    Page(s): 314 - 324
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    This paper deals with the problem of fire detection in the presence of obstacles that are nontransparent to visible or infrared wavelengths. Exploiting the obstacle penetration capability of microwaves, a solution based on passive microwave radiometry has been proposed. To investigate such a solution, a theoretical model of the scene sensed by a microwave radiometer is developed, accounting for the presence of both fire spot and wall-like obstacles. By reversing the model's equations, it is possible to directly relate the obstacle emissivity, reflectivity, and transmissivity to the antenna noise temperatures measured in several conditions. These temperatures have been sensed with a portable low-cost instrument. The selected 12.65-GHz operation frequency features good wall penetration capability to be balanced with a reasonable antenna size. In order to verify the aforementioned model, several fire experiments have been carried out, resulting in an overall good agreement between measurements and developed theory. In particular, a 2-cm-thick plasterboard wall, typically used for indoor building construction, shows a transmissivity equal to 0.86 and can easily be penetrated by a microwave radiometer in the X-band. View full abstract»

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  • Comparison of Two Bare-Soil Reflectivity Models and Validation With L-Band Radiometer Measurements

    Page(s): 325 - 337
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    The emission of bare soils at microwave L-band (1-2 GHz) frequencies is known to be correlated with surface soil moisture. Roughness plays an important role in determining soil emissivity although it is not clear which roughness length scales are most relevant. Small-scale (i.e., smaller than the resolution limit) inhomogeneities across the soil surface and with soil depth caused by both spatially varying soil properties and topographic features may affect soil emissivity. In this paper, roughness effects were investigated by comparing measured brightness temperatures of well-characterized bare soil surfaces with the results from two reflectivity models. The selected models are the air-to-soil transition model and Shi's parameterization of the integral equation model (IEM). The experimental data taken from the Surface Monitoring of the Soil Reservoir Experiment (SMOSREX) consist of surface profiles, soil permittivities and temperatures, and brightness temperatures at 1.4 GHz with horizontal and vertical polarizations. The types of correlation functions of the rough surfaces were investigated as required to evaluate Shi's parameterization of the IEM. The correlation functions were found to be clearly more exponential than Gaussian. Over the experimental period, the diurnal mean root mean square (rms) height decreased, while the correlation length and the type of correlation function did not change. Comparing the reflectivity models with respect to their sensitivities to the surface rms height and correlation length revealed distinct differences. Modeled reflectivities were tested against reflectivities derived from measured brightness, which showed that the two models perform differently depending on the polarization and the observation angle. View full abstract»

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  • Comparison of HOAPS, QuikSCAT, and Buoy Wind Speed in the Eastern North Atlantic and the North Sea

    Page(s): 338 - 348
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    A systematic investigation and comparison of near-surface marine wind speed obtained from in situ and satellite observations, a reanalysis, and a reanalysis-driven regional climate model (RCM) are presented for the eastern North Atlantic and the North Sea. Wind-speed retrievals from QuikSCAT Level 2B 12.5 km and HOAPS-3-S are analyzed. The root-mean-square error (rmse) between QuikSCAT and buoy 10-m equivalent neutral wind (EQNW) is 1.50 (1.87) m ?? s-1 using a colocation criteria of 0.1?? and 0.06?? (0.3?? and 0.2??) in longitudinal and latitudinal distances from buoy locations and within 10 (20) min, demonstrating that QuikSCAT's mission requirement of providing wind speed with an rmse of 2 m ?? s-1 is met for the investigated area. The influence of three different stability and anemometer height correction algorithms for buoy wind speed on the buoy/QuikSCAT error is assessed: EQNW gives the best agreement with QuikSCAT data; however, differences are smaller than the buoy measurement error. The rmse between HOAPS and buoy wind converted to 10 m by the logarithmic wind profile is 2.27 (2.36) m ?? s-1 using a colocation of 0.1?? ?? 0.06?? (0.3?? ?? 0.2??) and within 10 (20) min. QuikSCAT shows good agreement with buoy wind for speeds up to 20 m ?? s-1. HOAPS shows an underestimation of high wind speeds beyond 15-20 m ?? s-1 probably due to a saturation of the return signal. The rmse between buoy wind speed and the National Centers of Environmental Prediction/National Center for Atmospheric Research Reanalysis (NRA R1) and the spectrally nudged RCM REMO (SN-REMO) are 2.2 and 2.5 m ?? s-1, respectively. View full abstract»

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  • Three-Dimensional Coherent Radar Backscatter Model and Simulations of Scattering Phase Center of Forest Canopies

    Page(s): 349 - 357
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    A 3-D coherent radar backscatter model for forest canopies was developed and used to improve the understanding of synthetic aperture radar (SAR) interferometric data. The model was based on a realistic 3-D spatial structure of a forest stand, in which every scatterer has its deterministic location. A backscattering signal from a scatterer was mapped into a pixel according to its range or signal time delay. The range or the time delay also determines the phase of the scattered field. All scattering matrices within a pixel were coherently added to yield the total backscattering field of the pixel. The coherent radar backscatter model takes into account not only the scattering contribution from the scatterers in the forest canopy but also the direct backscattering of the ground surface. Forest stands with three different spatial structures were simulated using L-system and field measurements. The number and sizes of trees in these forest stands were identical, but the 2-D arrangements of the trees were different. The interferometric SAR (InSAR) signals of these scenes were simulated using the 3-D coherent SAR model, and the heights of scattering phase centers were estimated from the simulated InSAR data. The results reported in this paper show that the spatial structures of vegetation play an important role in the location of the scattering phase center. The height of scattering phase center depends on canopy height, attenuation of canopy, and the gaps within the canopy. This paper shows that the spatial structure needs to be considered when the InSAR data are used for the estimation of forest structural parameters. View full abstract»

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  • Compensation of Faraday Rotation in Multipolarization Scatterometry

    Page(s): 358 - 364
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    Spaceborne remote sensors operating at L-band and lower frequencies can significantly be affected by Faraday rotation (FR) as their signals pass through the Earth's ionosphere. A method of compensating for FR in multipolarization scatterometry is introduced, which utilizes an ancillary estimate of the FR angle to retrieve corrected polarized scattered powers. Simulation results are presented to demonstrate the behavior of the FR correction process when radar speckle, instrument errors, and errors in the ancillary FR angle estimate are introduced. View full abstract»

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  • Field Investigations of Ku-Band Radar Penetration Into Snow Cover on Antarctic Sea Ice

    Page(s): 365 - 372
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    Monitoring long-term, large-scale changes in the Antarctic sea ice thickness is not currently possible due to the sampling constraints of the ship-based and airborne observations which comprise most of the available thickness data. Satellite radar altimetry has been used to measure sea ice thickness variability in the Arctic where it is assumed that the highest amplitude radar return originates from the snow/ice interface as the Arctic snow is cold and dry; however, this may not be the case in the Antarctic due to more complex snow stratigraphy caused by warmer winter temperatures and frequent snow flooding. We present the first measurements of radar penetration into snow cover on Antarctic sea ice in the Ku-band at which satellite radar altimeters operate. Data were taken using a sled-borne radar on sea ice off East Antarctica during September and October 2007. Radar data and field measurements of snow density, thickness, wetness, and layers were taken over a range of locations including snow packs with flooding, hard crusts, and icy layers. Detailed snow pit studies showed that the snow/ice interface was the dominant scattering surface only for snow without morphological features or flooding. Analysis of transect data showed that the mean depth of the dominant scattering surface of the radar was only around 50% of the mean measured snow depth, indicating that the dominant scattering surface was not always the snow/ice interface for the Antarctic sea ice surveyed. View full abstract»

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  • Hypothesis Testing in Speckled Data With Stochastic Distances

    Page(s): 373 - 385
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    Images obtained with coherent illumination, as is the case of sonar, ultrasound-B, laser, and synthetic aperture radar, are affected by speckle noise which reduces the ability to extract information from the data. Specialized techniques are required to deal with such imagery, which has been modeled by the G 0 distribution and, under which, regions with different degrees of roughness and mean brightness can be characterized by two parameters; a third parameter, which is the number of looks, is related to the overall signal-to-noise ratio. Assessing distances between samples is an important step in image analysis; they provide grounds of the separability and, therefore, of the performance of classification procedures. This paper derives and compares eight stochastic distances and assesses the performance of hypothesis tests that employ them and maximum likelihood estimation. We conclude that tests based on the triangular distance have the closest empirical size to the theoretical one, while those based on the arithmetic-geometric distances have the best power. Since the power of tests based on the triangular distance is close to optimum, we conclude that the safest choice is using this distance for hypothesis testing, even when compared with classical distances as Kullback-Leibler and Bhattacharyya. View full abstract»

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  • Validation and Calibration of ASCAT Using CMOD5.n

    Page(s): 386 - 395
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    The Advanced Scatterometer (ASCAT) onboard the Metop-A satellite became operational shortly after launch in 2006, and an absolute calibration using three transponders was achieved in November 2008. In this paper, we describe how the CMOD5.n ocean backscatter geophysical model function (GMF), which was derived using data from previous scatterometers onboard the European Remote Sensing 1 and 2 satellites (ERS-1 and ERS-2), was used to derive backscatter bias correction factors. The purpose is to remove the bias between ASCAT backscatter data and the CMOD5.n GMF output which allows these data to be used in place of ERS data in existing wind processing algorithms. The ASCAT Wind Data Processor, developed at the Royal Netherlands Meteorological Institute (KNMI), applies the bias correction factors to ASCAT data and uses CMOD5.n to retrieve wind vectors in order to produce an operational wind product. This resulted in a stable and high-quality ASCAT wind product since February 2007. We validate this product by comparing it to the European Centre for Medium-range Weather Forecasts (ECMWF) winds and buoy measurements. The bias correction factors indicate that ASCAT data and the GMF differ by roughly 0.3 dB below 55 ? and up to 0.8 dB above 55 ?. A possible explanation lies in CMOD5.n which has been poorly validated in this incidence angle regime. Validation of ASCAT data using the ocean calibration method confirms this result and also indicates that bias-corrected data are everywhere within 0.3 dB of CMOD5.n. The wind product validation shows an rms error of 1.3 m ?s-1 in wind speed and 16 ? in wind direction when compared to ECMWF winds. This is better than the results achieved using ERS scatterometer data. Against buoy winds, we find an rms error wind component error of approximately 1.8 m ?s-1 . These results show that the ASCAT wind product is of high quality and satisfies its wind component accuracy requirement of - - 2 m ?s-1. View full abstract»

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  • Magnetic Sensor Design for Femtotesla Low-Frequency Signals

    Page(s): 396 - 402
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    A system for detecting magnetic fields in the femtotesla range at low frequencies is developed, including the antenna, transformer, and amplifier. Each component is described with relevant tradeoffs which allow a large variety of receivers to be easily designed for any magnetic field sensing in the very low frequency range. A system using a 1-??-1-mH impedance antenna is developed further as an example that has been used extensively in measurements all over the world in the frequency range of 50 Hz-30 kHz. It has a gain of 0.58 V/nT and a sensitivity of below 1 fT/Hz1/2 above 250 Hz. View full abstract»

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  • Monitoring Subglacial Volcanic Eruption Using Ground-Based C-Band Radar Imagery

    Page(s): 403 - 414
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    The microphysical and dynamical features of volcanic clouds, due to Plinian and sub-Plinian eruptions, can be quantitatively monitored by using ground-based microwave weather radars. In order to demonstrate the unique potential of this remote sensing technique, a case study of a subglacial volcanic eruption, occurred in Iceland in November 2004, is described and analyzed. Volume data, acquired by a C-band ground-based weather radar, are processed to automatically classify and estimate ash particle concentration. The ash retrieval physical-statistical algorithm is based on a backscattering microphysical model of fine, coarse, and lapilli ash particles, used within a Bayesian classification and optimal regression algorithm. A sensitivity analysis is carried out to evaluate the overall error budget and the possible impact of nonprecipitating liquid and ice cloud droplets when mixed with ash particles. The evolution of the Icelandic eruption is discussed in terms of radar measurements and products, pointing out the unique features, the current limitations, and future improvements of radar remote sensing of volcanic plumes. View full abstract»

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  • Derivation and Discussion of the SAR Migration Algorithm Within Inverse Scattering Problem: Theoretical Analysis

    Page(s): 415 - 422
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    The analysis of synthetic aperture radar (SAR) migration developed by Gilmore has been refined within the context of the inverse scattering problem, particularly the distorted-wave Born approximation (DWBA). The SAR migration algorithm can be deduced from the DWBA-based inversion formulation when the following assumptions are satisfied: 1) homogeneous and nonfrequency-dependent background medium; 2) the exploding source model; and 3) the well-resolved targets described by an orthogonal relation derived in this paper. In addition, the other contributions of this paper are as follows: 1) The removal of the ??2 term has been clarified by the derived orthogonal relation; 2) a scale factor that balances the near-far field has been derived; and 3) a novel SAR migration algorithm for the imaging of targets embedded in a layered medium has been proposed. View full abstract»

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  • A Novel Autofocusing Approach for Real-Time Through-Wall Imaging Under Unknown Wall Characteristics

    Page(s): 423 - 431
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    A novel real-time through-wall imaging (TWI) algorithm with autofocusing ability in the presence of wall ambiguities is proposed in this paper. The spectrum Green's function is employed to formulate the TWI algorithm, where the fast Fourier transform can be used to reconstruct the image in a very short computation time. The complex scattering process due to the presence of the wall is automatically included in the imaging formulation through the multilayer Green's function. The autofocusing is achieved by introducing a time factor in the TWI formulation to get a dynamic image at different focusing time. The image at the time instant when the defined entropy is minimized is stored as the output of the TWI result. Simulation results show that the proposed method can provide high-quality focused image in a short computation time regardless of the estimated value of the wall parameters. View full abstract»

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  • A New Slant-Range Velocity Ambiguity Resolving Approach of Fast Moving Targets for SAR System

    Page(s): 432 - 451
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    This paper describes an ambiguity resolving approach for slant-range velocity estimation which utilizes the wideband characteristic of the transmitted signal (multiple wavelengths). Based on the wavelength dual-wavelength radar data. Then, two effective approaches are introduced to focus the moving target no matter the Doppler ambiguity exists or not. The slant-range velocity is estimated by the number of azimuth cell displacements between the two focused images. Both imaging methods have different properties and advantages. A performance analysis is made, and deleterious factors in practice are analyzed in detail. The effectiveness of the unambiguous slant-range velocity estimation approach is demonstrated with the use of simulated and real data. View full abstract»

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  • Focusing Bistatic SAR Data in Airborne/Stationary Configuration

    Page(s): 452 - 465
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    This paper presents a frequency-domain-based focusing algorithm for the bistatic synthetic aperture radar (BiSAR) data in airborne/stationary configuration. In this bistatic configuration, only the moving platform contributes to the azimuth modulation, whereas the stationary platform introduces a range offset (RO) to the range migration trajectories of targets at the same range. The offset is determined by the azimuth position of different targets with respect to the stationary platform. Since the RO is position dependent, monostatic SAR imaging algorithms are not able to focus the bistatic data collected in this configuration. In this paper, an analytical bistatic point-target reference spectrum is derived, and then, a frequency-domain-based algorithm is developed to focus the bistatic data. It uses an interpolation-free wavenumber-domain algorithm as a basis and performs a range-variant interpolation to correct the position-dependent RO in the image domain after coarse focusing. The proposed algorithm is validated by the simulated data and the real BiSAR data acquired by the Forschungsgesellschaft fu??r Angewandte Naturwissenschaften's airborne SAR system, PAMIR, in December 2007. In this BiSAR experiment, an X-band transmitter was stationary operated on a hill with PAMIR as the receiver mounted on a Transall C-160. View full abstract»

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  • Applying the Freeman–Durden Decomposition Concept to Polarimetric SAR Interferometry

    Page(s): 466 - 479
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    In this paper, the Freeman-Durden polarimetric decomposition concept is adapted to polarimetric SAR interferometry (PolInSAR) data. The covariance matrix obtained from PolInSAR observations is decomposed into the three scattering mechanisms matrices proposed by Freeman and Durden for polarimetric SAR (PolSAR) data. The objective is to describe each interferometric cross correlation as the sum of the contributions corresponding to direct, double-bounce, and random volume scattering processes. This procedure enables the retrieval not only of the magnitude associated with each mechanism but also of their location along the vertical dimension of the scene. One of the most important features of this algorithm is the potential to isolate more accurately the direct and volume contributions which usually cannot be correctly separated by means of PolSAR measurements. In addition, it is also possible to distinguish between direct scattering responses originated either at ground or produced by upper layers of vegetation. The proposed algorithm has been tested with simulated data from PolSARProSim software, laboratory data from maize and rice samples, and airborne data from a test site with different scenarios. View full abstract»

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  • Equivalence Analysis of Accuracy of Geolocation Models for Spaceborne InSAR

    Page(s): 480 - 490
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (596 KB) |  | HTML iconHTML  

    There are two main geolocation models for spaceborne synthetic aperture radar (SAR) interferometry (InSAR): range Doppler (RD) and direct geocoding (DG) models. The RD model gets the target position by combining and solving the Doppler equation, slant-range equation, and the modified Earth model equation. The DG location model gets the target position through its 3-D coordinates by the Doppler equation and two slant-range equations. Usually, the geolocation accuracy analyses of these two models are discussed separately. It is confused which one is more precise and we should use during InSAR system designing. This paper deduced and compared the geolocation accuracies of these two models in the same frame-matrix. According to the matrix theory, the explicit expressions of the geolocation uncertainty of RD and DG models were deduced through the use of parameters in matrix form. After defining a new slant-range plane coordinate system, the precision of RD geolocation model and that of geocoding location model were compared quantificationally. It was presented that the geolocation uncertainty formulas between RD and DG models were the same. Then, the conclusion that these two models would lead to the same precision in geolocation measurement was obtained. At last, computer simulation results were employed to confirm the mathematical analysis. View full abstract»

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  • Land Surface Brightness Temperature Modeling Using Solar Insolation

    Page(s): 491 - 498
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (535 KB) |  | HTML iconHTML  

    Retrieval of land surface emissivity and temperature from microwave brightness temperature data is a complex problem. The diurnal variation of temperature due to the diurnal cycle of solar radiation and weather conditions makes this problem even more challenging. In this paper, we use solar radiation in modeling the temporal variation of the brightness temperature state of the surface. Solar insolation modeling is used to estimate the diurnal variation of land surface brightness temperature. Solar radiation and brightness temperature are linked through temperature of the surface which is derived based on the radiation balance equation. The temperature state model behaves consistent to the measured temperature data. The root-mean-square (rms) error of the model and measured temperature during 1999 is 1.47 K with a correlation of 0.98. Brightness temperature is calculated as a product of physical temperature and emissivity. This relationship is used to transform the temperature state model into the temporal model of the brightness temperature. The model is validated using Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) brightness temperature observations at 10.65-GHz vertical polarization. The rms error of the modeled and measured brightness temperature during 1999 is 2.15 K with a correlation of 0.98. Physical and brightness temperature models are ordinary differential equations that are solved numerically to estimate model parameters. The model parameters are related to geophysical characteristics that modulate the temporal variation of the physical and brightness temperature. These parameters provide new insight into the thermal characteristics of the land surface. Brightness temperature model is used to retrieve emissivity from TMI measurements. Images of emissivity and other model parameters are spatially coherent and reflect ground geometrical and dielectric conditions. The results confirm that incident solar radiation is an important input in - - modeling the temporal variation in the physical temperature and brightness temperature. View full abstract»

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  • Mapping Forest Background Reflectance in a Boreal Region Using Multiangle Compact Airborne Spectrographic Imager Data

    Page(s): 499 - 510
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    Forest background, consisting of understory, moss, litter, and soil, contributes significantly to optical remote sensing signals from forests in the boreal region. In this paper, we present results of background reflectance retrieval from multiangle high-resolution Compact Airborne Spectrographic Imager sensor data over a boreal forest area near Sudbury, ON, Canada. Modifications of the background by white and black plastic sheets at two sites provide two extreme limits for the development and testing of an algorithm for retrieving the background information from multiangle data. Measured background reflectances in red and near-infrared bands at six sites in the vicinity of these modified sites are used to validate the algorithm. We also explore the effect of uncertainties in the input forest structural parameters on this retrieval. The results document: 1) capability of the algorithm to retrieve meaningful background reflectance values for various forest stand conditions, particularly in the low to intermediate canopy density range; 2) the effect of background bidirectional reflectance distribution function on retrieved values; 3) performance of the algorithm using data with different cross angle values; and 4) verification of the internal consistency of the geometric-optical 4-Scale model used. The results provide an important platform for the operational estimation of the vegetation background reflectance from the bidirectional reflections observed by the Multiangle Imaging Spectroradiometer instrument. View full abstract»

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  • Automatic Mapping of Linear Woody Vegetation Features in Agricultural Landscapes Using Very High Resolution Imagery

    Page(s): 511 - 522
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    Automatic mapping and monitoring of agricultural landscapes using remotely sensed imagery has been an important research problem. This paper describes our work on developing automatic methods for the detection of target landscape features in very high spatial resolution images. The target objects of interest consist of linear strips of woody vegetation that include hedgerows and riparian vegetation that are important elements of the landscape ecology and biodiversity. The proposed framework exploits the spectral, textural, and shape properties of objects using hierarchical feature extraction and decision-making steps. First, a multifeature and multiscale strategy is used to be able to cover different characteristics of these objects in a wide range of landscapes. Discriminant functions trained on combinations of spectral and textural features are used to select the pixels that may belong to candidate objects. Then, a shape analysis step employs morphological top-hat transforms to locate the woody vegetation areas that fall within the width limits of an acceptable object, and a skeletonization and iterative least-squares fitting procedure quantifies the linearity of the objects using the uniformity of the estimated radii along the skeleton points. Extensive experiments using QuickBird imagery from three European Union member states show that the proposed algorithms provide good localization of the target objects in a wide range of landscapes with very different characteristics. View full abstract»

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  • Quantifying the Uncertainty of Land Surface Temperature Retrievals From SEVIRI/Meteosat

    Page(s): 523 - 534
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    Land surface temperature (LST) is estimated from thermal infrared data provided by the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat Second Generation (MSG), using a generalized split-window (GSW) algorithm. The uncertainty of the LST retrievals is highly dependent on the input accuracy and retrieval conditions, particularly the sensor view angle and the atmospheric water vapor content. This paper presents a quantification of the uncertainty of LST estimations, taking into account error statistics of the GSW under a globally representative collection of atmospheric profiles, and a careful characterization of the uncertainty of input data, particularly the surface emissivity and forecasts of the total water vapor content. Such analysis is the basis for LST uncertainty estimation, also distributed to users, in the form of error bars, along with the LST retrievals. Moreover, the spatial coverage of SEVIRI LST is essentially determined by the LST expected uncertainty, instead of being restricted to view zenith angles below a given threshold (e.g., 60??). Within the MSG disk, the atmosphere is often dry for clear-sky conditions where angles are large (e.g., Northern and Eastern Europe and Saudi Arabia). By considering several factors that contribute to LST inaccuracies, it is possible to increase the spatial coverage to regions such as those mentioned earlier. Retrieved values are also compared with in situ observations collected in Namibia, covering a seasonal cycle. The two data sets are in good agreement with root-mean-square differences ranging between 1??C and 2??C, which is well below the average error estimated for the satellite retrievals. View full abstract»

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  • On-Orbit Calibration and Performance of Aqua MODIS Reflective Solar Bands

    Page(s): 535 - 546
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    Aqua MODIS has successfully operated on-orbit for more than six years since its launch in May 2002, continuously making global observations and improving studies of changes in the Earth's climate and environment. Twenty of the 36 MODIS spectral bands, covering wavelengths from 0.41 to 2.2 ?m, are the reflective solar bands (RSBs). They are calibrated on-orbit using an onboard solar diffuser (SD) and an SD stability monitor. In addition, regularly scheduled lunar observations are made to track the RSB calibration stability. This paper presents Aqua MODIS RSB on-orbit calibration and characterization activities, methodologies, and performance. Included in this paper are characterizations of detector signal-to-noise ratio, short-term stability, and long-term response change. Spectral-wavelength-dependent degradation of the SD bidirectional reflectance factor and scan mirror reflectance, which also varies with the angle of incidence, is examined. On-orbit results show that Aqua MODIS onboard calibrators have performed well, enabling accurate calibration coefficients to be derived and updated for the Level 1B production and assuring high-quality science data products to be continuously generated and distributed. Since launch, the short-term response, on a scan-by-scan basis, has remained extremely stable for most RSB detectors. With the exception of band 6, there have been no new RSB noisy or inoperable detectors. Like its predecessor, i.e., Terra MODIS, launched in December 1999, the Aqua MODIS visible spectral bands have experienced relatively large changes, with an annual response decrease (mirror side 1) of 3.6% for band 8 at 0.412 ?m, 2.3% for band 9 at 0.443 ?m, 1.6% for band 3 at 0.469 ?m, and 1.2% for band 10 at 0.488 ?m. For other RSB bands with wavelengths greater than 0.5 ?m, the annual response changes are typically less than 0.5%. In general, Aqua MODIS optics degradation is smaller than Terra MODIS, and the mirror-side differences are much smaller. Overall- - , Aqua MODIS RSB on-orbit performance is better than that of Terra MODIS. 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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Antonio J. Plaza
University of Extremadura