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Geoscience and Remote Sensing, IEEE Transactions on

Issue 5 • Date Sept. 1996

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Displaying Results 1 - 25 of 27
  • GRSS Awards Presented at IGARSS'95

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  • Accurate and efficient determination of the shoreline in ERS-1 SAR images

    Page(s): 1243 - 1253
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    Extraction of the shoreline in SAR images is a difficult task to perform using simple image processing operations such as grey-value thresholding, due to the presence of speckle and because the signal returned from the sea surface may be similar to that from the land. A semiautomatic method for detecting the shoreline accurately and efficiently in ERS-1 SAR images is presented. This is aimed primarily at a particular application, namely the construction of a digital elevation model of an intertidal zone using SAR images and hydrodynamic model output, but could be carried over to other applications. A coarse-fine resolution processing approach is employed, in which sea regions are first detected as regions of low edge density in a low resolution image, then image areas near the shoreline are subjected to more elaborate processing at high resolution using an active contour model. Over 90% of the shoreline detected by the automatic delineation process appear visually correct View full abstract»

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  • Designing optimal spectral indexes for remote sensing applications

    Page(s): 1254 - 1265
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    Satellite remote sensing data constitute a significant potential source of information on our environment, provided they can be adequately interpreted. Vegetation indexes, a subset of the class of spectral indexes, represent one of the most commonly used approaches to analyze data in the optical domain. An optimal spectral index is very sensitive to the desired information (e.g. the amount of vegetation), and as insensitive as possible to perturbing factors (such as soil color changes or atmospheric effects). Since both the desired signal and the perturbing factors vary spectrally, and since the instruments themselves only provide data for particular spectral bands, optimal indexes should be designed for specific applications and particular instruments. This paper describes a rational approach to the design of an optimal index to estimate vegetation properties on the basis of the red and near-infrared reflectances of the AVHRR instrument, taking into account the perturbing effects of soil brightness changes, atmospheric absorption and scattering. The rationale behind the Global Environment Monitoring index (GEMI) is explained, and this index is proposed as an alternative to the Normalized Difference Vegetation Index (NDVI) for global applications. The techniques described here are generally applicable to any multispectral sensor and application View full abstract»

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  • Temporal change in the extinction coefficient of snow on the Greenland ice sheet from an analysis of Seasat and Geosat altimeter data

    Page(s): 1066 - 1073
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    The extinction coefficient of snow ke along the central portion of the Greenland ice sheet is mapped using data from the Seasat (1978) and Geosat (1985-1989) altimeters. The extinction coefficient is obtained by fitting altimeter waveforms with a surface/volume scattering model. The authors find that in the lower latitudes the Seasat and Geosat extinction coefficients are very nearly the same, while in a specific higher latitude region of the ice sheet the Seasat ke values exceed the Geosat values by over 100%. By analyzing 18 months of the Geosat data, the author quantified the variability inherent in the extinction coefficient measurements. The results show that the observed temporal variation in the extinction coefficient from 1978 to 1985 is three times larger than the measured variability. This indicates that the average grain size of the near surface snow in this region may have decreased during the time span between the two altimeter datasets. The temporal change in extinction coefficient found in this study demonstrates the important contributions that time-series analysis of satellite datasets can make to the study of the polar ice sheets. In addition, these results have important implications for the study of long-term elevation change over the ice sheets using altimeter data. The author's study demonstrates that significant biases could be introduced into ice-sheet elevation change estimates because of temporal variations in the surface conditions of the ice sheet. Future investigations of ice-sheet mass balance using altimetry data should be aware of this possibility View full abstract»

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  • Measurement of topography using polarimetric SAR images

    Page(s): 1266 - 1277
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    A processing technique for polarimetric synthetic aperture radar (SAR) data has been developed which produces profiles of terrain slopes and elevations in the azimuthal (or along-track) direction. This technique estimates the average shift in orientation angle of copolarization backscatter caused by azimuthal tilts of the scattering plane. Using P-band data, tests of this technique have been made for an area in the Black Forest near Villingen/Schwenningen in Baden-Wurttemberg, Germany. The radar measured slope and derived elevation profiles have low rms errors and high correlation values when compared with a stereo-photograph digital-elevation map (DEM) for the area. This algorithm is capable of adaptively making transitions from the forested areas to nearby regions with open-terrain. Subsequent tests of the algorithm have been conducted using polarimetric SAR L-band data for a mountainous, nonforested, region in the Mojave Desert (Ft. Irwin, CA) where an accurate DEM also was available. Complete elevation and slope mapping of the terrain in two dimensions using this technique is possible when azimuthal elevation profiles are produced throughout the range extent of the SAR image View full abstract»

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  • A simplified scheme for obtaining precipitation and vertical hydrometeor profiles from passive microwave sensors

    Page(s): 1213 - 1232
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    Presents a computationally simple technique for retrieving the precipitation and vertical hydrometeor profiles from downward viewing radiometers. The technique is computationally much less expensive than previous profiling schemes and has been designed specifically to allow for tractability of assumptions. In this paper, the emphasis is placed upon passive microwave applications, but the combination of passive with active microwave sensors, infrared sensors, or other a priori information can be adapted easily to the framework described. The technique is based upon a Bayesian approach. The authors use many realizations of the Goddard Cumulus Ensemble model to establish a prior probability density function of rainfall profiles. Detailed three-dimensional radiative transfer calculations are used to determine the upwelling brightness temperatures from the cloud model to establish the similarity of radiative signatures and thus the probability that a given profile is actually observed. In this study, the authors show that good results may be obtained by weighting profiles from the prior probability density function according to their deviation from the observed brightness temperatures. Examples of the retrieval results are shown for oceanic as well as land situations. Microwave data from the Advanced Microwave Precipitation Radiometer (AMPR) instrument are used to illustrate the retrieval structure results for high-resolution data while SSM/I is used to illustrate satellite applications. Simulations are performed to compare the expected retrieval performance of the SSM/I instrument with that of the upcoming TMI instrument aboard the Tropical Rainfall Measuring Mission (TRMM) to be launched in August 1997. These simulations show that correlations of ~0.77 may be obtained for 10-km retrievals of the integrated liquid water content based upon SSM/I channels. This correlation increases to ~0.90 for the same retrievals using the TMI channels and resolution. Due to the lack of quantitative validation data, hydrometeor profiles cannot be compared directly but are instead converted to an equivalent reflectivity structure and compared to existing radar observations where possible View full abstract»

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  • Comparison of backscattered signal statistics as derived from indoor scatterometric and SAR experiments

    Page(s): 1074 - 1083
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    A comparison between indoor scatterometric and SAR measurements of distributed targets is presented. In particular, the backscattered signal statistics (in amplitude and phase) obtained in these two types of measurements are compared for different surface parameters (roughness and correlation length) and varying spatial resolutions in the SAR images. Experimental data have been taken in the European Microwave Signature Laboratory (EMSL) at JRC, an indoor facility which is able to measure extended targets both in scatterometric and linear SAR mode under the same conditions. The results confirm that, with spatial resolutions in the SAR image smaller than two autocorrelation lengths (ACL), the statistics of the backscattered signal depends significantly on the resolution cell size. For larger resolution cells, the experimental data are consistent with the Rayleigh model. This experimental finding substantially confirms the results based on numerical simulations, previously presented in the literature View full abstract»

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  • First SIR-C ScanSAR results

    Page(s): 1278 - 1281
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    During the two space shuttle missions in 1994, the Space-borne Imaging Radar-C (SIR-C) acquired several experimental datatakes: using the ScanSAR mode. Same of these ScanSAR data have been processed into images of swath wider than 200 km. Example images are shown. These early results demonstrate that ScanSAR operation is not only feasible, but also represents a potentially useful tool for regional and global scale imaging by the future SAR missions View full abstract»

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  • Supervised classification of K-distributed SAR images of natural targets and probability of error estimation

    Page(s): 1233 - 1242
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    A radiometric and textural classification method for the single-channel synthetic aperture radar (SAR) image is proposed, which explicitly takes into account the probability density function (pdf) of the underlying cross section for K-distributed images. This method makes extensive use of adaptive preprocessing methods (e.g. Gamma-Gamma MAP speckle filtering), in order to ensure good classification accuracy as well as fair preservation of the spatial resolution of the final result. Error rates can be estimated during the training step, allowing one to select only relevant reflectivity classes and to save computation time in trials. The classification method is based on a maximum likelihood (ML) segmentation of the filtered image. Finally, a texture criterion is introduced to improve the accuracy of the classification result View full abstract»

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  • Effects of spatial inhomogeneities and microwave emission enhancement in random media: an experimental study

    Page(s): 1084 - 1089
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    The effects of spatial distribution of the scattering elements on microwave emission from random media have been investigated using an experimental model composed of long, thin vertical dielectric cylinders on a reflecting screen. The measurements have been carried out at 10 and 37 GHz, H, V polarizations and different nadir and azimuth angles. Random “uniform” and “cluster” distributions as well as periodic “row” configurations with different volume fractions have been studied. When the cylinders are distributed in nearly circular clusters or in parallel rows significant emission enhancement with respect to uniform distribution is detected at 10 GHz and incidence angle lower than 50°, whereas, at 37 GHz the emission is lower for clusters and strongly dependent on azimuth angle for the “row” configuration View full abstract»

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  • Extended chirp scaling algorithm for air- and spaceborne SAR data processing in stripmap and ScanSAR imaging modes

    Page(s): 1123 - 1136
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    Presents a generalized formulation of the extended chirp scaling (ECS) approach for high precision processing of air- and spaceborne SAR data. Based on the original chirp scaling function, the ECS algorithm incorporates a new azimuth scaling function and a subaperture approach, which allow an effective phase-preserving processing of ScanSAR data without interpolation for azimuth geometric correction. The azimuth scaling can also be used for automatic azimuth coregistration of interferometric image pairs which are acquired with different sampling distances. Additionally, a novel range scaling formulation is proposed for automatic range coregistration of interferometric image pairs or for improved robustness for the processing of highly squinted data. Several simulation and processing results of air- and spaceborne SAR data are presented to demonstrate the validity of the proposed algorithms View full abstract»

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  • Aperiodic transmitter waveforms for spectrum estimation of moderately overspread targets: new codes and a design rule

    Page(s): 1285 - 1287
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    Presents several new classes of aperiodic waveforms suitable for FFT-based spectral analysis of moderately overspread radar targets. In previous work, the authors explained the principle of operation with a particular code. They now provide codes with higher performance as well as a design rule for the user to apply these codes to real targets View full abstract»

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  • Generation of digital elevation models by using SIR-C/X-SAR multifrequency two-pass interferometry: the Etna case study

    Page(s): 1097 - 1114
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    The authors exploit the interferometric multifrequency potentiality of the SIR-C/X-SAR system which is equipped with an L-, C-, and X-band sensor. They present a solution to improve the unwrapping performance of the C- and X-band data by considering the L-band unwrapped pattern. A new algorithm for the generation of a single digital elevation model (DEM) combining L-, C-, and X-band information is presented. This solution is based on the fusion of the unwrapped phase patterns by using a Kalman filter. The proposed fusion operation also accounts for the coherence characteristics of the three data sets. The selected test site is the Mt. Etna region in Italy which is very interesting from the volcanological and geological point of view. Numerical assessments of the achieved results are provided by evaluating the height accuracy with respect to a reference DEM View full abstract»

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  • The concept of hyperimage in wide-band radar imaging

    Page(s): 1144 - 1150
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    A method is proposed to construct radar (or sonar) hyperimages from the knowledge of a backscattering function. Compared to usual imaging, the new feature is to treat on equal footing all parameters characterizing the image: frequency, directivity and position of the bright points in the two-dimensional static case; frequency, velocity, position, and date of existence in the one-dimensional imaging of nonstationary targets. The main interest of such a formulation is to allow the control of the reciprocal relations that are always present in microwave imaging. This possibility can be useful for processing data obtained by today's wide-band radars View full abstract»

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  • Ocean wave dispersion surface measured with airborne IR imaging system

    Page(s): 1282 - 1284
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    Image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system. The images are geographically registered, and 3D frequency-wavenumber spectra are calculated and shown to have a strong 2D dispersion surface that is characteristic of wind waves and swell. Wave directions compare well with in situ measurements, and their speeds are consistent with the water depth. The authors conclude that temporal sequences of IR images acquired from long distance can provide information for extracting surface wave parameters View full abstract»

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  • Semi-empirical model for radar backscatter from snow at 35 and 95 GHz

    Page(s): 1059 - 1065
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    Radar backscatter experiments were conducted at 35 and 95 GHz to measure the response of snow-covered ground to snow depth, liquid water content, and ice crystal size. The measurements included observations over a wide angular range extending between normal incidence and 60° for all linear polarization combinations. A numerical radiative transfer model was developed and adapted to fit the experimental observations. Next, the radiative transfer model was exercised over a wide range of conditions and the generated data were used to develop relatively simple semi-empirical expressions that relate the backscattering coefficient (for each linear polarization) to incidence angle, snow depth, crystal size, and liquid water content View full abstract»

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  • Estimation of heat and mass fluxes from IR brightness temperature

    Page(s): 1184 - 1190
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    Soil-vegetation-atmosphere transfer models have been developed to simulate mass and energy exchanges between vegetation canopies, the soil, and the atmosphere. They may be used in conjunction with remote sensing data through inversion procedures. In this study, the inversions of two soil-vegetation-atmosphere transfer models are compared on the same data set. Hourly evolutions of stomatal conductance and evapotranspiration are retrieved from the midday measurement of thermal infrared brightness temperature. Seasonal evolution of evapotranspiration and midday stomatal conductance are monitored with a good accuracy with both models. However, the simpler model underestimates evapotranspiration because it does not include a realistic description of hourly evolution of stomatal conductance, and then underestimates morning and afternoon evapotranspiration. The other model gives a better description of hourly evolutions of stomatal conductance and evapotranspiration. This model gives also better estimates of hourly canopy photosynthesis. However, it requires more parameters and computer time than the simpler model, two unfavorable factors for inversion View full abstract»

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  • Simulation of ATSR-2 optical data and estimates of land surface reflectance using simple atmospheric corrections

    Page(s): 1204 - 1212
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    The author presents simulations of ATSR-2 optical data using a combined orbit and scan model and model atmospheres. The atmospheric profiles are taken from LOWTRAN, but the author has converted these into a form suitable for a discrete ordinates radiative transfer code. All scattering processes are taken into account. The major approximations used are that the atmosphere is plane parallel, and that polarization is not taken into account. Cloud-free conditions are assumed. The author assumes ATSR-2 observations over the Earth with a constant atmosphere, and different types of reflecting ground in order to see what the implications are for retrieving ground reflectances. It was found that in the winter months, surface reflectance estimation will be difficult except at low latitudes. In the northern winter, the ATSR-2 forward scan sees large reflectances from aerosols in the atmosphere. These are not seen in the southern winter, but at 55° S, the Sun is more than 80° from the zenith point at ATSR-2 crossing times, and the atmosphere is still bright. The results from two simple reflectance retrieval algorithms and their region of validity are also examined . First, the author looks at a simulated retrieval for a Lambertian reflectance covering the globe at all times of year. Then the author looks at a simulated retrieval of a BRDF at a particular location using pairs of overpasses over a period of one year. It is predicted that the simple algorithms work well for ATSR-2 Sun/Sensor configurations except in the far northern and southern winters. The failure in the southern winter might be avoided if the Rayleigh/aerosol interaction is taken into account. It is als predicted that the forward scan data for the northern winter is unlikely to be useful for land surface studies, but might be useful for aerosol sounding, provided this is restricted to areas where there is no snow or frost. In this study of the BRDF at one location, it was found that if certain types of model BRDF are used, Sun synchronous orbits will severely limit the kinds of information that can be estimated View full abstract»

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  • Dependence of attenuation in a vegetation canopy on frequency and plant water content

    Page(s): 1090 - 1096
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    Theory is presented to provide insight into the observation that attenuation through vegetation is proportional to vegetation water content. In this analysis, the canopy is modeled as a sparse layer of randomly oriented particles (leaves, stalks, etc.) over a flat, homogeneous ground plane (soil) and an expression is obtained for the “optical depth”. The formulas developed by Ulaby and El Rayes are used to relate this expression to the water content of the canopy. In the low frequency extreme (Rayleigh scatterers), the attenuation varies almost linearly with water content and inversely with wavelength. In contrast, in the high frequency limit, the attenuation is independent of both water content and frequency, in between, geometry dependent “resonances” occur even at the low frequency end of the microwave spectrum (e.g. L-band) making the dependence of attenuation on frequency and water content specific to canopy architecture View full abstract»

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  • Fusion of satellite active and passive microwave data for sea ice type concentration estimates

    Page(s): 1172 - 1183
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    Young first-year sea ice is nearly as important as open water in modulating heat flux between the ocean and atmosphere in the Arctic. Just after the onset of freeze-up, first-year ice is in the early stages of growth and will consist of young first-year and thin ice. The distribution of sea ice in this thickness range impacts heat transfer in the Arctic. Therefore, improving the estimates of ice concentrations in this thickness range is significant. The NASA Team Algorithm (NTA) for passive microwave data inaccurately classifies sea ice during the melt and freeze-up seasons because it misclassifies multiyear ice as first-year ice. We developed a hybrid fusion technique for incorporating multiyear ice information derived from synthetic aperture radar (SAR) images into a passive microwave algorithm to improve ice type concentration estimates. First, we classified SAR images using a dynamic thresholding technique and estimated the multiyear ice concentration. Then we used the SAR-derived multiyear ice concentration to constrain the NTA and obtained an improved first-year ice concentration estimate. We computed multiyear and first-year ice concentration estimates over a region in the eastern-central Arctic in which field observations of ice and in situ radar backscatter measurements were performed. The fused estimates of first-year and multiyear ice concentration appear to be more accurate than NTA, based on ice observations that were logged aboard the US Coast Guard Icebreaker Polar Star in the study area during 1991 View full abstract»

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  • A phase matrix for a dense discrete random medium: evaluation of volume scattering coefficient

    Page(s): 1137 - 1143
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    In the derivation of the conventional scattering phase matrix of a discrete random medium, the far-field approximation is usually assumed. In this paper, the phase matrix of a dense discrete random medium is developed by relaxing the far-field approximation and accounting for the effect of volume fraction and randomness properties characterized by the variance and correlation function of scatterer positions within the medium. The final expression for the phase matrix differs from the conventional one in two major aspects: there is an amplitude and a phase correction. The concept used in the derivation is analogous to the antenna array theory. The phase matrix for a collection of scatterers is found to be the Stokes matrix of the single scatterer multiplied by a dense medium phase correction factor. The close spacing amplitude correction appears inside the Stokes matrix. When the scatterers are uncorrelated, the phase correction factor approaches unity. The phase matrix is used to calculate the volume scattering coefficients for a unit volume of spherical scatterers, and the results are compared with calculations from other theories, numerical simulations, and laboratory measurements. Results indicate that there should be a distinction between physically dense medium and electrically dense medium View full abstract»

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  • Enhancement of low spatial resolution image based on high resolution-bands

    Page(s): 1151 - 1158
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    Thermal infrared measurements of Band 6 acquired by Landsat TM sensor have lower spatial resolution than those of the other six bands. The authors propose a statistical approach to enhance the resolution of low spatial resolution image by using remaining bands. They employ a multivariate normal distribution for the seven band values. The values of Band 6 are predicted by the conditional expectations. Validity of their procedure is examined by mean squared errors based on actual images View full abstract»

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  • Radiometric slope correction of synthetic-aperture radar images

    Page(s): 1115 - 1122
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    The brightness in a SAR image is affected by topographic height variations due to (1) the projection between ground and image coordinates, and (2) variations in backscattering coefficient with the local scattering geometry. This paper derives a new equation for (1), i.e. the radiometric slope correction, based on a calibration equation which is invariant under a coordinate transformation. An algorithm is described to obtain the slope correction from a SAR interferogram, which also enables retrieval of the full scattering geometry. Since the SAR image and interferogram are derived from the same data set, there is no need to match the image with the calibration data. There is also no need for phase unwrapping since the algorithm only uses the fringe frequencies. A maximum-likelihood estimator for the fringe frequency is analyzed and the algorithm is illustrated by processing ERS-1 SAR data. The example demonstrates that the spatial resolution and calibration error are adequate for most applications View full abstract»

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  • Monitoring detailed land surface changes using an airborne multispectral digital camera system

    Page(s): 1191 - 1203
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    Airborne multispectral digital camera systems provide the potential for flexible and inexpensive monitoring of land surface changes at high spatial resolutions. The goal of this study was to characterize the radiometric properties and processing requirements of Airborne Data Acquisition and Registration (ADAR) System 5500 data in the context of environmental monitoring applications. This airborne multispectral digital imaging system, manufactured and operated by Positive Systems Inc., consists of four digital cameras and an on-board digital capture system. Sensor lab tests of the digital cameras were conducted to determine spatial uniformity of image brightness, high frequency noise and signal-noise ratio (S/N), linearity of radiometric response, spectral response, and radiometric normalization and calibration. Submeter resolution ADAR 5500 data were acquired over constructed and natural coastal marshes, concurrent with ground-level radiometric and plant measurements, ADAR 5500 data were found to have high radiometric fidelity and were successfully normalized and calibrated using simple procedures and knowledge that the radiometric response is linear. Noise levels were low, and S/N was about 120 for lab test images and 90 for airborne images of the marsh complex. Vignetting effects were evident in lab images, which were inverted and low-pass filtered to generate a correction mask. Spatial and temporal differences of spectral reflectance between important marsh vegetation cover types represent “environmental signals” in the context of monitoring habitat restoration projects 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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Meet Our Editors

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