Showing papers by "Robert T. Shin published in 1995"

Application of neural networks for sea ice classification in polarimetric SAR images

Abstract: Several automatic methods have been developed to classify sea ice types from fully polarimetric synthetic aperture radar (SAR) images, and these techniques are generally grouped into supervised and unsupervised approaches. In previous work, supervised methods have been shown to yield higher accuracy than unsupervised techniques, but suffer from the need for human interaction to determine classes and training regions. In contrast, unsupervised methods determine classes automatically, but generally show limited ability to accurately divide terrain into natural classes. In this paper, a new classification technique is applied to determine sea ice types in polarimetric and multifrequency SAR images, utilizing an unsupervised neural network to provide automatic classification, and employing an iterative algorithm to improve the performance. The learning vector quantization (LVQ) is first applied to the unsupervised classification of SAR images, and the results are compared with those of a conventional technique, the migrating means method. Results show that LVQ outperforms the migrating means method, but performance is still poor. An iterative algorithm is then applied where the SAR image is reclassified using the maximum likelihood (ML) classifier. It is shown that this algorithm converges, and significantly improves classification accuracy. The new algorithm successfully identifies first-year and multiyear sea ice regions in the images at three frequencies. The results show that L- and P-band images have similar characteristics, while the C-band image is substantially different. Classification based on single features is also carried out using LVQ and the iterative ML method. It is found that the fully polarimetric classification provides a higher accuracy than those based on a single feature. The significance of multilook classification is demonstrated by comparing the results obtained using four-look and single-look classifications. >

FD-TD analysis of electromagnetic radiation from modules-on-backplane configurations

Abstract: The electromagnetic radiation from a modules-on-backplane configuration found in computers is analyzed by means of the finite-difference time-domain (FD-TD) technique. The issues pertaining to the effects of the modules in influencing the overall radiating capacity of the configuration are addressed. Analyses are facilitated by using simplified modules-on-backplane models and by using a voltage source as the source of electromagnetic energy at a connector between a module and the backplane. The potential for enhancement of spurious emissions by the module-on-backplane configuration is demonstrated. For modules of typical dimensions, resonance is observed at the hundreds of MHz frequency range. Comparisons between numerical predictions and measurements are conducted to validate the numerical method. >

Polarimetric scattering from layered media with multiple species of scatterers

Abstract: Geophysical media are usually heterogeneous and contain multiple species of scatterers. In this paper a model is presented to calculate effective permittivities and polarimetric backscattering coefficients of multispecies-layered media. The same physical description is consistently used in the derivation of both permittivities and scattering coefficients. The strong permittivity fluctuation theory is extended to account for the multiple species of scatterers with a general ellipsoidal shape whose orientations are randomly distributed. Under the distorted Born approximation, polarimetric scattering coefficients are obtained. These calculations are applicable to the special cases of spheroidal and spherical scatterers. The model is used to study effects of scatterer shapes and multispecies mixtures on polarimetric signatures of heterogeneous media. The multispecies model accounts for moisture content in scattering media such as snowpack in an ice sheet. The results indicate a high sensitivity of backscatter to moisture with a stronger dependence for drier snow and ice grain size is important to the backscatter. For frost-covered saline ice, model results for bare ice are compared with measured data at C band and then the frost flower formation is simulated with a layer of fanlike ice crystals including brine infiltration over a rough interface. The results with the frost cover suggest a significant increase in scattering coefficients and a polarimetric signature closer to isotropic characteristics compared to the thin saline ice case.

Small aperture modeling for EMI applications using the finite-difference time-domain technique

Abstract: A method for modeling small aperture scattering in the Finite-Difference Time-Domain (FDTD) method is described and evaluated. The method replaces a small aperture in a perfectly conducting screen with pairs of oppositely directed electric and magnetic dipoles on either side of the short-circuited screen. Equations are derived which determine the electric and magnetic currents induced by arbitrary fields within the FDTD computational domain. These currents are induced by the short-circuit fields at the aperture. To determine the short-circuit fields it is necessary to subtract out the fields of the electric and magnetic dipoles. An analytical solution for dipole radiation is used to determine the dipole fields, and it is shown that the FDTD dipole fields deviate from the continuum dipole fields at distances close to the dipole. The method is evaluated for an isolated circular aperture and the results are compared with other methods. An analysis of the various errors inherent in the method is given. The an...