Springtime C-band SAR backscatter signatures of LabradorSea marginal ice: measurements versus modeling predictions

During the March 1987 Labrador Ice Margin Experiment (LIMEX '87) two independent investigations were conducted to determine the C-band backscattering cross section of the marginal pack ice along the Newfoundland coast. In one experiment, data from a recently calibrated C-band airborne scatterometer were combined with C-band synthetic aperture radar (SAR) data to measure the normalized scattering cross section of the ice at incidence angles from 10° to 74° to within ±2 dB. In the other experiment, detailed measurements of ice surface roughness and surface properties were made and the radar cross sections were predicted from a scattering model. In the present study, measured and model results are combined and shown to be fully compatible. By extension, the results are expected to apply to any rubbled sea-ice surface when surface scattering dominates     

Seasat over-land scatterometer data. I. Global overview of theKu-band backscatterer coefficients

Statistics on the backscatter coefficient σ⁰ from the Ku-band Seasat-A Satellite Scatterometer (SASS) collected over the world's land surfaces are presented. This spaceborne scatterometer provided data on σ⁰ between latitude 80° S and 80° N at incidence angles up to 70°. The global statistics of vertical (V) and horizontal (H) polarization backscatter coefficients for 10° bands in latitude are presented for incidence angles between 20° and 70° and compared with the Skylab and ground spectrometer results. Global images of the time-averaged V polarization σ⁰ at a 45° incidence angle and its dependence on the incidence angle are presented and compared to a generalized map of the terrain type. Global images of the differences between the V an H polarization backscatter coefficients are presented and discussed. The most inhomogeneous region, which contains the deserts of North Africa and the Arabian Peninsula, is studied in greater detail and compared with the terrain type     

Scattering from the perfectly conducting cube

The scattering cross sections in the E-plane, H-plane, and 45°-plane of the perfectly conducting cube illuminated broadside by an incident plane wave are computed using both a uniform high-frequency diffraction solution and magnetic-field integral equations. The computed cross sections are compared with measured cross sections for cube perimeters of 3, 6, 12, and 20 wavelengths. The total scattering cross section versus the perimeter of the cube is also computed and compared to that of the sphere     

An empirical model and an inversion technique for radar scatteringfrom bare soil surfaces

Polarimetric radar measurements were conducted for bare soil surfaces under a variety of roughness and moisture conditions at L -, C-, and X-band frequencies at incidence angles ranging from 10° to 70°. Using a laser profiler and dielectric probes, a complete and accurate set of ground truth data was collected for each surface condition, from which accurate measurements were made of the rms height, correlation length, and dielectric constant. Based on knowledge of the scattering behavior in limiting cases and the experimental observations, an empirical model was developed for σ°_hh, σ°_vv, and σ° _hv in terms of ks (where k=2π/λ is the wave number and s is the rms height) and the relative dielectric constant of the soil surface. The model, which was found to yield very good agreement with the backscattering measurements of the present study as well as with measurements reported in other investigations, was used to develop an inversion technique for predicting the rms height of the surface and its moisture content from multipolarized radar observations     

The polarization-dependent relation between radar backscatter fromthe ocean surface and surface wind vector at frequencies between 1 and18 GHz

A series of airborne scatterometer measurements carried out with the DUTSCAT multifrequency airborne scatterometer are discussed. This study deals with the first results obtained from the analysis of these measurements. The objective of this activity is to establish a multifrequency dual-polarization radar signature database, and with it a multidimensional version of the current CMOD1 model. The main features of the data set are the following. The wind exponent of the upwind normalized radar cross section (NRCS) increases with frequency and incidence angle in the case of HH polarization. The upwind/downwind ratio is mainly negative at 20° of incidence angle, always at C-, X-, and Ku1-bands     

Applicability of physical optics thin plate scattering formulas forremote sensing

The authors evaluate the applicability of simple formulas for scattering from flat plates that were developed using an extended physical optics (PO) procedure. These formulas take on especially simple forms (denote here as TPO) when the plates are electrically very thin. The authors consider circular plates (disks) and show that when the radius-to-thickness ratio (a/t) is large, the TOP formulas give accurate backscatter cross sections for all incident angles. The PO formulas are not usable for angles of incidence near or at edge-on to the flat surfaces of the disk. On the other hand, complex polarization rate information is lost with TPO. It has been argued elsewhere that TOP should hold even for electrically small disks provided a/t is large. The authors show that TPO gives results accurate (with some exceptions) to ~4% when a/t ~200 for Rayleigh disks. These results are obtained primarily by comparing TPO computational results with an exact numerical procedure     

Monte Carlo simulation of scattering from a layer of verticalcylinders

Electromagnetic scattering by a collection of randomly distributed vertical cylinders over a half-space dielectric is considered, using two approaches. In the first approach, a Monte Carlo simulation that takes into account scattering terms up to second order is used. Closed-form expressions for the second-order scattering terms are derived for cylinders that are in each other's near field. The second approach is based on the radiative transfer equations, which are solved by an iterative method up to and including the second-order terms. Radar backscatter measurements at X-band for a collection of metallic cylinders over a conducting ground plane are compared with the Monte Carlo and radiative transfer solutions. The data were acquired polarimetrically from 144 independent spots of the cylinder layer at incidence angles ranging from 20° to 60°. The simulation results agree well with the measured data and are used to check the validity of the radiative transfer results for a medium with large particles. It is shown that both the phase function computed for the cylinders and the extinction matrix of the layer are overestimated in the radiative transfer solution     

Scattering by a conducting spheroidal object with dielectriccoating at axial incidence

An analytic solution of a plane electromagnetic wave scattering by coated prolate spheroidal bodies is obtained, for axial incidence, by expanding the incident and scattered fields in terms of prolate spheroidal vector wave functions. The unknown expansion coefficients are determined by an infinite system of equations derived using appropriate boundary conditions. To solve for the unknown coefficients, the system of equations is truncated by retaining only the first N equations in N unknowns, where N depends on the size of the body and the desired accuracy. Numerical results for the scattering cross section are presented to show the effect of different coatings on the magnitude of the scattered field     

Radiation field consideration of biconical horn antenna withdifferent flare angles

The radiation fields of a biconical horn antenna with different flare angles are computed from both the fields of an equivalent Huygens source on the spherical-surface aperture and the infinite biconical horn antenna with spherical transverse electromagnetic wave. Numerical data and experimental results are first presented for the symmetrical biconical horn antenna ka=1.57 and 3.14 (where k is the propagation constant and a=cone length) with equal flare angle. Curves of the beamwidth versus cone length are then given for various flare angles. At flare angles above 75°, the beamwidth increases with the cone length, while at flare angles below about 75°, the beamwidth decreases with the cone length in this region of ka. For the asymmetrical biconical horn antenna, with two different flare angles (including a conical coaxial type and a discone type), the numerical and the experimental patterns are both presented. For the asymmetrical biconical horn antenna, the pattern main lobe direction, with respect to the antenna axis, may be adjusted by using suitable flare angles     

A numerical model for electromagnetic scattering from sea ice

A numerical model for scattering from sea ice based on the finite difference time domain (FDTD) technique is presented. The sea ice medium is modeled as consisting of randomly located spherical brine scatterers with a specified fractional volume, and the medium is modeled both with and without a randomly rough boundary to study the relative effects of volume and surface scattering. A Monte Carlo simulation is used to obtain numerical results for incoherent υυ backscattered normalized radar cross sections (RCSs) in the frequency range from 3 to 9 GHz and for incidence angles from 10° to 50° from normal incidence. The computational intensity of the study necessitates an effective permittivity approach to modeling brine pocket effects and a nonuniform grid for small scale surface roughness. However, comparisons with analytical models show that these approximations should introduce errors no larger than approximately 3 dB. Incoherent υυ cross sections backscattered from sea ice models with a smooth surface show only a small dependence on incidence angle, while results for sea ice models with slightly rough surfaces are found to be dominated by surface scattering at incidence angles less than 30° and by scattering from brine pockets at angles greater than 30°. As the surface roughness increases, surface scattering tends to dominate at all incidence angles. Initial comparisons with measurements taken with artificially grown sea ice are made, and even the simplified sea ice model used in the FDTD simulation is found to provide reasonable agreement with measured data trends. The numerical model developed ran be useful in interpreting measurements when parameters such as surface roughness and scatterer distributions lie outside ranges where analytical models are valid     

Absolute radiometric calibration of the CCRS SAR

Determining the radar scattering coefficients from SAR (synthetic aperture radar) image data requires absolute radiometric calibration of the SAR system. The authors describe an internal calibration methodology for the airborne Canada Centre for Remote Sensing (CCRS) SAR system, based on radar theory, a detailed model of the radar system, and measurements of system parameters. The methodology is verified by analyzing external calibration data acquired over a six-month period in 1988 by the C-band radar using HH polarization. The results indicate that the overall error is ±0.8 dB (1σ) for incidence angles ±20° from antenna boresight. The dominant error contributions are due to the antenna radome and uncertainties in the elevation angle relative to the antenna boresight     

Electrooptic modulation in an arbitrary cross section waveguide

The electromagnetic field of an optical wave of frequency ω, traveling through a long, electrooptic waveguide of arbitrary cross section and composition in the presence of an arbitrary external field of strength E^ext and radio frequency Ω, is derived by a perturbative argument, assuming Ω/ω≪1 and | r_ijkE^ext|≪1, where r _ijk are the linear electrooptic coefficients. An idealized model is solved exactly in the context of rigorous perturbation theory, and the solution is shown to be valid whenever (r_ijk E^ext)²ω/Ω≪1, without restriction on r_ijkE^extω/Ω. The arbitrary cross section formula, when evaluated in the model case, agrees exactly with the rigorous result, so it is argued that the arbitrary cross section formula should also be valid whenever (r_ijkE^ext)² ω/Ω≪1     

Electromagnetic scattering from an infinite circular metalliccylinder coated by an elliptic dielectric one

The scattering from an infinite, circular, perfectly conducting cylinder coated by an elliptic dielectric cylinder is considered. The electromagnetic field is expressed in terms of both circular and elliptical cylindrical wave functions, which are connected with one another by well-known expansion formulas. In the special case of small h=ka/2 (a is the interfocal distance of the elliptic dielectric and k its wavenumber), exact, closed-form expressions of the form S(h)=S(0)[1+gh ²+O(h⁴)] are obtained for the scattered field and the various scattering cross sections. Both polarizations are considered for normal incidence. Graphical results for various values of the parameters are given     

Diffraction coefficients and field patterns of obtuse angledielectric wedge illuminated by E-polarized plane wave

The diffraction problem is treated for the incidence of an E -polarized plane wave on both interfaces of an obtuse dielectric wedge. Based on the dual integral equation, the total field is obtained by the sum of the physical optics solution and the edge-diffracted correction term. Calculated diffraction coefficients and field patterns are plotted in figures for a wedge angle of 120°, incident angles of 60° and 70°, and relative dielectric constants of 2 and 10. It is shown that the Neumann expansion to the nonuniform currents provides a more accurate correction to the physical optics currents than the multiple expansion as the angle of dielectric wedge increases     

Electromagnetic scattering from an infinite elliptic metalliccylinder coated by a circular dielectric one

The scattering from an infinite elliptic metallic cylinder coated by a circular dielectric one is considered. The electromagnetic field is expressed in terms of both circular and elliptical cylindrical wave functions, which are connected with one another by well-known expansion formulas. In the special case of small h=ka/2 (a being the interfocal distance of the elliptic conductor and k the wavenumber of the dielectric coating), exact, closed-form expressions of the form S (h)=S(0)[1+g "h²+O(h⁴)] are obtained for the scattered field and the various scattering cross sections of the problem. Both polarizations are considered for normal incidence. Graphical results for various values of the parameters are given     

Calibration of polarimetric radar systems with good polarizationisolation

A practical technique is proposed for calibrating single-antenna polarimetric radar systems using a metal sphere plus any second target with a strong cross-polarized radar cross section. This technique assumes perfect isolation between antenna ports. It is shown that all magnitudes and phases (relative to one of the like-polarized linear polarization configurations) of the radar transfer function can be calibrated without knowledge of the scattering matrix of the second target. Comparison of the values measured (using this calibration technique) for a tilted cylinder in the X-band with theoretical values shows agreement within ±0.3 dB in magnitude and ±5° in phase. The radar overall cross-polarization isolation was 25 dB. The technique is particularly useful for calibrating a radar under field conditions, because it does not require the careful alignment of calibration targets     

Second harmonic generation and sum frequency mixing of dualwavelength Nd:YAlO3 laser in flux grown KTiOPO4crystal

On the basis of accurately measured refractive indexes, the authors have obtained the Sellmeier's equations for flux grown KTiOPO ₄ (KTP) crystal and used them to calculate the phase matched angles (&thetas;_m, φ_m) and effective nonlinear coefficients (d_eff) for type I and III second harmonic generation (SHG) and sum frequency mixing (SFM) of radiations at 1.0795 and 1.3414 μm. The optimum phase matching conditions for 1.0795 and 1.3414 μm SHG are that &thetas;_m=86.88 and 58.88°, respectively, in an XZ plane (φ=0) and for SMF of 1.0795 and 1.3414 μm in the same plane 76.02°. The corresponding d_eff values calculated from &thetas; _ms are 18.07×10^-9 and 17.42×10^-9 esu     

GaAs MESFET device dependences on ion-implant tilt and rotationangles

Lightly Cr-doped liquid-encapsulated Czochralski (LEC) GaAs wafers were implanted with 5×10¹² 100-keV Si²⁹ ions/cm² at tilt angles between 0 and 13° and at rotation angles between 0 and 45°C. Capacitance-voltage measurements were then made to determine electron profiles. It was found that cross-wafer device uniformity can be improved using implant tilt angles greater than 9°. For microwave MESFET devices, the maximum transconductances at low I_DS are achieved using tilt angles greater than 6° and rotation angles greater than 30°     

Extrapolation of near-field RCS measurements to the far zone

An algorithmic procedure for extrapolating near-field radar cross-section (RCS) measurements to the far zone has been derived, coded, and experimentally validated. The deviation of the extrapolation algorithm uses an optical model to estimate the surface currents induced on the scattering body by the incident field, and a specially weighted version of the Fourier transform to calculate the near-field scattering amplitudes associated with such surface currents. The extrapolation entails three steps. First, near-field measurements of the scattered electric and/or magnetic field are used to infer the monostatic vector potential. Next, the inverse Fourier transform of the inferred vector potential is multiplied by a special weighting function to estimate an equivalent obliquity factor. Finally, the far-field scattering pattern is estimated by taking the Fourier transform of the reweighted obliquity factor. This extrapolation procedure has been validated using anechoic-chamber data taken on a right-circular aluminium cylinder 25 λ high and 2.5 λ in radius at near-field range of 19% of 2 D²/λ where D is the nominal target diameter and λ the radiation wavelength. The extrapolated RCS pattern for this target was compared with an analytical estimate of its far-zone pattern and good amplitude and phase agreement was observed over a 20° cone of scattering angles     

Direct evidence of gate oxide thickness increase in tungstenpolycide processes

The increase of the effective gate oxide thickness for W-polycide processes is studied. The samples with as-deposited and annealed W polycide were analyzed by secondary ion mass spectrometry, transmission electron microscopy (TEM), and high-frequency CV measurements. The TEM cross section shows that the gate oxide thicknesses are ~244 and ~285 Å for as-deposited and 1000°C annealed samples, respectively. The TEM results agree with those from CV measurements. The TEM analyses provide direct physical evidence of an additional oxide thickness (~41 Å) during the W-polycide annealing