95-GHz scattering by terrain at near-grazing incidence

This study, consisting of three complimentary topics, examines the millimeter-wave backscattering behavior of terrain at incidence angles extending between 70 and 90°, corresponding to grazing angles of 20° to 0°. The first topic addresses the character of the statistical variability of the radar backscattering cross section per unit area σ_A. Based on an evaluation of an extensive data set acquired at 95 GHz, it was determined that the Rayleigh fading model (which predicts that σ_A is exponentially distributed) provides an excellent fit to the measured data for various types of terrain covers, including bare surfaces, grasses, trees, dry snow, and wet snow. The second topic relates to the angular variability and dynamic range of the backscattering coefficient σ⁰, particularly near grazing incidence. We provide a summary of data reported to date for each of several types of terrain covers. The last topic focuses on bare surfaces. A semi-empirical model for σ⁰ is presented for vertical (VV), horizontal (HH), and cross (HV) polarizations. The model parameters include the incidence angle &thetas;, the surface relative dielectric constant ϵ, and the surface roughness ks, where k=2π/λ and s is the surface root mean square (RMS) height     

Wind vector retrieval using ERS-1 synthetic aperture radar imagery

An automated algorithm intended for operational use is developed and tested for estimating wind speed and direction using ERS-1 SAR imagery. The wind direction comes from the orientation of low frequency, linear signatures in the SAR imagery that the authors believe are manifestations of roll vortices within the planetary boundary layer. The wind direction thus has inherently a 180° ambiguity since only a single SAR image is used. Wind speed is estimated by using a new algorithm that utilizes both the estimated wind direction and σ ₀ values to invert radar cross section models. The authors show that: 1) on average the direction of the roll vortices signatures is approximately 11° to the right of the surface wind direction and can be used to estimate the surface wind direction to within ±19° and 2) utilizing these estimated wind directions from the SAR imagery subsequently improves wind speed estimation, generating errors of approximately ±1.2 m/s, for ERS-1 SAR data collected during the Norwegian Continental Shelf Experiment in 1991     

An empirical model for interpreting the relationship betweenbackscattering and arid land surface roughness as seen with the SAR

The nature of the surface in a flat arid zone located in the western Sahara desert was studied and its roughness parameters defined. The main geological units were compared with backscattering data from ERS-1 images. Results showed a close correlation between the backscattering coefficient, σ₀, and the maximum height, h_max, of the rocks making up the arid landscape. The simplified relationship σ₀=4.63.1og h_max-15.29 (with h_max in cm and σ₀ in dB) was established from field measurements, giving an r² of about 88%. This empirical relationship is confirmed by another close relation between σ₀ and the statistical parameter s, the height standard deviation. The advantage of h_max is that it can be extracted for any site, whatever its roughness may be. The spatial distribution of the rock facets and the geometric characteristics of the incidence angle of the radar signal seem to explain the results     

QuikSCAT geophysical model function for tropical cyclones andapplication to Hurricane Floyd

The QuikSCAT radar measurements of several tropical cyclones in 1999 have been studied to develop the geophysical model function (GMF) of Ku-band radar σ₀ values (normalized radar cross section) for extreme high wind conditions. To account for the effects of precipitation, the authors analyze the co-located rain rates from the Special Sensor Microwave/Imager (SSM/I) and propose the rain rate as a parameter of the GMF. The analysis indicates the deficiency of the NSCAT2 GMF developed for the NASA scatterometer, which overestimates the ocean σ₀ for tropical cyclones and ignores the influence of rain. It is suggested that the QuikSCAT σ₀ is sensitive to the wind speed of up to about 40-50 m s^-1. The authors introduce modifications to the NSCAT2 GMF and apply the modified GMF to the QuikSCAT observations of Hurricane Floyd. The QuikSCAT wind estimates for Hurricane Floyd in 1999 was improved with the maximum wind speed reaching above 60 m s^-1. The authors perform an error analysis by comparing the QuikSCAT winds with the analyses fields from the National Oceanic and Atmospheric Administration (NOAA) Hurricane Research Division (HRD). The reasonable agreement between the improved QuikSCAT winds and the HRD analyses supports the applications of scatterometer wind retrievals for hurricanes     

Effect of tree structure on X-band microwave signature ofconifers

Experimental studies are performed on some coniferous trees, Austrian pine, Nordmann spruce, and Norway spruce, to investigate the relation between the tree architecture and radar signal at X-band. For a single tree, the radar cross section (RCS) is measured as a function of the scatterer location at 90° incidence. It is found that the main scatterers are the leafy branches, and the difference between σ_vv and σ_hh is significant at the upper portion of the tree. At the lower portion of the tree σ^o_vv and σ°_hh have almost the same level. For a group of trees the angular trends of σ°_vv and σ°_hh are measured. It is found that the levels of σ°_vv and σ° _hh are of the same order, but their angular trends vary from one tree species to the other, depending on the tree species structure. The experimental results are interpreted with the help of a theoretical model which accounts for the structure of the tree     

Viscosity of GeO2-doped silica glasses

The viscosity of GeO₂-doped silica glasses is experimentally evaluated and found to be expressed as a function of the monoexponential form η₀ exp(-AΔe), where η₀ is the viscosity of pure silica glass, A is a positive constant, and Δe is an equivalent relative-index difference that is introduced for a silica glass with nonuniform dopant distribution in its cross section. The constant A for GeO₂-doped silica glasses is found to be about 0.5, which is much smaller than that for fluorine-doped silica glasses and has almost no temperature dependence between 1400°C and 2200°C     

Dependence of the normalized radar cross section of water waves onBragg wavelength-wind speed sensitivity

Measurements of the normalized radar cross section (σ°) made by the YSCAT ultrawideband scatterometer during an extended deployment on the Canada Centre for Inland Waters (CCIW) Research Tower located at Lake Ontario are analyzed and compared with anemometer wind measurements to study the sensitivity of σ° to the wind speed as a function of the Bragg wavelength. This paper concentrates on upwind and downwind azimuth angles in the wind speed range of 4.5-12 m/s. While YSCAT collected measurements of σ° at a variety of frequencies and incidence angles, this paper focuses on frequencies of 2.0, 3.05, 5.30, 10.02, and 14.0 GHz and incidence angles within the Bragg regime, 30-50°. Adopting a power law model to describe the relationship between σ° and wind speed, both wind speed exponents and upwind/downwind (u/d) ratios of σ° are found using least squares linear regression. The analysis of the wind speed exponents and u/d ratios show that shorter Bragg wavelengths (Λ<4 cm) are the most sensitive to wind speed and direction. Additionally, vertical polarization (V-pol) σ° is shown to be more sensitive to wind speed than horizontal polarization (H-pol) σ, while the H-pol u/d ratio is larger than the V-pol u/d ratio     

Emission cross sections and spectroscopy of Ho3+ laserchannels in KGd(WO4)2 single crystal

The spectroscopic properties of Ho³⁺ laser channels in KGd(WO₄)₂ crystals have been investigated using optical absorption, photoluminescence, and lifetime measurements. The radiative lifetimes of Ho³⁺ have been calculated through a Judd-Ofelt (JO) formalism using 300-K optical absorption results. The JO parameters obtained were Ω₂=15.35×10^-20 cm², Ω ₄=3.79×10^-20 cm², Ω₆ =1.69×10^-20 cm². The 7-300-K lifetimes obtained in diluted (8·10¹⁸ cm^-3) KGW:0.1% Ho samples are: τ(⁵F₃)≈0.9 μs, τ( ⁵S₂)=19-3.6 μs, and τ(⁵F₅ )≈1.1 μs. For Ho concentrations below 1.5×10²⁰ cm^-3, multiphonon emission is the main source of non radiative losses, and the temperature independent multiphonon probability in KGW is found to follow the energy gap law τ_ph ^-1(0)=βexp(-αΔE), where β=1.4×10^-7 s^-1, and α=1.4×10³ cm. Above this holmium concentration, energy transfer between Ho impurities also contributes to the losses. The spectral distributions of the Ho³⁺ emission cross section σ_EM for several laser channels are calculated in σ- and π-polarized configurations. The peak a σ_EM values achieved for transitions to the ⁵I₈ level are ≈2×10^-20 cm² in the σ-polarized configuration, and three main lasing peaks at 2.02, 2.05, and 2.07 μm are envisaged inside the ⁵I₇→⁵I₈ channel     

Off-nadir radar altimetry

The characteristics of nadir versus off-nadir altimetry are reviewed and contrasted and a potentially serious problem has been pointed out that has been overlooked by earlier investigators, who focused on the nongeophysical error sources in off-nadir altimetry. Spatial gradients of radar cross section on the sea surface, caused by wind or current gradients or the variation of radar cross section with incidence angle, could introduce significant range errors in off-nadir altimeter. This potentially crippling effect can be overcome by leaving the traditional 13-GHz frequency and implementing the multibeam altimeter at 36 GHz. A multibeam altimeter proposed for the Eos (Earth Observing System) is described as well as a multimode airborne radar altimeter being developed to study problems inherent in off-nadir altimetry     

The study of atmospheric turbulence with an acoustic locator

As a rule, sodar investigations of turbulent atmosphere are based on measurements of the scattered cross section σ. Measurements of the spectrum broadening of the scattered signal provides an additional information. The authors compare scattering cross section data with spectrum broadening. The atmosphere sodar sounding was carried out in Zimenki, 25 km east of Nizhny Novgorod. The bistatic sodar with narrow (3°-5°) antenna beams has allowed the authors to carry out simultaneous measurements of scattering cross section a of acoustic signals with a wavelength λ≃0.5 m and frequency broadening of the scattered signal OF. Results have shown an essential correlation (up to 0.85) between σ and ΔF²     

Deep field penetration into a conducting cylinder

In this paper we investigate the penetration of both E and H waves into a cylinder of arbitrary cross section (characteristic dimension L). The material of the cylinder is a nonmagnetic good conductor, and we examine the limits of deep penetration (small L/δ, where δ is the penetration depth) and small quality factor Q=(ωϵ₀/σ). These two asymptotic conditions imply a low-frequency situation, for which we derive formulas for the induced currents and associated Joule and radiation losses     

Error-erasure decoding of binary cyclic codes, up to a particularinstance of the Hartmann-Tzeng bound

It is shown that error-erasure decoding for a cyclic code allows the correction of a combination of t errors and r erasures when 2t+r<σ₀; the parameter σ₀ denotes a particular instance of the Hartmann-Tzeng bound. This procedure is an improvement on the error-erasure decoding algorithm developed by G.D. Forney (1965), which works when 2t+r<σ, where σ denotes the BCH-bound of the code     

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     

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     

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°     

Relationship between and measurement of differential scatteringcoefficient (σ0) and bidirectional reflectancedistribution function (BRDF)

The microwave and optical scattering formulations are reviewed, a relationship between them at the same wavelength is presented, and the measurement conditions for obtaining range-independent and equipment-independent values are discussed. It is shown that the physical mechanism of scattering of electromagnetic beams from a surface can be quantified by either a differential scattering coefficient (σ⁰) or a bidirectional reflectance distribution function (BRDF). For a diffuse surface, σ⁰=4π BRDF cos &thetas;_i cos &thetas;_r, where &thetas;_i and &thetas;_r are the incident and reflected angles, respectively, from the surface normal. The geometrical constraints inferred in the measurement of coherent electromagnetic wave scattering from relatively small objects are easily satisfied to obtain range- and equipment-independent values, but care in experimental design is required when dealing with large surfaces. More care is also required when dealing with surfaces entailing specular features in addition to diffuse features     

Cr4+:YAG as passive Q-switch and Brewster plate in apulsed Nd:YAG laser

We demonstrate the performance of a Nd:YAG laser, passively Q-switched with a Cr⁴⁺:YAG plate, which plays the double role of a passive Q-switch and a Brewster plate. The Brewster plate configuration contributes an intracavity loss of approximately 3.2-10 ^-3 cm^-1 along the cavity length. Losses contributed by the active Cr⁴⁺ ions in the plate relate to their excited state absorption. A freshly measured transmission saturation curve of Cr⁴⁺:YAG suggests a ground state absorption cross section σ_gs=(8.7±0.8)-10^-19 cm², and an excited state absorption cross section σ_es=(2.2±0.2)-10^-19 cm² of the Cr⁴⁺ ions at λ=1064 nm     

Electroless Ni-Cu-P barrier between Si/Ti/Al pad and Sn-Pbflip-chip solder bumps

This work investigates the barrier effect of the electroless Ni-Cu-P deposit between Al conductor and Sn-Pb solder bump, as well as the interfacial reaction with the Sn-Pb solder. For the Ni-Cu-P/63Sn-37Pb system, a (Ni, Cu)₃Sn₄ compound with three different morphologies: fine-grain, whisker, and polygon are formed at the Ni-Cu-P/63Sn-37Pb interface after reflow at 220°C for 15 s. These (Ni, Cu)₃Sn₄ crystals transform into polygon shape with smooth appearance during 150°C aging. For the Ni-Cu-P/95Pb-5Sn system, equiaxial (Ni, Cu)₃Sn₄ crystals are formed at the Ni-Cu-P/95Pb-5Sn interface after reflow at 350°C for 15 s, and they also transform into polygon shape during 150°C aging. In addition, the Ni-Cu-P deposit will crystallize to form Ni₅ P₂ during 350°C reflow. The growth of the (Ni, Cu)₃ Sn₄ compound by solid state reaction is a diffusion controlled process for both Ni-Cu-P/63Sn-37Pb and Ni-Cu-P/95Pb-5Sn systems. A 4 μm Ni-Cu-P deposit can provide adequate barrier function between an Al conductor and two Sn-Pb solders under 150°C aging for 1000 h     

A composite optical waveguide-based polarimetric interferometer forchemical and biological sensing applications

A new polarimetric interferometer has been developed on the basis of the phase difference between transverse electric (TE)₀ and transverse magnetic (TM)₀ modes in a composite optical waveguide (OWG). The composite OWG consists of a single-mode potassium ion-exchanged planar waveguide overlaid with a high-index thin film that has two tapered ends and supports only the TE₀ mode. Applying tapered velocity coupling theory, we found that the TE₀ and TM₀ modes coexisting in the potassium ion-exchanged layer were separated in the thin film region of the composite OWG: the TE₀ mode was coupled into the thin film while the TM₀ mode was confined in the potassium ion-exchanged layer. Interference occurs between TE- and TM-polarized output components when a single output beam is passed through a 45°-polarized analyzer. The phase difference φ between both orthogonal output components is very sensitive to the superstrate index n_c in the thin film region. Our experimental results indicate that a slight change of Δn_c=3.71×10^-6 results in the phase-difference variation of Δφ=1° for a 5-mm-long TiO ₂/K⁺ composite OWG with a 34-nm-thick TiO₂ film. Such a simple polarimetric interferometer can be applied to chemical or biological sensors by modifying the upper film surface of the composite OWG with a chemically or biologically active substance