Radar modeling of a boreal forest

The authors report on the use of microwave modeling, ground truth, and synthetic aperture radar (SAR) data to investigate the characteristics of forest stands. A mixed coniferous forest stand has been modeled at SAR frequencies (P-, L-, and C-bands). The extensive measurements of ground truth and canopy geometry parameters were performed in a 200 m-square hemlock-dominated plot inside a forest. Hemlock trees in the forest are modeled by characterizing tree trunks, branches, and needles (leaves) with randomly oriented, lossy dielectric cylinders whose area and orientation distributions are prescribed. The distorted Born approximation is used to compute the backscatter at P-, L-, and C-SAR frequencies     

Simplified calculation of coverage area for MF AM broadcaststations

It is shown that the coverage area of medium- and high-frequency stations using ground-wave propagation beyond the horizon, especially MF AM stations, can be calculated by a simplified method. Recent field measurements have shown that the cumbersome FCC and CCIR calculations for estimating service area can be simplified to allow calculations with a PC, or even a programmable pocket calculator. The ground-wave electrical field intensity calculations are simplified by the introduction of a shadow or diffraction factor in the Norton planar earth expression. This factor permits the ground E field to be calculated well beyond the geometric and radio horizon, where E-field values are close to the atmospheric noise level. The approach permits engineers or technicians to calculate their own E =field curves for any frequency and any kind of soil     

Calibration of radars using polarimetric techniques

A method that uses the properties of rain medium itself to obtain accurate weather radar system gain calibration is discussed. This technique is based on the principle that the rainfall rate measured using absolute reflectivity (Z) and differential reflectivity ( Z_DR) is the same as that obtained from specific differential phase (K_DP). The measurements required for this technique are Z, Z_DR, and K _DP. The rainfall rate estimates obtained from Z and Z_DR are compared with the estimates obtained from K_DP. The scatter plot between the two rainfall estimates should lie close to a 1:1 line, and any systematic deviation from this line can be removed by appropriately adjusting the system gain. It is noted that Z_DR can be calibrated accurately because it is a differential power measurement, and K_DP is obtained from differential phase measurement, which is unaffected by system calibration. The sensitivity and accuracy of this technique are studied, and theoretical and simulation results for C-band frequencies are presented     

Microwave systems design for high-performance moving targetindicators in radars

Clutter cancellation of 65 dB and better is directly proportional to good radar stability, and since many hardware areas produce instabilities at various levels, the architecture of a radar requires special design considerations to support this high stability. The noise character and generation methods of these instabilities in the various hardware areas are described, and design solutions are given to eliminate them. Microwave delay line, a reliable, accurate method of measuring radar stability in L- and S-band radars, is described. The longest microwave delay line available for use at L -band and S-band frequencies is a 15-μs sapphire bulk acoustic wave (BAW) delay line. For higher-frequency radars, smaller delays must be used to keep the insertion loss down to a usable level. The question is raised as to the adequacy of this delay time to provide sufficient visibility for stability measurements of the stable noise. For transmitter measurements, it is adequate for the more common pulse widths, which are less than 15 μs. For LO measurements, the analysis shows that this delay does provide sufficient decorrelation for accurate LO noise measurements     

Chebyshev power-series solution of the cutoff frequencies in W-type single-mode fibers with an arbitrary core profile

A numerical method for calculating the cutoff frequencies in W -type single-mode fibers, which is based on the Chebyshev power-series approach, is discussed. It is shown that the method is fast and accurate, and can be applied to arbitrary-core-profile W-type fibers. Calculated results are presented to show the accuracy and convergence of the method. The method can be applied to discrete numerical data obtained from index profile measurements     

Ocean imaging with two-antenna radars

Synthetic-aperture radar (SAR) measurements are affected both by the radar backscattering cross section (σ⁰) and by the radial velocity (V_r) of the surface. An analysis of the capabilities of two-antenna SARs using a method for forming radial-velocity images to provide a general framework for evaluating the performance of different possible measurements is presented. Several key results are derived. One can measure σ⁰ and V _r separately, given a properly designed SAR. The error in V_r depends on the thermal noise in the receivers, the spacing of the two antennas, the coherence time of the surface (τ_c) and the spatial resolution of the measurement. There is an optimal separation of the antennas proportional to vτ _c, where v is the speed of the aircraft. In cases where radial velocities are unimportant, two-antenna SARs can be used to image azimuth-traveling waves with wavenumbers larger than the usual azimuth cutoff. The authors show how phase errors affect the accuracy of the measurement of V_r. In this development, they also provide a simple explanation for velocity bunching, a subject that has caused much controversy     

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     

Potential impact of emerging semiconductor technologies on advancedpower electronic systems

It is shown that a simple expression for k_D= R_on×C_in of a power semiconductor device can be used to evaluate the optimum performance feasible from a given material technology. A high-density, high-frequency microelectronic power supply based on synchronous rectifier switching topology is used to illustrate the potential impact of emerging semiconductor technologies on advanced power electronic systems. It is shown that optimum power devices based on wide-energy-bandgap semiconductors such as silicon carbide and diamond provide the basis for power conversion at very high frequencies     

Reconciliation of methods for estimating fmaxfor microwave heterojunction transistors

An attempt is made to reconcile the various approaches that have recently been used to estimate the maximum frequency of oscillation f_max in high-performance AlGaAs/GaAs HBTs. f_max is computed numerically from the full expression for Mason's invariant gain using y-parameters derived from the different approaches, i.e., the hybrid-π equivalent circuit, the T-equivalent circuit, and the drift-diffusion equations. It is shown that the results for f_max are essentially the same, irrespective of the source of the y-parameters, provided that the phase delays due to transit of carriers across the base and the collector-base depletion region are properly accounted for. It is also shown, for the particular device studied, that the widely used analytical expression for f_max, involving f _T and effective base resistance and collector capacitance, is remarkably accurate for frequencies below those at which transit-time effects become important     

A new system model for radar polarimeters

The validity of the 2×2 receive R and transmit T model for radar polarimeter systems, first proposed by H. Zebker et al. (1987), is questioned. The model is found to be invalid for many practical realizations of radar polarimeters, which can lead to significant errors in the calibration of polarimetric radar images. A more general model is put forward, which addresses the system defects which cause the 2×2 model to break down. By measuring one simple parameter from a polarimetric active radar calibration (PARC), it is possible to transform the scattering matrix measurements made by a radar polarimeter to a format compatible with a 2×2 R and T matrix model. Alternatively, the PARC can be used to verify the validity of the 2×2 model for any polarimetric radar system. Recommendations for the use of PARCs in polarimetric calibration and to measure the orientation angle of the horizontal (H) and vertical (V) coordinate system are also presented     

A stepped-frequency delta-K microwave radar for oceanographicstudies

A stepped-frequency delta-K (SFDK) radar has been developed to remotely sense ocean surface characteristics. This C-band radar uses frequency diversity and real-time signal processing for improved ΔK measurements. It is shown how frequency diversity can be implemented to enhance the energy of the resonant peak of the cross-product spectrum relative to the background energy. This feature is essential for making real-time measurements of resonant peak frequency over extended time periods. SFDK was used in a month-long field experiment at North Truro, MA. The results presented show that the phase velocity of ocean surface waves could be precisely measured 87% of the time     

Resolution function of nonsinusoidal radar signals. I.Range-velocity resolution with rectangular pulses

A generalization of a previously published ambiguity function that applies to radar known as large-relative-bandwidth radar, carrier-free radar, impulse radar, or nonsinusoidal radar is discussed. This radar has attracted attention because of its ability to penetrate absorbing materials used in the stealth technology. Another good application is the detection of moving targets with a small radar cross section by a look-down radar, which calls for a thumbtack ambiguity function. Since a small radar cross section in this application is typically due to the small size of the target that is coated with absorbing material, the antistealth feature of the nonsinusoidal radar is implicitly being used. The principle is presented of a resolution function (tentatively called the range-velocity or the range-Doppler resolution function) based on processing a nonsinusoidal signal consisting of N characters with a time separation T_D and each character consisting of a sequence of L binary pulses of duration T . It is shown that range-velocity resolution functions approaching the ideal thumbtack function are easy to obtain. The blind speeds of the pulse-Doppler radar with sinusoidal carrier do not inherently occur, and all velocities are observed as true velocities rather than as velocities modulo the first blind speed (velocity ambiguity)     

Self-cohering large antenna arrays using the spatial correlationproperties of radar clutter

A technique for self-calibrating a large antenna array system in the absence of a beamforming point source is presented that uses the spatial correlation properties of radar clutter. The array could be real or synthetic. It is shown that if R(X), the spatial autocorrelation function of the field (as measured by adjacent element pairs), is ensured to be real and positive in the neighborhood of the origin, both periodic and aperiodic arrays can be synchronized, forming retrodirective beams pointing at the axis of symmetry of the radar transmitter, provided that the interelement spacing does not exceed some limit (the order of the size of the transmitting aperture). If the spatial autocorrelation function is complex but has a linear phase, it is shown that one can still synchronize both periodic and aperiodic arrays, while if the phase of R(X) is nonlinear, only periodic arrays can be synchronized. In both cases of complex R(X), a residual beam-pointing error occurs. Computer simulations and airborne sea clutter data are reported that verify the theory and practicality of the algorithm     

The voltage-doping transformation: a new approach to the modelingof MOSFET short-channel effects

It is shown that the influence of the drain-source field on the potential barrier height is physically equivalent to and can be replaced by a reduction in channel doping concentration according to a formula derived from the two-dimensional Poisson equation. The actual barrier height for any drain bias and channel length, on which the derived equation depends, can be calculated easily using well-known one-dimensional (long-channel) solutions. This simple but general procedure, called the voltage-doping transformation (VDT), is shown to lead to analytically calculated potential distributions in fairly good agreement with two-dimensional numerical simulation. An application of the VDT to threshold voltage (V_tj) calculations also is shown. The V_th model is compared with measurements taken on implanted n-MOSFETs with various channel lengths. Good agreement demonstrates the accuracy of both the VDT and the new V_th model     

Estimation of surface roughness parameters from dual-frequencymeasurements of radar backscattering coefficients

A simple model was developed for estimating the surface roughness parameters of a bare soil field. The model uses a set of dual-frequency measurements of the field's radar backscattering coefficients, which can be matched to calculated results obtained with assumed values for the surface roughness parameters, as represented by the surface height standard deviation σ and its correlation lengths. Scatter plots of measured and calculated radar backscattering coefficients at the C -band (4.25-GHz) frequency versus those at L-band (1.5 GHz) show that it is feasible to estimate the surface roughness parameters using this technique. The estimated values for σ are in excellent agreement with those of measurements. However, there are discrepancies between the estimated and measured values for the correlation length L. For a very rough field, the geometrical optics model could be more appropriate for modeling the C-band data     

Propagation in a two-dimensional periodic random medium withinhomogeneous particle distribution

The behavior of electromagnetic waves when propagating in a periodic random medium, such as a row-structured canopy, is considered. The semideterministic character of the particle distributions is represented by nonuniform extinction and phase matrices and the problem is formulated by the radiative transfer equation. Solution of the radiative transfer equation is pursued both iteratively and by using a numerical technique, based on the discrete-ordinate approximation and Taylor series expansion. It is shown that the numerical solution for the periodic canopy is computationally efficient, and a closed-form for the first-order solution (iterative approach) of the radiative transfer equation is obtained for periodic cases. The analytical and numerical results are compared with transmission measurements at L- and C-band frequencies for a corn canopy for a variety of canopy conditions, with good agreement     

The Cramer-Rao lower bound for signals with constant amplitude andpolynomial phase

The authors derive the Cramer-Rao lower bound (CRLB) for complex signals with constant amplitude and polynomial phase, measured in additive Gaussian white noise. The exact bound requires numerical inversion of an ill-conditioned matrix, while its O(N ^-1) approximation is free of matrix inversion. The approximation is tested for several typical parameter values and is found to be excellent in most cases. The formulas derived are of practical value in several radar applications, such as electronic intelligence systems (ELINT) for special pulse-compression radars, and motion estimation from Doppler measurements. Consequently, it is of interest to analyze the best possible performance of potential estimators of the phase coefficients, as a function of signal parameters, the signal-to-noise ratio, the sampling rate, and the number of measurements. This analysis is carried out     

A new resistance measurement technique applicable tohigh-temperature superconducting materials at microwave frequencies

A two-gap electrically floating resonant strip was used for surface resistance measurements of the high-temperature superconductor YBa₂Cu₃O_7-δ. The method is simple, has no electrical contact, operates at various resonant frequencies, and requires only a small sample. An analysis that allows for the accurate design of the strip dimensions to produce a desired resonant frequency was used. Experimental measurements on resonant frequencies in X- and Ku-band (8-18 GHz) agree well with the calculations. The method allows one to extract the normalized surface resistance of the sample from transmission coefficient measurements at the resonant frequency. These normalized values compare favorably to the Mattis-Baredeen theory taken in the local limit. The resonant strip in waveguide should have applications in high-temperature superconductive material characterization and in the development of waveguide superconductive filters     

The DC characteristics of GaAs/AlGaAs heterojunction bipolartransistors with application to device modeling

A complete DC model for the heterojunction bipolar transistor (HBT) is presented. The DC characteristics of the HBT are compared with the Ebers-Moll (EM) model used by conventional bipolar junction transistors (BJTs) and implemented in simulation and modeling programs. It is shown that although the details of HBT operation can differ markedly from those of a BJT, a model and a parameter extraction technique can be developed which have physical meaning and are exactly compatible with the EM models widely used for BJTs. Device I- V measurements at 77 and 300 K are used to analyze the HBT physical device performance in the context of an EM model. A technique is developed to extract the device base, emitter, and collector series resistances directly from the measured I-V data without requiring an ideal exp(qV_be/kT) base current as reference. Accuracies of the extracted series resistances are assessed. AC parameters of HBT are calculated numerically from the physical device structure. For modeling purposes, these parameters are shown to be comparable with those of conventional BJTs     

Neural computation of arithmetic functions

A neuron is modeled as a linear threshold gate, and the network architecture considered is the layered feedforward network. It is shown how common arithmetic functions such as multiplication and sorting can be efficiently computed in a shallow neural network. Some known results are improved by showing that the product of two n-bit numbers and sorting of n n-bit numbers can be computed by a polynomial-size neural network using only four and five unit delays, respectively. Moreover, the weights of each threshold element in the neural networks require O(log n)-bit (instead of n -bit) accuracy. These results can be extended to more complicated functions such as multiple products, division, rational functions, and approximation of analytic functions