Ice depolarization of the COMSTAR beacon at 28.56 GHz during low fades and correlation with radar backscatter

Radar correlation with significant ice depolarization events accompanied by low copolarization fades of the 28.56-GHz COMSTAR beacon signal are described for an experimental program at Wallops Island, VA. Using a Faraday switch at the front end of the receiver, the copolarization and cross-polarization levels of the 28.56-GHz beacon signal are sequentially monitored. A nearby high resolutionS-band radar pointing along the Earth-satellite path monitors the simultaneous ice and rain reflectivity. Excellent correlation is noted between the cross-polarization events and relatively large and extended ice reflectivities along a segment of the Earth-satellite path at altitudes near and above the 0°C isotherm. The radar and receiver data strongly suggest the cross-polarization mechanism is due to a hailshaft which intersects the path at altitudes well below the 0° isotherm. Since the intervening ice results in a cross-polarization signal which either adds or subtracts to the cross-polarization antenna residual, a method is described to remove the residual from the resultant measured cross-polarization level, without employing a phase measurement. Cumulative, month-of-year and time-of-day statistics associated with the depolarization signals are established.     

Variability in the measurement of radar backscatter

A variety of systems and platforms have been used over the past three decades to acquire radar backscatter data of terrain. The variability in the reported data was evaluated for agricultural crops under "similar" phenological conditions and for approximately the same sensor parameters (frequency, polarization, and angle of incidence). The evaluation reveals wide variations in the magnitude of the scattering coefficient among different measurement programs. While it is difficult to determine the exact causes of these variations it is quite evident that extreme care must be employed in 1) monitoring the measurement system transfer function, 2) calibrating the system on an absolute basis, and 3) acquiring and reporting detailed target parameter information.     

Design and fabrication of a wideband reflectarray antenna in Ku and K bands

The purpose of this paper is design of a wide band single layer reflectarray antenna using a new broadband cell. The proposed cell consists of two parts. The first part is a circular patch and the second one is a ring with some additional stubs, which are attached to the patch and ring in a symmetrical form. The circular patch and ring elements are designed at frequencies of 20 GHz and 15 GHz respectively. In order to increase the reflectarray antenna bandwidth, at first the dimensions of the patch, ring and stubs are optimized to attain uniform phase response in the frequency range from 11 GHz to 20 GHz. Then an air layer is considered under the dielectric substrate. The reflectarray antenna is designed based on this optimized wideband unit cell. A wideband horn antenna is also designed as a feed antenna for the proposed reflectarray structure. The reflectarray antenna with horn are simulated. The 1 dB gain-bandwidth of 4.66 GHz is obtained (27.4% fractional bandwidth) in the frequency band of 14.72 GHz to19.38 GHz. Finally, the reflectarray antenna is fabricated and tested. It can be seen that there is a good agreement between simulation and measurement results.     

Dynamical properties of rainfall for performance assessment of earth/space communication links at Ku and Ka bands

Knowledge about the dynamic characteristics of rainfall rate and rainfading is of vital importance both for studies on the physical interpretation and modelling of the phenomena and for ancillary studies to counteract rain-induced effects by incorporating some changes in communication system parameters. This paper presents the dynamical characteristics of rainfall observed over Guwahati, India. Using a statistical procedure, an empirical model which predicts characteristic features of rainfading at the Ku and Ka bands is described. The fade inter-event gap statistics as a function of attenuation are discussed for widespread rain and thunderstorms separately. The observed and predicted results are compared with observations made by other investigators. As future communication systems using the Ku and Ka bands are being planned in India and other parts of Southeast Asia (e.g. Measat-I, Thaicom3, AsiaSat3), the results of this study may be used to predict rainfading properties. This study is the first of its kind over India. © 1998 John Wiley & Sons, Ltd.     

近海面电波传播试验与损耗模型分析

介绍了青岛近海海域开展的不同微波波段（L波段、S波段、C波段、X波段）传播试验。给出了不同的海上传播模型,其中包括统计性的Hata模型、Longly-Rice模型以及确定性的抛物方程模型。实测值与模型值比较发现：在L波段,实测损耗数据与Hata模型较为接近,但大部分要大于Hata模型;在S波段,实测数据亦与Hata模型较为接近,但在远距离上,实测损耗数据要小于Hata模型计算值;在C波段,实测数据与确定性蒸发波导传播损耗模型极为一致,Hata模型不再适用;而在X波段,实测数据与标准大气传播模型以及Longly-Rice模型较为接近,但低估了实际传输损耗值,Hata模型亦不再适用。     

Ku波段高性能声体波微波延迟线的设计与优化

通过优化换能器拓扑结构、腔体结构和可调的微带匹配网络设计,研制出中心频率14 000 MHz、带宽1 500 MHz及延迟时间0.5 μs的Ku波段声体微波延迟线.该产品的插入损耗为-56 dB,直通抑制大于45 dB,三次渡越抑制大于55 dB,产品综合性能指标优异.     

Unavailability due to rain of VSAT networks operating in Ka and Ku bands in Brazil

The unavailability due to rain of VSAT star networks operating on Ku and Ka bands in Brazil is analysed in this paper. A large number of simulations performed over the Earth–space links resulting from combinations of four (real and hypothetical) satellites with six Brazilian Earth stations provide a good characterization of the rain unavailability in tropical and equatorial regions subject to heavy rainfall and thunderstorms. The simulations also compare the influence on the estimated unavailability of the use of two different ITU‐R rain attenuation models and two values (calculated and measured) for the rainfall rate exceeded during 10^?2% of the time, the climatic parameter in the models, in addition to the link polarization employed. The results obtained point to VSAT star networks as a practical solution to provide telecommunications services to remote communities. Copyright © 2006 John Wiley & Sons, Ltd.     

Estimation of soil microwave effective temperature at L and C bands

The soil microwave effective temperature (T_e) is an important parameter which improves the accuracy of the soil surface moisture derived from low frequency microwave radiometric observations. A new semi-empirical model of T_e at L- and C-bands is proposed. The model is based on the following inputs: the air temperature (T_a), a deep soil temperature (T_d), and the microwave brightness temperature measured at X-band (λ≃3 cm) and V polarization (T_BXV). Unlike other approaches based on the surface temperature (T_s), the proposed model can be implemented without being dependent on the clear sky conditions required to measure T_s with a spaceborne infrared radiometer. However, the proposed model may also use T_s when available. The model was designed from a large data set simulated by a physical model for smooth bare soil. The model of T_e was then successfully validated with experimental data acquired during a ground based experiment with the multifrequency PORTOS radiometer. This model designed from smooth soil data was successfully tested on rough bare soil using experimental data     

Development of rain-attenuation and rain-rate maps for satellite system design in the Ku and Ka bands in Colombia

The predictions of rain rate and rain attenuation are the most important steps when analyzing a satellite link operating in the Ku and Ka bands. It can be a time-consuming process, especially when the analyses are made on a large number of sites, as might be the case with the broadband satellite systems of today. In this work, tools for the prediction of rain rate and rain attenuation are given in the form of contour maps for a tropical region (Colombia). The maps presented use the rain rate predicted by different methodologies, and the attenuation caused by these rain rates, using ITU recommendations. The information from these maps can then be entered into attenuation-prediction or system-planning tools. Additionally, a review of the results of the most important rain-rate and rain-attenuation campaigns is presented.     

Adaptation of the MIMICS backscattering model to the agriculturalcontext-wheat and canola at L and C bands

Presents the results obtained from the MIMICS (Michigan Microwave Canopy Scattering) forest backscattering model which was modified to accommodate agricultural parameters. In the MIMICS model, the forest canopy is divided into three regions: the crown layer, the trunk region, and the underlying rough ground. The crop cover situation is simulated by a rough ground and a crown layer composed of scatterers with different forms, distributions, and dielectric constants. The authors omit trunks from the final agricultural representation because these components are of the same order as the wavelength, contrary to the model's implicit assumptions. The simulation results are compared to ground based scatterometer data of wheat and canola. The paper describes the simulation results for the two crops at L and C bands and the two like polarizations. An analysis of the different backscattering mechanisms is also given for each crop. Good simulation results were obtained at L and C bands for HH polarization for both these crops throughout the growing season. An error analysis indicates that the soil moisture can be predicted with a precision better than 0.03 g/cm³ for both crops, if all other model parameters are known. In addition, if the moisture is known, the height of the stems and the diameter of the leaves of the canola crop can be estimated with a precision better than ±5 cm and ±0.5 cm, respectively     

Relation between the ringing of resonances and surface waves in radar scattering

The exact normal-mode or Mie series for electromagnetic scattering from a conducting sphere has in the previous literature been transformed via the Watson transformation into components corresponding to specularly reflected and to creeping waves, or via the singularity expansion method (SEM) into a series of pole contributions in the complex frequency plane which in the time domain give rise to a series of damped sinusoidal signals. In this work, the connection between the two methods is established by using the Watson transformation for obtaining the specular wave, and by transforming the remainder into the SEM series. From the latter, we obtain the shapes of creeping-wave pulses for the case of an incidentdelta-function pulse by evaluating the series via the stationary-phase method.     

A numerical study of the separation wavenumber in the two-scalescattering approximation [ocean surface radar backscatter]

The modeling of radar backscatter from the ocean uses the two-scale scattering approximation. This approximation assumes that the continuous spectrum of the ocean can be separated at some wavenumber into large- and small-scale surfaces, allowing use of physical optics and small perturbation methods. The authors investigate the choice of the separation wavenumber by comparing two-scale calculations and exact numerical calculations for a randomly rough surface with a power law spectrum     

Modeling of geometric properties of loblolly pine tree and standcharacteristics for use in radar backscatter studies

An approach is presented to physically describe the tree and canopy components within loblolly pine forests in studies of microwave backscattering from forested canopies. The approach is based on a set of algorithms which describe the biomass characteristics of individual trees using measurements of tree diameter, height, and canopy depth. These algorithms predict bole, branch, and needle biomass; number, sizes, and orientations of primary and smaller branches; and number and density of needle clumps. Methods are discussed to aggregate the individual tree measurements into stand measurements (i.e., measurements per unit area). The results of the model clearly show how biomass distribution between tree components and canopy layers varies as a function of stand age for loblolly pines. The results also show the approach can be used to predict changes in the branch size and orientation within the canopy as a function of stand age, both on an individual tree basis as well as an entire stand basis     

Kerr型介质界面上弱非线性TM表面波的传播特性

本文研究Ｋｅｒｒ型介质界面上弱非线性ＴＭ表面波的性质。数值计算表明了沿线性与非线性介质界面传播的ＴＭ表面波的自散焦特性。     

基于多层PCB布线的Ku波段TR组件的设计

多层板布线技术作为一种重要的组件小型化的技术,应用越来越广泛,通过此技术,可以将TR组件的尺寸做到更小,重量更轻。文章介绍了多层PCB布线技术的相关知识,并利用此技术针对一个Ku波段的TR组件给出了相应的结构设计、电路设计和工艺设计,并最终测得发射端功率输出大于10W,杂散抑制40dB左右;接收端接收总增益大于100dB,噪声系数小于4dB。     

Sea backscatter at HF: Interpretation and utilization of the echo

Theories and concepts for utilization of HF sea echo are compared and tested against surface-wave measurements made from San Clemente Island in the Pacific in a joint NRL/ITS/NOAA experiment. The use of first-order sea echo as a reference target for calibration of HF over-the-horizon radars is established. Features of the higher order Doppler spectrum can be employed to deduce the principal parameters of the wave-height directional spectrum (i.e., sea state); and it is shown that significant wave height can be read from the spectral records. Finally, it is shown that surface currents and current (depth) gradients can be inferred from the same Doppler sea-echo records.     

Superheterodyne amplification of electromagnetic waves of optical and terahertz bands in gallium nitride films

Superheterodyne amplification of electromagnetic waves of optical and terahertz bands in the case of three-wave interaction in n-GaN films with the space change wave of millimeter band amplified due to negative differential resistance is studied. It is shown that amplification of the space change wave in n-GaN films may be achieved on higher frequencies f ≤ 500 GHz than when using GaAs. The case of three-wave resonant interaction of two counter-propagating waves with the space charge wave is considered for the waveguide on based on GaN film on dielectric substrate. It is shown that gain of electromagnetic waves of optical band may reach 20–40 dB on the waveguide lengths of up to 100 μm.     

Study of various TDMA schemes for wireless networks in the presenceof deadlines and overhead

The objective of this paper is to determine the minimum system dropping rate (or, equivalently, dropping probability) induced by time division multiple access (TDMA) schemes supporting time-constrained applications with common maximum cell delay tolerance. Expressions are derived for the induced system dropping rate for various TDMA schemes with different overhead and the maximum number of users than can be admitted in the network without violating the maximum dropping rate constraint is determined. The system dropping rate achieved by suboptimal TDMA schemes is compared against the optimal (although ideal) TDMA scheme performance. The performance limiting factors associated with the suboptimal schemes are identified, and the magnitude of their (negative) impact is evaluated. Based on this information it is possible to point to performance improving modifications which should be pursued to the extent permitted by technological constraints. Finally, based on this derivations a network designer may choose the best TDMA scheme-among realizable variations of those considered here-to use in a particular situation     

Impact of tropospheric scintillation in the Ku/K bands on the communications between two LEO satellites in a radio occultation geometry

A theoretical analysis of the impact of clear-air tropospheric scintillation on a radio occultation link between two low Earth orbit satellites in K- and Ku-bands is presented, with particular reference to differential approaches for the measure of the total content of water vapor. The troposphere is described as a spherically symmetric turbulent medium satisfying Kolmogorov theory. Rytov's first iteration solution for weak fluctuations is used to derive an expression for the variance of amplitude fluctuations of the wave as well as their spectrum and the correlation between fluctuations at different frequencies. The validity of the assumptions made and the influence of atmospheric parameters on the quantities of interest are also investigated and discussed. Finally, numerical results are presented to provide an estimate of the level of scintillation-induced disturbances.