电离层Faraday效应对远程预警雷达性能的影响

在对近地空间目标进行预警跟踪时,雷达发射的电磁波会在电离层中传播,电离层作为磁化等离子体,会导致电磁波产生Faraday旋转,从而对雷达探测性能造成影响。通过数值计算的方式,仿真了Faraday旋转效应导致的雷达极化失配损耗,并对仿真结果进行了分析。通过仿真结果可以看出:太阳活动高年,Faraday旋转角比太阳活动低年大;雷达工作频率越高,受Faraday旋转影响越小;目标仰角较低时,线极化失配损耗较小;椭圆极化方式下,极化失配损耗随椭圆轴比增大而增大;用圆极化方式可以最大程度地减小由Faraday旋转导致的雷达探测性能降低。     

有耗表面反射波的极化特性研究

文中对有耗媒质界面反射波的极化特性进行了较详细的分析与计算，指出此时反射波的极化与入射波的极化是不同的，给出这种变极化特性随媒质参数及入射角的各种关系曲线，这些曲线对分析雷达目标的散射极化特性是很有意义的。     

Application of synthetic-aperture radars for measuring the Faraday rotation angle

The influence of the ionosphere on polarization characteristics of the electromagnetic radiation of a spacebome synthetic-aperture radar is analyzed. The value of the Faraday rotation angle of a linearly polarized wave radiated by this radar into the ionosphere is estimated for a comparatively long-wave band. A technique is proposed for determining the Faraday rotation angle by measuring the reflectivity of the Earth’s surface with a polarimetric radar. This technique is independent of the mapped geometry and orientation of reflected elements of the Earth’s surface.     

A survey of ionospheric effects upon earth-space radio propagation

The frequency dependence is derived and the order of magnitude is presented for various ionospheric effects upon radio waves which have frequencies greater than the penetration frequency of the ionosphere. Among the phenomena considered are phase-path length change, refraction, frequency change, group-path delay, polarization rotation, and absorption. A detailed discussion is given of the mean value and variability of ionospheric absorption, refraction, scintillations, and polarization changes.     

Sense reversal of circularly polarized waves on earth-space links

On VHF earth-space links near the geomagnetic equator, the sense of rotation of circularly polarized waves may, under some conditions, be reversed when penetrating the ionosphere. More generally, circular polarization is deformed to elliptical polarization. The effect may cause large, unexpected fades on links with circularly polarized terminals.     

Line-of-sight indoor radio communication using circular polarizedwaves

This paper presents an anti-multipath fading scheme employing a circular polarized wave for line-of-sight (LOS) indoor radio communications. The circular polarized wave transmission/reception significantly mitigates the multipath fading caused by reflections from walls, ceiling, and floor since the energies of single-bounce reflected waves do not appear at the receiver, thereby resulting in significant amplitude fade and delay spread reduction. The received signal intensity variations and delay spread are investigated numerically using a 14-path propagation model and the usefulness is confirmed by comparing with a linear polarized wave transmission/reception under a same LOS condition     

Wave Polarization Effects on the Transmission and Reception of lonospheric Radio Waves

The effects of wave polarization on the transmission and reception of ionospheric radio waves have been discussed. Special attention was given to the effects of polarization on the excitation of characteristic waves in the ionosphere, on the absorption losses in the ionosphere, on the reception of arbitrarily polarized plane waves, and on the polarization coupling losses. In this work only one-hop paths were considered and it was assumed that the operating frequency is in the HF region so that ray theory treatment may be applied with high accuracy. The results of investigation have shown that wave polarization is an important and sometimes critical factor in the overall performance of ionospheric radio networks. It is believed that careful consideration of the wave polarization effects in the design of reliable and efficient communication and broadcasting networks can result in relatively lower transmitter power and in the reduction of the number of channels occupied by a particular station.     

用IGS发布的TEC数据提高法拉第旋转校正精度

电磁波穿过电离层时引入的法拉第旋转是导致全极化微波辐射计观测亮温产生交叉极化的重要原因.分析得知IRI（international reference ionosphere）模型产生的TEC（total electron content）数据在部分低纬地区无法满足全极化辐射计对法拉第旋转的校正精度要求.为提高校正精度,分析了沿观测路径积分法和应用IGS（International GPS Service）发布的TEC数据校正两种方法对校正精度的影响.结果表明,沿观测路径积分法不能有效改善校正精度,而应用IGS数据校正可大幅提高低纬地区法拉第旋转校正精度,满足全极化微波辐射计对极化旋转角的校正精度要求.     

A shipborne fading reduction antenna system for maritime satellite communications

Multipath fading due to reflected waves from a sea surface is serious in maritime satellite communications. A shipborne antenna system which reduces the fading by using the cross-polarized component of the reflected waves is presented.     

Dual-polarization frequency reuse with frequency band shiftingallocation

Spectrum is a limited and precious resource in wireless communications. Effective spectrum utilization becomes more and more important because of the expeditious increase of demand for wireless communications. In principle, the dual-polarization frequency reuse system, which employs two orthogonally polarized electromagnetic waves to carry information, can double the system capacity. However, the system performance degrades seriously and sometimes becomes unacceptable due to cross polarization and fading. In this paper, a new linear-polarized dual-polarization frequency reuse system that utilizes the concept of orthogonal frequency allocation to increase spectrum utilization is proposed. We analyze the performance of the dual-polarization system modulated by the quadrature phase-shift keying in the multipath fading channel with cross-polarization interference and Doppler effect. Because of the orthogonality of the polarized carriers and frequency spacing, the effect of cross polarization, which defeats the performance of the dual-polarization system severely, can be ignored. The proposed system has a bit error rate around 10^-7 at the bit energy-to-noise ratio of 14 dB, while the bit error rates of the dual-polarization cancellation with bootstrap system and the conventional orthogonal frequency-division multiplexing system are about 10^-4     

Focusing of radio waves reflected from the ionosphere at low angles of elevation

The mechanisms by which radio waves reflected from the ionosphere experience focusing are discussed. Calculations of this focusing are presented for oblique propagation at low angles of elevation via the E and F layers, and curves for use in prediction studies are given.     

Propagation characteristics on line-of-sight over-sea paths in Japan

The results of line-of-sight over-sea propagation measurements over 37.2- and 35.5-km paths near Tokyo, Japan, are presented. Measurements of the fading (0.25, 3.99, 4.15, 6.72, and 18.0 GHz) of the cross polarization distortion (linear polarized wave at 4.15 GHz and circular polarized wave at 6.72 GHz) of the space diversity effect at 4.15 GHz, of the height-gain patterns at 6.72 GHz, and of the frequency-sweep patterns from 18 GHz to 22 GHz are included. The following statistical results are presented: the frequency, seasonal, and path dependence of fading; the effect of different fading occurrence mechanisms, such as K-type, duct-type, and distorted raindrops, for the cross polarization distortion; the diversity improvement factor; the effective reflection coefficient of sea surface; and the path-length difference between direct and sea reflected waves.     

A fading reduction method for maritime satellite communications

A maritime satellite communication system has been established mainly for large vessels. In the near future it will be expected to service not only large vessels but also small ones. Studies on reducing the fading of the received signals have become very important because a compact antenna designed to be mounted on small vessels has a broad beamwidth which can be greatly influenced by reflected waves from the sea surface. A simple method and antenna equipment which can reduce the fading are proposed and, further, the effects by experiments are confirmed.     

张衡一号卫星三频信标载荷典型事件观测

“张衡一号(简称ZH-1)”卫星搭载的三频信标机(tri-band beacon,TBB)载荷,能够对站链上空的电离层进行高精度测量.为检验TBB载荷在电离层监测中的应用效果,本文利用2018—2019我国中低纬地区TBB观测数据,采用求均值及多项式拟合的方法对各月份F₂层峰值电子密度(N_mF₂)数据进行处理,分析N_mF₂的半年变化特征;将2018-08-26磁暴当天与宁静日的电子密度作差值,分析电子密度对磁暴事件的响应特征.结果表明：TBB观测数据能够有效监测电离层变化,正确反映站链上空电离层变化规律;在磁暴事件中,观测到了电子密度的变化过程,其准确地反映了正暴相的特征,证明在重要天气事件中,这一手段能够获得较为准确的观测数据.     

无线电通信测向中的极化误差分析

无线电通信测向中,因极化效应而产生测向误差不容忽视。通过论述测向中极化误差的各种相关因素,分析非正常极化波所引起的测向误差及地面反射波与直射波的相位滞后等问题,分析无线电测向中的极化误差产生基理,并结合实际应用提出减小极化误差的措施,提出采取取平均值法和相应的测向体制以解决在无线电测向中减小极化误差的方法。     

Spatial distribution of the demodulated low-frequency field in the low-latitude perturbed ionosphere

Features of propagation of low-frequency electromagnetic waves in the low-latitude ionosphere are considered in the case when such waves are generated by means of demodulation of the radiation of a high-power short-wave ground transmitter in the lower ionosphere. Calculations are performed in the ray approximation based on a simplified model (developed specially for the case under study) of the anomaly of the low-latitude equatorial electron density. It is demonstrated that latitude gradients of the equatorial anomaly may result in both additional focusing and defocusing of the beam of demodulated low-frequency waves.     

Covariance matrix properties and multiple symbol/soft decisiondetection in slow flat Rician fading

For a small number of symbols N and slow flat fading channels, it is shown that covariance matrices encountered in practice have two nonnegligible eigenvalues, the first much larger than the second, with a symmetric eigenvector associated with the first eigenvalue, and a skew symmetric eigenvector associated with the second eigenvalue. The first eigenvector is well approximated by a conditional mean, and the second eigenvector represents a small drift about the mean. The eigenvalues and eigenvectors of the slow flat fading channel covariance matrix are shown to be strongly related to those of a certain conditional covariance matrix. The maximum likelihood (ML) rules for block hard decision and symbol-by-symbol hard decision, and a rule for soft decision detection of M-DPSK, all using multiple symbol information, are obtained for the Rician channel as a function of N. The eigenvalue-eigenvector results lead to practical implementations of all rules. For small to moderate N, it is shown that a simple open-loop algorithm, of complexity N log N, attains the performance of the ML decision rules for an E_s/N ₀ range of interest for several land mobile satellite systems. The ML decision rules are seen to give rapidly diminishing returns as N increases, showing that simple noncoherent techniques can have very effective performance for the Rician fading channel. Lastly, several conclusions are drawn about the asymptotic channel behavior, including the Rayleigh channel. The work is directly applicable to the Australian and North American land mobile satellite systems     

Polarisation of waves reflected from the ionosphere

The effect of electron collisions on the polarisation of waves reflected from the ionosphere is calculated by an iteration procedure and shown to be relatively small at night for both normal and oblique incidence, at frequencies above 0.5 MHz.     

Multipath interference for in-flight antenna measurements

Formulations are presented to predict the pattern of interference (multipath) between direct rays and those reflected from the surface of the earth. These formulations can be used to assess the performance of ground-to-air, air-to-ground, ground-to-ground, and air-to-air communication and antenna measuring systems. Methods are also introduced to determine accurately the point of reflection and the path phase difference between the direct and reflected waves. Various divergence factors, used to account for energy spreading from a curved surface, are presented and compared. The formulations introduced can account for any polarization state (linear, circular, elliptical), sense of rotation (right hand, left hand), and tilt angle of the transmitting and receiving elements, and for polarization changes due to reflection.     

Tri‐orthogonal polarization diversity reception for non‐geosynchronous satellite orbit ionospheric channels

Electromagnetic signals propagating through the ionosphere are subject to path delay and the depolarizing effect of Faraday rotation. These are both dependent on global position and link geometry, which constantly vary for satellites in non‐geosynchronous orbits. These effects introduce performance error and reduce range resolution of remote sensing polarimetric measurements. Communication with ground receivers may be severely degraded by these effects. In this paper, a tri‐orthogonal approach at the receiver is introduced to enhance performance of conventional polarization diverse receive schemes. Performance is measured through a capacity metric. The work presented forms part of a large‐field‐of‐view, non‐geosynchronous satellite model exploiting tri‐orthogonal receive polarimetry as a means to enhancing link performance in a field‐of‐view. Copyright © 2016 John Wiley & Sons, Ltd.