共查询到20条相似文献,搜索用时 78 毫秒
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Interference from the daytime skywave signal of one broadcasting station to the groundwave signal of a co-channel station located several hundred kilometers away has been observed in certain parts of the world. The need for a better of skywave propagation at daytime, the Radiocommunications Sector of the International Telecommunications Union (ITU-R, formerly known as the International Radio Consultative Committee, CCIR) adopted Question 215/3 at its 1993 Radio Assembly. This paper is in direct response to this Question. Daytime measurements of more than 30 paths are believed to be skywave and have been studied for detailed information of these paths. The purpose of this paper is twofold: to discuss daytime skywave propagation in a qualitative manner and to present a simple procedure for estimating daytime skywave field strength and interference levels 相似文献
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Calculations of mediumwave skywave field strengths within 100 km of an AM broadcast station using a 186 degree tower are presented. The analysis is novel in that the high vertical elevation angle radiation from the tower is calculated using a method of moments technique. The results show that skywave field strengths are several times stronger than predicted when using the FCC tower radiation formula, and that within 100 km the 10 percent time skywave field strength can in some cases be of comparable amplitude to the groundwave signal amplitude. An analysis of the resulting signal envelope amplitude and phase fading statistics is also presented. 相似文献
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We propose an approximate maximum likelihood parameter estimation algorithm, combined with a model order estimator for superimposed undamped exponentials in noise. The algorithm combines the robustness of Fourier-based estimators and the high-resolution capabilities of parametric methods. We use a combination of a Wald (1945) statistic and a MAP test for order selection and initialize an iterative maximum likelihood descent algorithm recursively based on estimates at higher candidate model orders. Experiments using simulated data and synthetic radar data demonstrate improved performance over MDL, MAP, and AIC in places of practical interest 相似文献
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This paper presents a spectral density estimator based on a normalized minimum variance (MV) estimator as the one proposed by Lagunas. With an equivalent frequency resolution, this new estimator preserves the amplitude estimation lost in Lagunas one. This proposition comes from a theoretical study of MV filters that highlights this amplitude lost. Two signal types are taken into account: periodic deterministic signals (narrow-band spectral structures) and stationary random signals (broad-band spectral structures). Without selecting a smoothing window, the proposed estimator is an alternative to Fourier-based estimator and, without modeling the signal, it is a concurrent to high-resolution estimators. 相似文献
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针对增强型罗兰(eLORAN)系统在信号处理中的核心问题——周期识别,该文提出一种应对强天波干扰及低信噪比(SNR)等恶劣环境的联合算法。该方法首先在改进窗函数的基础上利用频谱相除技术估计信号特征参数,并根据大数理论的思想实现了天地波识别;其次,提出天地波时延差及幅度比的自适应2阶网格搜索匹配算法,在节省计算量的同时准确重构并去除天波;最后利用伪地波信号准确实现周期识别。仿真结果分析表明,该算法能够成功地克服现有技术中的一些弊端,实现小时延差及大强度天波干扰下的天波识别及分离,同时结合频谱相除技术的稳定性极大提高周期识别的正确率,进而为后续解调解码等流程提供保障。 相似文献
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《Broadcasting, IEEE Transactions on》1989,35(2):223-226
The author discusses the advantages and drawbacks of skywaves and why they are avoided by domestic medium-wave broadcaster. He reports some results of a computer investigation of problems encountered with medium-wave antennas coupled to a skywave propagation medium. Experimental results are given showing skywave patterns for broadcasts from Alaska, one using a horizontal and the other a vertical antenna. It is concluded for these two examples that remarkable medium-wave coverage could be obtained using available frequencies 相似文献
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A novel eigen-decomposition technique for estimating the delays of Loran-C skywave components is presented. This provides the basis for a new generation of receivers which implement adaptive skywave interference monitoring, enhancing reliability and positioning accuracy. Performance of the new technique is demonstrated using off-air data 相似文献
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The ground-loss profile along the path of a skywave for a sectionally homogeneous ground is investigated by means of the compensation theorem. A recurrence procedure, which is suitable for numerical work, has been satisfactorily tested against recent experimental results concerning m.f. skywave propagation over a path in South-East England, extending to 300 wavelengths from the shore. 相似文献
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低纬地区天波雷达系统时间可用度研究 总被引:1,自引:0,他引:1
讨论了天波雷达三个与时间有关的“可用度”的区别。强调这种新体制雷达传播媒质电离层是不均匀、各向异位、色散随机介质,雷达传播信道是非线性时变信道,以及天波雷达工作环境的十分复杂性。给出了低纬度地区天波雷达的信道可用度研究方法和实例。研究了给定一定设备能力的天波雷达系统探测不同目标的时间可用度的计算方法和实例,并指出环境噪声对可用度的影响。 相似文献
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Progress toward a practical skywave sea-state radar 总被引:2,自引:0,他引:2
Recent advances in propagation modeling, ionospheric diagnostics, and signal processing have helped overcome the limitations the ionosphere imposes on sea-state measurements with HF skywave radar. Wind-direction fields in tropical storms can be routinely mapped under most ionospheric conditions, but waveheight and wave-spectrum extraction is more sensitive to ionospheric distortions and requires care in signal processing and in selecting an ionospheric path. Spot measurements with a high-resolution radar have verified its ability to measure (in order of increasing difficulty) wind-direction fields, rms waveheight, and the scalar ocean-wave spectrum at ranges up to 3000 km using one ionospheric hop. Although such a radar can in principle map these quantities over millions of square kilometers of an ocean area, the time required to do so under various ionospheric conditions remains to be determined. A minimum objective of one map of rms waveheight per day seems attainable. 相似文献
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用于罗兰C接收机天波识别的IFFT频谱相除技术探讨 总被引:2,自引:0,他引:2
罗兰C无线电导航接收机所接收的是带有天波干扰的地波信号。为了有效的将天波分离,给出了一种简单但是有效的IFFT的频谱相除技术,应用于罗兰c接收机中采样点的适应性调节,并给出了一种会影响到整体性能的全面、综合的参数分析。仿真结果表明,运算能够在极高噪声环境下成功运行。 相似文献
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本文从天波超视距雷达的机理和特点出发,论述了国外天波超视距雷达的发展历史和现状.根据新时期的作战需求和特点,重点阐述了天波超视距雷达在战略预警中的地位和作用,包括防空、反导和对海监视.从系统设计、信号处理及数据处理等技术领域,对天波超视距雷达发展趋势和关键技术进行了评述,提出了当前主要研究方向和热点,并对相应的技术途径... 相似文献
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论述了天波超视距雷达及多输入多输出体制近年来的发展和研究状况.同时,从分布式天波超视距雷达所面临的任务需求和技术体制等方面展开分析,给出了雷达系统架构以及2种配置方式,并就其资源配置和信息融合进行了设计.最后,根据技术难点和成熟度提出了分布式天波超视距雷达"三步走"的技术发展路线及相应的阶段目标,对其中的关键技术还进行了评述. 相似文献
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Fourier-based forward and back-projection methods can reduce computation in iterative tomographic image reconstruction. Recently, an optimized nonuniform fast Fourier transform (NUFFT) approach was shown to yield accurate parallel-beam projections. In this paper, we extend the NUFFT approach to describe an O (N2 log N) projector/backprojector pair for fan-beam transmission tomography. Simulations and experiments with real CT data show that fan-beam Fourier-based forward and back-projection methods can reduce computation for iterative reconstruction while still providing accuracy comparable to their O (N3) space-based counterparts. 相似文献