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排序方式: 共有158条查询结果,搜索用时 138 毫秒
1.
Xiaoqing Wang Jinsong Chong Xiangzhen Yu Lei Liu 《International journal of remote sensing》2014,35(11-12):4064-4085
In previous works on current measured by along-track interferometric synthetic aperture radar (ATISAR), the decorrelation function of an ocean-surface backscattering signal was usually assumed to be a real Gaussian function, i.e. the phase term was omitted. In this study, it is proved that the omission of the phase term included in the decorrelation function results in a significant estimation bias that can be modelled by a higher-order function of time lag and that the coefficients of this higher-order function can be expressed as a series of higher-order Doppler spectral moments. This model is validated by the scatterometer data obtained from an experimental wind-wave tank. The estimation bias especially needs to be considered for the ATISAR system with a long time lag. Simulation results show that if the time lag is equal to the coherence time, the estimation bias of the current can reach about 0.2 m s?1, which is not insignificant in high-precision current-retrieving applications. However, because most real-life ATISAR systems, including TerraSAR-X, operate with time lags significantly shorter than the expected coherence time, the estimation biases in these systems are relatively small or even negligible. Finally, four possible compensation methods for the estimation bias are proposed and discussed. 相似文献
2.
直升机载雷达具有全视域成像、重访周期短等优点,能有效保障直升机等旋翼飞行器的低空飞行安全.针对现有的成像算法存在方位向失配问题,提出了直升机载雷达频域成像算法,通过对直升机载雷达回波进行频域特性分析,从而获取二维精确频谱,并构建频域校正函数和匹配函数,实现回波数据的匹配成像.仿真实验验证了文中算法的有效性. 相似文献
3.
在空间下行相干激光通信系统中,激光器的线宽大小会影响通信系统的性能,而窄线宽激光器能有效降低激光器线宽引起的激光器相位噪声,目前已经成为相干激光通信系统的首选。光信号在大气信道传输时,大气湍流会引起光信号强度和相位的波动,从而进一步影响系统的通信性能。针对上述问题,基于四相移键控(Quadri Phase Shift Keying, QPSK)通信系统的工作原理,进一步考虑窄线宽激光器线宽和大气湍流引起的光信号强度和相位的波动,给出了空间下行QPSK通信系统误码率模型。并基于该模型,数值仿真分析了窄线宽激光器对空间下行相干激光通信系统性能的影响。结果表明:大气湍流不仅严重影响系统性能,也会弱化激光器线宽对系统性能的影响。而对于大气湍流影响而言,其引起的相位波动要大于光强波动的影响。此外,随着通信速率增大,激光器线宽对于系统性能影响也会随之降低。文中对于空间下行相干激光通信系统的优化设计和调试具有一定的实际参考意义。 相似文献
4.
《Measurement》2014
In practical engineering applications, useful information is often submerged in strong noise and the feature information is difficult to be extracted. Aimed at the detection problem of multi-frequency signal under colored noise background, a novel weak signal detection method based on stochastic resonance (SR) tuning by multi-scale noise is proposed. Firstly, noisy signal is processed by orthogonal wavelet transform to decompose the signal into multi-scale ingredients. According to the orthogonal wavelet transform coefficients characteristics of 1/f distribution, multi-scale noise is constructed so as to make the frequency-band containing the driving frequency be enhanced through SR system. Thus multi-frequency weak signal is detected. The method is effective to detect multi-frequency weak signal under colored noise background. Experiment signal analysis results show that the proposed method is simple for multi-frequency weak signal detection, and has good prospects for engineering applications. 相似文献
5.
可变码率编码是现代移动通信系统的一项关键技术。在分组码中,截短(short-ening)技术是实现这一目的主要方法之一。截短是一种通过将一些信息比特用预先约定好的值置换,从而将这些比特的置信度设为无限大的技术。通过该技术,可以缩短码长并降低码率,同时提高码字的性能。针对非规则QC-LDPC码,结合PEXIT分析方法,给出了一种优化地选取截短比特的方法。仿真结果和分析表明,本文的方法使得截短后的码字取得良好的译码性能。 相似文献
6.
太阳风暴是一种常见的空间灾害天气现象,对人类的航天活动有着巨大的影响。对太阳风暴的产生原因和航天器电子产品在太阳风暴发生期间可能遭受的影响,作了全面分析。针对这些影响,提出了提高航天器电子产品抗太阳风暴能力的建议以及相应的防护措施。 相似文献
7.
8.
本文介绍一种高抑制能力梳状滤波器的设计。给出一种用于谐波雷达中的滤波器结构和电气性能,其指标满足谐波雷达的整机要求。 相似文献
9.
10.
《Ceramics International》2019,45(10):12757-12763
Dense silicon nitride (Si3N4) ceramics were prepared using Y2O3 and MgF2 as sintering aids by spark plasma sintering (SPS) at 1650 °C for 5 min and post-sintering annealing at 1900 °C for 4 h. Effects of MgF2 contents on densification, phase transformation, microstructure, mechanical properties, and thermal conductivity of the Si3N4 ceramics before and after heat treatment were investigated. Results indicated that the initial temperature of liquid phase was effectively decreased, whereas phase transformation was improved as increasing the content of MgF2. For optimized mechanical properties and thermal conductivity of Si3N4, optimum value for MgF2 content existed. Sample with 3 mol.% Y2O3 and 2 mol.% MgF2 obtained optimum flexural strength, fracture toughness and thermal conductivity (857 MPa, 7.4 MPa m1/2 and 76 W m−1 K−1, respectively). It was observed that excessive MgF2 reduced the performance of the ceramic, which was caused by the presence of excessive volatiles. 相似文献