For radar targets flying at low altitude, multiple pathways produce fade or enhancement relative to the level that would be expected in a free-space environment. In this paper, a new detec- tion method based on a wide...For radar targets flying at low altitude, multiple pathways produce fade or enhancement relative to the level that would be expected in a free-space environment. In this paper, a new detec- tion method based on a wide-ranging multi-frequency radar for low angle targets is proposed. Sequential transmitting multiple pulses with different frequencies are first applied to decorrelate the cohe- rence of the direct and reflected echoes. After receiving all echoes, the multi-frequency samples are arranged in a sort descending ac- cording to the amplitude. Some high amplitude echoes in the same range cell are accumulated to improve the signal-to-noise ratio and the optimal number of high amplitude echoes is analyzed and given by experiments. Finally, simulation results are presented to verify the effectiveness of the method.展开更多
Dual-frequency multi-constellation(DFMC) satellitebased augmentation system(SBAS) does not broadcast fast correction, which is important in reducing range error in L1-only SBAS.Meanwhile, the integrity bound of a sate...Dual-frequency multi-constellation(DFMC) satellitebased augmentation system(SBAS) does not broadcast fast correction, which is important in reducing range error in L1-only SBAS.Meanwhile, the integrity bound of a satellite at low elevation is so loose that the service availability is decreased near the boundary of the service area. Therefore, the computation of satellite clockephemeris(SCE) augmentation parameters needs improvement.We propose a method introducing SCE prediction to eliminate most of the SCE error resulting from global navigation satellite system GNSS broadcast message. Compared with the signal-inspace(SIS) after applying augmentation parameters broadcast by the wide area augmentation system(WAAS), SIS accuracy after applying augmentation parameters computed by the proposed algorithm is improved and SIS integrity is ensured. With global positioning system(GPS) only, the availability of category-I(CAT-I)with a vertical alert level of 15 m in continental United States is about 90%, while the availability in the other part of the WAAS service area is markedly improved. With measurements made by the stations from the crustal movement observation network of China,users in some part of China can obtain CAT-I(vertical alert limit is 15 m) service with GPS and global navigation satellite system(GLONASS).展开更多
Traditional multi-band frequency selective surface （FSS） approaches are hard to achieve a perfect resonance response in a wide band due to the limit of the onset grating lobe frequency determined by the array. To so...Traditional multi-band frequency selective surface （FSS） approaches are hard to achieve a perfect resonance response in a wide band due to the limit of the onset grating lobe frequency determined by the array. To solve this problem, an approach of combining elements in different period to build a hybrid array is presented. The results of series of numerical simulation show that multi-periodicity combined element FSS, which are designed using this approach, usually have much weaker grating lobes than the traditional FSS. Furthermore, their frequency response can be well predicted through the properties of their member element FSS. A prediction method for estimating the degree of expected grating lobe energy loss in designing multi-band FSS using this approach is provided.展开更多
Multi-radar signal separation is a critical process in modern reconnaissance systems. However, the complicated battlefield is typically confronted with increasing electronic equipment and complex radar waveforms. The ...Multi-radar signal separation is a critical process in modern reconnaissance systems. However, the complicated battlefield is typically confronted with increasing electronic equipment and complex radar waveforms. The intercepted signal is difficult to separate with conventional parameters because of severe overlapping in both time and frequency domains. On the contrary, time-frequency analysis maps the 1D signal into a 2D time-frequency plane, which provides a better insight into the signal than traditional methods. Particularly, the parameterized time-frequency analysis (PTFA) shows great potential in processing such non stationary signals. Five procedures for the PTFA are proposed to separate the overlapped multi-radar signal, including initiation, instantaneous frequency estimation with PTFA, signal demodulation, signal separation with adaptive filter and signal recovery. The proposed method is verified with both simulated and real signals, which shows good performance in the application on multi-radar signal separation.展开更多
Considering that real communication signals corrupted by noise are generally nonstationary, and timefrequency distributions are especially suitable for the analysis of nonstationary signals, time-frequency distributio...Considering that real communication signals corrupted by noise are generally nonstationary, and timefrequency distributions are especially suitable for the analysis of nonstationary signals, time-frequency distributions are introduced for the modulation classification of communication signals： The extracted time-frequency features have good classification information, and they are insensitive to signal to noise ratio （SNR） variation. According to good classification by the correct rate of a neural network classifier, a multilayer perceptron （MLP） classifier with better generalization, as well as, addition of time-frequency features set for classifying six different modulation types has been proposed. Computer simulations show that the MLP classifier outperforms the decision-theoretic classifier at low SNRs, and the classification experiments for real MPSK signals verify engineering significance of the MLP classifier.展开更多
针对基于定向传输的飞行器自组织网络(flying ad-hoc network,FANET)中拓扑变化频繁、链路质量波动以及初始建网困难等问题,提出了一种基于多输入多输出(multiple-input multipleoutput,MIMO)正交时频空(orthogonal time frequency spac...针对基于定向传输的飞行器自组织网络(flying ad-hoc network,FANET)中拓扑变化频繁、链路质量波动以及初始建网困难等问题,提出了一种基于多输入多输出(multiple-input multipleoutput,MIMO)正交时频空(orthogonal time frequency space,OTFS)通感一体化波形的感知辅助快速邻居发现方法,借助感知机制实时获取邻节点信息以加速建网效率,并采用新型的通感一体波形OTFS以对抗快变信道中的多普勒效应,提升链路质量。针对FANET场景研究物理层基于MIMOOTFS通感一体化波形的多目标检测技术;将物理层感知方案映射到上层网络中,设计感知辅助的高效邻居发现算法;最后提出一种多点通感协同机制,通过邻节点间交互感知信息和邻居发现表以间接感知及发现潜在目标,提升FANET初始建网的效率。仿真结果表明,所提方案相比传统通信组网协议可以极大降低FANET的初始建网耗时,增加目标感知精度,提升组网的整体性能。展开更多
对换流站的控制能够实现多端柔直系统在电网调频过程中的相互支撑。但不同扰动下,需考虑换流站的功率裕度及频率、电压之间的耦合性,以提升多端柔性直流输电(multi-terminal flexible direct current transmission systems,MTDC)系统的...对换流站的控制能够实现多端柔直系统在电网调频过程中的相互支撑。但不同扰动下,需考虑换流站的功率裕度及频率、电压之间的耦合性,以提升多端柔性直流输电(multi-terminal flexible direct current transmission systems,MTDC)系统的调频能力。该文对此展开研究,首先,根据各区域频率、直流电压及传输功率等信息,以各VSC在分担不平衡功率时的参与度一致、频率偏差变化率最小为目标设计触发规则,作为是否进行改进下垂控制策略切换的依据。其次,触发事件发生后,为减小传统P-Udc-f下垂控制中电压、频率的耦合,控制受扰区域换流站工作在P-f下垂控制模式,未受扰区域换流站工作在P-Udc下垂控制模式;并在常规下垂系数设置基础上,设计考虑换流站(voltage source converter,VSC)的功率裕度因子等因素的下垂系数调整举措。仿真结果表明,所提策略能有效均衡各VSC分担不平衡功率时的参与度,减小直流电压及各区域频率偏差。展开更多
绝缘油是电抗器内部重要的绝缘介质,击穿电压是评估其绝缘特性的关键指标,与绝缘油的品质状态密切相关。本文共选取155组电抗器绝缘油进行实验,分别进行击穿电压的测定和多频超声信号在油样中传播衰减后信号的采集,分析多频超声声学参...绝缘油是电抗器内部重要的绝缘介质,击穿电压是评估其绝缘特性的关键指标,与绝缘油的品质状态密切相关。本文共选取155组电抗器绝缘油进行实验,分别进行击穿电压的测定和多频超声信号在油样中传播衰减后信号的采集,分析多频超声声学参数和击穿电压之间的幅频响应、相频响应之间的关系,并基于多频超声检测技术提出结合灰狼优化算法(grey wolf optimizer,GWO)优化随机森林算法(random forest algorithm,RF)的击穿电压预测方法。结果表明:GWO-RF绝缘油击穿电压预测模型的预测值与实际值的平均相对误差为4.04%,预测准确率达到95.96%,相较于优化前的RF绝缘油击穿电压预测模型准确率提升了20.25%。结合多频超声检测技术和GWO-RF建立的并联电抗器绝缘油击穿电压预测模型,对击穿电压的预测具有可行性。展开更多
基金supported by the National Natural Science Foundation of China(6137213661372134+2 种基金61172137)the Fundamental Research Funds for the Central Universities(K5051202005)the China Scholarship Council(CSC)
文摘For radar targets flying at low altitude, multiple pathways produce fade or enhancement relative to the level that would be expected in a free-space environment. In this paper, a new detec- tion method based on a wide-ranging multi-frequency radar for low angle targets is proposed. Sequential transmitting multiple pulses with different frequencies are first applied to decorrelate the cohe- rence of the direct and reflected echoes. After receiving all echoes, the multi-frequency samples are arranged in a sort descending ac- cording to the amplitude. Some high amplitude echoes in the same range cell are accumulated to improve the signal-to-noise ratio and the optimal number of high amplitude echoes is analyzed and given by experiments. Finally, simulation results are presented to verify the effectiveness of the method.
文摘Dual-frequency multi-constellation(DFMC) satellitebased augmentation system(SBAS) does not broadcast fast correction, which is important in reducing range error in L1-only SBAS.Meanwhile, the integrity bound of a satellite at low elevation is so loose that the service availability is decreased near the boundary of the service area. Therefore, the computation of satellite clockephemeris(SCE) augmentation parameters needs improvement.We propose a method introducing SCE prediction to eliminate most of the SCE error resulting from global navigation satellite system GNSS broadcast message. Compared with the signal-inspace(SIS) after applying augmentation parameters broadcast by the wide area augmentation system(WAAS), SIS accuracy after applying augmentation parameters computed by the proposed algorithm is improved and SIS integrity is ensured. With global positioning system(GPS) only, the availability of category-I(CAT-I)with a vertical alert level of 15 m in continental United States is about 90%, while the availability in the other part of the WAAS service area is markedly improved. With measurements made by the stations from the crustal movement observation network of China,users in some part of China can obtain CAT-I(vertical alert limit is 15 m) service with GPS and global navigation satellite system(GLONASS).
基金supported by the National Natural Science Foundation of China(90305026).
文摘Traditional multi-band frequency selective surface （FSS） approaches are hard to achieve a perfect resonance response in a wide band due to the limit of the onset grating lobe frequency determined by the array. To solve this problem, an approach of combining elements in different period to build a hybrid array is presented. The results of series of numerical simulation show that multi-periodicity combined element FSS, which are designed using this approach, usually have much weaker grating lobes than the traditional FSS. Furthermore, their frequency response can be well predicted through the properties of their member element FSS. A prediction method for estimating the degree of expected grating lobe energy loss in designing multi-band FSS using this approach is provided.
文摘Multi-radar signal separation is a critical process in modern reconnaissance systems. However, the complicated battlefield is typically confronted with increasing electronic equipment and complex radar waveforms. The intercepted signal is difficult to separate with conventional parameters because of severe overlapping in both time and frequency domains. On the contrary, time-frequency analysis maps the 1D signal into a 2D time-frequency plane, which provides a better insight into the signal than traditional methods. Particularly, the parameterized time-frequency analysis (PTFA) shows great potential in processing such non stationary signals. Five procedures for the PTFA are proposed to separate the overlapped multi-radar signal, including initiation, instantaneous frequency estimation with PTFA, signal demodulation, signal separation with adaptive filter and signal recovery. The proposed method is verified with both simulated and real signals, which shows good performance in the application on multi-radar signal separation.
文摘Considering that real communication signals corrupted by noise are generally nonstationary, and timefrequency distributions are especially suitable for the analysis of nonstationary signals, time-frequency distributions are introduced for the modulation classification of communication signals： The extracted time-frequency features have good classification information, and they are insensitive to signal to noise ratio （SNR） variation. According to good classification by the correct rate of a neural network classifier, a multilayer perceptron （MLP） classifier with better generalization, as well as, addition of time-frequency features set for classifying six different modulation types has been proposed. Computer simulations show that the MLP classifier outperforms the decision-theoretic classifier at low SNRs, and the classification experiments for real MPSK signals verify engineering significance of the MLP classifier.
文摘针对基于定向传输的飞行器自组织网络(flying ad-hoc network,FANET)中拓扑变化频繁、链路质量波动以及初始建网困难等问题,提出了一种基于多输入多输出(multiple-input multipleoutput,MIMO)正交时频空(orthogonal time frequency space,OTFS)通感一体化波形的感知辅助快速邻居发现方法,借助感知机制实时获取邻节点信息以加速建网效率,并采用新型的通感一体波形OTFS以对抗快变信道中的多普勒效应,提升链路质量。针对FANET场景研究物理层基于MIMOOTFS通感一体化波形的多目标检测技术;将物理层感知方案映射到上层网络中,设计感知辅助的高效邻居发现算法;最后提出一种多点通感协同机制,通过邻节点间交互感知信息和邻居发现表以间接感知及发现潜在目标,提升FANET初始建网的效率。仿真结果表明,所提方案相比传统通信组网协议可以极大降低FANET的初始建网耗时,增加目标感知精度,提升组网的整体性能。
文摘对换流站的控制能够实现多端柔直系统在电网调频过程中的相互支撑。但不同扰动下,需考虑换流站的功率裕度及频率、电压之间的耦合性,以提升多端柔性直流输电(multi-terminal flexible direct current transmission systems,MTDC)系统的调频能力。该文对此展开研究,首先,根据各区域频率、直流电压及传输功率等信息,以各VSC在分担不平衡功率时的参与度一致、频率偏差变化率最小为目标设计触发规则,作为是否进行改进下垂控制策略切换的依据。其次,触发事件发生后,为减小传统P-Udc-f下垂控制中电压、频率的耦合,控制受扰区域换流站工作在P-f下垂控制模式,未受扰区域换流站工作在P-Udc下垂控制模式;并在常规下垂系数设置基础上,设计考虑换流站(voltage source converter,VSC)的功率裕度因子等因素的下垂系数调整举措。仿真结果表明,所提策略能有效均衡各VSC分担不平衡功率时的参与度,减小直流电压及各区域频率偏差。
文摘绝缘油是电抗器内部重要的绝缘介质,击穿电压是评估其绝缘特性的关键指标,与绝缘油的品质状态密切相关。本文共选取155组电抗器绝缘油进行实验,分别进行击穿电压的测定和多频超声信号在油样中传播衰减后信号的采集,分析多频超声声学参数和击穿电压之间的幅频响应、相频响应之间的关系,并基于多频超声检测技术提出结合灰狼优化算法(grey wolf optimizer,GWO)优化随机森林算法(random forest algorithm,RF)的击穿电压预测方法。结果表明:GWO-RF绝缘油击穿电压预测模型的预测值与实际值的平均相对误差为4.04%,预测准确率达到95.96%,相较于优化前的RF绝缘油击穿电压预测模型准确率提升了20.25%。结合多频超声检测技术和GWO-RF建立的并联电抗器绝缘油击穿电压预测模型,对击穿电压的预测具有可行性。
