期刊文献+
共找到22篇文章
< 1 2 >
每页显示 20 50 100
基于非极化分量的SAR图像海洋锋特征增强方法 被引量:1
1
作者 赵亚威 孙凯 种劲松 《光学精密工程》 EI CAS CSCD 北大核心 2021年第6期1440-1447,共8页
受到海面整体运动、海面后向散射系数相对低以及海况的影响,海洋锋特征在SAR图像中表现较弱,给后续的应用带来困难,亟需开展海洋锋特征增强方法的研究。SAR海面回波可以分解成Bragg散射分量和非极化分量,本文提出了一种基于非极化分量的... 受到海面整体运动、海面后向散射系数相对低以及海况的影响,海洋锋特征在SAR图像中表现较弱,给后续的应用带来困难,亟需开展海洋锋特征增强方法的研究。SAR海面回波可以分解成Bragg散射分量和非极化分量,本文提出了一种基于非极化分量的SAR图像海洋锋特征增强方法。首先对双极化(VV和HH极化)数据进行辐射定标,为解决非极化分量提取过程中噪声增强的问题,对定标结果进行滤波;然后计算Bragg散射极化比;最后借助于极化差和极化比提取非极化分量,实现海洋锋特征增强。本文使用ALOSPALSAR数据进行处理验证,并通过计算峰值-背景比(PBR)和等效视数(ENL)进行定量分析。实验结果表明,经过本文方法处理后PBR较双极化图像可以提升15%以上,ENL较双极化图像可以提升30%以上。实验结果证明了方法的有效性,为后续的应用奠定基础。 展开更多
关键词 海洋锋 极化分量 特征增强
下载PDF
天线极化参数的正确计算
2
作者 杨建良 《湘潭大学自然科学学报》 CAS CSCD 1991年第3期31-33,共3页
0 引言分析极化椭圆的矢量正交分解法有线极化分量法、圆极化分量法与斜极化分量法三种.本文指出了现有一般资料关于线极化分量法与斜极化分量法中两个重要公式的错误,给出了修改后的正确公式。
关键词 天线 极化 线极化分量 极化分量
下载PDF
抛物面反射天线的空域瞬态极化特性研究 被引量:17
3
作者 戴幻尧 罗佳 +3 位作者 王雪松 李永祯 李金梁 何密 《电波科学学报》 EI CSCD 北大核心 2009年第1期126-131,共6页
以天线的空域指向为变化因素来分析研究抛物面天线的空域极化特性,采用表面电流法计算了天线远场的主极化和交叉分量,进而利用空域极化比分布,瞬态极化投影集等空域瞬态极化描述子研究了雷达坐标系下不同方位角和俯仰角下典型抛面天线... 以天线的空域指向为变化因素来分析研究抛物面天线的空域极化特性,采用表面电流法计算了天线远场的主极化和交叉分量,进而利用空域极化比分布,瞬态极化投影集等空域瞬态极化描述子研究了雷达坐标系下不同方位角和俯仰角下典型抛面天线极化特性的变化规律。通过计算和分析比较,结果表明,在空域指向的不同位置,抛物面天线的极化特性服从一定规律变化,这对抛物面天线空域极化特性的应用提供了理论依据和支持。 展开更多
关键词 抛物面天线 空域瞬态极化特性 瞬态极化描述子 交叉极化分量
下载PDF
典型抛面天线的空域极化特性及散射矩阵测量方法 被引量:3
4
作者 戴幻尧 罗佳 +2 位作者 李金梁 李永帧 王雪松 《信号处理》 CSCD 北大核心 2009年第9期1347-1352,共6页
单极化雷达天线的极化特性在空域中并非一成不变,而是按一定规律变化的。本文研究了抛面反射天线的空域极化特性,通过数值计算验证了天线极化状态在不同空域指向上分布的规律。并以此为基础,讨论了立足于现有的单极化雷达,仅需一个极化... 单极化雷达天线的极化特性在空域中并非一成不变,而是按一定规律变化的。本文研究了抛面反射天线的空域极化特性,通过数值计算验证了天线极化状态在不同空域指向上分布的规律。并以此为基础,讨论了立足于现有的单极化雷达,仅需一个极化通道对天线接收到雷达回波电压进行处理,完成雷达目标极化散射矩阵估计的方法。该法将可以大幅度的降低生产成本和设备复杂度,具有一定的理论参考价值和应用前景。 展开更多
关键词 抛面反射天线 空域极化特性 交叉极化分量 极化 目标散射矩阵测量
下载PDF
偏置抛物面天线的空域瞬态极化特性及仿真分析 被引量:1
5
作者 戴幻尧 罗佳 +3 位作者 王雪松 李永祯 李金梁 何密 《信号处理》 CSCD 北大核心 2009年第8期1215-1222,共8页
在天线空域瞬态极化特性的表征的基础上,研究了偏置抛物面天线在雷达坐标系下的天线主极化和交叉极化方向图,探讨了利用天线主瓣内和主瓣外扫描时不同方位和俯仰向上的空域极化特性,充分诠释了天线极化状态的变化规律。通过计算机仿真... 在天线空域瞬态极化特性的表征的基础上,研究了偏置抛物面天线在雷达坐标系下的天线主极化和交叉极化方向图,探讨了利用天线主瓣内和主瓣外扫描时不同方位和俯仰向上的空域极化特性,充分诠释了天线极化状态的变化规律。通过计算机仿真和分析比较,结果表明,在空域指向的不同位置,不同的尺寸和不同的焦径比的影响下,天线的极化特性服从一定规律变化,有共性也有差异,这对天线空域极化特性的应用提供了理论依据和支持。 展开更多
关键词 偏置抛物面天线 空域极化特性 瞬态极化描述子 交叉极化分量
下载PDF
基于RPIM法的圆极化天线特性参数快速测试 被引量:2
6
作者 张晓燕 李利卫 +1 位作者 展爱云 赵艺萌 《微波学报》 CSCD 北大核心 2019年第4期66-71,共6页
为提高圆极化天线特性参数的测试精度,线极化分量法往往需要对线分量场进行多次测量。文中在天线测试系统中引入插值算法,减少采样点,提高圆极化天线测试效率。将径向基函数的点插值无网格(RPIM)法引入圆极化天线参数合成算法中,首先使... 为提高圆极化天线特性参数的测试精度,线极化分量法往往需要对线分量场进行多次测量。文中在天线测试系统中引入插值算法,减少采样点,提高圆极化天线测试效率。将径向基函数的点插值无网格(RPIM)法引入圆极化天线参数合成算法中,首先使用RPIM拟合出测试场的幅值和相位,然后再使用解析的方式合成圆极化天线的轴比和方向图,算法性能与基于分段线性插值技术的模型参数估计法(MBPE)和基于三次样条插值的移动最小二乘法(MLS)进行比较,数值实验显示,当采样点数仅为测试点数的4%时,MLS失效,MBPE和RPIM误差均为10-3量级;而当采样点数减少为测试点数的2%时,MBPE和MLS均失效,RPIM的误差虽然增大到10-1量级,但仍能保持稳定。数值实验证明,在采样点数较少情况下,RPIM性能优于其它两种算法。 展开更多
关键词 线极化分量 径向基函数点插值无网格法 极化天线 特性参数
下载PDF
线极化天线接收短波天波信号的应用研究 被引量:1
7
作者 王豪 韩仿仿 《中国无线电》 2018年第9期57-60,共4页
文章对天线的极化概念作了一般描述,介绍了天线的极化分类,以及相应的极化天线接收多种极化波的衰减特性,并对垂直极化和水平极化天线接收短波天波信号的实际效果进行了对比测试。
关键词 天线的极化 极化特性 垂直极化分量 水平极化分量
下载PDF
基于物理散射模型的全极化SAR图像增强滤波算法 被引量:2
8
作者 单子力 谷宏志 +1 位作者 陈金勇 王超 《计算机应用研究》 CSCD 北大核心 2013年第10期3152-3154,共3页
针对传统的极化SAR滤波方法图像中城镇区域和植被区域地物在滤波中易被混淆,导致滤波后图像中地物边缘保持效果下降的问题,提出了一种增强的保持极化散射特性的滤波算法。利用一种增强的四分量极化分解方法获取更加精确的地物散射机制,... 针对传统的极化SAR滤波方法图像中城镇区域和植被区域地物在滤波中易被混淆,导致滤波后图像中地物边缘保持效果下降的问题,提出了一种增强的保持极化散射特性的滤波算法。利用一种增强的四分量极化分解方法获取更加精确的地物散射机制,并将散射机制信息引入滤波方法中,使滤波算法中像素的散射机制更精确。增强的四分量极化分解方法引入了极化SAR数据的定向角补偿技术、一种新的体散射模型以及两种散射功率限制条件,来改进Freeman-Durden分解的结果。理论分析和实验结果表明,改进后的方法获取了比传统的极化SAR图像滤波算法更加理想的计算结果。 展开更多
关键词 极化SAR图像 极化SAR图像滤波 增强的四分量极化分解
下载PDF
基于ADS的全极化微波辐射计建模仿真 被引量:1
9
作者 党鹏举 陈文新 李浩 《现代电子技术》 2013年第15期14-16,19,共4页
全极化微波辐射计是一种用于海洋表面风场测量的新型被动微波遥感器,主要对水平极化和垂直极化分量进行自相关和互相关处理。针对全极化微波辐射计接受到的数字信号高带宽高采样率的特点,基于ADS软件平台对全极化微波辐射计进行了系统仿... 全极化微波辐射计是一种用于海洋表面风场测量的新型被动微波遥感器,主要对水平极化和垂直极化分量进行自相关和互相关处理。针对全极化微波辐射计接受到的数字信号高带宽高采样率的特点,基于ADS软件平台对全极化微波辐射计进行了系统仿真,重点针对通道和相位不平衡对极化分量的测量影响进行了理论分析和仿真测试,并对结果误差进行了简要分析。 展开更多
关键词 极化 ADS 极化分量 通道和相位不平衡
下载PDF
天线非理想特性对极化方向测量的误差分析
10
作者 尹程 吴嘉诚 +1 位作者 陈安军 马小雅 《现代电子技术》 2022年第23期14-18,共5页
某型装备需要对来波的极化方向进行高精度实时测量,传统的单天线极化方向测量方法实时性差且精度不高,采用正交双极化天线和线极化分量法测量时,测量天线的非理想特性会引入交叉极化分量,进而对测量结果造成影响。传统研究中交叉极化经... 某型装备需要对来波的极化方向进行高精度实时测量,传统的单天线极化方向测量方法实时性差且精度不高,采用正交双极化天线和线极化分量法测量时,测量天线的非理想特性会引入交叉极化分量,进而对测量结果造成影响。传统研究中交叉极化经常被忽略或者避免,文中根据线极化分量法测量原理,给出了来波极化方向测量的数学模型,通过推导重点分析了天线的交叉极化分量对测量误差的影响,建立了测量误差数学模型,分析和仿真结果对类似需求具有理论指导意义。分析仿真结果表明,天线主极化与交叉极化的相位差、正交天线主极化增益比、交叉极化鉴别率等因素均会影响最终的测量误差。天线主极化与交叉极化的相位差的影响可通过外辐射校准予以消除,正交天线主极化增益比的影响可以通过天线配对的方式尽量减小。不考虑前两者影响的情形下,当测量天线自身的交叉极化鉴别率优于-35 dB时,线极化分量法对来波极化方向的测量误差小于1°。 展开更多
关键词 线极化分量 极化方向测量 交叉极化干扰 交叉极化鉴别率 非理想特性 极化 正交双极化天线
下载PDF
基于方向极化调制的安全可靠传输技术研究
11
作者 齐帅 张邦宁 +2 位作者 郭道省 张晓凯 李晓光 《信息技术与网络安全》 2018年第7期27-32,共6页
由于极化调制和方向调制单一调制方式的安全性不够,在信息传输过程中,容易被窃听者窃听。将方向调制和极化调制相结合,利用方向调制中天线阵列的方向性和在理想方向信道的零空间上加上干扰激励,可以使窃听者接收的信息产生畸变,将极化调... 由于极化调制和方向调制单一调制方式的安全性不够,在信息传输过程中,容易被窃听者窃听。将方向调制和极化调制相结合,利用方向调制中天线阵列的方向性和在理想方向信道的零空间上加上干扰激励,可以使窃听者接收的信息产生畸变,将极化调制(PM)代替方向调制的PSK后,可以进一步增大窃听者接收信息的误码率。仿真实验证明,利用本方法,在非期望方向上,每一个信息符号产生的畸变更剧烈,各点的最小欧氏距离更小,误码率更大,因此可以提高信息传输的安全性。 展开更多
关键词 方向调制 极化调制 人工噪声 极化分量 误码率
下载PDF
Theoretical Study on Stereodynamics of Reactions of N(^2D)+H2→NH+H and N(^2D)+D2→ND+D
12
作者 岳现房 程杰 +2 位作者 冯海冉 李宏 Emilia L. Wu 《Chinese Journal of Chemical Physics》 SCIE CAS CSCD 2010年第4期381-386,I0001,共7页
The vector correlations between products and reagents for the title reactions have been calculated by the quasi-classical trajectory method at a collision energy of 21.32 kJ/mol on an accurate potential energy surface... The vector correlations between products and reagents for the title reactions have been calculated by the quasi-classical trajectory method at a collision energy of 21.32 kJ/mol on an accurate potential energy surface of Ho et al. (J. Chem. Phys. 119, 3063 (2003)). The peaks of the product angular distribution are found to be in both backward and forward directions for the two title reactions. The product rotational angular momentum is not only aligned, but also oriented along the negative direction of y-axis. These theoretical results are in good agreement with recent experimental findings for the two title reactions. The isotopic effect is also revealed and primarily attributed to the difference of the mass factor in the two title reactions. 展开更多
关键词 STEREODYNAMICS Quasi-classical trajectory method Vector correlation Polarization-dependent differential cross-section Isotopic effect
下载PDF
Digital characterization and mathematic model of sodium penetration into cathode material for aluminum electrolysis 被引量:5
13
作者 吕晓军 李庆余 +1 位作者 赖延清 李劼 《Journal of Central South University》 SCIE EI CAS 2009年第1期96-100,共5页
The sodium expansion curves of semi-graphitic cathode measured with the improved Rapoport-Samoilenko apparatus. The and TiB2/C composite cathode with different TiB2 contents were mathematic model of the sodium expansi... The sodium expansion curves of semi-graphitic cathode measured with the improved Rapoport-Samoilenko apparatus. The and TiB2/C composite cathode with different TiB2 contents were mathematic model of the sodium expansion was deduced on the basis of the experimental results. The sodium expansion parameter (a) and penetration rate factor (Q), were introduced into the model The model was validated with the experimental sodium expansion curves self-measured and reported. The results show that the variation tendency of the sodium expansion parameter (a) and penetration rate factor (Q) is consistent with that of the experimental curves. The model is capable of not only conveniently judging the cathode quality, but also favorably establishing a unified standard of the resistance to sodium penetration of cathode. 展开更多
关键词 aluminum electrolysis CATHODE sodium expansion modeling CHARACTERIZATION
下载PDF
Vector transformation and reduction to the pole for regional magnetic data with variable field direction 被引量:3
14
作者 Luo Yao Wu Mei-Ping 《Applied Geophysics》 SCIE CSCD 2022年第1期107-116,145,146,共12页
The vector transformation and pole reduction from the total-field anomaly are signifi cant for the interpretation.We examined these industry-standard processing procedures in the Fourier domain.We propose a novel iter... The vector transformation and pole reduction from the total-field anomaly are signifi cant for the interpretation.We examined these industry-standard processing procedures in the Fourier domain.We propose a novel iteration algorithm for regional magnetic anomalies transformations to derive the vertical-component data from the total-field measurements with the variation in the core-fi eld direction over the region.Additionally,we use the same algorithm to convert the calculated vertical-component data into the corresponding data at the pole and realize the processing of diff erential reduction to the pole(DRTP).Unlike Arkani-Hamed’s DRTP method,the two types of iterative algorithms have the same forms,and DRTP is realized by implementing this algorithm twice.The synthetic model’s calculation results show that the method has high accuracy,and the fi eld data processing confi rms its practicality. 展开更多
关键词 magnetic anomaly diff erential reduction to the pole three-component magnetic data large-area magnetic survey
下载PDF
Stereodynamics of O(^3P)+H2 at Scattering Energies of 0.5, 0.75, and 1.0 eV 被引量:2
15
作者 Victor Wei-keh Wu 《Chinese Journal of Chemical Physics》 SCIE CAS CSCD 2010年第2期149-154,I0001,共7页
Quasiclassical trajectory calculation of the title reaction O(^3P)+H2→OH+H at three different scattering energies of 0.5, 0.75, and 1.0 eV on the lowest electronic potential energy surface 1^3A" has been done. D... Quasiclassical trajectory calculation of the title reaction O(^3P)+H2→OH+H at three different scattering energies of 0.5, 0.75, and 1.0 eV on the lowest electronic potential energy surface 1^3A" has been done. Distribution P(θr) of polar angles between the relative velocityk of the reactant and rotational angular momentum vector j' of the product, distribution P(φr) of the azimuthal as well as dihedral angles correlating k-k'-j', 3-dimensional distri-bution, and polarization-dependent differential cross sections (PDDCSs)dependent upon the scattering angle of the product molecule OH between the relative velocity k of the reactant and k' of the product at different scattering energies of 0.5, 0.75, and 1.0 eV are presented and discussed. 展开更多
关键词 Quasiclassical trajectory calculation Stereodynamics Alignment Rotational angular momentum Polarization-dependent differential cross section O+H2 reaction
下载PDF
Nondoped-type White Organic Light-Emitting Diode Using Star-Shaped Hexafluorenylbenzene as an Energy Transfer Layer
16
作者 Jun-sheng Yu Tao Ma +2 位作者 Shuang-ling Lou Ya-dong Jiang Qing Zhang 《Chinese Journal of Chemical Physics》 SCIE CAS CSCD 北大核心 2008年第5期500-504,共5页
White organic light-emitting diodes (WOLEDs) with a structure of indium-tin-oxide (ITO)/N,N'-bis- (1-naphthyl)-N,N'-diphenyl- (1, 1'-biphenyl)-4,4'-diamine (NPB)/1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluor... White organic light-emitting diodes (WOLEDs) with a structure of indium-tin-oxide (ITO)/N,N'-bis- (1-naphthyl)-N,N'-diphenyl- (1, 1'-biphenyl)-4,4'-diamine (NPB)/1,2,3,4,5,6-hexakis(9,9-diethyl-9H-fluoren-2- yl)benzene (HKEthFLYPh)/5,6,11,12-tetraphenylnaphtacene (rubrene)/tris(8-hydroxyquinoline) aluminum (Alq3)/Mg:Ag were fabricated by vacuum deposition method, in which a novel star-shaped hexafluorenyl- benzene HKEthFLYPh was used as an energy transfer layer, and an ultrathin layer of rubrene was inserted between HKEthFLYPh and Alq3 layers as a yellow light-emitting layer instead of using a time-consuming doping process. A fairly pure WOLED with Commissions Internationale De L'Eclairage (CIE) coordinates of (0.32, 0.33) was obtained when the thickness of rubrene was 0.3 nm, and the spectrum was insensitive to the applied voltage. The device yielded a maximum luminance of 4816 cd/m2 at 18 V. 展开更多
关键词 White organic light-emitting diode diethyl-9H-fluoren-2-yl)benzene Energy transfer Star-shaped hexafluorenylbenzene 1 2 3 4 5 6-hexakis(9 9- Ultrathin layer
下载PDF
Electrochemical mechanism of rusticyanin (Rus.) isolated from A.ferrooxidans measured by Rus.-ZnS-QDs/L-Cys/Au electrode
17
作者 孙静 余润兰 +3 位作者 苗雷 钟代立 刘杰 顾帼华 《Journal of Central South University》 SCIE EI CAS 2011年第5期1389-1394,共6页
Electrochimcal behaviors of rusticyanin (Rus.) isolated from Acidithiobacillus ferrooxidans were investigated through Rus.-ZnS-QDs/L-Cys/Au electrode. The cyclic voltammetric results indicate that rusticyanin immobi... Electrochimcal behaviors of rusticyanin (Rus.) isolated from Acidithiobacillus ferrooxidans were investigated through Rus.-ZnS-QDs/L-Cys/Au electrode. The cyclic voltammetric results indicate that rusticyanin immobilized on the surface of Rus.-ZnS-QDs/L-Cys/Au electrode can undergo a direct quasi-reversible electrochemical reaction. The immobilized rusticyanin is not denatured and still retains its activity in the temperature range of 19-43 ℃. The reduction ability of the protein increases and its oxidation ability becomes weak with the increase of pH from 6.0 to 7.8. Fe^2+ ions in the solution can promote the electron transfer kinetics of the immobilized rusticyanin and make its peak potentials (φp) markedly move negatively. 展开更多
关键词 rusticyanin acidithiobacillusferrooxidans cyclic voltammetry ELECTROCHEMISTRY
下载PDF
Quantitative Interpretation of Polarization SFG Vibrational Spectra of Air/Methanol Interface
18
作者 Hui Wu Wen-kai Zhang +2 位作者 Wei Gan Zhi-feng Cui Hong-fei Wang 《Chinese Journal of Chemical Physics》 SCIE CAS CSCD 北大核心 2006年第3期187-189,共3页
Even though in IR and Raman spectra of liquid methanol there is always an apparent feature for the asymmetric stretching mode of the CH3 group around 2970 cm^-1, this feature has not been observed in the Sum Frequency... Even though in IR and Raman spectra of liquid methanol there is always an apparent feature for the asymmetric stretching mode of the CH3 group around 2970 cm^-1, this feature has not been observed in the Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) in any polarizations from the air/methanol interface. Here we present a treatment based on a corrected bond additivity model to quantitatively interpret the SFG-VS of the air/methanol interface from the IR and Raman spectra of liquid methanol. 展开更多
关键词 Sum frequency generation Bond additivity model Raman polarizability
下载PDF
Molecular Dipole Moment Computed with Ab Initio MKS Charges
19
作者 FAN Jian-Fen WANG Qu-Xia +2 位作者 XIA Qi-Ying XIAO He-Ming B.van de Graaf 《Chinese Journal of Structural Chemistry》 SCIE CAS CSCD 北大核心 2002年第2期139-141,共3页
Molecular dipole moments computed at the levels of HF/STO-3G, HF/6-31G(d, p), HF/6-311+G(2d, 2p), MP2/6-31G(d, p) and MP2/6-311+G(2d, 2p) have been investigated. HF/6-311+G(2d, 2p) was found to be the relatively good ... Molecular dipole moments computed at the levels of HF/STO-3G, HF/6-31G(d, p), HF/6-311+G(2d, 2p), MP2/6-31G(d, p) and MP2/6-311+G(2d, 2p) have been investigated. HF/6-311+G(2d, 2p) was found to be the relatively good choice to compute MKS charges for reproducing the experimental values of molecular dipole moments. Root mean square deviation of computed dipole moments for 21 small polar molecules is about 0.1969 D. 展开更多
关键词 molecular dipole moment MKS charge HF/6-311+G(2d 2p)
下载PDF
SUSY QCD Impact on Top-Pair Production Associated with a Z^0-Boson at a Photon-Photon Collider
20
作者 DONG Chuan-Fei MA Wen-Gan ZHANG Ren-You GUO Lei WANG Shao-Ming 《Communications in Theoretical Physics》 SCIE CAS CSCD 2009年第8期302-310,共9页
The top-pair production in association with a Z^0-boson at a photon-photon collider is an important process in probing the coupling between top-quarks and vector boson and discovering the signature of possible new phy... The top-pair production in association with a Z^0-boson at a photon-photon collider is an important process in probing the coupling between top-quarks and vector boson and discovering the signature of possible new physics. We describe the impact of the complete supersymmetric QCD (SQCD) next-to-leading order (NLO) radiative corrections on this process at a polarized or unpolarized photon collider, and make a comparison between the effects of the SQCD and the standard model (SM) QCD. We investigate the dependence of the lowest-order (LO) and QCD NLO corrected cross sections in both the SM and minimal supersymmetric standard model (MSSM) on colliding energy √s in different polarized photon collision modes. The LO, SM NLO, and SQCD NLO corrected distributions of the invariant mass of tt^--pair and the transverse momenta of final Z^0-boson are presented. Our numerical results show that the pure SQCD effects in γγ →tt^- Z^0 process can be more significant in the ++ polarized photon collision mode than in other collision modes, and the relative SQCD radiative correction in unpolarized photon collision mode varies from 32.09% to -1.89% when √s goes up from 500 GeV to 1.5 TeV. 展开更多
关键词 supersymmetric QCD photon-photon collider top-gauge boson coupling
下载PDF
上一页 1 2 下一页 到第
使用帮助 返回顶部