﻿ 波束跃度对星载SAR滑聚模式成像影响分析
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Impact of antenna beam granularity on sliding spotlight spaceborne SAR image quality
Zeng Hongcheng, Chen Jie, Yang Wei, Zhang Haojie
School of Electronic and Information Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191, China
Abstract:Antenna beam in sliding spotlight spaceborne synthetic aperture radar (SAR) cannot continuously scan the illuminated scene, leading to paired-echo in SAR images. A method quantitatively analyzing the impact of antenna beam granularity was presented. Firstly, mathematical mode of sliding spotlight spaceborne SAR was proposed based on the azimuth antenna beam granularity. Then, the qualitative and quantitative studies about the impacts of antenna beam granularity on image quality were proposed. And the expression for determining both amplitudes and positions of paired-echo and the minimum beam granularity was illuminated. Moreover, the impacts of paired-echo on azimuth resolution, integrated side lobe ratio (ISLR), peak side lobe ratio (PSLR) and system gain were discussed, at different operation frequencies, e.g. X-band and C-band. Therefore, the analysis provides a theoretical basis for minimum antenna beam granularity design. Finally, computer simulations testify to the validity of theoretical analysis.
Key words: synthetic aperture radar (SAR)     sliding spotlight     beam granularity     paired-echo     ghost target

1 波束跃度下的回波信号数学建模

 图 1 星载SAR滑动聚束模式空间几何关系Fig. 1 Geometry of spaceborne sliding spotlight SAR

 图 2 波束跃度对方位向等效天线方向图的影响Fig. 2 Impact of beam granularity on azimuth equivalent antenna

g(t)=t－MΔT

g(t)相当于把周期函数进行傅里叶级数展开,即

2 天线波束跃度影响分析

1) 不同于星载SAR聚束模式[11],滑动聚束模式方位向天线波束跃度控制引入的成对回波为双峰,而VkrRRSΔΦφYLV2λRRSkr,虚假目标sp(t,1)的峰值将出现在±VkrRRSΔΦφ附近,即虚假目标位置受信号波长λ影响较小.此外天线最小波束跃度ΔΦφ越高,成对回波离主瓣越远.RRS决定了滑动聚束模式的天线扫描速度与方位向分辨率,当RRS变小,天线扫描速度越快,方位向分辨率越高,成对回波位置离主瓣也越远.

2) 成对回波的幅度Ep1只与方位向天线长度D、天线最小波束跃度ΔΦφ以及信号波长λ相关.Ep1与D成正比,为降低天线最小波束跃度精度要求,需要缩短方位向天线长度.Ep1与λ成反比,较长的信号波长将降低天线波束控制精度要求,即最小波束跃度一定的条件下,C波段成对回波幅值要低于X波段的成对回波幅值.

1) |t0|<时,其中Ts为合成孔径时间,sp(t,1)峰值落在so(t)的主瓣内,使得主瓣展宽,分辨率下降,同时减小积分旁瓣比与系统增益,而对峰值旁瓣比影响较小.

2) |t0|=时,sp(t,1)峰值位置落在第一零点处,此时积分旁瓣比增大,系统增益减小,分辨率影响较小,而且当Ep1大于so(t)的峰值旁瓣时,峰值旁瓣比增大.

3) <|t0|≤时,sp(t,1)峰值位置落在so(t)的副瓣内,此时积分旁瓣比增大,系统增益减小,分辨率影响较小,而且当sp(t,1)峰值位置与so(t)峰值旁瓣重合或sp(t,1)峰值幅度大于so(t)的峰值旁瓣时,峰值旁瓣比增大.

4) |t0|>时,sp(t,1)峰值位置落在第10个旁瓣之外,此时对积分旁瓣比、峰值旁瓣比以及分辨率的影响基本可以忽略,主要考虑虚假目标对成像质量的影响.为满足系统成像指标,需要控制虚假目标的幅度,即最小波束跃度需要足够小.

3 计算机仿真实验

 仿真参数 数值 波长/m 0.03(X波段),0.056(C波段) 方位向天线长度/m 6.0 卫星高度/km 700 旋转点离地面距离/km 130 中心视角/(°) 35 脉冲重复频率/Hz 4 000 信号带宽/MHz 300 信号采样率/MHz 400 方位向分辨率/m 0.5

 图 3 X波段不同控制精度下的脉冲压缩结果Fig. 3 Pulse compression results for different beam control accuracy in X-band

 图 4 虚假目标位置和幅度与波束跃度关系Fig. 4 Relationship between the position and amplitude of ghost target and beam granularity

 图 5 C/X波段峰值位置和峰值幅度与控制精度的关系Fig. 5 Relationship between the peak position and amplitude and beam control accuracy in C/X-band
4 结 论

1) 本文通过星载SAR滑动聚束模式天线波束跃度控制回波信号建模,并结合成对回波理论,证实了天线波束跃度控制将导致回波信号的幅度调制,引起无穷多对成对回波的出现,从而影响成像质量.

2) 本文通过天线波束跃度控制数学模型的建立,定性与定量化分析了虚假目标位置与幅度同最小波束跃度之间的关系.在此基础上,对不同波段下,虚假目标对分辨率、积分旁瓣比、峰值旁瓣比以及系统增益的影响展开了讨论,并得出在相同的指标条件下,X波段相比于C波段系统实现需要更高的最小波束跃度精度.

3) 本文通过计算机仿真实验验证了理论推导的有效性,为合理设计天线最小波束跃度指标提供了可靠的理论基础.

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#### 文章信息

Zeng Hongcheng, Chen Jie, Yang Wei, Zhang Haojie

Impact of antenna beam granularity on sliding spotlight spaceborne SAR image quality

Journal of Beijing University of Aeronautics and Astronsutics, 2014, 40(11): 1549-1554.
http://dx.doi.org/10.13700/j.bh.1001-5965.2013.0669