星载合成孔径雷达俯仰向数字波束形成处理方法研究

High resolution and wide swath (HRWS) are important goals for the next generation of space-borne synthetic aperture radar (SAR) remote sensing satellites. In recent years, multichannel in azimuth and variable pulse repetition interval (VPRI) have been regarded as the most promising candidate for simultaneously obtaining wide swath and high resolution. But the performance of system sensitivity and ambiguity for the mentioned schemes have constrained the further development. So digital beam forming (DBF) with multiple channels in elevation is proposed, which could significantly increase the receiver gain and signal to ambiguity ratio so as to enhance the performance of SAR system. However, It is generally believed that on board processing and high precision algorithms is challenging for spaceborne DBF-SAR due to heavy computation load and complex imaging scenes. To solve the above mentioned problems, this project will devote to systematic research on processing algorithm and scheme for spaceborne DBF-SAR system, which includes 1) constructing generic signal model of DBF-SAR; 2) proposing robust signal processing method and implementation scheme; 3) validating the theory and processing approach based on spaceborne and airborne SAR experiments. The research results of this project can be utilized to provide theoretical basis for the techniques deveopment and application of DBF for future spaceborne SAR system.

克服传统星载SAR分辨率和测绘幅宽指标之间矛盾从而实现高分宽幅的途径主要包括方位多波束与变脉冲重复间隔（VPRI）技术，但较低系统灵敏度和距离模糊性能限制着上述体制在星载SAR系统中的发展与实际应用。俯仰向多通道DBF技术应运而生，该技术能够显著提高系统接收增益并改善其距离模糊性能。但俯仰向DBF技术的星上处理运算复杂度很高，这对星载系统的实时处理能力提出严峻考验，而且实际成像场景的复杂性与测绘带幅宽的加大也使得地面目标回波的空变性变得愈发严重。针对上述问题，本项目将系统地开展星载SAR俯仰向DBF处理方法与架构研究，构建通用的高精度俯仰向DBF信号模型；形成稳健的星载SAR-DBF信号处理方法和高效高精度的俯仰向DBF星上实时处理架构；通过理论分析、仿真实验和星载/机载试验完成原理和性能验证；为俯仰向DBF技术在星载SAR系统中的持续发展和系统应用奠定理论基础和提供技术支撑。

本项目深入研究了基于俯仰向多通道的星载SAR相关技术。推导了基本的俯仰向多通道数字波束形成(digital beamforming, DBF)的处理原理与方法。针对俯仰向DBF需要耗费大量硬件资源实现星上实时处理的问题，提出了一种改进的DBF处理架构，该架构可大大降低系统资源占有率并得到与传统方案等效的DBF输出结果。随着测绘带幅宽的逐步加大，DBF接收的扫描角度范围也不断加大，此时通道间的斜距误差随距离向的空变性不可忽略，测绘带边缘会出现接收增益下降的现象。本文提出了一种基于变标技术的DBF处理方案，该方案可有效地补偿通道间斜距误差的空变性，提升回波接收增益。之后详细分析了俯仰向DBF对SAR系统性能的影响并给出了与方位向多通道结合的高分宽测星载SAR系统设计流程与实例。最后研究了基于俯仰向DBF波形分离技术的一种多发多收 (multiple-input multiple-output, MIMO) SAR体制，该体制的多个发射通道的信号基于短时正交理论而建立，借助俯仰向DBF零点指向技术实现回波分离，最后可以得到比单发多通道更多的相位中心，进一步提高SAR系统性能。