合成孔径雷达(SAR)技术 中国科学院电子所 2002/12/09

SAR 技术 SAR 是一种脉冲雷达技术，具有较高的分辨率，获得区域目标的图像。 SAR 具有广泛的应用领域，它有两种模式 机载SAR

应用领域 地形测绘与地质研究中的应用 农业和林业中的应用 海洋研究和监测方面的应用 军事方面的应用 减灾防灾方面的应用

SAR的特点 I 为什么使用雷达成像技术 全天候，穿透云雾能力 全天时工作 穿透植被和树叶 目标与频率的相互关系 运动检测

SAR的特点 II 方位分辨率: 实例: 星载SAR距离850km，工作频率1.276GHz，像素分辨率25m  需要 8km 合成孔径

合成孔径原理 8km 的孔径长度由小天线实现 原理

SAR合成孔径原理

SAR 基本概念 最大聚焦合成孔径长度: 天线尺寸的减小导致更长的聚焦合成孔径长度 SAR 聚焦分辨率: 分辨率的改善与天线尺寸有关，与距离和波长无关

星载SAR

机载SAR Consider an airborne SAR imaging perpendicular to the aircraft velocity as shown in the figure below. Typically, SARs produce a two-dimensional (2-D) image. One dimension in the image is called range (or along track) and is a measure of the "line-of-sight" distance from the radar to the target. Range measurement and resolution are achieved in synthetic aperture radar in the same manner as most other radars: Range is determined by precisely measuring the time from transmission of a pulse to receiving the echo from a target and, in the simplest SAR, range resolution is determined by the transmitted pulse width, i.e. narrow pulses yield fine range resolution.   The other dimension is called azimuth (or cross track) and is perpendicular to range. It is the ability of SAR to produce relatively fine azimuth resolution that differentiates it from other radars. To obtain fine azimuth resolution, a physically large antenna is needed to focus the transmitted and received energy into a sharp beam. The sharpness of the beam defines the azimuth resolution. Similarly, optical systems, such as telescopes, require large apertures (mirrors or lenses which are analogous to the radar antenna) to obtain fine imaging resolution. Since SARs are much lower in frequency than optical systems, even moderate SAR resolutions require an antenna physically larger than can be practically carried by an airborne platform: antenna lengths several hundred meters long are often required. However, airborne radar could collect data while flying this distance and then process the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna. Achieving fine azimuth resolution may also be described from a doppler processing viewpoint. A target's position along the flight path determines the doppler frequency of its echoes: Targets ahead of the aircraft produce a positive doppler offset; targets behind the aircraft produce a negative offset. As the aircraft flies a distance (the synthetic aperture), echoes are resolved into a number of doppler frequencies. The target's doppler frequency determines its azimuth position. While this section attempts to provide an intuitive understanding, SARs are not as simple as described above. Transmitting short pulses to provide range resolution is generally not practical. Typically, longer pulses with wide-bandwidth modulation are transmitted which complicates the range processing but decreases the peak power requirements on the transmitter. For even moderate azimuth resolutions, a target's range to each location on the synthetic aperture changes along the synthetic aperture. The energy reflected from the target must be "mathematically focused" to compensate for the range dependence across the aperture prior to image formation. Additionally, for fine-resolution systems, the range and azimuth processing is coupled (dependent on each other) which also greatly increases the computational processing.

SAR成像模式 Stripmap, Spotlight, Scan, ISAR (not pictured)

SAR成像模式 Stripmap(条带式): Spotlight(聚束式): 最早的成像模式，1950’s 低分辨率成像的最有效方法 获得较高的分辨率 一次飞行中，通过不同视角改变对同一区域成像

SAR成像模式 Scan(扫描模式): 信号处理非常复杂 ISAR(逆SAR) 雷达静止, 目标运动

SAR信号处理 图像分辨率不高 聚焦处理 距离关系 点目标提取: 聚焦 数据距离校正 处理算法: 匹配滤波 Example: Received raw stripmap data from point targets 图像分辨率不高 聚焦处理 距离关系 点目标提取: 聚焦 数据距离校正 处理算法: 匹配滤波

SAR信号处理 匹配滤波 计算速度 频域相乘方法 时域卷积方法

SAR信号处理 运动补偿问题 多普勒频率漂移问题 数据采样 距离向 方位向

SAR信号处理 脉冲宽度(T) 与带宽成反比 带宽增加，距离分辨率提高 线性调频信号 脉冲重复周期(PRI)或频率(PRF) 采样定理的限制 脉冲重复频率增加，方位分辨率提高

SAR信号处理 距离采样 满足采样定律 方位采样(PRF) 必须满足: Rfar = 远距点, Rnear =近距点

SAR使用的波段 VHF/UHF 125 to 950 MHz C band 5.3 GHz X band 7.5 to 10.2 GHz Ku band 14 to 16 GHz Ka band 32.6 to 37.0 GHz

SAR图像例 美国加州洛杉矶的卫星雷达图像

南极卫星图像

参考文献 Tony Freeman, Jet Propulsion Laboratory. What Is Imaging Radar. Fitch, J. P. Synthetic Aperture Radar. Springer-Verlag, New York, 1988. Soumekh, M. Synthetic Aperture Radar Signal Processing. Wiley, New York, 1999. Carrara, W. G., et al. Spotlight Synthetic Aperture Radar Signal Processing Algorithms. Artech House, Boston, 1995. The Alaska SAR Facility. http://www.asf.alaska.edu/