(a) High Resolution
High resolution is a lasting pursuit of the SAR system. Typical high-resolution SAR system includes Lynx [35, 36], MiniSAR , RAMESE , PAMIR [39-42] and so forth. The finest resolution of an airborne SAR in China is 0.1 m, which is achieved by band-composing. At present, the high resolution SAR systems are mainly airborne SAR systems. Spaceborne SAR systems have relatively lower resolutions than airborne SAR systems, which are higher than 1 m.
Along with the improvement of the image resolution, the detection, recognition, classification and description of the targets can be achieved. Therefore, high resolution SAR images can be used not only on mapping domain but also on moving target indication and recognition .
SAR system is originally designed as an imaging sensor. However, with the increasing requirements, SAR systems with multiple functions and working modes are becoming the research hotspot. The working mode includes stripmap SAR, scan-SAR, spotlight SAR [44-46], sliding spotlight SAR [47-49] and so forth. Different working modes can satisfy different requirements of resolutions and observation swath. Besides imaging, other functions such as GMTI and InSAR [50-52] are required. Phased-array antenna technique is also used in SAR system, which makes the multifunction and fast switch possible [53-55].
Along with the development of micro-electronics and antenna techniques, small volume, light weight, and low power consumption SAR systems are becoming a tendency of the SAR system. Mini SAR system can be applied on UAV, which is a burgeoning area in military applications. Typical systems include MiSAR [56, 57], MiniSAR, ^AR [58, 59] with FMCW technique and so forth.
(d) Multi-band, Multi-polar
Microwaves of different bands have different transmission capabilities. Therefore, more information can be derived by the fusion of images of different bands. Material and surface structure information can be extracted from the polarized SAR images, and is used in target recognition and classification . Therefore, multi-band, multi-polar SAR is an important tendency of the SAR technique.