## 1 Introduction  
Synthetic aperture radar (SAR) technology is an active microwave remote sensing technique, capable of day/night, all-weather operation, used to detect and acquire electromagnetic information about objects without physical contact [1; 2; 3]. The enormous potential of SAR--invented by [PERSON] in 1951--has been evident since the first demonstration of the concept. This potential has facilitated extensive research into SAR technology [4], including avenues for improving its operation. For instance, in the case of conventional single aperture SAR, it is difficult to obtain a high-resolution wide swath image, due to the trade-off between resolution and image size [5]. This problem was solved by adopting multi-channel processing techniques to create a high-resolution wide swath (HRWS) SAR system [6; 7; 8; 9; 10]. By adding multiple transmit antennas to the HRWS SAR system using multiple-input multiple-output (MIMO) technology, it is possible to operate a single SAR system in multiple modes, such as HRWS, interferometric SAR (InSAR), and polarimetric SAR (PolSAR). These modes can be operated simultaneously, without performance degradation [11; 12]. By applying MIMO and multi-channel azimuth processing techniques to video synthetic aperture radar (ViSAR), improvements to the frame rate and image size of SAR video can be achieved, without compromising other aspects of the system's performance. ViSAR is a SAR imaging mode which has recently gained increased research interest. In this mode, images can be generated at a much higher rate than in conventional SAR; thus, they can be viewed continuously, just like watching a video [13; 14; 15]. Since ViSAR systems can provide high resolution SAR images at a high frame rate regardless of adverse weather conditions, they can be utilized in many day/night, all-weather military and civilian applications, including fire control support [16], as a replacement for electro-optical (EO)/infrared (IR) sensors, which can be only operated in clear weather, land and maritime traffic monitoring, and surveillance [17]. There are two kinds of SAR video synthesis methods associated with ViSAR, full aperture synthesis, which uses a circular SAR mode [13; 15], and sub-aperture synthesis, which uses a spotlight SAR mode [14; 16]. In the full aperture synthesis mode, an overlapped processing method is typically used between SAR video frame updates to obtain high frame rates, since the synthetic aperture time is very long at relatively low frequencies. Due to its parallel nature and ease of motion compensation, the backprojection algorithm (BPA) is suitable for use with full aperture synthesis. The frame rate in this case can be adjusted easily by changing the overlapping ratio between SAR video frames. Due to the large amount of computation power required, the BPA uses sub-images generated from sub-apertures, which are partial sections of the full aperture. In the sub-aperture synthesis mode, the frame rate can be improved by increasing the transmitted frequency and the radar velocity at a given resolution. When operated as a partial measurement, circular SAR is the same as spotlight SAR. Therefore, on a circular path, SAR video can be generated from the sub-aperture data, using the polar format algorithm (PFA) in spotlight SAR mode. However, as the radar velocity increases, the Doppler bandwidth of the antenna beamwidth increases, thus, azimuth ambiguities occur. These azimuth ambiguities can be suppressed effectively, using multi-channel azimuth processing techniques. The X band video SAR system (XWEAR) [13], generates SAR video using the BPA on a circular path. The NanoSAR, operating in the X band, and the MiniSAR, operating in the Ku band, can also perform the video SAR function using a circular path. The Defense Advanced Research Projects Agency (DARPA) is currently developing a ViSAR system capable of achieving a high frame rate at an extremely high frequency (233 GHz), to replace EO/IR sensors used for fire control on maneuvering ground targets [15; 16].