## 1 Introduction
The Agency for Defense Development (ADD)--a defense research center in the Republic of Korea--is currently in the process of developing a 94-GHz airborne MIMO video SAR system, as a possible concept for surveillance sensors in unmanned aerial vehicles (UAVs). Frequency-modulated continuous wave (FMCW) technology is suitable for small size, lightweight and low cost systems. Various technologies and algorithms related to FMCW-based SAR systems have been studied in [11; 18; 19]. Several FMCW SAR systems have historically been used successfully in ice measurement, environment monitoring, and three-dimensional applications [20]. The combination of FMCW technology and ViSAR mode gives birth to a light-weight, cost-effective, high-resolution, active microwave remote sensing instrument, which is suitable for small platforms such as UAVs [20]. In 1994, [PERSON] and [PERSON] patented the use of multiple antennas for both transmission and reception in wireless communications, in order to increase channel capacity [21]. This invention has shown important potential for deployment in various wireless communication applications, as well as for radar sensor technology. The potential applications of this technology have promoted extensive research on the use of multiple antennas. The MIMO radar, the combination of multiple antennas with radar sensors, is currently one of the most interesting topics in radar communities [22]. In this case, the MIMO function is implemented using a coding technique to apply a pseudo-orthogonal waveform. This solution is not suitable for SAR operation, because, unlike the point target case, the received signal consists of distributed targets from large swaths, in the time-frequency domain [23]. Orthogonal frequency division multiplexing (OFDM) chirp [24; 25], short-term shift orthogonal (STSO) waveforms [26], and OFDM chirp diverse waveforms [27] have been studied with pulsed MIMO SAR. MIMO technology can be combined with ViSAR systems using orthogonal waveforms for FMCW radars such as the beat frequency division (BFD) waveform [28], the chirp rate division waveform [28], and the OFDM (or interleaved OFDM, I-OFDM) chirp waveform [29]. This MIMO technology can be used for various objectives, for example, the virtual array can be used to increase the equivalent number of phase centers or receiving channels, for the reduction of the aperture size. When reconfigurable transmitting (Tx) antennas are employed, MIMO technology can also be used for multi-mode operation within a single system [12; 25], such as along-track interferometry, ground moving target indication (GMTI), and polarimetric SAR. In this paper, we use MIMO technology to increase the equivalent number of receive channels of a virtual array, and to reduce aperture size, by combining it with a ViSAR system. The stop-and-go approximation used in pulsed SAR is not valid for FMCW SAR. Since an FMCW SAR has a long sweep duration, the motion of the target object within the sweep must be considered [11]. Doppler shift is more sensitive in OFDM chirp waveforms [24; 25], than in linear frequency modulation (LFM) [29]. It is therefore difficult to apply Doppler compensation techniques when the sweep duration is long, as in FMCW SAR. This problem can be solved by applying the BFD FMCW, and then compensating the Doppler shift. As such, the proposed MIMO ViSAR system is based on this orthogonal FMCW. A multi-channel azimuth processing technique is utilized for improving the frame rate and image size of SAR video by suppressing the azimuth ambiguities in the receiving channels. Applying the multi-channel reconstruction algorithm (MCRA) to the ViSAR system allows SAR image formation without Doppler ambiguities, which are due to the simultaneous increase of the Doppler bandwidth and the radar velocity [6; 7; 8; 9]. As a result, the frame rate and image size of the SAR video can be increased. There exist several different HRWS algorithms and MCRAs for reconstructing a stationary scene.