Multi UAV imaging system is the one of most popular application fields of UAVs. Currently, they are widely used in bridge crack detection, air pollution monitoring, cooperation track, rescue work, and so on. However, there are many issues to be resolved in these applications. Imagery measurement, crack recognition , the optical and image processing technology are the basis to use the aerial photography; communication networks is important to transmit photography and realize flight control; The

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... ooperation and stable control of a fleet of UAV to ensure acquiring the reliable image; flying in formation is an effective way to enlarge the coverage area. These issues are proposed and reviewed in the paper. And the main challenge for the application is concluded in the end. Meanwhile, multi UAV system has the following communication feature: shadowing, fading model, multiple-input-multiple output (MIMO) systems, the effect of Doppler shifts and spread, and so on [57] . In this multi imaging applications review, the communication application model, the communication propagation model (Doppler shifts and spread, fading channel model, etc.) are discussed. Fig 4: The main communication issue of multi-UAV The communication application models In Multi UAV application system, the widely used communication model is the "disk model". The disk model assumes that there is a clear communication distance d (or received SNR) between UAVs. If the distance of two UAVs is less than d (or the received SNR large than a value), communication between them is 100% successful; otherwise, they cannot communicate with each other [60]. In [61], the concept iJournals: International Journal of Software & Hardware Research in Engineering