The paper presents a synthetic information on a UAV-based radar terrain imaging system, its purpose, structure and working principle as well as terrain images obtained from flight experiments. A SAR technology demonstrator has been built as a result of a research project conducted by the Military University of Technology and WB Electronics S.A. under the name WATSAR. The developed system allows to obtain high resolution radar images, both in on-line and off-line modes, independently of the light conditions over the observed area. The software developed for the system allows to determine geographic coordinates of the imaged objects with high accuracy. Four LFM-CW radar sensors were built during the project: two for S band and two for Ku band, working with different signal bandwidths. Acquired signals were processed with the TDC algorithm, which allowed for a number of analyses in order to evaluate the performance of the system. The impact of the navigational corrections on a SAR image quality was assessed as well. The research methodology of the in-flight experiments of the system is presented in the paper. The projects results show that the developed system may be implemented as an aid to tactical C4ISR systems.