This paper describes the development and implementation of FMCW GPR system mounted on a mini UAV to detect and generate underground images using SAR techniques as processing engine. This FMCW GPR system consists of a portable FMCW radar, two LPDA antennas and one Raspberry Pi model 2. This radar operates at 745 MHz, with a bandwidth of 510 MHz. The modulated sweep signal is a sawtooth type and the chirp length is 8.5 ms. The system is capable of acquiring, digitalizing and storing data 'onboard' in order to do a post-processing. First evaluations of this system show the detection of a metal object buried at 0.4 m underground.