Abstract: Small targets moving at low altitude are great threatens to national air defense, and in this case radars usually have poor performances because of the strong clutter and low Signal to Noise Ratio (SNR). However, Spaceborne bistatic radar has special advantages in detecting such kind of targets because of large coverage and long integration time and so on.In this paper, a kind of bistatic radar based on Geosynchronous (GEO) SAR was proposed, which used GEO SAR as the transmitter and put the receivers on the ground or on the airship.The relationship between receiving range and bistatic angles as well as receiving antenna area was studied, and the key technologies were discussed, including long time integration, and multipath interference suppression, and time and phase synchronization, and beam scanning and so on. Finally, preliminary solutions were given aiming to these key technologies.