I. Introduction

Because the electro-magnetic wave travels along straight lines and there are many tall buildings and mountains on the earth, so it is very difficult to detect low altitude targets for ground based radars. Besides that, some low-flying targets (such as helicopters and cruise missiles) can fly among buildings and mountains at a height of about 50 meters. The clutter of the echo is so strong that ground based radars can't detect these low-flying targets. So detecting low-flying targets is one of the most difficult tasks in modern radar detection. Displaced Phase Center Antenna (DPCA) technique can be used to suppress clutter and detect moving targets by moving the phase centers. But researchers tend to study DPCA technique based on monostatic radars. Because of the development of Electronic Warfare (EW), monostatic radars are threatened by ‘Four Dangers’: Electronic Countermeasures (ECM), ultra-low altitude penetration, stealth weapon, Anti-Radiation Missile (ARM). All of these challenge the ability of monostatic radars to exist.