Abstract: The monostaic passive location systems based on FM broadcast signals are actually bistatic Radar systems , but having the characteristics that the ground clutters and the target signal propagate at low altitudes with small grazing angle. The propagation at low altitudes and the ADC noise in the RF direct sampling system taken are considered, and the main factors which affect the coverage of low altitude detection is analyzed. The results of simulation validate the above analysis above and show that the low altitude observation area increases with a higher transmitter antenna.