Abstract: Enhanced techniques to declare Mode S preamble pulses in high Mode A/C fruits environment are discussed in this paper. Basis and roles to detected preamble are analyzed first. The improved preamble detection algorithm includes three steps: preamble detection, preamble validation, and reference level generation. Leading edge detection method and pulse matching method are employed to acquire pulse positions, then detect preamble by position matching. The PPM encoding used in the data block is difficult to generate with Mode A/C fruit replies. Counting the sample amplitudes within 2 dB in four sub-pulses, the reference level is the most amplitude. At last, efficiency of algorithms is verified by simulation results.