Abstract: Under the complex electromagnetic environment, LPI (low probability of intercept) airborne radar can effectively reduce the distance between the passive detection system and combat aircraft, and increase the tracking guidance accuracy, which can effectively improve the survivability and penetration ability of combat aircraft. An airborne radar of low intercept probability usually uses broadband modulation signal, by expanding the energy of the signal in the frequency domain, to reduce the probability that the radar radiation signal is intercepted by the enemy. An airborne radar signal of low probability of intercept is usually a linear FM signal (including linear FM signal and phase encoding signal) and a nonlinear FM signal (including Costas code and Barker code signal). In this paper, the inherent characteristics of Costas and Barker signal in nonlinear frequency modulation signal are analyzed, and the method to detect the nonlinear frequency modulation signal by using fractional Fourier transform discussed. Since Costas codes and Barker codes can be regarded as approximate linear frequency modulation signals, and fractional Fourier transform is the most suitable method for linear frequency modulated signal detection, the signal energy of the nonlinear frequency modulation signal was focused by using fractional Fourier transform so that the signal can be detected. Because in the common time-frequency analysis methods, such as Choi-Williams time-frequency analysis, the signal energy can not be concentrated after the signal is changed, the short time fractional Fourier transform is better than the nonlinear frequency modulation signal for the energy accumulation, which is good or the signal detection. The simulation analysis shows that the intercept receiver has different detection performance for different nonlinear frequency modulation signals.

Key words: LPI, nonlinear FM signal, short-time fractional Fourier transform, interceptors, Costas codes, Barker codes