Time-frequency Analysis of Time-Varying Plasma’s Influences on PCM-FM Signal

In the process of hypersonic flight vehicle reentry, plasma sheath form on the aircraft surface because of atmospheric friction. Due to the changes of vehicle speed, posture and height, the atmospheric turbulence and non-uniform ablation, the influence of such factors makes the formed plasma sheath has time-varying characteristics. Statistical properties of telemetry PCM-FM signal change with the plasma's parameters after the signal propagate through the time-varying plasma, and the PCM-FM signal become non-stationary random signal. In order to study the influence of time-varying plasma to PCM-FM signal, The time-frequency analysis is carried out on the output signal using short-time Fourier transform. Analysis results show that the time-varying plasma's influence on the PCM-FM signal perform as amplitude attenuation and phase stray. Increasing the carrier frequency can reduce the influence of amplitude attenuation and improve the quality of the signal to a certain extent. But phase-stray phenomenonstill exist, and can be solved by other methods such as phase filtering.