Improved Type3-PLL to Mitigate Parasitic Amplitude Modulation Effects Caused by Time-Varying Plasma Sheath

Time-varying plasma sheath likely causes parasitic amplitude and phase modulation effects on communication signals. It can increase the carrier tracking error of tracking telemetry and command systems for hypersonic vehicles flying in near space, and it may lead to loss of lock under serious conditions. This paper comprehensively analyzes the influence mechanism of the amplitude modulation effects on the loss threshold of Type3-phase-locked loop (PLL) in phase-locked receivers. Analysis result shows that amplitude fading in time-varying plasma sheaths is similar to the Gaussian distribution unrelated to the fluctuant densities. The standard deviation of amplitude fading increases with the fluctuant intensity. The 8% fluctuant intensity of electron density begins to have a significant impact on the lock probability of Type3-PLL. On the basis of the analysis results and the amplitude modulation effects, Type3-PLL with a prefiltering (PF-Type3-PLL) method is proposed to mitigate the amplitude modulation effects. PF is placed before the discriminators of Type3-PLL to restrict the amplitude variation within a reasonable range. The tracking loop PF-Type3-PLL is tested with spatial-temporal fluctuant plasma sheath model and compared with the traditional Type3-PLL. Tracking results show that PF-Type3-PLL is more robust than the traditional Type3-PLL. The phase error jitter is significantly reduced when the fluctuant intensity is 8%, and the loss threshold is reduced by 4 dB. The robustness is also enhanced under different loop noise bandwidths (5-15 Hz).