Improved FLL-assisted PLL with in-phase pre-filtering to mitigate amplitude scintillation effects

Ionospheric scintillation can increase carrier tracking error of the Global Positioning System (GPS) receiver and may lead to loss of lock under serious conditions. It is a threat to reliable and accurate GPS applications. Kalman filter-based phase lock loop, frequency lock loop-assisted phase lock loop (FAP) and vector-based tracking algorithms are considered as solutions to improve the robustness of carrier tracking. However, their performances are sensitive to loop parameters: noise bandwidth and integration period. We analyze the effects of amplitude scintillation on inputs signals, in-phases and quadra-phases, of carrier tracking loop. Analysis result shows amplitude scintillations affect in-phase much more seriously than quadra-phase. Based on the results, the FAP with in-phase pre-filtering (FAP_IPF) method is proposed to mitigate the amplitude scintillation effect on in-phase. IPF is placed before discriminators of FAP to restrict the in-phase variation within a reasonable range. The tracking loop FAP_IPF is tested using both simulated and real-world GPS intermediate frequency (IF) data with amplitude scintillation and compared to the traditional third-order PLL and FAP without IPF. The tracking results show that FAP_IPF is more robust than FAP and traditional PLL. In 10 simulation scintillation cases (S₄ = 0.6–1.0 and C/N₀ = 40 dB–Hz), FAP_IPF can maintain tracking in 6–9 cases even if its bandwidth Bn varies in a large range 7.5–15 Hz. FAP has the best performance at bandwidth Bn = 10 Hz, but its performance is degraded dramatically at other bandwidths. PLL shows the worst performance among the three algorithms. In real-world GPS IF data with real-world scintillation S₄ = 1.11 and σ_ϕ = 1.49 rad, FAP_IPF can successfully maintain signal tracking within bandwidth Bn = 7.5–12.5 Hz, while both FAP and traditional PLL lose signal trackings.