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

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37 Citations (Scopus)


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 σϕ = 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.
Original languageEnglish
Pages (from-to)263-276
Number of pages14
JournalGPS Solutions
Issue number2
Publication statusPublished - 1 Jan 2015


  • FLL-assisted PLL
  • In-phase pre-filtering
  • Ionospheric amplitude scintillation

ASJC Scopus subject areas

  • General Earth and Planetary Sciences


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