Multi-PLL with two-stage fusion to mitigate ionospheric scintillation effects on GPS receivers

Ionospheric scintillation poses a great threat to the reliability and accuracy of Global Positioning System (GPS) in various applications. It can increase tracking errors of the phase-locked loop (PLL) in a GPS receiver and even cause the PLL loss of lock under severe scintillations. To mitigate the effect of scintillation on GPS receivers, especially to reduce the occurrence of loss of lock, a multi-PLL with two-stage fusion (i.e., tracking fusion and output fusion) is proposed in this paper. This algorithm integrates several parallel sub-PLLs with different loop parameters into one channel to track one GPS satellite's signal. Every sub-PLL has its own discriminator, loop filter, carrier numerical controlled oscillator, and a tracking fusion (i.e., the first stage fusion). The tracking fusion of each sub-PLL integrates the Doppler frequency measurements from all other sub-PLLs to detect the state of its own sub-PLL and feeds back reliable Doppler frequency measurements. Simultaneously, the tracking fusion outputs the Doppler frequency measurements to the second stage fusion (i.e., output fusion), which integrates the outputs from all tracking fusions to provide continuous and accurate Doppler frequency measurements for the following positioning/navigation estimator. Performances of the proposed algorithm are tested using real-world GPS data with different levels of scintillations and compared with results from single-PLLs. For three real-world scintillation cases (S₄=0.26-1.1, σ_φ=0.05-1.49rad, and average C/N₀=41.2-45.7dBHz), the multi-PLL algorithm performs more robustly than the single-PLLs and is able to keep tracking in all scintillation cases.