Improving the robustness and accuracy of GPS software receiver under ionospheric scintillation conditions

Abstract

Ionospheric scintillation is a random fluctuation in the amplitude and phase of radio signal that passes through irregularities of electron density distribution in the ionosphere. It can lead to loss of lock on the carrier tracking loop of GPS receiver and hence a requirement of re-tracking and reacquisition of the lost signal. This results in cycle slips, increased navigation errors and even navigation interruption. Ionospheric scintillation poses a threat to the reliability and accuracy of various GPS applications. To reduce the occurrence of scintillation-caused loss of lock, several improved tracking algorithms such as Frequency Lock Loop (FLL)-assisted Phase Lock Loop (PLL), Kalman Filter based PLL (KFPLL) and vector based tracking algorithms are conducted. These methods commonly involve adjusting loop parameters (i.e. the equivalent noise bandwidth and integration time) to scintillation intensity. However, one limitation is the dependence of tracking loop performances on its parameters that are difficult to estimate under scintillation conditions. To understand the process of loss of lock due to scintillation, the effects of separate and combined amplitude and phase scintillation on PLL inputs (i.e. in-phase, quadra-phase and phase error) are first analyzed. The analysis reveals amplitude scintillation and phase scintillation have different effects on PLL inputs. Thus, scintillation effects on carrier tracking loop can be mitigated through dealing with in-phase and quadra-phase. In addition, the analysis results imply the effects of amplitude scintillation and phase scintillation can be decoupled from each other and mitigated separately. Based on the analysis results, a new approach called Frequency Lock Loop (FLL)-assisted Phase Lock Loop (PLL) with In-phase Pre-Filtering (IPF) is proposed. In the method, FLL-assisted PLL is used to mitigate phase scintillation effects because it has high dynamic performances and can fast track phase variations. IPF is used to mitigate the effects of amplitude scintillation. It is realized using a homomorphic filter and a novel segmented filter since scintillation amplitude is non-additive error. Their performances are evaluated under different levels of simulated scintillation cases and the segmented filter is suggested. To reduce the dependence of tracking performances on the integration time, a parallel multiple PLL architecture called multi-PLL is designed in the carrier tracking loop. In the multi-PLL, each channel contains several sub-PLLs with different loop parameters to track one satellite signal. Realization of the multi-PLL involves two issues: sub-PLL selection and integration algorithm. In the study, the degree of correlation between two sub-PLLs is introduced to choose the sub-PLLs parameters. A tracking fusion algorithm and an output fusion algorithm are designed to integrate the multiple sub-PLLs. In the study, both FLL-assisted PLL with IPF and multi-PLL were tested using simulated and real-world GPS scintillation data. The results verified the effectiveness of the FLL-assisted PLL with IPF and multi-PLL.