Vector Tracking Aiding for Carrier Phase Estimation in the Presence of Ionospheric Scintillation

In GNSS receivers, the ability of guaranteeing continuous position, velocity and time (PVT) solutions is linked with their capacity of maintaining lock with the satellite signals. However, this is not always possible and in degraded environments the received signals can be strongly attenuated or affected by errors that cause the tracking loops to lose lock and the navigation processor to be unable to produce a useful positioning solution. A particularly challenging environment is represented by the presence of ionospheric scintillations that combine signal fading and fast phase variations. These effects can severely test receiver tracking loops and their capacity to follow the received signal changes. Carrier tracking through phase lock loops (PLL) is especially susceptible to losing lock. Amplitude scintillations may in fact generate deep fades in the received signal power that in worst cases can cause the signal to drop below the lock threshold; on the other hand, phase scintillations cause rapid carrier phase changes that might produce cycle slips and even cause loss of lock. This lack of robustness strongly impacts receiver performance as it reduces carrier phase measurements availability, the capability to demodulate the navigation message data and the ability to perform carrier-smoothing. This is the motivation behind the study of innovative architectures to improve receiver robustness and aid the phase tracking process. Ionospheric scintillation mitigation can in fact be achieved by improving carrier tracking ability to sustain rapidly changing received signals. Different tracking schemes could be envisaged; in this paper a Vector Frequency Lock Loop (VFLL) assisted PLL is described. Vector tracking loops are interesting as they allow implementing cross-channel aiding by linking together all received signals through the receiver position and velocity. The aim of this paper is therefore to present the VFLL-assisted PLL receiver architecture describing the system model and feedback generation process and to test its feasibility in scintillation scenarios. To reproduce the scintillation effect, the Cornell Scintillation Model (CSM) has been used and simulations that quantify the performance in terms of phase tracking error variance, cycle slips and loss of lock are conducted.