Unscented Kalman Filter based Carrier Tracking to Mitigate Ionospheric Scintillation Threat in High latitude for Maritime PNT

摘要

Small-scale time-varying electron density irregularities in the ionosphere distort the wave-front and result in wavelet interference at the receiver, manifested as variations in amplitude and phase observed at the receiver. Fast phase changes and/or deep amplitude fades can present a considerable problem to the tracking loop of the receiver PLL which can result in a significant positional error. Rapid random fluctuations in amplitude and/or phase of received transionospheric radio signals are termed as ionospheric scintillation, which occurs mainly at low latitudes and in the auroral and polar regions. Generally, in the GPS receiver, fading due to scintillation causes the In-phase and Quadrature-phase components of the received signal to be altered, resulting in large phase jitter in the tracking loop as it attempts to track the perturbed phase. Intense scintillation not only degrades the signal quality but also results in the receiver PLL losing phase lock. In the auroral region, phase scintillation can be very intense resulting in huge phase errors; thus, the Costas-type PLL takes a long time to minimize the phase error, which can then result in a cycle slip. In order to implement methods to maintain the GNSS services during strong scintillation conditions, it is proposed to investigate the GNSS raw data at high latitude using GPS frontend devices and a scintillation monitoring device can help in investigating the best approach to mitigate ionospheric scintillation. Thus, in this study, the GNSS signal with scintillation is received using both USRP X310 and NovAtel GISTM receivers with a RF splitter simulated by SPLN based ionospheric scintillation and a GNSS RF signal using the Spirent simulator. The data campaign including the collection from the NovAtel receiver in parallel with the GNSS software receiver helps in measuring and thus providing information of the scintillation indices and any loss of lock. The GNSS software receiver employs an Unscented Kalman Filter approach in the tracking loop to understand and mitigate ionospheric phase scintillation. The results obtained from both receivers are compared and found that when the COTS (Commercial Off The Shelf) receiver loses lock, the adaptive unscented Kalman filter-based carrier tracking loop maintains lock during demodulation of the navigation data frame.