Delay Lock Loop Assisted Phase Lock Loop for GNSS Signal Tracking

Abstract

For positioning with Global Navigation Satellite System (GNSS) in urban canyon area, besides the weak signal power, the satellite signal may also be frequently sheltered and no power can be received. It is a great challenge for the GNSS receiver to keep positioning continuously. If the tracking loop in GNSS receivers can recover locking the signal soon after the signal appears again, it will help a lot for the improvement of the positioning continuity since the acquisition stage is omitted. However, when signal is shortly interrupted, the local frequency estimation error increases during the period without signal. The tradition Phase Lock Loop (PLL) cannot directly lock the signal again. Hence, we propose the Delay Lock Loop (DLL) assisted PLL. In the method, when the signal is detected to be recovered, the DLL is firstly started up. Once the DLL is locked, the code phase measurements are extracted from the DLL to estimate the carrier frequency. Afterwards, it is utilized to correct the initial frequency of the PLL. By making use of the DLL's insensitivity to the frequency variation, it will help the PLL quickly and accurately lock the signal when the signal is shortly sheltered. The theoretical analysis results inform that the accuracy of the carrier frequency estimated from the DLL is sufficient for the PLL to recover locking. The simulation results show that the proposed DLL assisted PLL can recover tracking immediately after the signal is shortly interrupted. And it will help a lot for the positioning availability of GNSS receiver.