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Session A4: GNSS in Challenging Environments 2

Phase Locked Loop Performance Metrics Based on the Distribution of the Discriminator Output
Stefan Stevanovic and Boris Pervan, Illinois Institute of Technology
Location: Grand Ballroom G
Date/Time: Thursday, Feb. 1, 8:35 a.m.

Peer Reviewed

This work introduces a new phase-lock loop (PLL) performance metric derived from the distribution of the discriminator output. In order to track weak GPS signals, or track through wideband interference events, the PLL pre-detection averaging time must often be extended beyond the length of a navigation data bit. However, we show that the existing techniques used to extend averaging introduce biases in the discriminator output (DO) distributions, which cause biases in the reconstructed carrier Doppler and can lead to cycle slips. For extended averaging times, we show that the DO variance is an insufficient metric for PLL design. In this paper, we propose novel PLL performance metric based on the bias in the discriminator output probability distribution. In addition, we show that the DO bias is directly useful for comparing candidate methods (e.g. coherent and non-coherent methods) of extending the averaging time beyond 20 ms. The relationship between the DO bias and the variance-based metric is studied using simulation and experimental results. The experiments are performed using an RF signal generator to directly simulate a GPS signal with wideband interference, which is realized as a deteriorated carrier-to-noise ratio. The simulated signal enters into a GPS front-end kit, which utilizes an external clock input from a commercially available rubidium clock. A software receiver is used for the tracking tests. Experimental results are consistent with theory. The DO bias is superior to the variance metric in its ability to predict loss of phase-lock.



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