Modified Costas Loop for Carrier Phase Tracking in GPS Receivers

Abstract

The carrier phase received at the receivers of the Global Positioning System (GPS) links is used to detect navigation data and to precisely determine the position, speed and time corresponding to the user's equipment. Therefore, subsystems for carrier phase tracking are crucial parts in all GPS receivers. When the propagation conditions are favorable, the method frequently used for phase tracking is based on Digital Phase-Locked Loop (DPLL)) and implemented through the discrete Costas loop operating under the modulated L1 carrier, in the case of a GPS receiver. This technique is quite simple, well known and very suitable for implementation in low-cost receivers. In this article, we revisit the traditional Costas loop design and point out some issues that affect the phase tracking performance of this loop. In order to overcome these problems, we propose some modifications to the traditional Costas loop. The resulting architecture presents better performance and complexity equivalent to the original loop. Another contribution of this work is the mathematical analysis to evaluate the performance of the new architecture when operating on an Additive White Gaussian Noise (AWGN) channel. Various results from computational simulations carried out with the two architectures, in different operating scenarios, including AWGN, dynamic stress and ionospheric scintillation are presented and discussed. We conclude that the new architecture outperforms the traditional Costas loop in terms of the variance of the estimated phase error, root mean squared error of the estimated phase and robustness to cycle-slip and loss of lock.

Keywords

• carrier phase tracking
• gps receiver
• costas loop
• phase recovery