Design and Performance Evaluation of an Adaptive Hybrid Coherent and Non-coherent GNSS Vector Tracking Loop

摘要

The GNSS vector tracking loop can be divided into two categories, coherent and non-coherent. In coherent algorithms, the GNSS accumulated correlator outputs are chosen as the Kalman filter's measurements directly, while discriminator outputs are used as measurements in non-coherent algorithms. The coherent algorithms is better for carrier-phase tracking and coherent code tracking is less noisy than non-coherent. However, it will not work well in weak-signal environments. On contrary, the non-coherent algorithms can maintain tracking in weaker signal environments than coherent algorithms because the code discriminator function is independent of the carrier phase. What's more, the non-coherent algorithms can abandon the carrier phase in order to keep the robustness when there is not sufficient carrier power to noise density (C/No) to track carrier phase. In order to maintain high position precision in weak-signal environments, this paper proposes an adaptive hybrid coherent and non-coherent GNSS vector tracking loop (AHVTL), which is implemented with coherent vector tracking loop used as the primary mode and non-coherent vector tracking loop as the reversionary. An adaptive hysteresis controller is designed to implement the tracking mode switch of the AHVTL so that the AHVTL can work in coherent, non-coherent aided coherent and non-coherent modes and switches the tracking mode adaptively according to the signal quality. Besides, the robust extended Kalman filter algorithm for the AHVTL is also included in this paper to further enhance the accurate estimates under weak-signal environments. It uses an adaptive factor to adjust extended Kalman filters so as to detect and reduce the influence of measurement outliers and noise. The experiment results show that the proposed AHVTL can operate under the applications where both high precision and weak-signal environments are required. The AHVTL also works well in weak-signal environments, where some signals are attenuated more than others. Compared to the traditional scalar tracking loop, single coherent and single non-coherent tracking loop, the proposed AHVTL can achieve accuracy and robustness tracking performance preferably under weak-signal environments.