Signal fading is the main disturbing phenomenon for Global Navigation Satellite System (GNSS) receivers in urban environment, which induces frequent loss of carrier lock in GNSS phase tracking loops. By taking advantage of the prevalent configuration of multi-frequency reception in emerging GNSS receivers, this paper proposes a Frequency Diversity combined GNSS carrier Tracking Loop (FDTL) for robust and centimeter-level positioning under harsh propagation conditions. Specifically, a bank of correlators is used to estimate the signal parameters of interest with a Maximum Likelihood Estimation (MLE) discriminator to avoid the need for a separate acquisition which normally requires the most processing time in GNSS receivers. The Doppler relationship between multifrequency signals offers an inherent constraint which is used to improve accuracy of MLE measurements. In the loop tuning process, a frequency diversity combined Adaptive Kalman Filter (AKF) is implemented for system dynamic propagation and measurement update. The primary advantages of the proposed FDTL algorithm include: 1) significantly improved carrier tracking robustness compared with traditional Phase Lock Loops (PLLs); 2) enhanced sensitivity obtained from the diversity combination. The effectiveness of the algorithm is demonstrated by simulated and real GNSS fading signals.