Hybrid Carrier Tracking and Position Determination using the Low Elevation Satellite Signals

This paper presents a hybrid carrier tracking algorithm applicable to low elevation GNSS satellite signals. Low elevation GNSS satellite signals are known to have attenuated amplitude and large carrier phase and frequency fluctuations due to multipath and propagation through atmospheric structures. The hybrid tracking algorithm utilizes the open loop (OL) Doppler frequency model to assist phase lock loop (PLL) to generate the local references and to enable accurate carrier parameters estimations with improved robustness. In addition, the accurate carrier phase estimations obtained by the hybrid carrier tracking are used to derive the atmospheric delays, and correct range measurements to improve position solution accuracy. The hybrid tracking algorithm and low elevation positioning method are validated by the airborne radio occultation (ARO) data collected to study the disturbance of hurricane Karl on September 13, 2010. The results show that hybrid tracking can maintain lock of the low elevation signals for at least 200s longer than the PLL. The cut-off elevation for positioning can be as low as 5 ? below the horizon. The accuracy of the hybrid tracking method is also improved when compared to OL tracking. The accurate carrier tracking results lead to a correction of ~300m in vertical position errors due to atmospheric bending.