A Joint High-Precision Frequency and Code Phase Estimation Algorithm for Iridium NEXT Satellites’ Signals of Opportunity

In recent years, Low Earth Orbit (LEO) satellite Signals of Opportunity (SOP) positioning technology has emerged a significant alternative to the Global Navigation Satellite System (GNSS), effectively addressing navigation and positioning challenges in GNSS-denied environments. Among various LEO constellations, Iridium NEXT has garnered considerable attention due to its well-developed constellation architecture and extensive coverage. To achieve high-precision positioning using Iridium NEXT SOP, it is crucial to fully analyze both time-domain and frequency-domain observation data. Existing time-frequency joint estimation methods for Iridium NEXT signals typically employ a grid-based search strategy, which suffers from low accuracy and high computational complexity. Additionally, the high dynamic of LEO satellites introduces frequency shifts that significantly degrade bit synchronization accuracy and reduce the precision of signal estimation. To address these challenges, this paper proposes a joint high-precision frequency and code phase estimation algorithm. By incorporating a phase adjustment strategy during the bit synchronization stage, it mitigates the impact of satellite dynamics on signal estimation, enhances frequency estimation accuracy, and provides a precise synchronization sequences for code phase estimation. Additionally, leveraging the Early Minus Late Amplitude (EMLA) principle, the algorithm achieves efficient and accurate code phase estimation. It further integrates Frequency of Arrival (FOA) measurements for positioning, ensuring the effective utilization of SOPs to improve positioning accuracy. Experimental signal analysis demonstrates that the proposed algorithm achieves a frequency measurement error of approximately 1Hz, approaching the Cramér-Rao Lower Bound (CRLB). For both long-frame and short-frame signals, the accuracy is 25 m and 37 m, respectively, outperforming existing Time of Arrival (TOA) estimation methods for Iridium NEXT signals. Furthermore, in a 176m baseline differential scenario, Time Difference of Arrival (TDOA) and Frequency Difference of Arrival (FDOA) fusion positioning accuracy reaches 10m, compared to existing frequency estimation methods, the proposed approach improves FDOA-based positioning accuracy 74%.