Joint High-Precision Frequency and Code Phase Estimation Algorithm for Iridium NEXT Signals

In recent years, Low Earth Orbit (LEO) satellite Signals of Opportunity (SOP) position technology has emerged as a crucial alternative to the Global Navigation Satellite System (GNSS), effectively addressing navigation and positioning challenges in the event of GNSS failure. Among numerous LEO constellations, Iridium NEXT garners significant attention due to its high-quality signals and extensive coverage. However, current signal estimation algorithms overlook the frequency shifts caused by satellite dynamics, thereby impacting the accuracy of signal demodulation results and Time-of-Arrival (TOA) estimation accuracy. Simultaneously, traditional TOA estimation methods face challenges of low accuracy and high complexity. To address these, the paper proposes a joint high-precision frequency and code phase estimation algorithm. By adjusting the phase deviation of synchronous signals, the algorithm mitigates the impact of high dynamics on signal estimation, and enhance estimation accuracy. Additionally, the algorithm utilizes the Early Minus Late Amplitude (EMLA) method for efficient TOA estimation. Experimental results demonstrate that, compared to existing signal estimation approaches, the proposed algorithm significantly enhances the positioning accuracy of Frequency Difference of Arrival (FDOA) by 36.1254% and 28.5289% in 220m short baseline and 20 km long baseline scenarios, respectively. Moreover, the algorithm allows for Time Difference of Arrival (TDOA) and FDOA fusion positioning, exhibiting markedly superior accuracy compared to FDOA positioning, with a maximum improvement of 53.0922%. The results also indicate a substantial enhancement in the stability and reliability of the differential position results.