Improved Doppler and Time-of-Arrival Estimation Algorithm Based on Iridium Signals with FPGA Implementation

In recent years, low earth orbit (LEO) satellites provide a promising solution in enhancing navigation capabilities. Among these, the Iridium satellite system stands out owing to its advantages of high signal power, and extensive coverage. This article focuses on addressing the challenges associated with time-frequency two-dimensional searching in Iridium non-cooperative signal positioning. To address these issues, an improved joint estimation algorithm is proposed, aiming to enhance the precision of Doppler frequency and time-of-arrival (TOA) estimations. First, a refined Doppler frequency estimation scheme based on the phase-time method is designed. This approach introduces a carrier phase compensation strategy to significantly reduce the impact of high-dynamic environments on Doppler estimation accuracy. Second, an enhanced TOA estimation algorithm utilizing early-late discriminator has been developed. This innovation not only reduces the computational complexity inherent in traditional TOA estimation methods but also improves estimation performance through optimized correlation coefficients. Experimental results demonstrate a notable improvement in signal processing real-time performance and reduced computational complexity. The root mean squared error (RMSE) of Doppler estimation is maintained below 1.2 Hz. Furthermore, the RMSE of TOA estimation is maintained below 120 ns. Static positioning accuracy achieves within 20 meters.