Phase-Time Method: Accurate Doppler Measurement for Iridium NEXT Signals

Opportunistic positioning utilizing low Earth orbit satellite (LEO) signals mostly adopts Doppler positioning, the performance of which largely depends on the accuracy of Doppler measurement at the receiver. Traditional Doppler measurement methods for Iridium NEXT signals are limited and mostly implemented in frequency domain, which cannot avoid the limitation of fast Fourier transform operation. Aiming at this problem, this article proposes a method achieving accurate Doppler measurement of Iridium NEXT signal in time domain, namely phase-time method. This method achieves accurate Doppler measurement by measuring the change rate of the signal phase over time. Experiments were implemented using real Iridium NEXT signals, and the results have demonstrated that compared with the existing method, the Doppler measurement value obtained by the phase-time method possesses higher accuracy. Taking the Doppler measurement values obtained by the phase-time method as observations, the stability and reliability of Doppler positioning can be significantly improved. The proposed phase-time method is of great significance to LEO-signal frequency estimation in time domain, and further contributes to opportunistic positioning using LEO constellations.