Carrier Phase Differential Positioning Using Iridium Signals: Feasibility Analysis and Preliminary Findings

This paper presents a differential carrier phase positioning framework utilizing non-cooperative Iridium satellite signals, with support for GNSS integration. Carrier phase and Doppler measurements are extracted from the tone and BPSK bursts of Iridium downlink transmissions using a phase-locked loop (PLL) tracker. A Doppler-matching approach is employed to estimate the signal transmission time and reconstruct satellite geometry based on TLE and orbital propagation, enabling differential processing without synchronized clocks. Two positioning models are formulated: a conventional GNSS-style model assuming the same carrier wavelength, and a LEO-specific model that accounts for different carrier wavelengths and non-parallel signal paths. Simulations using a Starlink-like constellation show that both models achieve comparable 3D positioning accuracy under various noise conditions, validating the generality of the LEO-specific formulation. Further simulations with increasing LEO satellite numbers confirm that GNSS/LEO fusion enhances positioning performance, reducing 3D RMSE from 2.82 meters (GNSS-only) to 1.52 meters with 15 LEO satellites. Experimental validation is conducted using real Iridium signal captured at the reference station and simulated user-side data. Carrier phase tracking is shown to be stable, and the integrated solution achieves accuracy similar to GNSS-only. These results demonstrate the feasibility and compatibility of incorporating non-cooperative LEO signals into differential positioning frameworks.