Double-Difference Iridium/GNSS Hybrid Positioning: Signal Processing and Experimental Evaluation

Abstract

Global navigation satellite systems (GNSSs) suffer from signal blockage and multipath effects in dense urban environments, leading to degraded positioning performance. Low Earth orbit (LEO) satellites provide stronger received signals and larger Doppler variations, making them promising complementary navigation sources. Among existing LEO constellations, Iridium-NEXT is attractive because of its global coverage and well-defined burst structure. However, Iridium signals lack explicit timing information and are affected by large pseudorange noise and orbital errors, while most existing methods rely on accurate local time or external clock synchronization. This article proposes a double-difference (DD) Iridium/GNSS hybrid positioning framework that does not require Iridium message decoding or precise clock synchronization. A GNSS-aided emission time difference (ETD) method is used to align the same Iridium burst at two receivers. Doppler matching at the base station is then used to recover satellite states and signal emission times from Two-Line Element (TLE)/SGP4 data. Sliding-window outlier rejection and multiepoch smoothing are further introduced to suppress ranging noise. Experiments show that single-epoch Iridium fusion does not degrade accuracy under weighted least squares (WLSs), while multiepoch smoothing progressively improves the solution as Global Positioning System (GPS) geometry weakens: with four GPS satellites, vertical root-mean-square error (RMSE) improves by 33 % with improved stability; with three GPS, horizontal RMSE improves by 70 % with substantially enhanced stability; and with a single GPS, the fusion still achieves a coarse fix of 143 m (4-min smoothing). These results demonstrate the potential of Iridium burst signals as complementary ranging sources, improving both accuracy and stability in GNSS-challenged environments, particularly for static or quasi-static applications, such as initial positioning in urban surveying, emergency rescue operations, and environments with potential GNSS spoofing.