In order to get rid of the dependence of the navigation and positioning system on the global navigation satellite system (GNSS), radio, television, satellite, and other signals of opportunity (SOPs) can be used to achieve receiver positioning. The space-based SOPs based on satellites offer better coverage and availability than ground-based SOPs. Based on the related research of Iridium SOPs positioning in the open environment, this paper mainly focuses on the occluded environment and studies the Iridium SOPs positioning technique in weak signal environment. A new quadratic square accumulating instantaneous Doppler estimation algorithm (QSA-IDE) is proposed after analysing the orbit and signal characteristics of the Iridium satellite. The new method can improve the ability of the Iridium weak signal Doppler estimation. The theoretical analysis and positioning results based on real signal data show that the positioning based on Iridium SOPs can be realized in a weak signal environment. The research broadens the applicable environment of the Iridium SOPs positioning, thereby improving the availability and continuity of its positioning.

The global navigation satellite system (GNSS) system has a number of disadvantages such as weak signal strength and high cost, while signals of opportunity (SOPs) can make up for these shortcomings. Non-GNSS satellite SOPs offer some advantages in terms of high signal strength, low cost that belongs to ground-based SOPs and better coverage than ground-based SOPs. We investigate a new method of instantaneous Doppler positioning with Iridium signals that are treated as satellite SOPs. Initially, the Iridium SOPs positioning system is introduced through a description of its characteristics and components. Next, the instantaneous Doppler positioning algorithm is introduced, and the effects of measurement errors and satellite orbital errors on the positioning solution are analyzed by a new method. Furthermore, the influence of constellation geometrical distribution on positioning performance is analyzed based on the relative position relationship between discrete distribution points of the receiver position. Finally, the simulation data are used for theoretical verification, and the real data experiment is implemented with IRIDIUM NEXT signals. The results show that the two-dimensional (2-D) position accuracies of approximately 22 m (1σ) can be achieved with the height aiding when the static receiver has a complete view of the sky. INDEX TERMS GNSS, signals of opportunity, non-GNSS satellite, Iridium, instantaneous Doppler positioning, Doppler geometric dilution of precision.