Precise point positioning for ground-based navigation systems without accurate time synchronization

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Despite the broad range of navigation, positioning, and timing services offered by the global navigation satellite system (GNSS), its signals are vulnerable to blockage and interference, leading to a degraded performance or even unavailability. In contrast to GNSS, ground-based navigation systems exhibit better layout flexibility and more efficient terrain adaptability in shaded areas, with their additional advantages of accurate station coordinates and the absence of ionospheric delay effects enabling precise point positioning. However, accurate inter-station time synchronization is thought to be necessary for the ground-based precise point positioning. Locata utilizes a proprietary TimeLoc technology for tight time synchronization, allowing the clock difference in the carrier phase measurement equation to be ignored. However, for other ground-based navigation systems, accurate time synchronization requires accurate calibration of equipment delays and thus significantly increases the difficulty of engineering implementation. Herein, we propose a new precise point positioning method that does not rely on accurate inter-station time synchronization. According to this method, the uncalibrated clock difference and the integer carrier phase ambiguity are combined into a new ambiguity parameter, with the ambiguity and position coordinates subsequently solved in two steps. Further, in order to overcome the limitations of the Locata on-the-fly algorithm and the conventional “known-point initialization” method, a new “dynamic key point initialization” method is proposed. Finally, a simulation test and an indoor field trial are conducted, demonstrating that precise point positioning can be achieved by sharing a single clock source without relying on accurate inter-station time synchronization. An atomic clock is not needed and an inexpensive crystal oscillator is sufficient to act as the shared clock. The results of indoor positioning tests reveal real-time centimeter-level accuracy.