Range estimation method of LEO communication opportunity signals for alternative PNT

Positioning, Navigation, and Timing (PNT) services have delivered significant benefits to modern society. Currently, much of our PNT needs are fulfilled by Global Navigation Satellite Systems (GNSS). However, GNSS is facing a series of challenges, including signal blockage in dense urban areas, bush land and indoors and susceptivity to radio frequency interference. There is a urgent global demand for backup solutions to address the reliability of GNSS. The substantial expansion and massive deployment of small satellites have catalyzed the development of Low Earth Orbit (LEO) communication constellations have been rapidly developed, such as SpaceX’s Starlink and China Satellite Network communication system. In the near future, these constellations are projected to comprise tens of thousands of satellites. These satellites present significant advantages over GNSS, such as operating at higher frequencies (over 10 GHz) and providing wider signal bandwidths (several hundred MHz), along with superior signal quality. Such attributes make them viable candidates for PNT solutions and open up opportunities to utilize their signals as alternative sources for PNT applications.

Due to unknown structure of commercial LEO communication signals, most positioning methods based on LEO communication satellites rely on Doppler measurements. However, Doppler-based positioning is challenging for high-dynamic objects and high-precision positioning, as it requires integrating over significant periods for range difference measurements and imposing height constraints. Previous experience suggests that positioning with ranging measurements significantly outperforms Doppler-based methods in terms of accuracy and applicability. Unlike GNSS, which has publicly available signal structures for obtaining range, LEO communication signal structures are not disclosed. Therefore, this study aims to develop algorithms for estimating ranging measurements derived from LEO communication signals with unknown signal structures.

The downlink signals of LEO communication systems (i.e. Starlink) are simulated. By employing the Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS) sequences as ranging sequences, we facilitate the coarse acquisition of communication satellites. Then, a Delay Lock Loop (DLL) is developed to track the PSS and SSS sequences to continuously estimate signal delays. Additionally, we exploit the benefits of Orthogonal Frequency-Division Multiplexing (OFDM) modulation in LEO communication signals by designing the multiple Phase Lock Loops (PLLs) to track various subcarriers. By applying the phase difference across different frequencies, we can construct artificial wavelengths at the meter level, akin to wide-lane combinations in GNSS. This approach can reduce the ambiguity in the integer number of wavelengths between the satellite and the receiver, which is a notable challenge in carrier measurements of high-frequency LEO communication signals. This study introduces two ranging schemes: one based on time delay estimation via synchronization sequences and the other on carrier phase tracking using multiple PLL. When combined with two-line element (TLE) files, these schemes enable a positioning service based on LEO communication satellites.