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Session F3c: Beyond GNSS: Emerging Trends in LEO-Based and Terrestrial Signals of Opportunity for PNT

Carrier Phase Properties of the Starlink Downlink Signal
Wenkai Qin, Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin; Zacharias M. Komodromos, Electrical and Computer Engineering, The University of Texas at Austin; Todd E. Humphreys, Aerospace Engineering and Engineering Mechanics, The University of Texas at Austin
Alternate Number 2

Researchers interested in augmentation methods for global navigation satellite systems have shown interest in the development of methods for position, navigation, and timing (PNT) via opportunistic exploitation of low Earth orbit (LEO) telecommunications signals. However, despite the typical importance of signal carrier phase in such estimation algorithms, little is known about the carrier phase transmitted by the most commonly available signals of opportunity. With a focus on the SpaceX Starlink constellation, this paper builds on existing knowledge regarding the Starlink signal structure and timing characteristics to develop methods for investigating the Starlink Ku-band downlink’s phase carrier properties. It contributes key findings regarding the behavior of: (1) intra- and inter-frame carrier phase coherence,
(ii) frame and carrier clock decoupling, and (iii) mid- and inter-channel tone stability and persistence.
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INTRODUCTION
Advancements in satellite and orbital launch vehicle technology have supported a major maturation of the low Earth orbit (LEO) satellite ecosystem in the past decade, with technology companies such as Amazon and SpaceX investing billions of dollars to launch thousands of LEO communications satellites in the name of global connectivity. Since 2010, the number of active satellites has grown from 1,000 to 7,000 in 2023, a number forecasted to increase dramatically over the next decade. Although these megaconstellations already are already connecting millions of users, they are also poised to revolutionize global positioning, navigation, and timing (PNT).
The use of LEO megaconstellations for PNT offers several unique benefits. Compared to traditional global navigation satellite systems (GNSS), LEO PNT solutions offer mitigation against both situational unavailability (in deep urban canyons, under dense foliage) and malicious attacks (jamming, anti-satellite warefare) [1]–[4]. However, as no LEO PNT service is publicly available yet, researchers have instead exploited LEO telecommunications signals as signals of opportunity. Not only are these signals readily available in many areas, but they are cost-free to observe and record. Currently, several papers have already proposed and demonstrated opportunistic LEO PNT solutions using pseudorange-based, Doppler-based, and carrier-phase-based estimators [5]–[7].
To unlock the potential benefits of opportunistic LEO PNT, researchers must overcome several unique obstacles. The veiled nature of the received signals warrants particular attention. Unlike traditional GNSS satellites that use published sequences to aid signal acquisition and tracking, a signal of opportunity’s structure is typically proprietary – known only to the companies that designed them. This complicates the usual process of extracting PNT information from such signals, which fundamentally relies on correlation of a received signal against a local replica.
Researchers are addressing the veiled signal structure issue using two constrasting approaches. The first approach accepts the limited a priori knowledge regarding the signal of opportunity’s structure, leveraging basic signal aspects such as band occupancy and signal modulation scheme. This method is quickly adaptable and readily applicable to signals originating from a wide variety of sources, ranging from competing telecommunications providers to planetary observation satellites [7]–[9]. The second approach more comprehensively explores the signals of opportunity transmitted by a single source constellation, identifying not only major design principles of said signal but also the minutia of its implementation methods and transmission patterns. While this method is relatively costly to research and develop, key insights afforded by such close examination are expected to enable greater fundamental understanding. Consequently, particular strengths of the received
signal behavior may be exploited for PNT while latent weaknesses may be mitigated, ultimately resulting in a precise, efficient receiver implementation.
The authors of this paper take the latter approach with a focus on Starlink, the megaconstellation that enjoys the most mature deployment of LEO broadband
communications networks to date. Starlink signals are widely available for observation, particularly in urban areas with high communications traffic, making the Starlink megaconstellation a desirable target for opportunistic LEO PNT. This paper builds upon recent work that revealed much of the timing behavior and internal structure of the Starlink downlink, including its frame timing properties, OFDM nature, band occupancy, and known sequences [10], [11]. It focuses on methods to identify details of the Starlink signal structure related the signal carrier phase, a core observable relied upon by the majority of radionavigation methods.
Carrier phase characteristics of any transmitted signal are generally impacted by the quality of clocks used during signal generation. As the clocks present on LEO megaconstellation satellite vehicles (SVs) are typically relatively economical and unpredictable when compared to their traditional GNSS counterparts, the carrier phase of signals transmitted by LEO megaconstellation SVs can become erratic as well. The resulting inconsistent carrier phase fundamentally degrades the precision of any navigation solution that uses Doppler or carrier phase as an observable. To build effective navigation algorithms and properly evaluate the quality of the provided
navigation solution, typical carrier phase behaviors and inconsistencies must be well understood.
This paper uses recently-published findings regarding the Starlink signal structure and timing properties to provide the first detailed examination of the Starlink carrier phase behavior. It presents findings regarding the Starlink phase coherence in four portions: First, it investigates and characterizes both the intra- and inter-frame carrier phase coherence, or lack thereof. Second, it describes discoveries made regarding episodic decoupling of the frame and carrier clocks. Third, it presents findings about the availability, persistence, and stability of mid- and inter-channel tones, features that existing opportunistic PNT methods already rely upon for signal
acquisition and tracking [6]. The paper concludes by predicting the impact these errors could have on the precision afforded by a opportunistic Doppler-based PNT solution using Starlink signals and offers mitigation recommendations.
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REFERENCES
[1] P. A. Iannucci and T. E. Humphreys, “Economical fused LEO GNSS,” in Proceedings of the IEEE/ION PLANS Meeting, 2020.
[2] R. T. Ioannides, T. Pany, and G. Gibbons, “Known vulnerabilities of global navigation satellite systems, status, and potential mitigation techniques,”
Proceedings of the IEEE, vol. 104, no. 6, pp. 1174–1194, 2016.
[3] T. E. Humphreys, “Interference,” in Springer Handbook of Global Navigation Satellite Systems. Springer International Publishing, 2017, pp. 469–503.
[4] E. C. Dolman, “New frontiers, old realities,” Strategic Studies Quarterly, vol. 6, no. 1, pp. 78–96, 2012.
[5] M.L.Psiaki,“Navigation using carrier Doppler shift from a LEO constellation: TRANSIT on steroids, ”Navigation, Journal of the Institute of Navigation, vol. 68, no. 3, pp. 621–641, 2021.
[6] C. Yang and A. Soloviev, “Starlink Doppler and Doppler rate estimation via coherent combining of multiple tones for opportunistic positioning,” in 2023 IEEE/ION Position, Location and Navigation Symposium (PLANS). IEEE, 2023, pp. 1143–1153.
[7] S. E. Kozhaya and Z. M. Kassas, “Positioning with Starlink LEO satellites: A blind Doppler spectral approach,” in 2023 IEEE 97th Vehicular Technology Conference (VTC2023-Spring), 2023, pp. 1–5.
[8] S. Kozhaya, H. Kanj, and Z. M. Kassas, “Blind Doppler tracking and positioning with NOAA LEO satellite signals,” in Proceedings of the 36th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2023), 2023, pp. 363–372.
[9] S. Kozhaya, H. Kanj, and Z. M. Kassas,“Multi-constellation blind beacon estimation, Doppler tracking, and opportunistic positioning with OneWeb, Starlink, Iridium NEXT, and Orbcomm LEO satellites,” in 2023 IEEE/ION Position, Location and Navigation Symposium (PLANS), 2023, pp. 1184–1195.
[10] T. E. Humphreys, P. A. Iannucci, Z. M. Komodromos, and A. M. Graff, “Signal structure of the Starlink Ku-band downlink,” IEEE Transactions on Aerospace and Electronic Systems, pp. 1–16, 2023.
[11] W. Qin, Z. M. Komodromos, A. M. Graff, and T. E. Humphreys, “Timing properties of the Starlink Ku-band downlink, ”IEEE Transactions on Aerospace and Electronic Systems, 2024, in preparation.

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