Session A4a: Applications Using Communication Technologies and Collaborative Positioning

A Look at the Stars: LEO PNT with Non-cooperative Satellites
Zak (Zaher) M. Kassas, Sharbel Kozhaya, Joe Saroufim, Samer Hayek, Will Barrett, and Paul El-Kouba; The Ohio State University
Location: Holiday 2-3 (Second Floor)
Date/Time: Thursday, Sep. 11, 2:58 p.m.

LEO PNT (low Earth orbit positioning, navigation, and timing) has captivated in recent years the research community, government agencies, and private industry [1]. The concept of LEO PNT, however, is not new. In fact, Transit, the first satellite-based navigation system, was LEO-based. Nevertheless, it took nearly an hour to produce a position estimate, necessarily waiting for enough satellites to pass overhead [2]. GPS, whose space vehicles (SVs) reside in medium Earth orbit (MEO), alleviated the inherent limitation of LEO PNT, whereby one needs orders of magnitude more satellites in LEO to achieve comparable PNT performance to what could be achieved with MEO-based SVs [3]. This paved the way for MEO-based global navigation satellite systems (GNSS) to dominate satellite-based PNT. And so, LEO PNT has been “shelved.”
The past few years have heralded a new era in LEO. The number of LEO SVs orbiting Earth increased by an order of magnitude, from about 800 in 2019, to about 8,400 by the end of 2023. This is mainly attributed to the birth of so-called megaconstellations. Each of these constellations comprises hundreds to thousands of SVs launched into LEO with the purpose of providing high-speed internet connectivity virtually everywhere on Earth. At this point, SpaceX’s Starlink leads the megaconstellation “race,” with the most number of SVs in orbit (~6,000) and the most planned SVs (~12,000 with possible extension to ~42,000). Other megaconstellations include Amazon’s Kuiper (3,236), OneWeb (648), and Telesat (198). Similar to past “space races,” as the idea of LEO megaconstellations picked up steam in the late 2010’s, several players jumped on the promise of launching their own constellations, but then retracted and their promised constellations faded away. For example, Boeing initially planed a constellation of 2,956 LEO SVs, which was scaled down to 147, but eventually bowed down and relinquished its license in late 2023. Still, the LEO space race is just heating up. Despite some players dropping out, others are joining the race, which is expanding to beyond private industry to include government agencies. The European Union approved in early 2024 the Infrastructure for Resilience, Interconnectivity and Security by Satellite (IRIS2) LEO megaconstellation, comprising 450 SVs, while China is planning its “own version of Starlink,” comprising about 12,000 LEO SVs.
Signals from LEO SVs offer attractive PNT attributes, compared to MEO SVs. First, LEO SVs transmit their signals in a wide swath of the frequency spectrum, making them more resilient to interference than MEO signals. Second, with a significantly higher relative velocity than MEO SVs, LEO SVs offer more informative Doppler measurements, leading to increased positioning accuracy. Third, LEO SVs are around twenty-times closer to Earth compared to GNSS SVs, which results in less spreading loss, improving the carrier-to-noise ratio by about 30 dB. Fourth, the sheer number of LEO SVs that will cover the Earth in diverse orbits offers favorable position dilution of precision (PDOP), particularly in “harsh GNSS environments,” in which there may not be clear line-of-sight from the receiver to the SVs [4].
LEO PNT concepts can be classified into four categories:
1. LEO-augmented GNSS: signals from LEO are fused with MEO GNSS signals for improved PNT [5]
2. Dual-purposed LEO: the hardware already designed and the spectrum already allocated for the SVs’ primary mission is dual-purposed for PNT [6]
3. PNT-dedicated LEO: the LEO constellation is launched for the sole purpose of providing PNT services [7]
4. Opportunistic LEO PNT: signals from whichever LEO constellation, whether PNT-dedicated or otherwise, are exploited for PNT [8]
The opportunistic LEO PNT approach offers the most space and spectrum sustainability compared to the first three approaches, alleviating space congestion, generation of space debris, and allocation of scarce spectrum. Nevertheless, one needs to address three challenges associated with opportunistic LEO PNT with non-cooperative SVs, namely the unknown nature of the LEO SVs’: (i) downlink signal, (ii) clock error and synchronization schemes, and (iii) ephemerides.
This presentation will overview advances in addressing the aforementioned challenges. Specifically, the presentation will show:
- A cognitive opportunistic navigation LEO receiver, capable of producing navigation observables (pseudorange, Doppler, and carrier phase) from unknown LEO SVs, regardless of the modulation and multiple access scheme adopted by the LEO provider. The efficacy of this receiver is demonstrated by blindly acquiring and tracking several uncooperative LEO constellations: Starlink, OneWeb, Orbcomm, Iridium, and National Oceanic and Atmospheric Administration (NOAA), without assuming prior knowledge about the signal’s structure.
- An approach to disambiguate ephemeris and timing errors of LEO SVs. The approach utilizes a model parametrizing the ephemeris and timing errors by the three-dimensional (3-D) ephemeris error magnitude and its direction angle from the in-track axis, which can be estimated by a reference receiver. The two parameters can be communicated to any unknown receiver (located hundreds of kilometers away) listening to the same LEO SVs to correct for ephemerides ranging error, leading to improved PNT. Two sets of experimental results are presented where the ephemeris correction method was applied to carrier phase measurements: (i) ephemeris parameters were estimated at a reference receiver from 3 Starlink and 7 OneWeb LEO SVs, and used to localize the same receiver, achieving a horizontal final position error of 0.85 m, compared to 7 km utilizing TLE+SGP4 ephemerides; and (ii) the two parameters of 7 Starlink SVs were estimated and communicated from St. Louis, Missouri, to an unknown receiver in Columbus, Ohio, over a baseline distance of 635 km, achieving a horizontal final position error of 8.78 m, compared to 2.41 km utilizing TLE+SGP4 ephemerides.
- A framework enabling warm start navigation with uncooperative LEO satellites whose ephemerides are poorly known. In this framework, a standalone terrestrial receiver with poor knowledge about its states makes pseudorange and/or Doppler measurements to overhead LEO satellites, simultaneously estimating via a marginalized particle filter (MPF) its own states and the LEO satellites' orbit and clock errors, in a completely GNSS-denied fashion. Experimental results of stationary receiver positioning are presented, demonstrating the efficacy of the MPF with Doppler measurements extracted from 6 Starlink and 7 OneWeb LEO satellites, where the receiver's position is estimated along with the orbit and clock errors that are inherited from publicly available two-line element (TLE) files. Starting from an initial estimate 1.7 km away, it is shown that using erroneous TLE ephemerides yields a positioning error of 1.37 km, while the proposed online ephemeris estimation with MPF yields an error of 57.6 m. Finally, the LEO ephemeris error correction strategy is shown experimentally via a LEO-aided inertial navigation system (INS), yielding a ground vehicle position root-mean squared error (RMSE) of 21 m over a 1.5 km trajectory.
- Navigation results with the Starlink and OneWeb constellations on a high-altitude balloon, ascending to an altitude exceeding 80,000 ft above ground level.

References
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[2] T. Stansell, “Transit, the navy navigation satellite system,” NAVIGATION, Journal of the Institute of Navigation, vol. 18, no. 1, pp. 93–109, 1971.
[3] T. Reid, T. Walter, P. Enge, D. Lawrence, H. Cobb, G. Gutt, M. O’Conner, and D. Whelan, “Position, navigation, and timing technologies in the 21st century,” vol. 2, ch. 43: Navigation from low Earth orbit – Part 1: concept, current capability, and future promise, pp. 1359– 1379, Wiley-IEEE, 2021.
[4] R. Morales and E. Lohan, “Comparison of MEO, LEO, and terrestrial IoT configurations in terms of GDOP and achievable positioning accuracies,” IEEE Journal of Radio Frequency Identification, vol. 5, no. 3, pp. 287–299, 2021.
[5] A. Nardin, F. Dovis, and J. Fraire, “Empowering the tracking performance of LEO-based positioning by means of meta-signals,” IEEE Journal of Radio Frequency Identification, vol. 5, no. 3, pp. 244–253, 2021.
[6] Y. Liao, S. Li, X. Hong, J. Shi, and L. Cheng, “Integration of communication and navigation technologies toward LEO-enabled 6G networks: A survey,” Space: Science & Technology, vol. 3, pp. 1–19, October 2023.
[7] T. Reid, B. Chan, A. Goel, K. Gunning, B. Manning, J. Martin, A. Neish, A. Perkins, and P. Tarantino, “Satellite navigation for the age of autonomy,” in Proceedings of IEEE/ION Position, Location and Navigation Symposium, pp. 342–352, 2020.
[8] Z. Kassas, J. Morales, and J. Khalife, “New-age satellite-based navigation – STAN: simultaneous tracking and navigation with LEO satellite signals,” Inside GNSS Magazine, vol. 14, no. 4, pp. 56–65, 2019.