Previous Abstract Return to Session A6 Next Abstract

Session A6: Signals of Opportunity (SoOp)

Unveiling Starlink for PNT
Zak M. Kassas, The Ohio State University
Location: Ballroom B
Date/Time: Tuesday, Jun. 3, 11:10 a.m.

The birth of low Earth orbit (LEO) satellite megaconstellations has resulted in tremendous interest in exploring the use of their signals for positioning, navigation, and timing (PNT). To this end, numerous studies over the past few years have explored various challenges in opportunistic PNT with LEO, from addressing space vehicle orbit, clock, and propagation errors, to receiver and signal design, to analyzing PNT performance.
With over 6,000 satellites already in LEO, SpaceX’s Starlink is currently the leading provider of LEO-based broadband communication. Indeed, SpaceX is reshaping the communication sector with its commercial Starlink and military Starshield LEO; however, as of today, little to no effort is being invested to expand the con- stellation’s capabilities to offer PNT services. Shortly after the first batch of Starlink satellites was launched, Starlink-related studies began to populate the literature, ranging from studies of the different aspects of the constellation to studies of how the constellation could be opportunistically exploited in applications other than those it was originally intended for.
The first works of note exploited the constellation for positioning purposes by tracking the carrier phase and Doppler shift of one of Starlink’s persistent signals (referred to as the “beacon”). The beacon that these papers considered was a trail of nine unmodulated, data-less, pilot tones at the center of each Ku-band user downlink channel. Later, other studies examined the structure of the uplink and downlink signals transmitted by Starlink to reveal salient information about their transmission, modulation, and multiple access schemes. These papers determined that Starlink employs orthogonal frequency division multiplexing (OFDM) for data modulation and exposed many signal parameters, from the frame duration to some decoded sequences transmitted by Starlink for synchronization purposes.
While some researchers focused on exploiting Starlink’s beacons (pilot tones and OFDM) and turning them into signals of opportunity for PNT purposes, other efforts demonstrated the use of Starlink satellites as illuminators of opportunity for passive radar applications. Other recent work has evaluated the performance of the constellation in its primary application domain, communication, by studying the physical layer transmission rates, throughput, and protocols used at the user equipment terminals.
These previous studies have only scratched the surface of using Starlink LEO satellite signals for PNT. This presentation will be present the first comprehensive study to-date showcasing the intricacies of exploiting Starlink’s signals for PNT: from the full OFDM frame, to the achievable carrier-to-noise density ratio (C/N0), to the feasibility of sustainable code, carrier, and Doppler tracking.
The presentation will answer the following questions: (1) Can the pilot tones still be tracked using low-gain antennas? (2) How reliable is the OFDM beacon published in the literature in low SNR regimes? (3) Are Starlink’s pseudorange observables useful for positioning? (4) Is there more to exploit in the Ku-band downlink OFDM signals? (5) Will the navigation observables generated using the OFDM beacon unlock more precise positioning compared to its pilot tone-based counterpart?
Although some of these questions seem to have obvious answers based on the available literature, this presentation shares some surprising results concerning the use of highly optimized communication signals for PNT. Specifically, the presentation will:
• Reveal for the first time, the full Starlink OFDM beacon, which spans the whole time-frequency resource grid. This is achieved through blind beacon estimation, which shows that the Starlink sequences published in the literature only comprise 0.66% of Starlink’s full OFDM. Exploiting the full OFDM beacon is shown to increase the receiver’s process gain by nearly 18 dB compared to only using the signals published in the literature. This process gain, in turn, unlocks a higher effective signal-to-noise ratio (SNR) at the receiver’s correlator output, enabling practical exploitation of received signals for PNT in low SNR regimes with a commercial off-the-shelf low-noise block downconverter with feed horn (LNBF).
• Provide a comprehensive theoretical and experimental description for how to exploit Starlink satellites for PNT. The maximum achievable received C/N0 is studied and compared for different scenarios, namely: (i) pilot tones versus OFDM-based beacons and (ii) low-gain versus high-gain reception setups.
• Show the first experimental results of extracting navigation observables using the OFDM signal transmitted by Starlink satellites, namely the carrier phase, Doppler shift, and code phase. Moreover, the quality of the OFDM-based observables is discussed extensively and compared against the pilot tone- based observables. Aligned with the primary application of the constellation, which is providing broadband communication, the results show that, before the year 2024, step-like corrections contaminate all the OFDM-based navigation observables, rendering their raw integration a challenge for precise positioning. After 2024, fewer OFDM-related corrections are being recorded, promoting the constellation’s ability to provide positions with meter-level accuracy.
• Show how the consistent corrections made to the OFDM carrier frequency offset (CFO) can be estimated on-the-fly, with a good degree of fidelity, within the tracking loop of the software-defined receiver.
• Showcase the first full Ku-band downlink channel capture and examine the number of simultaneously active Starlink satellites. The presentation will show that, with an average of only three active Starlink satellites, a positioning solution with meter-level 3D position error can be achieved in just 10 seconds.

Previous Abstract Return to Session A6 Next Abstract