The interest in Low Earth Orbit (LEO) mega constellation systems providing either high data-rate communications (high-speed internet) or Position Navigation and Timming (PNT) services, which is commonly referred to as LEO PNT, has grown in the last years with a significant acceleration of the research and development of such systems [1]. To provide some examples, China is planning on deploying 3 LEO mega constellation systems; Geospace, CentiSpace and Satnet [2]. US is planning on deploying several LEO mega constellation systems, Blackjack, XONA, TrustPoint, Satelles and Starlink [2]. Finally, Europe is preparing two projects, LEO PNT (following ELCANO project [1]) completely focused on PNT services and IRIS2, funded by the EC and is ddesigned to tackle a wide variety of services [2].

The advantages to use LEO mega constellation for PNT services are quite numerous and the driving force of the constant interest growth in these systems [1]. First of all, the increase of satellites in view, even in constrained environments such as urban canyons, increase the DOP of the navigation solution with the consequence increase on the final accuracy performance. Moreover, the fast change in the LEO satellites position makes the propagation channel to vary very fast and, for example, to quicly pass from obstructed Line-Of-Sight (LOS) to unobstructed LOS signals in comparison with MEO GNSS signals. Second, LEO satellites being closer to Earth have low Free Space Losses (FSL) in comparison to MEO GNSS satellites; for example a LEO satellite have about 20dB less of FSL at the 600km (Edge-of-Coverage). Third, the improved link budget can allow for the use of non-typical RNSS frequency bands to provide more specifically targeted PNT services, such as Ka/Ku frequency bands for high precision PNT services due to the large available bandwidth and such as UHF frequency band to obtain deep signal penetration for indoor, canopy or urban canyons positioning. Fourth, the high dynamics of the LEO satellites increase the importance of Doppler frequency/speed measurements and thus the overall quality of the PVT solution; moreover, the high dynamics of the satellite will also change the correlation time of the signal distortions, such as the ionosphere, and will allow reduced convergence time for high-accuracy GNSS positioning. Fifth, the high number of LEO satellites expected in a mega constellation and the proximity of the satellites allow for two-way PNT signals enhancing their timing and ranging capabilities.

The goal of this thesis thus is to conduct the analysis of LEO mega constellations focusing on the Radio-Frequency Front End (RFFE) block and mainly the signal processing block of a receiver. The proposed thesis will focus on the LEO PNT being currently developed by ESA:

First a selection of 2 or 3 signal candidates wil be made depending on the targeted applications. For example, one typical band (e.g. L band) and one experimental band (Ka band) could be selected.

Second, the thesis will analyze in-depth the identified previous signals with the following tasks:

a) main analysis will focus on the identification of advanced acquisition and tracking algorithms specially tailored to the LEO constellation specifications (LEO satellites high dynamics, higher clock noise jitter) as well as the ESA LEO PNT individual signals (Chirp-Spread-Spectrum (CSS) waveforms [1] or 2-way PNT signals)

b) Theoretical C/N0 thresholds, minimum C/N0 necessary to obtain the desired performance of a process, will be derived for the proposed acquisition and tracking algorithms.

c) Derivation of mathematical models for nominal and abnormal code delay and phase delay pesudorange measurements errors associated to the proposed algorithms (at least for AWGn distortions)

d) Radio Frequency compatibility between LEO PNT signals and existing MEO GNSS signals or other signals in the selected frequency band will be computed; existing RF computation methodologies will be reviewed in order to determine its applicability to LEO PNT signals [3].

e) Innovative 2-way ranging signal will be analyzed, specific signal processing methods will be proposed and theoretical thresholds will be derived.

Practical analysis to validate theoretical developed results will be conducted through simulated IQ samples or through the exploitation of collected IQ samples from broadcasting satellites (if available).

[1] L. Ries et al., "LEO-PNT for Augmenting Europe's Space-based PNT Capabilities," 2023 IEEE/ION Position, Location and Navigation Symposium (PLANS), Monterey, CA, USA, 2023, pp. 329-337.

[2] Sébastien Trilles, TAS "De la Terre à la Lune : les challenges des futurs systèmes de navigation", Les Rendez-vous aéro de l’innovation 2023 « Systèmes de communication et de navigation par satellites », ISAE-SUPAERO/ENAC

[3] Radio Technical Commission for Aeronautics (2022) DO 235C - Assessment of Radio Frequency Interference Relevant to the GNSS L1 Frequency Band