Starlink Doppler and Doppler Rate Estimation via Coherent Combining of Multiple Tones for Opportunistic Positioning

Large low earth orbit (LEO) systems or mega constellations exemplified by Starlink and OneWeb hold the promise for LEO-based positioning, navigation, and timing (PNT). Each Starlink signal has characteristic 9 tones within the center 1 MHz of each downlink channel, differentiating it from OneWeb that also broadcasts in the same Ku band. From the point of view of using Starlink signals for opportunistic PNT, these distinct tones are the simplest waveforms to receive despite the fact that the tones do not carry a satellite ID, so it is ambiguity as to from which satellite the received signal originates. Furthermore, simple tracking of one tone or two, albeit the strongest, is also ambiguous regarding the carrier center frequency per downlink channel for Doppler estimation. In this paper, we set forth a general baseband signal processing scheme with two steps for Doppler and Doppler rate estimation without any prior information about a receiver's location and time and the appearance (disappearance) of a Starlink signal. The first step performs satellite detection, coarse frequency estimation, and tone grouping per satellite while the second step performs refined frequency and frequency rate estimation and tracking. Specifically, the second step consists of three operations, namely, channelized downconversion, chirp (frequency rate) estimation and dechirping, and refined frequency estimation. In particular, the frequency rate estimation is based on a novel idea of coherent combining of all detected tones. The processing steps are illustrated in the paper with actual Starlink signals captured in the 11,325 MHz and 11,950 MHz channels, respectively. The rationale for designing signal processing parameters and tone grouping criteria are discussed. The ability of the proposed method to recover all 9 tones per satellite and unambiguously estimate Doppler (the middle tone off the channel center frequency) and Doppler rate is shown for a case with a single Starlink satellite and a case with two Starlink satellites that are closely-spaced in frequency, respectively.