Timing Properties of the Starlink Ku-Band Downlink

TL;DR

This paper analyzes Starlink Ku-band downlink timing, revealing short-term stability suitable for PNT but also identifying timing irregularities and drift that challenge GPS-like positioning accuracy, suggesting software fixes could improve performance.

Contribution

It provides a detailed characterization of Starlink frame timing, highlighting both its potential for PNT and the software-related issues affecting timing stability.

Findings

Starlink timing shows ns-level jitter, supporting short-term stability for PNT.

Timing exhibits unpredictable large adjustments and drift exceeding 20 ppm.

Software design choices cause timing irregularities that could be mitigated with corrections.

Abstract

We develop signal capture and analysis techniques for precisely extracting and characterizing the frame timing of the Starlink constellation's Ku-band downlink transmissions. The aim of this work is to determine whether Starlink frame timing has sufficient short-term stability to support pseudorange-based opportunistic positioning, navigation, and timing (PNT). A second goal is to determine whether frame timing is disciplined to a common time scale such as GPS time. Our analysis reveals several timing characteristics not previously known that carry strong implications for PNT. On the favorable side, periods of ns-level jitter in frame arrival times across all satellite versions indicate that Starlink hardware is fundamentally capable of the short-term stability required to support GPS-like PNT. But there are several unfavorable characteristics that, if not addressed, will make GPS-like PNT impractical: (1) The v1.0 and v1.5 Starlink satellites exhibit once-per-second abrupt frame timing adjustments whose magnitude (as large as 100s of ns) and sign appear unpredictable. Similar discontinuities are also present in the v2.0-Mini frame timing, though smaller and irregularly spaced. (2) Episodic 15-s periods of high frame jitter routinely punctuate the nominal low-jitter frame arrival timing. (3) Starlink frame timing is disciplined to GPS time, but only loosely: to within a few ms by adjustments occurring every 15 s; otherwise exhibiting drift that can exceed 20 ppm. These unfavorable characteristics are essentially incompatible with accurate PNT. Fortunately, they appear to be a consequence of software design choices, not hardware limitations. Moreover, they could be compensated with third-party-provided corrections.