An Analysis of the Short-Term Time Stability of the Starlink Ku-Band Downlink Frame Clock

We develop a signal capture and analysis technique for extracting precise timing information from the Starlink communications megaconstellation’s downlink transmissions. Several characterizations of the Starlink frame clock adjustment pattern and stability are presented. The frame clock is adjusted at a regular 1 Hz cadence, and the adjustments are nearly discontinuous in nature. A composite clock Allan deviation analysis indicates that the Starlink frame clock has best-case stability charactersitic of a temperature-controlled crystal oscillator. A further high-frequency clock instability analysis is conducted via polynomial trend removal, and indicates that the Starlink frame clock could hypothetically support a global position, navigiation, and timing (PNT) mission when performing nominally, but manifests episodic oscillatory and excursive behavior that would severely degrade opportunistic positioning and timing based on pseudoranges formed from the frame clock. Examples of such oscillatory and excursive patterns are shown and other aspects of the phenomena are discussed.