On-Orbit GNSS SV Time Management: Automated Time Projection and Control

GPS nanosecond-level time accuracy is limited by the fixed bit structure of the legacy navigation message (LNAV) defined in the IS-GPS-200 that provides the space vehicle’s (SV) clock coefficient phase, frequency, and frequency drift states. Due to the intrinsic frequency drift of the Rubidium Atomic Frequency Standards (RAFS) used in today’s GPS satellites, time error slowly accumulates over time and deviates onboard SV time from GPS time. If the SV time offset exceeds the LNAV clock coefficient upload limit, then the SV will broadcast inaccurate clock coefficients. Therefore, it is imperative to monitor SV clock offset from GPS time and perform onboard clock frequency drift adjustments to maintain the SV clock offset within the LNAV clock coefficient upload limits while providing continuous and uninterrupted SV operation. In this paper, we demonstrate an automated SV time projection and control script that utilizes the publicly available National Geospatial-Intelligence Agency (NGA) GPS Precise Ephemeris data to monitor SV clock offsets and autonomously derives optimal SV clock adjustments. First, we derive the ISGPS-200 LNAV clock coefficient upload limits to GPS time and evaluate the limits with respect to the GPS III RAFS performance. Then we apply our script to estimate and project SV clock states using the NGA data and introduce our minimization algorithm used to compute the recommended SV clock adjustments to steer SV time within the LNAV clock coefficient upload limits. Finally, we demonstrate the script using clock data of a newly powered on RAFS to illustrate how we can optimally maintain SV time with the RAFS’s beginning-of-life, nonlinear frequency drift.