Design of a high-stability heterogeneous clock system for small satellites in LEO

Abstract

We describe our investigation into the performance of low-power heterogeneous timing systems for small satellites, using real GPS observables from the GRACE Follow-On mission. Small satellites have become capable platforms for a wide range of commercial, scientific and defense missions, but they are still unable to meet the needs of missions that require precise timing, on the order of a few nanoseconds. Improved low-power onboard clocks would make small satellites a viable option for even more missions, enabling radio aperture interferometry, improved radio occultation measurements, high altitude GPS navigation, and GPS augmentation missions, among others. One approach for providing improved small satellite timekeeping is to combine a heterogeneous group of oscillators, each of which provides the best stability over a different time frame. A hardware architecture that uses a single-crystal oscillator, one or more Chip Scale Atomic Clocks (CSACs) and the reference time from a GPS receiver is presented. The clocks each contribute stability over a subset of timeframes, resulting in excellent overall system stability for timeframes ranging from less than a second to several days. A Kalman filter is used to estimate the long-term errors of the CSACs based on the CSAC-GPS time difference, and the improved CSAC time is used to discipline the crystal oscillator, which provides the high-stability reference clock for the small satellite. Simulations using GRACE-FO observations show time error standard deviations for the system range from 2.3 ns down to 1.3 ns for the clock system, depending on how many CSACs are used. The results provide insight into the timing performance which could be achieved on small LEO spacecraft by a low power timing system.

Position, Navigation, and Timing (PNT) Through Low Earth Orbit (LEO) Satellites: A Survey on Current Status, Challenges, and Opportunities

“…sumption of stable atomic clocks is to adapt heterogeneous clock systems and exploit the reference time from a GNSS receiver onboard a LEO satellite. In this regard, Van Buren et al [73] designed an architecture that uses a single-crystal oscillator, one or more chip scale atomic clocks (CSACs), and a spaceborne GPS receiver. This heterogeneous group of oscillators is combined in order to discipline the crystal oscillator and obtain overall system stability for timeframes ranging from less than a second to several days.…”

Simulations of Dedicated LEO-PNT Systems for Precise Point Positioning: Methodology, Parameter Analysis, and Accuracy Evaluation

“…However, our simulations are performed considering atomic clocks as precise as the GPS atomic clocks. This assumption is grounded in the fact that time synchronization in LEO satellites can be improved by using on-board GNSS receivers [74]. Hence, the clock model presented in Section B can be employed.…”