A Hybrid Timing System for Small Satellite Missions

Small-satellite missions in low-Earth, geostationary, and cislunar orbits increasingly demand low-SWaP (size, weight, and power) positioning, navigation, and timing (PNT) solutions that remain functional despite limited ground contact and intermittent GNSS coverage. Continuous two-way steering from Earth is often constrained by Deep Space Network scheduling, communication geometry, and limited access windows, motivating the development of autonomous onboard timing architectures. Chip-Scale Atomic Clocks (CSACs) offer milliwatt-level power consumption and compact volume, enabling spacecraft to maintain a local time-base, but their frequency drifts accumulate without external steering. To address this limitation, we propose a hybrid timing architecture that fuses a CSAC with a Pulsar Ensemble Timescale (PET) using a Vondrak– ´ Cepek filter (VCF) to optimally blend ? the short-term stability of the CSAC with the long-term stability of pulsars. We use the International Pulsar Timing Array second data release (IPTA DR2) to construct PET using a classical weighted average algorithm across three millisecond pulsars. The resulting hybrid timescale, CSAC-Pulsar Ensemble Timescale (CPET) exhibits the intended complementarity: the CSAC dominates at short averaging times, while pulsar disciplining improves stability at longer intervals. The ?z(? ) analysis shows a transition around a week, beyond which the hybrid provides an improvement approaching an order of magnitude at long-term.