Stability Assessment of Spire and PlanetiQ Receiver Clocks and Its Implications for GNSS-RO Atmospheric Profiles

Abstract

Commercial SmallSats offer cost-effective alternatives to traditional GNSS radio occultation (GNSS-RO) missions through scalable constellation deployments. In GNSS-RO processing, the short-term stability of the LEO receiver clock is a key constraint on the feasibility and accuracy of undifferenced (UD) retrievals. Specifically, limited stability in compact receivers can introduce clock noise that degrades retrieval profiles and increases uncertainty. This study evaluated Spire and PlanetiQ onboard clock stability and quantified the impact on bending-angle and refractivity retrievals. Spire exhibited lower short-term clock stability, with 1-s clock stability exceeding $10^{-6}$, making UD infeasible, while its single-differenced (SD)-derived profiles remained consistent with UCAR and ECMWF reference datasets. In contrast, PlanetiQ exhibited better short-term stability, with the 1-s clock stability typically better than $10^{-9}$. Subsequent analyses were confined to clock segments with 1-s clock stability better than $10^{-12}$, sufficient for accurate SD and UD processing. For PlanetiQ, refractivity derived from both methods was in agreement between 10 and 25 km (mean bias $< 0.05\%$, STD $< 1\%$); above 25 km, SD showed slightly larger deviations due to reference-link noise. Across constellations, GPS showed the lowest deviations while GLONASS had the highest. Sensitivity tests with injected clock noise targeting 1-s clock stability over the range of $10^{-12}$ to $10^{-10}$ showed that UD and SD were statistically comparable when 1-s clock stability was about $\text{3.0}\times \text{10}^{-11}$, and the retrieval deviations increased with both altitude and noise amplitude. These results confirm that PlanetiQ’s high clock stability supports accurate SD and UD retrievals and provide valuable insights for oscillator selection, quality control, and processing strategy in cost-effective GNSS-RO missions.