Clock Calibration and Aging Compensation

Purpose

Extend the useful life of the master oscillator by modeling and compensating for predictable drift (aging) and environmental dependencies (temperature, radiation dose), so external sync (L3-COM-TIME-SYNC) can be less frequent and clock holdover during outages is more accurate.

Context

Complementary to L3-COM-TIME-SYNC: while SYNC uses external references for absolute corrections, CAL maintains an internal aging model of the local clock, fitting drift coefficients from historical comparison data. Outputs continuous predictive corrections to L3-COM-TIME-USOC even when external sync unavailable.

Principles

• ▸Quartz aging follows a logarithmic curve early in life and asymptotes to ~10⁻¹⁰/year for high-quality SC-cut crystals
• ▸Rubidium clock aging is dominated by ⁸⁷Rb lamp degradation and cell buffer gas migration — ~5×10⁻¹¹/year typical
• ▸Temperature dependence of crystal frequency follows a third-order polynomial (turnover temperature ~80°C for SC-cut)
• ▸Radiation-induced frequency shift: quartz shifts by ~10⁻¹¹ per krad; characterised in test reactors and used to predict on-orbit drift
• ▸Linear-fit aging models are simple but inadequate over decades — polynomial or exponential decay fits provide better long-term prediction
• ▸Holdover performance: ability to maintain time accuracy when external references are lost — directly determined by aging model quality

Typical implementations

• ▸Microchip TimeProvider 4100 — terrestrial PTP grandmaster with sophisticated holdover algorithms
• ▸GPS-disciplined oscillators with adaptive aging fits
• ▸NASA NTRS papers on long-duration clock characterization (DSAC, JUNO, etc.)
• ▸Telecom industry SyncE clock quality classification (G.811 PRC, G.8272 etc.)
• ▸ESA TimSat in-flight clock aging characterization studies

Lunar considerations

• ▸100-year mission span means aging models must extrapolate beyond any flight-tested calibration data
• ▸Cumulative radiation dose contributes a slow but measurable frequency shift — model parameter updated from in-situ dosimetry (L1-RAD)
• ▸Thermal environment is much more stable than Earth surface (no day/night air convection) — temperature dependence dominates only during anomalies
• ▸Replaceable clock units mean aging history transfers via cold start — calibration state must be preserved across hardware swaps
• ▸Long-term clock comparison logs from previous units inform aging model priors for newly installed clocks

Specifications

Interfaces

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