External Time Synchronization

Purpose

Periodically reset the accumulated drift of the local time source by comparing to authoritative external time, keeping the Ark on UTC (or future Coordinated Lunar Time, LTC) within a defined tolerance over decades.

Context

Consumes timestamped messages from external sources (via L3-COM-EARTH-XBAND, L3-COM-MESH-NODE) and outputs steering corrections to L3-COM-TIME-USOC. Implements Kalman filtering of multiple references with weights based on confidence.

Principles

• ▸Two-way time transfer (TWSTFT-like) with DSN exchanges time tags in both directions; symmetric path delay cancellation gives ~1 ns accuracy
• ▸GNSS sidelobe reception on the Moon (using high-sensitivity receivers like NASA NavCube, ESA NaviMoon) provides ~100 ns time accuracy and 100 m positioning
• ▸Lunar Pathfinder demonstrates Galileo + GPS sidelobe reception at the Moon, with PVT solution validated by ESA
• ▸Coordinated Lunar Time (LTC) is a proposed lunar surface time scale, accounting for relativistic effects on the Moon (~58.7 µs/day faster than Earth UTC)
• ▸Kalman filter merges multiple time references, weighting by their predicted uncertainty (state-space estimation)
• ▸Frequency steering vs. time-jam: smooth frequency offset corrections preserve clock continuity; abrupt time jumps disrupt logs and ordering

Typical implementations

• ▸DSN two-way time transfer service (used by all NASA deep-space missions)
• ▸NavCube (NASA) — software-defined GNSS receiver flight-tested in lunar orbit
• ▸ESA NaviMoon receiver for Lunar Pathfinder — GPS+Galileo at lunar distances
• ▸GMV Magnetic-VHF / autonomous orbit determination for Moon
• ▸IAU/CCTF working group on Lunar Coordinated Time (LTC) — recommendations expected 2026–2027

Lunar considerations

• ▸DSN time transfer requires Earth visibility — unavailable during occultation at south pole site
• ▸GNSS sidelobe signal at Moon is ~30 dB weaker than at Earth surface; requires high-sensitivity acquisition (cold start may take hours)
• ▸Lunar relativistic time offset (~58.7 µs/day fast) must be applied if maintaining UTC, or accepted if migrating to LTC
• ▸Long mission means several generations of GNSS constellation upgrades — receiver must support firmware updates
• ▸Solar conjunctions periodically degrade DSN performance — backup sources critical during these periods

Specifications

Interfaces

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