Ground-to-Space Time Transfer for Lunar Navigation: Comparing Synchronous and Asynchronous Methods in ESA’s Moonlight System

Measuring time desynchronization between distant clocks requires comparing their readings at the same coordinate time, a procedure that requires relativistic transformations between proper time and coordinate time. Here we adopt a simplified formulation in which these effects are treated as deterministic corrections to be applied operationally, so that only the finite speed of light is considered explicitly. Within this framework, we compare two methods for ground-to-space time transfer. The first is an asynchronous technique analogous to the well-established two-way satellite time and frequency transfer (TWSTFT) method used for terrestrial clock comparisons and UTC generation. The second is a novel, synchronous approach that combines time transfer with ranging measurements and orbit determination, without interrupting the two-way coherent link used for navigation and TT&C, and thus enabling a much higher measurement cadence. This is especially important for ESA’s Moonlight radio navigation system, which relies on Doppler and range measurements from ground for the orbit determination and time synchronization (ODTS) functions. We focus on the synchronous method and show that the achievable time-transfer accuracy is essentially identical to that of the asynchronous approach.