Time-Transfer from Terrestrial GPS for Distributed Lunar Surface Communication Networks

Reliable lunar communication infrastructure will be instrumental in building a sustainable human presence on the Moon. Recently, NASA has awarded a $14.1 million contract to Nokia for developing a 4G Long-Term Evolution (LTE) network on the Moon. In particular, there has been a great emphasis on designing a low-power and an ultra-compact LTE solution for the future Lunar Surface Communication Network (LSCN) due to its ease of transportation and potential for scalable deployment on the lunar surface. Ensuring a reliable LSCN design requires the base stations to be time-synchronized, and to maintain a precise globally referenced timing standard. We propose an LSCN design with distributed time-transfer from Earth-GPS to maintain precise timing across the network. We design a diffusion Kalman filter framework that utilizes the intermittently available Earth-GPS signals and the crosslink signals exchanged among the LSCN base stations to facilitate global time synchronization, while alleviating the timing stability and SWaP requirements for the on-site clocks. We validate our proposed technique by simulating the Earth-GPS satellites and multiple base stations near the Moon's Haworth Crater using the Systems Tool Kit (STK) software. We simulate the visibility and carrier-to-noise-density ratio of the Earth-GPS satellites at each LSCN base station, while examining the timing accuracy performance when each base station is equipped with a low-SWaP Chip Scale Atomic Clock (CSAC). We demonstrate the robustness of our distributed time-transfer technique as the error covariance of the transmission delay is varied.