RNSS Time Transfer and Synchronization Payload

Stefano Rossi*, Daniele Cretoni, Salvatore Corvo, Sung Soo Jang, Deok Won Lim

A Regional Navigation Satellite System (RNSS) provides Positioning, Navigation and Timing (PNT) Services over specific areas of the Earth surface. One of the core functionalities of a Satellite Navigation system is the Time Synchronization capability. All the elements of the system providing a local time scale require a time transfer technique in order to observe the behaviour of the local clock with respect to an external reference. The aspect of Time Synchronization is particularly important for RNSS, due to limitations in the system's geometry. The aim of the Time Synchronization Payload (TSP) presented in this paper is to support the system in achieving time synchronization between ground and space nodes. The TSP enables time synchronization between any pair of remote ground station clocks as well as between ground station clocks and satellite clocks. To achieve these objectives, the TSP performs two main functions: a transparent telecommunication functionality, required to implement the end-to-end Two Way Time Transfer (TWTT) technique for comparing pairs of remote ground station clocks and synchronize these; a transceiver functionality, necessary to implement the end-to-end TWTT technique for comparing satellite clocks to ground station clocks and synchronize these. This paper presents first a RNSS mission context and the main TSP design drivers; on the basis of these, the architecture of the TSP is described, together with the main functional and technological challenges and solutions; in particular, the TSP should be able to manage simultaneously more overlapped Code Division Multiple Access (CDMA) channels, relevant to both transparent and transceiver TWTT links, so allowing to compare and synchronize the clocks of more ground stations and satellites simultaneously , continuously, and using an optimized service bandwidth; in addition, the Time Comparison Unit (TCU), which implements the payload transceiver function, should use a digital-based architecture, offering the best solutions in terms of on-board tracking capabilities, modulation formats, and flexibility.

Keywords: RNSS, TMS, TWTT, TSP, CDMA, time-comparison, synchronization