In-orbit demonstration of optical links for highly accurate time synchronization and ranging: the OpSTAR mission

Abstract

The Optical Synchronized Time and Ranging (OpSTAR) mission, part of the European Space Agency’s “FutureNAV” activities, aims at in-orbit validation and verification of optical links integrating highly accurate synchronization and ranging functions. This key optical technology is an enabler for next-generation Global Satellite Navigation Systems (GNSSs), and their evolution as part of global multi-layer Position, Navigation and Timing (PNT) “system-of-systems”. The cornerstone of the OpSTAR mission is the in-orbit testing of optical inter-satellite and satellite-to-ground links to autonomously synchronize remote clocks, obtain accurate ranging measurements to enhance satellite orbit determination, and exchange measurement and mission data with low latency and high throughput across all mission elements. To achieve this objective, two satellites, equipped with multiple optical terminals, are supported by several Optical Ground Stations (OGSs) providing an optical interface between space and ground segments. In order to ensure compatibility across all optical terminals realizing the links, open specifications are produced to ensure interoperability, transparency, and long-term compatibility. A subset of OGSs interfaces directly with a purposefully-built system synchronization testbed supporting validation of novel approaches to system synchronization enabled by optical observables. Specifically, a distributed space and ground synchronization system is in focus, in which all satellites and ground elements contribute to the definition of a common system time, to which all assets are kept synchronized with unprecedented accuracy. A system simulator, built drawing from the capabilities under verification in the OpSTAR mission, is used to extrapolate system and end-user performance for GNSSs integrating optical links in their system and processing architectures. An OpSTAR-dedicated (optical) ground PNT test range addresses system exploitation by implementing a number of actual end-user cases, focusing on L-band, optical and hybrid (joint L-band and optical) users. The PNT test range enables the validation of diverse use cases under realistic conditions, assessing the potential of hybrid (optical-radiofrequency) signal processing, and system performance from the user perspective. The OpSTAR mission is operationally supported by a ground control segment overseeing the operations of both spacecraft, from launch to decommissioning, as well as the operational interface between space and ground segments via traditional radiofrequency links, handling part of the mission data flow (mission commands, telemetry, download and dissemination of mission-related data). A dedicated orbit and time reference system provides the necessary capability to accurately estimate and predict both spacecraft orbits and onboard clocks, in order to support the execution of real-time operations (scheduling, pointing for establishing optical links, generation and distribution of a geodetic reference and a system time, etc.) and all verification activities (e.g. ranging and time-synchronization verifications). The OpSTAR mission overarching objective is to prove optical technologies for PNT system operators and end-users, quantifying the overall benefits offered. To this purpose, the OpSTAR mission is a testbed for validating architectures integrating optical links in next-generation GNSSs for more accurate, autonomous, robust and resilient global position, navigation and timing services. This work presents the mission overall OpSTAR mission architecture from a technical standpoint, focusing on experimental elements and their coordinated operations for achieving the mission objectives.