OPTICAL SYNCHRONIZED TIME AND RANGING: IN-ORBIT DEMONSTRATOR FOR NEXT-GENERATION PNT

Kurzfassung

The Optical Synchronized Time and Ranging (OpSTAR) mission is an ESA In-Orbit Demonstrator (IOD) aiming to validate the application of optical ranging, time synchronization and data relay in support of next-generation navigation satellite systems. As global demands for more accurate, secure and resilient positioning, navigation and timing (PNT) services grow, optical inter-satellite and ground-to-space links offer a promising leap in overall PNT performance and robustness at system and user level.

The use of optical technologies has the potential to become a game changer for PNT, in the same way it has been for satellite communications. Besides the inherent properties to increase resilience and robustness, optical technologies may allow system time synchronization with an improvement of at least one order of magnitude with respect to current GNSS capabilities, while enabling inter-satellite ranging in support of more accurate satellite orbit determination. The integration of optical technologies in GNSS will bring upon changes on the system architecture as we know it; this is why such concept requires to be demonstrated. ESA and European industry key players are working towards this goal.

The OpSTAR IOD currently under definition will consist of two Medium Earth Orbit (MEO) satellites equipped with optical terminals and navigation payloads, complemented by dedicated optical ground stations and test facilities. The mission will demonstrate precise synchronization between satellites at picosecond level and inter-satellite ranging within millimeter precision, far exceeding current GNSS capabilities. The key system concepts—such as autonomous network timekeeping via a distributed clock ensemble and robust operation with reduced ground contact—will be tested under realistic conditions. OpSTAR also includes a comprehensive experimentation programme, using an optical system testbed and a hybrid optical/RF user test range on the ground to assess new user equipment and use case scenarios. These experiments will evaluate improvements in end-user positioning accuracy, timing stability, and resilience against outages or threats.

Figure 1 shows the baseline OpSTAR mission concept. Two satellites, each equipped with two Laser Communication and Ranging Terminals (LCRTs), establish optical-inter-satellite links (OISLs) and, when visible from optical ground stations (OGSs), also establish optical satellite-to-ground links (OSGLs). On ground, a dedicated optical PNT system testbed is tasked with all experimental activities focused on validating and verifying the exploitation of optical observables in the provision of a PNT function: inter-platform time synchronization and ranging, overall system synchronization approaches, and performance extrapolation (e.g. Orbit Determination Time Synchronization, Signal-in-Space accuracy, PPP convergence properties, etc.) to a complete optical GNSS system. For these tasks, the testbed is equipped with a) two co-located OGSs establishing simultaneous optical links to both satellites to support the verification of the system synchronization performance at ps-level; b) a number of ground clocks emulating a wider satellite clock constellation and inter-satellite measurements, in order to validate and verify novel approaches to overall system synchronization enabled by optical observables; and c) an accurate and comprehensive GNSS simulator capable of emulating and processing optical observables jointly with legacy RF observables to assess orbit determination, time system synchronization and high-accuracy end-user navigation for a number of diverse GNSS system and processing architectures exploiting optical links. A hybrid optical and RF-based PNT test user range is foreseen to assess a number of relevant use-case scenarios envisioned for the optical system under development. These include, but are not limited to, high-accuracy time and frequency distribution over (inter-)continental distances via optical satellite relay, data access to/from satellites at low latency and high bandwidth, enhanced synchronization of legacy navigation signals broadcast for PNT end-users, improved constellation operability by system operators. Special focus in the OpSTAR IoD is given to the definition of open specifications for optical links enabling PNT functions over the optical layer, as a primary contribution to the development of European open standards for optical links in space systems (e.g. ESTOL). This effort serves two purposes: to establish a future open market for the provision of optical terminals and OGSs with communication and PNT services for both space and ground segments, and to pave the way to a system-of-system -- e.g. multi-layer PNT -- in which different constellations can be interfaced to enhance the accuracy of the respective services, autonomy, robustness and resilience. In this context, a dedicated testbench will be operated before and during the mission to verify on-hardware and in controlled environment both accuracy and inter-operability across diverse optical elements. The OpSTAR IoD mission is supported by a launch and satellite control system, and by an appropriate orbit and time reference system provision to guarantee the necessary support to achieve the overall mission goals. The results of the OpSTAR IOD will serve as a crucial foundation for the design of future PNT architectures by de-risking optical PNT technology and providing a blueprint for integrating optical links into operational PNT systems. This paper presents the OpSTAR mission concept, architecture, experimentation plans, and the expected benefits for Europe’s “system of PNT systems” vision, as well as a roadmap for transitioning from today’s radio-frequency based GNSS towards an optical future PNT.