054 - Monitoring timing signals from space. A novel approach for a worldwide robust time and synchronisation capability

Many systems (including critical infrastructure) rely on GNSS receivers for synchronization purposes nowadays, for instance telecom networks, power grids and financial markets. Due to the vulnerability of GNSS to interference and spoofing, there is a growing need on alternative (non-GNSS-dependent) timing services, as shown by several investigations and initiatives on this topic (e.g. European Radio Navigation Plan, experimentations under the initiative of US Departments of Transport).

The use of terrestrial transmitters (e.g. cellular signals, SOOP, Digital TV) as timing alternatives to GNSS has already been considered and demonstrated. However, despite good robustness and coverage, including indoor, these options share a strong limitation when it comes to resilience: they all mostly rely on terrestrial GNSS receivers to create the reference time source, which may be interfered or even spoofed.

The use of terrestrial transmitters to distribute timing and synchronisation requires finding an efficient workaround to GNSS reference stations on ground, without requiring the deployment of additional ground infrastructure, which would be very expensive and long to deploy if the provision of a worldwide service is targeted. An attractive workaround to the above issue could be to transfer the referencing function into orbit, leveraging on the enhanced resilience of GNSS space receivers and the ability to monitor terrestrial signals from space.

LEO satellites have been already shown to be able to monitor signals from non-cooperative ground transmitters in order to estimate their location. This concept can be extended to estimate the clock offsets of terrestrial transmitters in order to synchronise them. Thanks to their wide coverage, multiple receivers can be simultaneously synchronized. An on-board GNSS receiver will still need to be used, but being at LEO altitude it will be robust to interference originating from terrestrial sources and benefit from additional space sources to enhance further the resilience of the on-board time reference and its link to UTC (e.g. using EDRS, satcom, ISL).

The objectives of this activity are to:

• study a novel system concept for the provision of accurate and resilient timing, relying on terrestrial RF transmitters (Digital TV) synchronized to GNSS using space monitoring infrastructure;
• investigate optimal architectures and implementation, in particular for the space monitoring functions;
• validate performance in terms of achieved accuracy and resilience; accuracies in the order of at least micro-seconds and lower shall be targeted.

The tasks to be performed include:

• identification of the most suitable techniques for the proposed concept (techniques for monitoring terrestrial signals from space, use and processing of SOOP for timing purposes);
• identification and analysis of architectures, techniques and technologies for the candidates solutions, in particular for the space-based monitoring component (including constellation sizing, coverage, targeted accuracy, payload and algorithms), trading-off different options and end-to-end performance;
• implementation and simulation of the critical processing algorithms used for signal monitoring, synchronisation, and validation of the associated models and performance using real signals;
• assessment of timing performance achieved for the end-user with different architectures and techniques, via simulations;
• definition of a detailed proof of concept and assessment of a potential future demonstrations.

The main results of the activity will provide:

• a feasibility analysis of the proposed concept, including trade-off’s and a high-level system architecture;
• simulation results of critical algorithms associated to an end-to-end performances analysis;
• initial definition of future demonstrations.