PulChron: A Pulsar Time Scale Demonstration for PNT systems

Abstract

Galileo System Time (GST) is the cornerstone for the operations and performance of Galileo, the European GNSS (Global Navigation Satellite System). GST must be stable, traceable to UTC, and always available. GST is currently generated from an ensemble of physical on-ground clocks steered to UTC. This approach limits certain types of performance. For example, if a clock breaks down, a downtime is caused to the operations or a procedure needs to put in place to switch to a redundant clock. Moreover, for very long time periods (comparable to the design operational lifetime of GNSS, i.e., in the order of decades) the performance of the physical clock will degrade. A time scale built with pulsar measurements, i.e., measurements from celestial objects emitting radiation in pulses, would typically be less stable than one built using atomic or optical clocks in the short term, but could be competitive in the very long term (several decades, a period over which individual atomic clocks will cease to work). An additional justification for a pulsar time scale is that it would be independent of the clock technology for the generation of the oscillation mechanism (neutron star rotational period as opposed to atomic transitions in rubidium, caesium or hydrogen atoms). The objective of the PulChron project, an abbreviation built with the words “Pulsar” and “Chronos” (??????), which is the ancient Greek term for “Time”, is to demonstrate the effectiveness of a pulsar time scale for the generation and monitoring of system timing in Positioning, Navigation and Timing (PNT) in general, and of GST in particular. The PulChron project has been developed in the frame of NAVISP (Navigation Innovation and Support Programme), an ESA programme aiming at fostering innovation in the PNT field while supporting industry and ESA member states interests. In this context, it was considered interesting to demonstrate the implementation of a “real time clock” and a “paper time scale” based on pulsar measurements for PNT monitoring.