ESA began experimenting with a new type of clock called PulChron based on pulsars. Activated at the end of November 2018, this system is based on the impulses generated by these neutron stars that spin very rapidly in extremely precise times. 18 pulsars monitored with various radio telescopes were used for the measurements, which provide extremely accurate time references for various purposes such as verifying the performance of the Galileo European navigation system’s satellites.
Neutron stars represent a possible end to stars after the collapse of their core if it has enough mass to make it collapse to the point of degenerating the matter that forms it but not so high as to generate a black hole. A mass higher than the Sun’s is concentrated in a sphere of a dozen kilometers in diameter that in some cases can spin at very high speeds with emissions of electromagnetic radiation from its poles. From the Earth, a pulsar is detected with extremely regular emissions and this could help to measure the passage of time with an accuracy equal to or higher than that of atomic clocks.
Engineer Stefano Binda, who oversees the PulChron project, explained that in the short term a pulsar-based time measurement is typically less stable than an atomic clock but could be competitive over the long and very long term, beyond the expectation of operation of any atomic clock. For example, the navigation satellites of the Galileo constellation use atomic clocks that are extremely precise and yet need continuous corrections to maintain that precision in the order of very few billionths of a second.
18 pulsars get monitored by the 5 European Pulsar Timing Array (EPTA) radio telescopes: the Westerbork Synthesis Radio Telescope in the Netherlands, the Effelsberg Radio Telescope in Germany, the Lovell Telescope in the UK, the Nancay Radio Telescope in France and the Sardinia Radio Telescope in Italy. Those pulsars have been used for some time in the search for anomalies in which precise measurements of time are important such as gravitational waves, but now they have started being used as a kind of cosmic clock.
The Galileo Timing and Geodetic Validation Facility (TGVF) at ESA’s ESTEC (European Space Research and Technology Center) in the Netherlands was opened at the beginning of the Galileo program and keeps on being used to monitor the proper work of the satellites and the accuracy of their atomic clocks. Stefano Binda explained that it was a perfect opportunity to host the PulChron for the possibility of integrating new elements with little effort into a time measurement task.
In this test phase the time measurements performed by PulChron are monitored using another ESTEC laboratory, UTC, which helps to maintain the so-called universal coordinated time, the reference time zone for the whole world, which uses atomic clocks. The goal is to bring PulChron to an error within 0.2 billionths of a second per day.
Atomic clocks are very precise but there are some components that degrade after a certain number of years. Pulsars maintain their accuracy for very long periods and the radio telescopes that monitor them can operate for many years and can also be used for other purposes. In applications requiring extreme precision in measuring time, PulChron could become very useful.