Minimize PulChron

PulChron - Pulsar Timescale Demonstration

Navigation Innovative Support Program    PulChron, a Pulsar timescale demonstration    References

ESA's technical center in the Netherlands has begun running a pulsar-based clock. The ‘PulChron’ system measures the passing of time using millisecond-frequency radio pulses from multiple fast-spinning neutron stars. Operating since the end of November 2018, this pulsar-based timing system is hosted in the Galileo Timing and Geodetic Validation Facility of ESA’s ESTEC establishment, at Noordwijk in the Netherlands, and relies on ongoing observations by a five-strong array of radio telescopes across Europe. 1)

Neutron stars are the densest form of observable matter in the cosmos, formed out of the collapsed core of exploding stars. Tiny in cosmic terms, on the order of a dozen kilometers in diameter, they still have a higher mass than Earth’s Sun.

A pulsar is a type of rapidly rotating neutron star with a magnetic field that emits a beam of radiation from its pole. Because of their spin – kept steady by their extreme density – pulsars as seen from Earth appear to emit highly regular radio bursts – so much so that in 1967 their discoverer, UK astronomer Jocelyn Bell Burnell, initially considered they might be evidence of ‘little green men’.

PulChron_Auto6

Figure 1: Pulsar encased in supernova bubble. Massive stars end their lives with a bang: exploding as spectacular supernovas, they release huge amounts of mass and energy into space. These explosions sweep up any surrounding material, creating bubble remnants that expand into interstellar space. At the heart of bubbles like these are small, dense neutron stars or black holes, the remains of what once shone brightly as a star (image credit: ESA/XMM-Newton/ L. Oskinova/M. Guerrero; CTIO/R. Gruendl/Y. H. Chu)

Since supernova-carved bubbles shine for only a few tens of thousands of years before dissolving, it is rare to come across neutron stars or black holes that are still enclosed within their expanding shell. This image captures such an unusual scene, featuring both a strongly magnetized, rotating neutron star – known as a pulsar – and its cosmic cloak, the remains of the explosion that generated it.

This pulsar, named SXP 1062, lies in the outskirts of the Small Magellanic Cloud, one of the satellite galaxies of our Milky Way galaxy. It is an object known as an X-ray pulsar: it hungrily gobbles up material from a nearby companion star and burps off X-rays as it does so. In the future, this scene may become even more dramatic, as SXP 1062 has a massive companion star that has not yet exploded as a supernova.

Most pulsars whirl around incredibly quickly, spinning many times per second. However, by exploring the expanding bubble around this pulsar and estimating its age, astronomers have noticed something intriguing: SXP 1062 seems to be rotating far too slowly for its age. It is actually one of the slowest pulsars known.

While the cause of this weird sluggishness is still a mystery, one explanation may be that the pulsar has an unusually strong magnetic field, which would slow the rotation.

The diffuse blue glow at the center of the bubble in this image represents X-ray emission from both the pulsar and the hot gas that fills the expanding bubble. The other fuzzy blue objects visible in the background are extragalactic X-ray sources.

This image combines X-ray data from ESA’s XMM-Newton (shown in blue) with optical observations from the Cerro Tololo Inter-American Observatory in Chile. The optical data were obtained using two special filters that reveal the glow of oxygen (shown in green) and hydrogen (shown in red). The size of the image is equivalent to a distance of 457 light-years on a side.

Table 1: Legend to Figure 1: This image was first published on ESA’s Science and Technology website in 2011. It is based on data from the paper “Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the Wing of the Small Magellanic Cloud” by V. Hénault-Brunet, et al. 2012.

PulChron_Auto5

Figure 2: Dune-side aerial view of ESA’s ESTEC technical center (image credit: ESA - SJM Photography)

Legend to Figure 2: On the left of the image can be seen ESTEC’s Test Center for full-scale testing of satellites, equipped with a suite of simulation facilities to reproduce every aspect of the space environment. In the center is the main building, home to ESA laboratories and mission teams, distinguished by an almost 200-m long main corridor. To the right of the main building is the restaurant and tower complex built by renowned Dutch architect Aldo van Eyck in the late 1980s. On the other side of the car park is the two-tone square-shaped Erasmus building, focused on human spaceflight, and to its right is the T building, home to ESA’s Galileo team.

PulChron aims to demonstrate the effectiveness of a pulsar-based timescale for the generation and monitoring of satellite navigation timing in general, and Galileo System Time in particular,” explains navigation engineer Stefano Binda, overseeing the PulChron project.

“A timescale based on pulsar measurements is typically less stable than one using atomic or optical clocks in the short term but it could be competitive in the very long term, over several decades or more, beyond the working life of any individual atomic clock. In addition, this pulsar time scale works quite independently of whatever atomic clock technology is employed – it doesn’t rely on switches between atomic energy states but the rotation of neutron stars.”

PulChron sources batches of pulsar measurements from the five 100-m class radio telescopes comprising the EPTA (European Pulsar Timing Array) – the Westerbork Synthesis Radio Telescope in the Netherlands, Germany’s Effelsberg Radio Telescope, the Lovell Telescope in the UK, France’s Nancay Radio Telescope and the Sardinia Radio Telescope in Italy. 2)

The EPTA collaboration has categorized 18 pulsars as 'Priority 1', meaning that they offer the highest timing precision using our telescopes. They are the most promising candidates for gravitational wave detection, and accurate timing of some of these pulsars will also allow us to make improved studies of the pulsar systems themselves. The Priority 1 pulsars form the backbone of the array because they have already shown excellent timing precision and are, in most cases, visible to all five EPTA telescopes. Many more pulsars than these are observed as part of the EPTA timing program, assisting with gravitational wave detection and other research objectives.


For PulChron, these radio telescope measurements are used to steer the output of an active hydrogen maser atomic clock with equipment based in the Galileo Timing and Geodetic Validation Facility – combining its extreme short- and medium-term stability with the longer-term reliability of the pulsars. A ‘paper clock’ record is also generated out of the measurements, for subsequent post-processing checks.

PulChron_Auto4

Figure 3: Atomic clocks at ESTEC's Navigation Laboratory (image credit: ESA - Anneke Le Floc'h)

ESA established the Timing and Geodetic Validation Facility in the early days of the Galileo program, first to prepare for ESA’s two GIOVE test satellites and then in support of the world-spanning Galileo system, based on ‘Galileo System Time’ which needs to remain accurate to a few billionths of a second. The Facility continues to serve as an independent yardstick of Galileo performance, linked to monitoring stations across the globe, as well as a tool for anomaly investigation.

Stefano adds: “The TGVF (Time and Geodetic Validation Facility) of Galileo provided a perfect opportunity to host the PulChron because it is capable of integrating such new elements with little effort, and has a long tradition in time applications, having been used even to synchronize time and frequency offset of the Galileo satellites themselves.”

PulChron’s accuracy is being monitored down to a few billionths of a second using ESA’s adjacent UTC Laboratory, which harnesses three such atomic hydrogen maser clocks plus a trio of cesium clocks to produce a highly-stable timing signal, contributing to the setting of Coordinated Universal Time, UTC – the world’s time.

PulChron_Auto3

Figure 4: Setup of the PulChron system, setting an atomic clock using millisecond-scale pulses from fast-spinning pulsars. Radio telescope measurements are used to steer the output of an active hydrogen maser atomic clock with equipment based in ESA's Galileo Timing and Geodetic Validation Facility – combining its extreme short- and medium-term stability with the longer-term reliability of the pulsars. A ‘paper clock’ record is also generated out of the measurements, for later, post-processing checks (image credit: ESA) 3)




NAVISP (Navigation Innovative Support Program)

NAVISP is an important element for the overall European GNSS landscape, capable of leveraging both ESA expertise gained through Galileo, EGNOS and Navigation Programs and the existing industrial base of the European Navigation sector. The main NAVISP objective is to facilitate the generation of Satellite Navigation/PNT innovative propositions with participating States and their industry, in coordination with EU and its institutions. 4) 5)

Under NAVISP Element 1, ESA awarded the activity “Pulsar Timescale Demonstration” to a consortium led by GMV-UK with the support of the University of Manchester and the UK’s National Physical Laboratory. The objective of the activity is to set up a pulsar-based clock, called “PulChron”. The system measures the passing of time using millisecond-frequency radio pulses from multiple fast-spinning neutron stars. 6)

This pulsar-based timing system is operational as of Q4 2018. It is hosted in the TGVF (Timing and Geodetic Validation Facility) of ESA’s ESTEC establishment, at Noordwijk in the Netherlands, and relies on ongoing observations by a five-strong array of radio telescopes across Europe.

“PulChron aims to demonstrate the effectiveness of a pulsar-based timescale for the generation and monitoring of satellite navigation timing in general, and Galileo System Time in particular,” explains navigation engineer Stefano Binda, managing the PulChron activity. “A timescale based on pulsar measurements is typically less stable than one using atomic or optical clocks in the short term but it could be competitive in the very long term, over several decades or more, the kind of time period in which individual atomic clocks will cease to work. In addition, this pulsar time scale works quite independently of whatever atomic clock technology is employed – it doesn’t rely on switches between atomic energy states but the rotation of a neutron star.”

PulChron_Auto2

Figure 5: PulChron architecture (image credit: ESA)

For “PulChron”, these radio telescope measurements are used to steer the output of an active hydrogen maser atomic clock with an equipment based in the Timing and Geodetic Validation Facility – combining its extreme short- and medium-term stability with the longer-term reliability of the pulsars. A ‘paper clock’ is also generated out of the measurements, for later use in post processing.

Stefano adds: “The TGVF provided a perfect opportunity to host the PulChron because it is capable of integrating such new elements with little effort, and has a long tradition in time applications, having been used even to synchronize time and frequency offset of the Galileo satellites themselves.”

PulChron’s accuracy is being monitored down to a few billionths of a second using ESA’s adjacent UTC Laboratory, which harnesses three such atomic hydrogen maser clocks plus a trio of cesium clocks to produce a highly-stable timing signal, contributing to the setting of Coordinated Universal Time, UTC – the world’s time.

The gradual diversion of pulsar time from ESTEC’s UTC time can therefore be tracked – anticipated at a rate of around 200 trillionths of a second daily.




PulChron, a Pulsar timescale demonstration

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, cesium 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 Program), an ESA program 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. 7)

Pulsars are magnetized spinning neutron stars that emit a beam of electromagnetic radiation along their magnetic axis (Figure 6). 8)

Depending on their alignment, to an observer on Earth they may look like a beacon emitted by a lighthouse. Because of their compact size, of the order of 10 km, these objects rotate very fast with typical periods of less than a couple of seconds. Although the pulses emitted by a pulsar can be quite irregular and show a very clear deceleration trend, a class of pulsars with periods of the order of milliseconds and extremely stable pulses over periods of the order of billions of years has been identified. The details behind the mechanism involved in the formation of these so-called millisecond pulsars are still not completely understood, but it is suspected they are formed in binary systems when an old pulsar with very little of its rotational energy left, starts accreting large amounts of mass from its companion star. The accreted mass will thus transfer angular momentum to the neutron star, therefore “recycling” the pulsar and giving rise to a very stable rotation that can continue for billions of years. Millisecond pulsars are so stable that at some point it was believed they could compete for accuracy with the most accurate clocks on Earth. With the advance of clock technology this is no longer true (Figure 7) but, fortunately for us, that is not the end of the story regarding the use of pulsars as clocks.

PulChron_Auto1

Figure 6: PulChron, a Pulsar timescale demonstration. Artist's illustration of a spinning neutron star emitting a beam of electromagnetic radiation along their magnetic axis (image credit: ESA)

The use of pulsars as clocks still has some benefits:

• As they are completely independent from terrestrial clocks, they can be used as a check on conventional timescales;

• As opposed to atomic clocks whose principles of operation rely on quantum mechanics, pulsar clocks are based on astrophysics processes on stellar mass objects;

• Unlike atomic clocks whose useful lifetime hardly exceeds 10 years, pulsar clocks will continue to operate for millions to billions of years and they could therefore be used for navigation far from the Earth.

Pulsar timescales are therefore still worth investigating and that is the goal of PULCHRON, a project funded by NAVISP involving a partnership between GMV, the University of Manchester (UoM) and ESA.

PulChron_Auto0

Figure 7: Pulsar observations performed by 5 European radio telescopes participating in the European Pulsar Timing Array (EPTA) and provided by UoM, allow the PULCHRON team to investigate how to build a stable timescale combining pulsar data with conventional timescales, such as UTC (image credit: UoM)

Using the time of arrival (ToA) of the pulsar signal, frequencies and drifts for each pulsar are calculated with a model that takes into account pulsar physics, propagation of the signal in the interstellar medium and antenna characteristics. Once the frequencies are known, the PulChron team calculates the timing residuals, the differences between the ToAs and the values predicted by the model, for each pulsar.

With the pulsar characteristics and the timing residuals, PulChron will then investigate both the implementation of a physical timescale (using the pulsar data to steer the output of an hydrogen maser) and the construction of a composite time scale (using the pulsar timescale and satellite and ground based clocks). The first will demonstrate the feasibility of using a pulsar timescale to monitor a “conventional” clock, in this case an hydrogen maser. The second will investigate the generation of a timescale combining both the short term stability of earth bound atomic clocks and the long term availability and stability of pulsars.

Although PulChron has been running for less than 6 months, the results are already extremely encouraging and show how it is possible to explore the synergies between two apparently unconnected fields: astrophysics and navigation.



1) ”ESA sets clock by distant spinning stars,” ESA, 24 December 2018, URL: http://m.esa.int/Our_Activities/Navigation/ESA_sets_clock_by_distant_spinning_stars

2) ”The Large European Array for Pulsars Project,” URL: http://www.leap.eu.org/

3) ”PulChron setup,” ESA, 21 December 2018, URL: http://m.esa.int/spaceinimages/Images/2018/12/PulChron_setup

4) ”NAVISP, the program,” ESA, URL: https://navisp.esa.int/page/about-navisp

5) ”Navigation Innovation and Support Program (NAVISP): a new ESA program in Navigation to foster innovation and competitiveness of European industry,” 2018, URL: https://navisp.esa.int/uploads/files/documents/5a13efbb274d8322240807.pdf

6) ”Pulsar-based timing system “PulChron” now operational at ESA-ESTEC,” ESA, URL: https://navisp.esa.int/news/article/navisp-PulChron

7) Ricardo Píriz, Esteban Garbin, Pedro Roldán, Michael Keith, Benjamin Shaw, Setnam Shemar, Kathryn Burrows, John Davis, Stefano Binda, ”PulChron: A Pulsar Time Scale Demonstration for PNT systems,” Proceedings of the 50th Annual Precise Time and Time Interval Systems and Applications Meeting, January 28 - 31, 2019, Reston VA, USA, https://doi.org/10.33012/2019.16753

8) ”PULCHRON, a Pulsar timescale demonstration,” gnss science support centre, 8 April 2019, URL: https://gssc.esa.int/pulchron-a-pulsar-timescale-demonstration/


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).

Navigation Innovative Support Program    PulChron, a Pulsar timescale demonstration    References    Back to top