The European Galileo navigation system now has an additional role as a fundamental science platform, providing data that is helping us to push the boundaries of physics. RHEA is playing an essential supporting role at the European Space Agency’s Navigation Science Office.

About the Navigation Science Office

The multidisciplinary Navigation Science Office (NSO) is a joint initiative between the European Space Agency’s (ESA’s) Navigation and Science directorates.

The NSO’s main goal is to foster the use of Galileo data to perform science. Some of the science applications are what you might expect from a global navigation satellite system (GNSS), such as the study of the movement of tectonic plates, seismology, ionospheric studies, novel clocks and timescale generation and distribution. Others, however, seem more exotic – general relativity, dark matter detection and gamma ray burst detection.

ESTEC’s Galileo Payload Testbed Facility
ESTEC’s Galileo Payload Testbed Facility where engineers took apart an entire Galileo satellite to reassemble its navigation payload on a laboratory test bench to run it as though it were in orbit. © ESA–Cesar Miquel Espana

How Galileo became a fundamental physics tool

How did Galileo expand from being an eminently practical system we all use in our day-to-day lives into a tool for fundamental physics experiments?

The answer lies in the technology that enables it to provide unprecedented positioning accuracy on Earth – the precise clocks carried by Galileo satellites.

In a GNSS system, positioning accuracy depends on the precision with which we can measure the time it takes light to travel from the GNSS satellites to the earth. Extremely accurate time-keeping is also essential for fundamental physics experiments – remember all those small effects that crop up in Special Relativity for observers moving very fast?

Galileo satellites are equipped with passive hydrogen maser atomic clocks that are so accurate they may lose or gain only one second in 2.7 million years. By contrast, a normal digital wristwatch is accurate to around one second in a 24-hour period.

The reason this becomes valuable for scientists is that atomic clocks depend on the stability of the frequency of atomic transitions and these are determined by fundamental physics. Any small discrepancies between the real atomic frequencies in real atomic clocks and the theoretically predicted values may indicate something amiss in physical theories.

Galileo’s main fundamental physics achievement

The NSO participates in several fundamental physics projects focussing on testing General Relativity.

The most notable achievement so far has been a new measurement of the ‘gravitational redshift’. This is the shift in the frequency of light for electromagnetic radiation travelling between two points at different gravitational potentials that was predicted by Einstein’s Equivalence Principle, one of the keystones of Einstein’s Theory of General Relativity.

How is gravitational redshift measured?

The energy of a photon is determined by its frequency:

  • Any photon moving from a position of higher gravitational potential to a lower one gains energy and its frequency is increased – that is, shifted towards the blue end of the spectrum.
  • Any photon climbing a gravitational potential loses energy and its frequency is reduced, so shifted towards the red end of the spectrum.

To measure this effect, very accurate clocks are required, such as those at the heart of Galileo satellites. In an atomic clock, time is measured by counting the crests of electromagnetic radiation resulting from a well-defined atomic transition. If the frequency of this radiation is affected by gravitational redshift, the frequency of the clock will change and we will see a perturbation in the flow of time as measured by the clock.

How was Galileo used to measure gravitational redshift?

In 2014, two Galileo satellites were delivered to an incorrect elliptical (elongated) orbit due to a problem with the launch vehicle. The elongation of their eventual orbit means their clocks effectively fall and climb around 8000km every time they orbit the Earth. The result was that all the ingredients to measure gravitational redshift were now in place, even though this was not originally planned as part of their mission.

The resulting project – Galileo gravitational Redshift Experiment with eccentric sATellites (GREAT) – was a complete success.

Two teams at SYRTE, in France, and ZARM, the Center of Applied Space technology and Microgravity in Bremen, Germany, analyzed the data with the support of the NSO and the Galileo Navigation Office at ESA’s European Space Operations Centre (ESOC).

Their results produced the first reported improvement of one of the longest standing measurements in experimental gravitation – the Gravity Probe A (GPA) hydrogen maser rocket experiment in 1976 by Harvard-Smithsonian Center of Astrophysics. Even more impressive was that they improved the 40-year-old measurements by almost one order of magnitude. This was a brilliant result for a system that was not even built to test General Relativity!

Galileo case study graph showing the comparison of predicted and measured gravitational time shift
Comparison of predicted and measured gravitational time shift as predicted by GR and measured by GREAT

Galileo’s other fundamental physics projects

The NSO is participating in other projects aimed at probing the frontiers of physics.

Two of the most interesting ones are the search for a particular type of ‘dark matter’ – the mysterious substance believed to fill around 25% of the Universe but which has so far eluded detection – and a feasibility study for the measurement of the so-called ‘gravitomagnetic clock effect’. This is a very small modification to Kepler’s third law predicted by General Relativity that modifies the time measured by any object orbiting a rotating body by different amounts depending on the direction the orbiting object moves.

In both cases, the fundamental piece of hardware needed to perform the measurement is an accurate clock.

For the latter, the need for accurate clocks is obvious. For the former, this is because dark matter introduces very small perturbations in the clock frequencies that can be extracted from Galileo data. Although the chances of detecting dark matter are slim, non-detection is also interesting as experimental results provide useful constraints on the theories that purport to explain the properties of dark matter.

Next generation Galileo science opportunities

The next generation of Galileo satellites could have dedicated experimental hardware mounted on them, although this will only be done if it does not interfere with the normal functioning of the overall Galileo system.

Under the European Commission’s Horizon 2020 Framework Programme for Research and Innovation in Satellite Navigation, and with the active participation of the NSO, a feasibility study is assessing the possibility of mounting instruments on Galileo satellites to detect gamma ray bursts. These are very fast, energetic explosions that generate huge amounts of energy. They are believed to be caused by the merger of stellar mass objects in binary systems giving rise to a black hole or the collapse of very massive objects.

Studying such events gives us a huge amount of information about star formation rates in the Universe and how chemistry has evolved in galaxies like our own. This is because the heavy metals in the Universe are believed to have been formed during the events giving rise to gamma ray bursts.

The hope is that installing gamma ray burst detectors on at least four Galileo satellites could double the number of gamma ray bursts detected annually. This is an early assessment, but the final outcome looks promising.

The future of the NSO

Galileo was never intended to function as a fundamental science platform. However, it turns out to be ideally suited to the role for certain experiments – and at a reduced cost compared with dedicated science missions, because the hardware already exists and all there is left to do is perform data analysis.

The reduced cost and the nature of the data accessible to such experiments guarantees the NSO a bright future in pushing the boundaries of our knowledge of the Universe.

How RHEA is contributing to NSO activities

A RHEA employee works at the NSO, providing support to fundamental physics related activities. This includes submitting ideas for interesting projects that could be sponsored by the NSO, evaluating proposals for open calls and following the evolution of fundamental physics projects in which the NSO participates.

The skills required for this job are a background in theoretical physics at PhD level, good knowledge of navigation methods and familiarity with ESA management procedures for small projects. This is a multidisciplinary office involving a small group of people, so an understanding of other scientific areas and good communication skills are essential.

Main image: In 2014, the first two Galileo satellites launched by Soyuz-STB Fregat-MT rocket were placed into the wrong orbit due to a problem with the launch vehicle. However, this led to a project called Galileo gravitational Redshift Experiment with eccentric sATellites (GREAT), which was a complete success. © ESA–Pierre Carril, 2014

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