World's first cosmic-ray GPS can detect underground movement

A team of scientists have successfully demonstrated the world's first cosmic-ray GPS to detect movement underground and in volcanoes which can potentially aid in future search-and-rescue missions.
Tejasri Gururaj
An illustration of cosmic rays
An illustration of cosmic rays


Cosmic rays are high-energy particles originating from outer space, including sources such as the sun, distant galaxies, supernovae, and other celestial bodies. Although we can't see or feel cosmic rays directly, they constantly bombard the Earth from outer space. 

In fact, these particles are so abundant that scientists estimate one cosmic ray hits one square centimeter of the Earth's surface every minute! Scientists study cosmic rays to learn about the universe and how particles interact at high energies.

However, scientists have now used cosmic rays to devise a global positioning system (GPS) to track underground movements. This groundbreaking system is capable of revolutionizing the world of disaster response. 

The research team, led by Prof. Hiroyuki Tanaka from the University of Tokyo, has developed a wireless muometric navigation system or MuWNS, a novel wireless navigation technique. 

Using muon detection to develop positioning systems

When cosmic rays enter the Earth's atmosphere, they collide with the atoms and molecules in the air, producing a shower of secondary particles called muons. Muons are fundamental subatomic particles like electrons but weigh 207 times more. 

Muons can penetrate and pass through solid objects, with the degree of penetration depending on the density of the material. For example, rocks and buildings absorb a lot of muons due to their high density. 

In contrast, GPS, which relies on radio waves, is weaker at higher altitudes and scatters easily. This makes it hard to detect motion underground using GPS signals.

Tanaka and his team exploited this property of cosmic rays to map the interiors of hard-to-access places such as volcanoes, the core of nuclear reactors, and pyramids. 

How does the MuWNS work?

Tanaka and his team had previously developed a predecessor to the MuWNS called the muometric positioning system (muPS). The muPS was used to detect seafloor changes using muons. It involved four surface-level reference stations and a receiver station on the ocean floor. 

The MuWNS involves placing reference detectors on the surface and a receiver detector underground to detect the passage of muons. By analyzing the timing and direction of the muons, the MuWNS triangulates the relative position of the receiver detector underground concerning the reference detectors on the surface. 

The reference and receiver detectors are synchronized using high-precision quartz clocks to prevent timing discrepancies and ensure accuracy in positioning.

Once all the data is collected, it is used to reconstruct the paths of the muons to create a model or map of the underground area. The map can provide essential information, such as the composition and density of the materials that the muon encounters, thus allowing for the visualization of underground structures and geographical features. 

The team then tested their MuWNS system by giving a person the receiver detector in the basement of a building and placing four reference detectors on the sixth floor of the same building. The researchers successfully rebuilt the underground navigator's path by screening for cosmic rays picked up by the detectors and receiver. 

The team demonstrated the world's first cosmic-ray GPS that can aid in future search-and-rescue missions and also be used to monitor volcanoes. Their next step is to streamline the MuWNS to incorporate it into a smartphone. 

In a press release, Tanaka said, "The receiver's detector size will be a chip scale. We don't need precise time synchronization either; hence the atomic clock is not needed anymore. Therefore, it is definitely possible to fit [in] smartphones."

The findings of the study are published in the journal iScience.

Study abstract:

Navigation in indoor and underground environments have been extensively studied to realize automation of home, hospital, office, factory and mining services, and various techniques have been proposed for its implementation. By utilizing the relativistic and penetrative nature of cosmic-ray muons, a completely new wireless navigation technique called wireless muometric navigation system (MuWNS) was developed. This paper shows the results of the world first physical demonstration of MuWNS used on the basement floor inside a building to navigate (a person) in an area where GNSS/GPS signals cannot reach. The resultant navigation accuracy was comparable or better than the positioning accuracy attainable with single point GNSS/GPS positioning in urban areas. With further improvements in stability of local clocks used for timing, it is anticipated that MuWNS can be adapted to improve autonomous mobile robot navigation and positioning as well as other underground and underwater practical applications.

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