Japanese scientists test cosmic ray GPS that works through walls and underwater

Scientists from the University of Tokyo in Japan have conducted their first tests of a new navigation system that uses cosmic rays for location tracking rather than the radio waves relied on with GPS.

Ars Technica reports the researchers recently published their findings in a paper in the iScience journal.

While the global position system (GPS) has become a mainstay for navigation, tracking, and mapping across general-use applications, it fails to perform under certain conditions.

Because GPS relies on relatively weak satellite radio waves, it struggles with location tracking inside buildings, underground, or underwater.

Cosmic rays, in this case mainly muons, can pass through walls, roofs, the ground, rocks, and water.

These have previously been used to map archaeological structures and scan containers for nuclear materials.

“Cosmic-ray muons fall equally across the Earth and always travel at the same speed regardless of what matter they traverse, penetrating even kilometres of rock,” explained paper co-author Hiroyuki Tanaka.

“Now, by using muons, we have developed a new kind of GPS, which we have called the muometric positioning system (muPS), which works underground, indoors and underwater.”

Muon-based imaging typically requires using gases that result in a flash of light when cosmic rays move through them.

Detectors record these flashes and can be used to calculate a particle’s energy and trajectory.

This process is similar to X-ray imaging or ground-penetrating radar. However, those methods require emitting X-rays or radio waves, while muons occur naturally.

The researcher’s Muographix system uses four muon-detecting aboveground reference stations that act as coordinates for muon receivers, which can be deployed underground or underwater.

The Japanese team successfully trialled a wired version of its system in Tokyo Bay in 2021, using ten muon receivers in the Tokyo Bay Aqua-Line roadway, 45 metres below sea level.

Mapping water movements could help sense storm waves or tsunamis.

The team has also used aboveground sensors to map the vertical profile of a cyclone.

Used in conjunction with satellite weather systems, it could help improve cyclone predictions.

The latest version of their system —  the muometric wireless navigation system (MuWNS) — is entirely wireless and features high-precision quartz clocks to sync the ground stations with the receivers.

Tanaka said tests of MuWNS conducted in a building showed location accuracy of between 2 and 25 meters, with a range of as much as 100 meters.

“This is as good as, if not better than, single-point GPS positioning aboveground in urban areas,” he said.

“But it is still far from a practical level. People need one-meter accuracy, and the key to this is the time synchronisation.”

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Japanese scientists test cosmic ray GPS that works through walls and underwater