Cosmic-ray muons enable navigation where GPS can't reach

Monday, 26 June, 2023

A research team led by The University of Tokyo has used superfast, subatomic-sized particles called muons to wirelessly navigate underground, in what is reported to be a world first. Described in the journal iScience, the technology could be deployed in future search and rescue efforts to monitor undersea volcanoes and to guide autonomous vehicles underground and underwater.

GPS is a well-established navigation tool and offers an extensive list of positive applications, from safer air travel to real-time location mapping. However, it has some limitations. GPS signals are weaker at higher latitudes and can be jammed or spoofed (where a counterfeit signal replaces an authentic one). Signals can also be reflected off surfaces like walls, interfered with by trees and blocked by buildings, rock or water.

By comparison, muons have been making headlines in recent years for their ability to help us look deep inside volcanoes, peek through pyramids and see inside cyclones. Muons fall constantly and frequently around the world (about 10,000/m²/min), and they can’t be tampered with. They exist for only 2.2 µs, but because they travel at the speed of light in a vacuum (300,000 km/s), they have enough time to reach Earth from the atmosphere and penetrate deep into the ground.

“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 Professor Hiroyuki Tanaka, from Muographix at The University of Tokyo. “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.”

MuPS was initially created to help detect seafloor changes caused by underwater volcanoes or tectonic movement. It uses four muon-detecting reference stations above ground to provide coordinates for a muon-detecting receiver underground. Early iterations of this technology required the receiver to be connected to a ground station by a wire, restricting movement, but the latest research uses high-precision quartz clocks to synchronise the ground stations with the receiver. The four parameters provided by the reference stations, plus the synchronised clocks used to measure the muons’ ‘time of flight’, enable the receiver’s coordinates to be determined. This new system is called the muometric wireless navigation system (MuWNS).

To test the navigation ability of MuWNS, reference detectors were placed on the sixth floor of a building while a ‘navigatee’ took a receiver detector to the basement floor. They slowly walked up and down the corridors of the basement while holding the receiver. Rather than navigating in real time, measurements were taken and used to calculate their route and confirm the path they had taken.

“The current accuracy of MuWNS is between 2 metres and 25 metres, with a range of up to 100 metres, depending on the depth and speed of the person walking,” Tanaka said. “This is as good as, if not better than, single-point GPS positioning above ground in urban areas. But it is still far from a practical level. People need one-metre accuracy, and the key to this is the time synchronisation.”

Improving the system to enable real-time, metre-accurate navigation hinges on time and money. Ideally, the team wants to use chip-scale atomic clocks, or CSACs.

“CSACs are already commercially available and are two orders of magnitude better than the quartz clocks we currently use,” Tanaka said.

“They are too expensive for us to use now, but I foresee that they will become much cheaper as the global demand for CSAC for cellphones increases.”

MuWNS could someday be used to navigate robots working underwater or guide autonomous vehicles underground. Aside from the atomic clock, all the other electronic components of MuWNS can now be miniaturised, so the team hopes that eventually fitting it into handheld devices, like smartphones, will be feasible. In emergency situations like a building or mine collapse, this could be a future game changer for search and rescue teams.