Cosmic muons could provide a practical alternative to global navigation satellite systems (GNSSs) in places where radio signals cannot reach. That is the conclusion of the muPS collaboration, which has created a system that worked deep in the basement of a university building. The muPS team was led by Hiroyuki Tanaka at the University of Tokyo, and its new system could allow users to navigate in indoor, underground and underwater environments.
GNSSs such as GPS work by transmitting radio signals from a group of satellites to a receiver on the ground. While GNSSs have revolutionized how we get around, GNSS signals are rapidly attenuated by materials like metal, concrete, rock and water – limiting its use indoors, underground and underwater.
In 2020 Tanaka’s team introduced an entirely new approach that tracks the position of a receiver using cosmic muons. These particles are created when high-energy cosmic rays collide with Earth’s atmosphere, and they are constantly raining down on us.
Moving through mountains
“Cosmic muons are not intercepted like radio waves, since they can penetrate even through pyramids or mountains, making the technique suitable for universal indoor or underground navigation,” Tanaka explains.
Dubbed the muPS Wireless Navigation System (muWNS) by its inventors, the system replaces satellites with a network of three or more reference muon detectors, which are synchronized with a receiver detector. These reference detectors could be set up on roofs or higher floors for indoor navigation, or at ground or sea level for navigation through underground or underwater environments.
The system works by identifying muons that have passed through one of the reference detectors and then passed through the receiver. These muons travel at close to the speed of light, which allows muWNS to calculate the distance between the reference detector and receiver. By doing this several times using different reference detectors, the system uses triangulation to determine the receiver’s position.
While the concept is simple, Tanaka’s team had to overcome several challenges while developing muWNS. The initial design required the receiver to be wired to each reference detector to guarantee precise time synchronization, which severely restricted the range and usefulness of the system.
Precision timekeeping
To get around this problem, the team fitted the detectors with ultra-precise quartz crystal clocks, which were synchronized to allow them to compare muon arrival times wirelessly.
The researchers also managed to improve the accuracy of their initial system. “When muWNS was demonstrated for the first time about a year ago, the navigation accuracy was only down to 10 m,” Tanaka recalls. “This is far from the satisfactory level for practical implementation.”
By further improving the accuracy of the clocks, the team has now significantly reduced the errors that accumulate in the timings. In the latest demonstration, Tanaka’s team has shown that muWNS is now accurate enough to be useful for indoor navigation.
In a new study, the researchers used muWNS to track a user’s route across the basement floor of the University of Tokyo’s Institute of Industrial Science – an area that cannot be reached by conventional GNSS. This was done using reference detectors placed on the building’s sixth floor.
Noticeable improvement
When the user was in close range with the reference detectors, the system showed a noticeable improvement. “The current accuracy of muWNS is 2–25 m, with a range of up to 100 m, depending on the depth and speed of the person walking,” Tanaka explains. “This is as good as, if not better than, single-point GPS positioning aboveground in urban areas.”
Nuclear reactor reconstructed in 3D using muon imaging
However, Tanaka says there is still much room for improvement. “MuWNS is still far from practical. People need one-metre accuracy, and the key to this is the time synchronization.”
The researchers hope future improvements could be made by using chip-scale atomic clocks for timing. These clocks are an order of magnitude more precise than quartz crystals, but are too expensive today for practical use. Tanaka’s team also intends to miniaturize the system’s components, and believes it could eventually fit onto a handheld device.
The research is described in iScience.
