Scientists conduct the first test of a cosmic-ray wireless navigation system

GPS is now a mainstay of everyday life, helping us with location, navigation, tracking, mapping and timing in a wide spectrum of applications. But it has a few shortcomings, most notably not being able to go through buildings, rocks or water. That’s why Japanese researchers have developed an alternative wireless navigation system that relies on cosmic rays, or muons, instead of radio waves, according to a new paper published in the journal iScience. The team has completed their first successful test and the system could one day be used by search and rescue teams, for example to guide robots underwater or to help autonomous vehicles navigate underground.

“Cosmic ray muons fall evenly over the Earth, always traveling at the same speed regardless of the matter they traverse, even penetrating kilometers of rock,” said study co-author Hiroyuki Tanaka of Muographix at the University of Tokyo in Japan. by using muons we have developed a new kind of gps called the muometric positioning system (muPS) that works underground, indoors and underwater.”

As previously reported, there is a long history of using muons to image archaeological structures, a process made easier because cosmic rays provide a steady supply of these particles. Muons are also used to hunt for illegally transported nuclear material at border crossings and to monitor active volcanoes in hopes of detecting when they might erupt. In 2008, scientists at the University of Texas, Austin, repurposed ancient muon detectors to search for possible hidden Mayan ruins in Belize. Physicists at Los Alamos National Laboratory have developed portable versions of muon imaging systems to unlock the construction secrets of the dome (Il Duomo) atop the Cathedral of St. Mary of the Flower in Florence, Italy, designed by Filippo Brunelleschi in the early 15th century. century.

In 2016, scientists using muon imaging picked up signals pointing to a hidden passage behind the famous chevron blocks on the north side of the Great Pyramid of Giza in Egypt. The following year, the same team discovered a mysterious void in another part of the pyramid, believing it could be a hidden chamber, which was then mapped using two different muon imaging methods. And last month, scientists used muon imaging to discover a previously hidden chamber at the ruins of the ancient necropolis of Neapolis, some 10 meters (about 33 feet) below modern-day Naples, Italy.

Autonomous robots and vehicles could one day be commonplace in homes, hospitals, factories and mining operations, as well as for search and rescue missions, but there’s no universal means of navigation and positioning yet, according to Tanaka et al. As noted, GPS cannot penetrate underground or underwater. RFID technologies can achieve good accuracy with small batteries, but require a control center with servers, printers, monitors, and so on. Dead reckoning is plagued by chronic estimation errors without an external signal providing correction. Acoustic, laser scanner and Lidar approaches also have drawbacks. So Tanaka and his colleagues turned to muons in developing their own alternative system.

Muon imaging methods usually involve gas-filled chambers. As muons whiz through the gas, they collide with the gas particles and emit a telltale flash of light (scintillation), which is registered by the detector, allowing scientists to calculate the particle’s energy and trajectory. It is similar to X-ray imaging or ground-penetrating radar, except with naturally occurring high-energy muons instead of X-rays or radio waves. That higher energy makes it possible to visualize thick, dense substance. The closer the imaged object, the more muons are blocked. The Muographix system is based on four above-ground muon-sensing reference stations that serve as coordinates for the muon-sensing receivers, which are deployed underground or underwater.

The team conducted the first trial of a muon-based underwater sensor array in 2021 and used it to detect the rapidly changing tidal conditions in Tokyo Bay. They placed ten muon detectors in the service tunnel of the Tokyo Bay Aqua-Line roadway, which is some 45 meters (147 feet) below sea level. They were able to image the sea above the tunnel with a spatial resolution of 10 meters (nearly 33 feet) and a temporal resolution of one meter (3.3 feet), sufficient to demonstrate that the system is capable to observe strong storm waves or tsunamis.

The array was put to the test in September of that same year, when Japan was hit by a typhoon approaching from the south, causing mild ocean swells and tsunamis. The extra volume of water slightly increased muon scattering, and that variation was in good agreement with other measurements of ocean swell. And last year, Tanaka’s team reported that they had successfully imaged the vertical profile of a cyclone using muography, showing the cross-sections of the cyclone and revealing variations in density. They found that the warm core had a low density, unlike the high-pressure cold exterior. In combination with existing satellite tracking systems, muography could improve cyclone forecasts.

The team’s earlier iterations connected the receiver to the ground station with a wire, which significantly restricted movement. This new version, the Muometric Wireless Navigation System or MuWNS, as the name implies, is completely wireless and uses high-precision quartz clocks to synchronize the ground stations with the receiver. Together, the reference stations and synchronized clocks make it possible to determine the receiver’s coordinates.

For the test run, the ground stations were placed on the sixth floor of a building and a “navigee” holding the receiver walked through the basement corridors. The resulting measurements were used to calculate the navigee’s route and confirm the path traveled. According to Tanaka, MuWNS performed with an accuracy of between 2 and 25 meters (6.5 to 82 feet), with a range of as much as 100 meters (about 328 feet). “This is as good as, if not better than, single-point GPS positioning above ground in urban areas,” he said. “But it’s still far from a practical level. Humans need one-meter accuracy, and the key to this is time synchronization.”

One solution would be to incorporate commercially available chip-scale atomic clocks, which are twice as accurate as quartz clocks. But those atomic clocks are too expensive at the moment, though Tanaka foresees costs to fall in the future as the technology becomes more widely integrated into mobile phones. The rest of the electronics used in MuWNS will be miniaturized in the future to make it a handheld device.

DOI: iScience, 2023. 10.1016/j.isci.2023.107000 (About DOIs).