Shulou(Shulou.com)11/24 Report--

This article comes from the official account of Wechat: SF Chinese (ID:kexuejiaodian), author: SF

To navigate, we often rely on satellite navigation systems such as GPS and Beidou. But in some places, especially deep underground, satellite signals are difficult to reach. What are we going to do? Recently, scientists have proposed that this problem can be solved by using a particle produced when cosmic rays collide with the earth's atmosphere.

(by Chen Qiang / tr. by Robert Taylor)

The satellite navigation system has a blind area satellite navigation system, whether it is the Global Positioning system (GPS) in the United States or the Beidou Satellite Navigation system (BDS) in China, which works thanks to a technology called "trilateral measurement". Next, let's take car navigation as an example to illustrate its principle.

When a navigation satellite moves around the earth, it transmits radio signals to the ground, and the signal receiver on your car can receive this signal at every moment; record the time of transmission and reception of the signal, you can know the time of the signal transmission, multiplied by the speed of light, you can get the distance between the car and the satellite at this time. The distance is known, which means that your car is located on a sphere centered on this satellite and with a radius of that distance. If we want to know where the car is, we also need to refer to the distance from other satellites. Similarly, the sphere is drawn with each satellite as the center and the distance as the radius, and the intersection of these spheres is where the car is at this time. The coordinates of these satellites are known, so that the coordinates of the car can be given.

However, radio waves are a kind of electromagnetic waves that can be easily blocked. They cannot penetrate rocks or water, nor can they penetrate walls and thick woods. In addition, navigation satellite signals are usually weak at high latitudes, and there are currently no navigation satellites operating over the poles. Therefore, it is necessary to develop a new navigation system to overcome the blind area of the satellite navigation system.

With the help of highly penetrating particles, Hiroyuki Tanaka, a physicist at the University of Tokyo in Japan, and his colleagues have been studying the use of muons as a substitute for navigation satellite signals for years. A μ is an elementary particle with a unit negative charge and a spin of 1 to 2. It has similar properties to electrons and tauons, which also belong to leptons.

When cosmic rays from outer space collide with particles in the earth's atmosphere, they produce a large number of microns, which pour down to the ground like raindrops. Muons are almost everywhere, and there may be a large number of microns passing through your body right now. It is estimated that one mu falls on each square centimeter of the earth's surface every minute. However, you don't have to worry about it. μ is harmless to the human body.

Muons can travel at nearly the speed of light and can penetrate deep underground. In recent years, μ sub-imaging technology has been widely concerned and studied because of its super penetration ability. Scientists have learned to use muons to scan the internal structures of ancient city walls, pyramids and volcanoes.

Muons can penetrate into places where radio waves cannot penetrate, which makes Tanaka and others think that they can be used to make up for the shortcomings of satellite navigation systems.

μ sub is used to navigate with high precision clock, and its principle is similar to satellite navigation system. Four reference stations that can detect microns are placed on the ground, and the receiver of microns is placed on underground equipment or people. When the microns in the sky fall, they first pass through the detector of the reference station and then reach the receiver underground. When the time difference between the reference station and the receiver is recorded, the trilateral measurement can be carried out and the coordinates of the receiver can be given.

A few years ago, Tanaka and others developed a navigation system based on this principle, which is called μ Sub-Measurement Positioning system (muPS). However, muPS requires a wired connection, which is not very practical for people who walk around. Now, the team has made the system wireless, and the new system is called μ Sub-Measurement Wireless Navigation system (MuWNS).

The navigation system is not perfect because it cannot give the coordinates of the receiver in real time. However, the team did experiment with it.

They connected the reference station and receiver to a synchronized precision quartz clock. One of the reference stations was placed on the sixth floor of the building, while people carrying receivers walked around the basement. The receiver coordinates given by the system can be used to reconstruct the route taken by the person in the basement. The results show that the current accuracy of MuWNS is between 2m and 25m, and the detection range can be up to 100m, depending on the depth and speed of walking. This is similar to or even better than the GPS positioning effect of urban areas on the ground.

But this is far from reaching the level of practical application. The accuracy of the system needs to reach at least 1 meter, and can achieve real-time navigation.

The crux of the problem is time synchronization. In other words, MuWNS needs a better clock. The chip-level atomic clock is a very good choice, which uses the precise microwave signal emitted by the cesium atom in the transition between the two energy levels to measure time. But this kind of clock is too expensive now, and maybe its price will come down in the future.

At the same time, the team hopes to try to reduce the size of the receiver to something you can hold in your hand. In this way, through further improvement, MuWNS can be applied to all satellite navigation system failures in underground, underwater, high latitudes and other places in the future, and can assist us to complete search and rescue, underground mining, seafloor monitoring, radio quiet area navigation and other tasks.

References:

Https://doi.org/10.1016/j.isci.2023.107000

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