GPS is invalid, radio is too weak! Researchers develop underwater backscatter positioning system, deep-sea navigation may no longer be difficult
For precision guidance, some major countries have established their own satellite navigation systems. The United States has GPS, China has Beidou, and Russia has GLONASS. However, satellite navigation systems can only play a role on land and sea level, and have little effect on submarine navigation.
At the same time, due to the characteristics of radio waves, its ability to penetrate water is very poor, making it difficult to navigate with radio waves on the seabed. To track deep-sea creatures like whales, researchers usually rely on sound signals. However, devices that generate and send sound usually require batteries. These batteries are large, short in life, require regular use, and are cumbersome to use, which are very disadvantageous for long-term tracking.
Massachusetts Institute of Technology researchers have recently established a battery-free precision Positioning system. UBL does not emit sound signals, but reflects modulated signals from the environment, which can provide positioning information with zero energy consumption.
UBL in the test (source:MIT)
This research was conducted by the MIT Media Laboratory and the associate professor of MIT’s Department of Electrical Engineering and Computer Science, Adi Than (Adib) leadership. The team used a unique resource for low-power acoustic signals:piezoelectric materials.
Piezoelectric material refers to a crystalline material that has an electric charge between the two ends when it is subjected to pressure, just like being hit by a vibrating sound wave. Piezoelectric sensors can use these charges to selectively reflect some sound waves back into the environment. The receiver converts this sequence of reflections called backscatter into a pattern of 1 (reflected sound wave) and 0 (unreflected sound wave). The resulting binary code can carry information about ocean temperature or salinity.
In principle, the same technology can be used to provide location information. An observation unit can emit sound waves, and then calculate how long it takes for the sound waves to reflect from the piezoelectric sensor and return to the observation unit. The elapsed time can be used to calculate the distance between the observer and the piezoelectric sensor.
But in fact, determining the time for this backscatter is very complicated, because the ocean can be an echo chamber, and sound waves not only propagate directly between the observation unit and the sensor, but also between the water surface and the seabed. Shuttle back and forth between time and return to the observation unit at different points in time. This problem is particularly prominent in shallow waters. The shallower the depth, the more mixed the signal rebound.
Researchers use”frequency hopping” to solve this problem, that is, to send a series of signals in a frequency range. Each frequency has a different wavelength, so the reflected sound waves return to the observation unit with different phases. By combining information about time and phase, observers can pinpoint the distance to the tracking device.
After testing, the frequency hopping scheme is effective in deep water simulation, but the researchers said that the application in shallow water also needs to consider the problem of signal mixing.
In areas with complex signals in the ocean, the amount of information transmitted must be reduced. To this end, the researchers reduced the bit rate, that is, extended the interval between each signal sent by the observation unit. This spacing ensures that the echo of each bit disappears before disturbing the next bit.
In order to track moving objects, researchers actually have to increase the bit rate. For example, in order to accurately locate an object moving in deep water at a speed of 30 cm/sec, the speed must be increased to 10,000 bits per second.
It can be seen that UBL requires a low bit rate in complex areas of the signal, and a high bit rate in order to track high-speed objects. Researchers are also currently working to resolve this contradiction.
Although this technology is still under development, UBL may one day become an important technical means for seabed exploration.
Compilation/Foresight Economist APP Information Group