- Researchers develop translational acoustic radio frequency to enable seamless air-water communication.
- It only needs two components to work: an underwater transmitter and a receiver above the water surface.
Did you know that underwater machines (like submerged submarines) can’t communicate wirelessly with those on land? Both use different signals to send/receive data: Submarine uses SONAR (acoustic signals), whereas airplanes use GPS or radio signals.
Acoustic signals reflect off the water surface while radio signals cannot travel through a dense medium like water. Therefore, even today neither can share data across the water surface, causing inefficiencies in numerous applications.
To solve this longstanding challenge, researchers at MIT have developed a system named translational acoustic radio frequency communication (TARF). The idea is to utilize the air-water boundary as a communication medium.
How It Works?
They used a transmitter underwater to direct sonar signals towards the surface of the water. These signals are in the form of very small vibrations corresponding to 0s and 1s. The sensitive receiver placed above the water surface reads these tiny disturbances and decrypts the acoustic signal.
Photo credit: Christine Daniloff/MIT
Existing systems (like buoys) can pick up acoustic signal, process data, and transfer radio signals to flying devices. However, they can easily get lost and covers only a small area, and thus cannot be used in most practical applications like submarine-to-surface communications.
TARF, on the other hand, uses a small acoustic speaker to send sonar signals in the form of pressure waves with different frequencies. These frequencies correspond to data bits. For instance, the wave traveling at 200 hertz sends a binary bit ‘1’, while a 100-hertz wave sends a ‘0’.
These signals cause very small ripples (on micrometer scale) in the water whenever they hit the surface. The height of these small ripples depends on the signals’ frequencies. Multiple frequencies are transmitted simultaneously to attain high data rates.
A modulation technique named orthogonal frequency-division multiplexing allows scientists to transmit hundreds of bits at the same time. A new class of high-frequency radar processes these signals in the wave spectrum ranging from 30 to 300 GHz.
The radio signal reflecting off the vibrating surface and rebounding back to radar carries binary data from the water surface to radar. However, the signal comes back to the radar with a slight variation: the angle of the signal changes according to the binary bit sent by the acoustic signal. For example, a surface vibration of 0 bit would cause reflected angle of the radio signal to vibrate at 100 Hz. This way, researchers picked up the variations corresponding to the sonar signal.
As you might have guessed by now, the major challenge in this study was collecting micrometer waves that are usually surrounded by giant, natural waves. In extreme environments, the oceans can produce a million times larger waves, which interfere with extremely small sonar vibrations.
To overcome this issue, they created a signal processing algorithm to pick up acoustic vibrations. Typically, natural waves occur at up to 2 Hz, whereas sonar vibrations are nearly 100 times faster (100 to 200 Hz). The algorithm eliminates/filters all slower waves.
Testing and Application
Courtesy of researchers
The researchers performed 500 tests in a swimming pool and tank. In the pool, they placed the transmitter 3.5 m below the water surface, and the radar 30 cm above surface. In the tank, the transmitter was immersed between 5 and 70 cm below the surface, while the radar was placed between 20 and 40 cm above the water surface. They also had swimmers generating waves of nearly 16 cm.
In both configurations, the system accurately decoded several data at hundreds of bits per second. For example, it successfully transmitted a sentence, “Hello! from underwater”.
However, the disturbances caused by higher than 16 cm waves affected the system’s performance. The technology is still in its infancy: In the future work, researchers will improve the TARF to make it work in extreme water conditions.
This new technology could greatly enhance air-water communication, for instance, submarines would not need to come to the surface (or change their location) every time they send signals to airplanes. And, deep diving underwater drones would not need to regularly resurface to transmit data. TARF can also aid searches for planes that mysteriously vanish underwater.