MIT researchers are working on solving a longstanding challenge in wireless communication: direct data transmission between underwater and airborne devices. “Today, underwater sensors cannot share data with those on land, as both use different wireless signals that only work in their respective mediums. Radio signals that travel through air die very rapidly in water,” explained MIT, in a statement.
In a paper being presented at this week’s SIGCOMM conference, MIT Media Lab researchers presented a system that uses an underwater transmitter directs a sonar signal to the water’s surface, causing tiny vibrations that correspond to the 1s and 0s transmitted. A highly sensitive receiver above the surface reads these minute disturbances and decodes the sonar signal.
Until recently, there was no method of sending signals between air and water. Fadel Adib and Francesco Tonolini of MIT Media Lab have developed a way to connect these seemingly dissonant mediums through Translational Acoustic-RF communication, or TARF. Using sound waves from underwater, and Radar from the air, messages can be transmitted by creating faint ripples on the surface of the water.
Military submarines, for example, could use the system and not surface to communicate with airplanes, thereby avoid compromising their location. Similarly underwater drones that monitor marine life wouldn’t need to constantly resurface from deep dives to send data to researchers. Another promising application is aiding searches for planes that go missing underwater. “Acoustic transmitting beacons can be implemented in, say, a plane’s black box,” Adib says.
TARF includes an underwater acoustic transmitter that sends sonar signals using a standard acoustic speaker. The signals travel as pressure waves of different frequencies corresponding to different data bits. When the signal hits the surface, it causes tiny ripples in the water, only a few micrometers in height, corresponding to those frequencies. To achieve high data rates, the system transmits multiple frequencies at the same time, building on a orthogonal frequency-division multiplexing. This lets the researchers transmit hundreds of bits at once.
The researchers took TARF through 500 test runs in a water tank and in two different swimming pools on MIT’s campus. In the tank, the radar was placed at ranges between 20 centimeters to 40 centimeters above the surface, and the sonar transmitter was placed from 5 centimeters to 70 centimeters below the surface. In the pools, the radar was positioned about 30 centimeters above surface, while the transmitter was immersed about 3.5 meters below. In these experiments, the researchers also had swimmers creating waves that rose to about 16 centimeters.
In both settings, TARF was able to accurately decode various data at hundreds of bits per second — such as the sentence, “Hello! From underwater”. The research was supported, in part, by the National Science Foundation and researchers hope that their system will enable an airborne drone or plane flying across water’s surface to constantly pick up and decode the sonar signals as it zooms by.