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. Acoustic signals, or sonar, sent by underwater devices mostly reflect off the surface without ever breaking through. This causes inefficiencies and other issues for a variety of applications such as ocean exploration and submarine-to-plane communication.
A system was developed that tackles this problem. An underwater transmitter directs a sonar signal to the water’s surface, causing tiny vibrations that correspond to the 1s and 0s transmitted. Above the surface, a highly sensitive receiver reads these minute disturbances and decodes the sonar signal. The system, called translational acoustic-RF communication (TARF), would not require military submarines to surface to communicate with airplanes, compromising their location. And 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 a plane’s black box. If it transmits a signal occasionally, the new system could pick up that signal.
Today’s technological workarounds to this wireless communication issue suffer from various drawbacks. Buoys, for instance, have been designed to pick up sonar waves, process the data, and shoot radio signals to airborne receivers. But these can drift away and get lost. Many are also required to cover large areas, making them impracticable for submarine-to-surface communications.
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 transmitter wants to send a 0, it can transmit a wave traveling at 100 hertz; for a 1, it can transmit a 200-hertz wave. 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 modulation scheme used in wireless communication called orthogonal frequency-division multiplexing. This lets the researchers transmit hundreds of bits at once.
Positioned in the air above the transmitter is a new type of extremely-high-frequency radar that processes signals in the millimeter wave spectrum of wireless transmission, between 30 and 300 gigahertz. The radar, which looks like a pair of cones, transmits a radio signal that reflects off the vibrating surface and rebounds back to the radar. Due to the way the signal collides with the surface vibrations, the signal returns with a slightly modulated angle that corresponds exactly to the data bit sent by the sonar signal. A vibration on the water surface representing a 0 bit, for instance, will cause the reflected signal’s angle to vibrate at 100 hertz.
A key challenge was helping the radar detect the water surface. To do so, a technology was used that detects reflections in an environment and organizes them by distance and power. As water has the most powerful reflection in the new system’s environment, the radar knows the distance to the surface. Once that’s established, it zooms in on the vibrations at that distance, ignoring all other nearby disturbances.