Until recently, there has never been a way to send communication signals between air
and water.
MIT researchers have taken a step toward solving it i-e enabling 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.
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.
MIT Media Lab researchers have designed a system that tackles this problem in a novel
way.
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), is still in its early
stages.
But it represents a “milestone” that could open new capabilities in water-air communications.
Using the system, military submarines, for instance, wouldn’t need 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.
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
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.
For example, 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, the researchers employed a technology 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.
The next major challenge was capturing micrometer waves surrounded by much larger, natural waves.
The smallest ocean ripples on calm days, called capillary waves, are only about 2 centimeters
tall, but that’s 100,000 times larger than the vibrations.
Rougher seas can create waves 1 million times larger.
This interferes with the tiny acoustic vibrations at the water surface.
To solve this, the researchers developed sophisticated signal-processing algorithms.
Natural waves occur at about 1 or 2 hertz — or, a wave or two moving over the signal
area every second.
The sonar vibrations of 100 to 200 hertz, however, are a hundred times faster.
Because of this frequency differential, the algorithm zeroes in on the fast-moving waves
while ignoring the slower ones.
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 from 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 — such as the sentence,
“Hello! from underwater” — at hundreds of bits per second, similar to standard data
rates for underwater communications.