No matter where you are on land or in the sky, you can be connected wirelessly. New research from MIT hopes to extend your connection even as you go deep.
Historically, submarine communication has used sonar. Above the surface, radio waves are used. There has been no connection between the two methods. Since World War I, engineers have been trying to find a way for submarines and airplanes to communicate while the submarine is underwater. Submarines rely on remaining underwater for stealth and protection. They have only been able to communicate with the outside world when surfaced, which exposes them to detection and threats.
Another problem has been the recovery of airplane black boxes. They are found using sonobuoys dropped in the vicinity where an airplane is thought to have gone down. It’s a rather hit or miss affair that was painfully brought to light with Malaysian Airlines flight 370. It has still not been found despite one of the most extensive searches in history.
Both radio and sonar (acoustic) emissions only seem to work in their respective medium. Radio signals that travel through the air die rapidly in water. Sonar signals sent by underwater devices mostly reflect off the surface without ever breaking through.
Researchers from MIT’s Media Lab may have finally bridged the barrier.
“Trying to cross the air-water boundary with wireless signals has been an obstacle. Our idea is to transform the obstacle itself into a medium through which to communicate,” said Fadel Adib, assistant professor and lead researcher.
Instead of trying to find one type of signal to transcend the air-water wall, they are using an airborne radio wave sensitive enough to pick up the imprint that underwater sonar signals leave on the sea surface. The team developed Translational Acoustic-RF communication(TARF), which works by applying telecom’s new favorite bandwidth for IoT and 5G networks: millimeter wave, sometimes known as mmWave.
These extremely high-frequency waves are sensitive to tiny vibrations on the surface of the water in the form of very small waves. The millimeter waves are emitted from the airborne vehicle, such as an airplane or a drone, via radar so that messages picked up from the water vibration are returned to the vehicle’s sensor.
Beneath the surface of the water, a special type of sonar is used that can produce tiny ripples for the sky-borne radar waves to pick up. The sonar signals are also responsible for transforming messages into water ripples.
“The signals travel as pressure waves of different frequencies corresponding to different data bits,” according to the MIT Media Lab press release. “For example, when the transmitter wants to send a 0, it can transmit a wave traveling at 100 Hz; for a 1, it can transmit a 200-Hz wave. When the signal hits the surface, it causes tiny ripples in the water, only a few micrometers in height, corresponding to those frequencies.”
Considering that the signal-bearing water ripples are quite small and delicate, Adib’s team faced somewhat of a challenge in getting them to be heard amidst the chaos of a stormy sea. The concern is that the sensitive millimeter waves will not be able to pick up the delicate sonar ripples.
“It’s as if someone’s screaming, and you’re trying to hear someone whispering at the same time,” Adib said.
To solve this, the researchers developed sophisticated signal-processing algorithms. Natural waves occur at about 1 or 2 Hz—a wave or two moving over the signal area every second. The sonar vibrations of 100 to 200 Hz, 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.
Of course, the technology is still in its infancy and nowhere near perfect. For one thing, it’s only one-way communication. Submarines may be able to send vital messages to airplanes but still have no way to receive them.
Despite this drawback, the team is adamant that TARF will prove to be valuable, including for civilian applications. One potential use lies in the monitoring of marine life. Sensors that monitor temperature, pressure and current can emit continual updates using TARF to be picked up by drones.
The research was presented at the 2018 SIGCOMM conference.