Is this the new wave of submerged communications?

By: Kelsey Atherton

The ocean hides what it contains, and it is in that hiding that submarines have their power. Lurking under seas, at first with just enough capability for an attack run and now with the ability to lurk for months at a time, submarines remain power out of reach, unseen until engaged in combat or resupplying in a friendly port. That stealth comes at a cost, however, besides the simple perils of existing underwater.

When submerged, submarines are more or less on their own until they resurface again, since radio waves do not travel well through seawater. Or they are for now. New research by MIT, presented at a conference in late August, devised a way for submerged submarines to communicate wirelessly with people on the surface by combining hydroacoustics and acoustic radars.

Presently, submarines communicate either across normal radio frequencies when surfaced or through hydroacoustic signals and listening posts underwater that can transmit the messages back to counterparts on shore. Very and extremely low-frequency radio waves can be transmitted in a way that submarines can listen to below the surface, but it's a one-way form of communication, from stations on land to submarines. To get something responsive, with the flexibility to communicate away from static seabed hydrophones, needs something else.

Specifically, it needs a way to combine hydroacoustic transmission from the submarine through water that can then be converted into a useful data.

“We present a new communication technology, translational acoustic-RF communication (TARF),” write paper authors Francesco Tonolini and Fadel Adib of the MIT Media Lab.

“TARF enables underwater nodes to directly communicate with airborne nodes by transmitting standard acoustic signals. TARF exploits the fact that underwater acoustic signals travel as pressure waves, and that these waves cause displacements of the water surface when they impinge on the water-air boundary. To decode the transmitted signals, TARF leverages an airborne radar which measures and decodes these surface displacements.”

In testing, they demonstrated that the communication technique can transfer data at standard underwater bitrates up to 400bps, and even do so with surface waves 6.3 inches crest-to-crest, or 100,000 times larger than the surface perturbations made by the acoustic transmitter.

Right now, this communication is one-way. While the signal transmitted up from the water produces useful information at the boundary with the air, a signal transmitted through the air downwards would disintegrate on integration with water. This one-way is distinct from previous forms of communication with submarines, however, as it lets the submarine talk without revealing its position to surface sensors.

Despite the limitations, and the earlierness of the research, Tonolini and Adlib see a bright future for the technology, as a way to enable a host of new technology in machines. The technology, they write, can enable “many applications including submarine-to-drone communication, deep-sea exploration, and subsea IoT (Internet of Things).

https://www.c4isrnet.com/c2-comms/2018/08/28/mit-discovers-way-for-submarines-to-talk-to-drones