Submarines and aircraft can't talk to one another, but that may one day be routine if a new MIT technology pans out. The Translational Acoustic-RF (TARF) communication system uses a combination of acoustics and radar to allow messages transmitted underwater to be directly received in the air without the need for any sort of physical link. The technology holds the promise of making the "silent world" not so silent.

In terms of communications, the underwater world is almost the ultimate in getting away from it all. Submerge a few yards beneath the surface and radio messages are left far behind as the water effectively blocks off all conventional transmissions.

The only practical form of radio communications are the land-based Very Low Frequency and Extremely Low Frequency systems used by some of the major naval powers to communicate with their submarines while at depth. But these are only one-way systems where the boats can only receive at the painfully low rate of a few words a minute and cannot reply.

For most communications, submarines, remote controlled drones, and sensor packs must either ascend to antenna depth, release a relay buoy, or use a tether to send messages. This is because not only does water prevent radio signals from propagating, but the only practical form of underwater communications today is through sonar and other acoustical systems that rely on sound. This means we are dealing with two entirely incompatible forms of communication with no common ground – until now.

The TARF system aims to change that by turning the interface between air and water into a translation medium that will allow acoustic systems to talk to radio systems – effectively making it possible for submarines to talk to airplanes. Developed by MIT Media Lab, TARF is still in its infancy, but the basic technology has already proven itself.

Currently, TARF consists of a standard underwater acoustic speaker that sends sonar signals that travel through the water as pressure waves at different frequencies for different kinds of data, like 1,000 Hz for 0 and 200 Hz for 1. These frequencies can be multiplexed to allow for high data rates in a way similar to digital television broadcasts and 4G, using a system called orthogonal frequency-division multiplexing.

When these acoustic signal reach the surface, they create tiny ripples in the water. These are monitored by a radar system in the air above the transmitter operating between 30 and 300 gigahertz that sends down radio beams that are reflected back, where an algorithm analyzes the signal for distance and power. This allows the system to weed out any nearby clutter and focus on the true sonar signal. Other algorithms eliminate the much larger natural waves by looking for the smaller, faster sonar waves and ignoring the larger, slower natural ones.

According to MIT, the team has conducted over 500 tests of TARF in a laboratory water tank and in two swimming pools with swimmers splashing about. Despite this, the system was able to accurately read the sonar signals that included messages like "Hello from underwater." However, a swimming pool is very different to an ocean and the researchers say that TARF still only works on calm days. What is needed now is to find ways to make it work in all seas and weather.

Once the technology is made practical, the team sees it has having a wide variety of applications. Not only will it allow submarines, remote submersibles, and sensors to send messages to aircraft while still submerged, but it could also be used to find crashed airplanes by means of acoustic beacons that could be detected by search aircraft.

"Acoustic transmitting beacons can be implemented in, say, a plane's black box," says project leader Fadel Adib. "If it transmits a signal every once in a while, you'd be able to use the system to pick up that signal."

The research was presented in a paper at the SIGCOMM conference in Budapest.

The video below shows TARF being tested.

Source: MIT