Radio waves don’t penetrate water. The exception is what is know as ELF, extremely low frequencies, which the major navies use to talk to their submarines. But using ELF isn’t really practical for the rest of us due to the antenna sizes, which need to be thousands of miles long, which also means that communication is one way. You can’t fit a thousand mile long antenna on a submarine. Which is where a new system called Translational Acoustic-RF (TARF), being worked on by the Signal Kinetics group at the MIT Media Lab, might prove useful.
“A TARF transmitter sends standard sound (or SONAR signals). Sound travels as pressure waves; when these waves hit the surface, they cause it to vibrate. To pick up these vibrations, a TARF receiver in the air uses a very sensitive radar. The radar transmits a signal which reflects off the water surface and comes back. As the water surface vibrates, it causes small changes to the received radar signal, enabling a TARF receiver to sense the tiny vibrations caused by the underwater acoustic transmitter.”
The problem with communication through the surface of the water is also why, although some high end commercial systems use laser communications, most ROVs operate using tethers.
TARF may open up the the ability to build ROVs that can at least report back without tethers, although they’ll need to be autonomous because we won’t be able to transmit commands due to the one-way nature of the TARF system. While vessels underwater can talk to those above the surface, aircraft and other vessels using TARF to receive the messages can’t talk back.
Like ELF the system TARF is low bandwidth, and right now the system does not work if wave height is greater than 16 cm (around six inches). Something most mariners would consider “flat calm.” However the research team is hoping to develop algorithms to filter the wave “noise” out from the SONAR signal to allow transmission is slightly choppier conditions.
If you want to know more about TARF details can be found in the project’s latest research paper, which was presented at this year’s ACM’s SIGCOMM meeting, held this week in Budapest.