A group of MIT boffins have successfully tested what they said is the first practically useful ultra-low power underwater networking and communication technology, and all it took was some nearly 70-year old technology to get it going. 

In a pair [PDF] of papers [PDF] set to be presented in the coming months, the researchers detailed how, with a combination of piezoelectric materials (which create electrical charge when exposed to mechanical energy) and an old-school Van Atta array they created devices able to transmit acoustic signals at kilometer-plus ranges without even needing a battery. 

Van Atta arrays are a series of connected nodes able to triangulate and redirect signals back toward their source rather than simply reflecting them. In underwater applications, this makes them far more efficient than acoustic devices that transmit data omnidirectionally, which leads to energy loss and shorter comms ranges from signal scatter.

By building Van Atta arrays consisting of piezoelectric sensors (with transformers between nodes to avoid energy loss), the team was able to develop acoustic sensors that are roughly three feet squared, orientation independent, and able to reflect multiple simultaneous signals at distances of up to four or five kilometers, MIT associate professor and MIT Media Lab Signal Kinetics group director Fadel Adib told The Register. 

The system can theoretically reach that four or five-kilometer range, as Adib and his team demonstrated via models it was forced to build once the team ran out of dock space to test the system in the field. 

"The dock we were testing on is only so long," Adib told us. "We have docks in the river and the ocean, but if we want to test at further ranges we're going to have to head out on a boat," which Adib said his team has plans to do soon. 

In the meantime, actual field tests of the system have been limited to 300 meters, which is still 15 times further than previous designs using the same throughput and power that Adib's team had been working on. This latest development on the 1959 design, dubbed Van Atta Acoustic Backscatter (VAB) was only invented five years ago and "is just now becoming practical," Adib said. 

Despite the physical limits placed on their tests by the size of the docks near MIT, the team has still managed to test the VAB's efficiency.

"Instead of trying to get longer distances we've been reducing power" applied to the nodes, Adib told us. Power was dialed all the way back to "a tenth of a Watt so we're pretty certain it'll work over longer distances." 

While more power may be required at longer ranges, it'd still be a fraction of what's needed for other underwater comms systems, the team claimed.

Once they can validate the models showing four to five kilometers of range, the team said their battery-free sensors could be used to measure the vital signs of the ocean (temperature, pressure, etc.), improve climate modeling, be used for coastal monitoring and the tracking of marine life, and could even find a use exploring aquatic exoplanets or moons like Titan. 

"There are still a few interesting technical challenges to address, but there is a clear path from where we are now to deployment," Adib said. ®

https://www.theregister.com//2023/09/07/mit_underwater_communications/