MIT Creates Battery-Free Underwater GPS

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If you need to know where you are, there’s a network of GPS satellites in orbit that can tell you with a high degree of accuracy. That is, unless, you’re underwater. The GPS radio signal dissipates quickly when it hits water, causing a headache for scientific research at sea. The only alternative is to use acoustic systems that chew through batteries. A team from MIT has devised a battery-free tracking technology called Underwater Backscatter Localization (UBL) that could end this annoyance. 

Currently, scientists who want to track a drone or a tagged animal have to use power-hungry acoustic location technologies. These devices require batteries, which add bulk and limit the useful lifespan of trackers. Recharging batteries is often difficult, verging on impossible. If you’re trying to follow a tagged whale, for example, you probably won’t get close enough to swap the battery. 

The technology was developed under the supervision of lead study author Reza Ghaffarivardavagh and Fadel Adib, who leads the research team. UBL still relies on sound waves, but it’s much more efficient about it. Adib and his team leveraged piezoelectric materials, a technique they previously used to create battery-free sensors (see below). These materials generate an electric charge in response to mechanical stress. In this case, the mechanical stress is the vibration from soundwaves. 

The charge generated by the piezoelectric sensor allows the system to selectively reflect some soundwaves back into the underwater environment. Meanwhile, a receiver translates those reflections (the backscatter) into either a 1 (reflected) or a 0 (not reflected). Put that together, and you have a low-bitrate binary code. To turn this into location technology, an observation unit simply emits soundwaves and tracks how long it takes for the piezoelectric sensor to return the signal. 

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There’s a kink in this plan, though. Sound waves propagate in all directions, causing a messy acoustic environment that would be computationally expensive to analyze. The team devised a solution with frequency hopping similar to the approach that helps wireless networks avoid interference. The observation unit emits several different frequencies so the waves bounce back in phases, making the signal easier to parse. 

The approach showed promise off the bat, but there were some additional challenges. To combat echoes in shallow water, the researchers found they could slow the bitrate from 2,000 bits/second to 100 bits/second. That’s still plenty for a location lock on slow-moving or stationary objects, but the team is still experimenting with higher bitrates (around 10,000 bits/second) for moving objects. Finding the balance between echoes and bitrate will take some time, but UBL could eventually lead to a boom in ocean exploration.

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