MIT Develop Battery Free Underwater Navigation System

MIT Develop Battery Free Underwater Navigation System

GPS signals which are radio waves fail to provide navigation in waters because radio waves quickly deteriorate as they move through the water.

To navigate and track undersea objects like drones or whales, researchers rely on acoustic signals.  Acoustic signals or sound waves travel faster and further underwater than through air, making them an efficient way to send data. However, to generate acoustic signals devices need power which is generated by on board batteries. And batteries need frequent charging and replacement over the period of time.

This makes it hard to precisely track objects or animals for a long time-span – changing a battery is no simple task when it’s attached to a migrating whale. So, the team sought a battery-free way to use sound.

MIT researchers have built a battery-free underwater navigation system dubbed Underwater Backscatter Localization (UBL).

The system does not emit its own acoustic signals rather UBL reflects modulated signals from its environment. That provides researchers with positioning information, at net-zero energy. Though the technology is still developing, UBL could someday become a key tool for marine conservationists, climate scientists, and the U.S. Navy.

These advances are described in a paper being presented this week at the Association for Computing Machinery’s Hot Topics in Networks workshop, by members of the Media Lab’s Signal Kinetics group. Research Scientist Reza Ghaffarivardavagh led the paper, along with co-authors Sayed Saad Afzal, Osvy Rodriguez, and Fadel Adib, who leads the group and is the Doherty Chair of Ocean Utilization as well as an associate professor in the MIT Media Lab and the MIT Department of Electrical Engineering and Computer Science.

The Principle of Underwater Navigation System – Underwater Backscatter Localization (UBL)

Adib’s group turned to a unique resource they’d previously used for low-power acoustic signaling: piezoelectric materials. These materials generate their own electric charge in response to mechanical stress, like getting pinged by vibrating soundwaves. Piezoelectric sensors can then use that charge to selectively reflect some soundwaves back into their environment. A receiver translates that sequence of reflections, called backscatter, into a pattern of 1s (for soundwaves reflected) and 0s (for soundwaves not reflected). The resulting binary code can carry information about ocean temperature or salinity.

In principle, the same technology as GPS could provide location information. In the underwater navigation system, an observation unit could emit a soundwave, then clock how long it takes that soundwave to reflect off the piezoelectric sensor and return to the observation unit. The elapsed time could be used to calculate the distance between the observer and the piezoelectric sensor. But in practice, timing such backscatter is complicated, because the ocean can be an echo chamber.

Like GPS signals which have multipath errors, the sound waves don’t just travel directly between the observation unit and sensor. They also careen between the surface and seabed, returning to the unit at different times.

Underwater Navigation System - Underwater Backscatter Localization (UBL)
Image source: MIT

The researchers overcame the reflection issue with “frequency hopping.” Rather than sending acoustic signals at a single frequency, the observation unit sends a sequence of signals across a range of frequencies. Each frequency has a different wavelength, so the reflected sound waves return to the observation unit at different phases. By combining information about timing and phase, the observer can pinpoint the distance to the tracking device. Frequency hopping was successful in the researchers’ deep-water simulations, but they needed an additional safeguard to cut through the reverberating noise of shallow water.

Where echoes run rampant between the surface and seabed, the researchers had to slow the flow of information. They reduced the bitrate, essentially waiting longer between each signal sent out by the observation unit. That allowed the echoes of each bit to die down before potentially interfering with the next bit. Whereas a bitrate of 2,000 bits/second sufficed in simulations of deep water, the researchers had to dial it down to 100 bits/second in shallow water to obtain a clear signal reflection from the tracker. But a slow bitrate didn’t solve everything.

To track moving objects, the researchers actually had to boost the bitrate. One thousand bits/second was too slow to pinpoint a simulated object moving through deep water at 30 centimeters/second. At a speedy 10,000 bits/second, they were able to track the object through deep water.

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