MIT Team Develops Underwater GPS to Map Ocean Floors

The underwater navigation system uses sound instead of batteries.
Fabienne Lang
MIT's Underwater Backscatter LocalizationMIT

Oceans make up approximately three-quarters of our Earth, yet a large part of them is still inaccessible to us. Funnily enough, we're able to explore space more easily. Why is that? 

One of the reasons lies in safe and long-lasting GPS systems. We rely on them regularly on the surface of the Earth, and even above it in the stratosphere and beyond, but they've been lacking in the water department. 

A team of MIT scientists has developed an underwater navigation system that uses sound instead of batteries to move around, called the Underwater Backscatter Localization (UBL) system. Using a two-way pattern, the UBL can move around underwater without any batteries, which will undoubtedly improve ocean exploration.

The team presented its research in a paper at the Association of Computing Machinery's Hot Topics in Networks workshop.


Water and GPS

Once radio waves enter water they become scattered, rendering any GPS useless. This is why sonar is widely used underwater, however, this method of using acoustics is very energy-intensive. 

So, MIT researchers set themselves the task of finding a way of minimizing the use of batteries to power underwater navigation systems, and they came up with the UBL. This system is especially useful for smaller devices, such as those that follow animals underwater. 

The team looked into piezoelectric materials, these materials generate an electric charge under mechanical stress, which includes being subjected to sound waves. For their system, the researchers used piezoelectric sensors to selectively reflect back sound waves from the environment as backscatter, all while using the sound waves themselves for power. 

These sound waves were picked up by a receiver as a binary pattern with 1 standing for reflected sound waves, and 0 for unreflected ones. 

This binary signal is then used by the UBL system to carry information that could be used to create a location fix simply by timing the length of time a sound wave takes to reflect off the sensor and come back to the observation unit. 

The team has demonstrated how tricky this system is, especially in shallow waters where rebounding signals off of the ocean's floor create a complex pattern. So, the researchers turned to frequency hopping. This means that signals of varying frequencies are sent across in a pattern, returning at different times. 

Using both the timing and the phase data allowed for a more specific location fix. 

Currently, the team has a proof-of-concept UBL system that can go up to distances of 20 inches (50 cm). The team's next steps include increasing this range, and ultimately, creating a navigation system that allows autonomous vehicles to map out the ocean's floor.