Science

Underwater GPS system powered by sound could open up ocean exploration

Underwater GPS system powered ...
A sound-powered acoustic sensor sealed in plastic
A sound-powered acoustic sensor sealed in plastic
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A sound-powered acoustic sensor sealed in plastic
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A sound-powered acoustic sensor sealed in plastic

MIT scientists have developed an acoustic system that acts like an underwater GPS, yet doesn't need batteries to operate. The Underwater Backscatter Localization (UBL) system is powered by reflecting modulated audio signals to generate binary impulses.

GPS navigation has been so successful that not only do many people take it for granted, they also couldn't function without it. Besides helping motorists to get from point A to point B, the technology has found a bewildering variety of applications, from the battlefield to the warehouse.

It's therefore surprising that three quarters of the Earth's surface are inaccessible to GPS by simply submerging underwater. This is because water impedes and scatters the radio waves GPS depends on, making it useless. This is also the reason why submarines use sonar rather than radar to probe their surroundings. Where a radar beam would be swallowed up within a few yards, acoustic signals can travel for thousands of miles under the right conditions.

According to MIT scientists, the problem with using acoustics to create an underwater equivalent to GPS is that acoustic signal generators are very power hungry. That might not matter to a nuclear submarine, but for small devices that rely on batteries for missions like tracking animals this can be a real problem.

To help overcome this, MIT researchers with the support of the Office of Naval Research turned to piezoelectric materials that generate an electric charge under mechanical stress, including being subjected to sound waves. For the UBL system, the team used piezoelectric sensors to selectively reflect back sound waves emitted from the environment as backscatter while using the sound waves themselves as the power source. These sound waves were then picked up by a receiver as a binary pattern with 1 being reflected sound waves and 0 being unreflected sound waves.

This binary signal allows the UBL system to carry information that could be used to make a location fix by timing how long it takes a sound wave to reflect off the sensor and then return to the observation unit. However, the team points out that the underwater environment is extremely complex, with sound waves bouncing off the surface and the sea bottom. This is especially difficult in shallow waters where the rebounding signals are stronger.

To solve this problem the team used frequency hopping, where the signals were sent across a range of frequencies in a pattern, so they return at different phases. Combining the timing data and the phase data allows for a more precise fix. In shallow water, the bit rate of the signals was slowed down to allow the echoes time to subside and not interfere with the signals.

So far, the UBL system has passed a proof-of-concept test in shallow waters, where it estimated distances up to almost 50 cm (20 in). The next step will be to increase the range before starting field tests in collaboration with the Wood Hole Oceanographic Institution. The end goal is a navigation technology that will allow for autonomous vehicles that can make detailed maps of the ocean floor.

"Why can’t we send out unmanned underwater vehicles on a mission to explore the ocean? The answer is: We will lose them," says team leader Reza Ghaffarivardavagh.

The research was presented in a paper at the Association for Computing Machinery’s Hot Topics in Networks workshop.

Source: MIT

5 comments
Bob809
Will any research be done to see how this great technology for humankind affects the occupants of the oceans and seas? I don't know the history of sea life beaching themselves and dying, but it seems to me that in my 59 years it has been getting much worse and more common. And please, don't say that is because more people possess a means to record it, as that is what idiots say when they can't accept the truth. Don't we have enough emissions surrounding us daily without placing the same disruption amongst sea life? We are not the only living beings on this planet, how about considering others too.
Heckler
RFID chips are also known to transmit once energized, but they are designed to only work at a few millimeters distance. If only they had a way to increase the transmission distance of low power sonar, then they could make communicating the phase delay between low and high sound frequencies into a cost effective underwater positioning system. These selective wave repeaters could be tethered from buoys at known depths along a path of interest and allow robots to navigate alongside them to scan the ocean floor.
Karmudjun
David - are there any risks posed to marine wildlife with yet another acoustic wave producer in the oceans? I know there have been studies of SONAR advancements which negatively impact whales, given that we do use dolphins on occasion I would hope that MIT & ONR have considered the marine impact. But very nice write-up, Thanks.
paul314
Once the water gets deeper, it's not just the reflections off the bottom and the surface and any other features, it's also refraction and reflection off different layers of water.
aki009
@Karmudjun -- I seriously doubt that energy harvested sound is going to be harmful to any wild life. The underwater environment is very noisy (as noted in the article itself), and adding some low power spread spectrum noise to it is not going to make a difference. As to the device itself, that's pretty neat, but I'd like to see it in action in a real environment with complex reflective surfaces and other things to mess things up. I'm also not convinced that piezo energy harvesting will obtain enough power to make the thing work at reasonable distances, but I'd love to be surprised on that one. If they make it work for real, I'll be one of the first to buy one (assuming the price is not out of sight).