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
