MIT researchers have developed a battery-free technique for positioning underwater. The Underwater Backscatter Localization (UBL) developed at MIT’s Media Lab reflects modulated signals from its environment rather than generating its own sonar signals. That provides researchers with positioning information without using a battery.
“Sound is power-hungry and batteries can drain very quickly,” said Fadel Adib, who leads the Signal Kinetics 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 team used a piezoelectric sensor that generates charge from the movement of the water, and uses charge to selectively reflect some soundwaves back into the environment. A receiver translates the backscatter reflections 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 could provide location information. 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 as the system has to handle multiple reflections.
“You start running into all of these reflections,” saids Adib. “That makes it complicated to compute the location.” Accounting for reflections is an even greater challenge in shallow water — the short distance between seabed and surface means the confounding rebound signals are stronger.
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.
Reducing the bitrate allows each signal sent out by the observation unit top die down before interfering with the next one. A bit rate of 2Mbit/s worked in simulations of deep water, but this had to be reduced to 100 bit/s in shallow water to obtain a clear signal reflection from the tracker. However, a higher bit rate is needed to track the position of an object using the frequency hopping. “By the time you get enough information to localize the object, it has already moved from its position,” said researcher Sayed Saad Afzal. At 10Mbit/s they were able to track the object through deep water.