GPS depends on radio waves, which break down rapidly in liquids including seawater. To track undersea objects like drones or whales, researchers rely on acoustic signaling. But devices that generate and send sound usually require batteries that need regular changing.
Researchers have built a battery-free pinpointing system dubbed Underwater Backscatter Localization (UBL). Rather than emitting its own acoustic signals, UBL reflects modulated signals from its environment, providing positioning information at net-zero energy. The team used piezoelectric materials that generate their own electric charge in response to mechanical stress such as 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 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 because the ocean can be an echo chamber.
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. 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.
Where echoes run rampant between the surface and seabed, the team had to slow the flow of information. They reduced the bit rate, 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 bit rate 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 bit rate 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 10,000 bits/second, they were able to track the object through deep water.
The proof-of-concept was tested in a shallow-water environment. UBL estimated the distance between a transmitter and backscatter node at various distances up to nearly half a meter.
UBL could one day help autonomous vehicles stay found underwater, without spending battery power. The technology could also help subsea robots work more precisely and provide information about climate change impacts in the ocean.