Slashdot: GPS and Water Don't Mix. So Scientists Have Found a New Way To Navigate Under the Sea

GPS and Water Don’t Mix. So Scientists Have Found a New Way To Navigate Under the Sea
Published on November 04, 2020 at 08:00AM
An anonymous reader quotes a report from ZDNet: [E]ven today’s most sophisticated GPS systems are still unable to map a huge chunk of the Earth: that which is located under oceans, seas, or rivers. The technology, in effect, doesn’t mix well with water, which breaks down the radio waves GPS relies on to function. MIT scientists have been looking at ways to create a new type of underwater GPS, which could be used to better understand the mysteries that lie between surface and seabed. The researchers have now unveiled a device called an underwater backscatter localization (UBL) that reacts to acoustic signals to provide positioning information, even when it is stuck in oceanic depths. All of this, without even using a battery.

Underwater devices already exist, for example to be fitted on whales as trackers, but they typically act as sound emitters. The acoustic signals produced are intercepted by a receiver that in turn can figure out the origin of the sound. Such devices require batteries to function, which means that they need to be replaced regularly — and when it is a migrating whale wearing the tracker, that is no simple task. On the other hand, the UBL system developed by MIT’s team reflects signals, rather than emits them. The technology builds on so-called piezoelectric materials, which produce a small electrical charge in response to vibrations. This electrical charge can be used by the device to reflect the vibration back to the direction from which it came. In the researchers’ system, therefore, a transmitter sends sound waves through water towards a piezoelectric sensor. The acoustic signals, when they hit the device, trigger the material to store an electrical charge, which is then used to reflect a wave back to a receiver. Based on how long it takes for the sound wave to reflect off the sensor and return, the receiver can calculate the distance to the UBL.

In practice, piezoelectric materials are no easy component to work with: for example, the time it takes for a piezoelectric sensor to wake up and reflect a sound signal is random. To solve this problem, the scientists developed a method called frequency hopping, which involves sending sound signals towards the UBL system across a range of frequencies. Because each frequency has a different wavelength, the reflected sound waves return at different phases. Using a mathematical theorem called an inverse Fourier transform, the researchers can use the phase patterns and timing data to reconstruct the distance to the tracking device with greater accuracy. Frequency hopping showed some promising results in deep-sea environments, but shallow waters proved even more problematic. Because of the short distance between surface and seabed, sound signals uncontrollably bounce back and forth in lower depths, as if in an echo chamber, before they reach the receiver — potentially messing with other reflected sound waves in the process. […] While the scientists acknowledged that addressing these challenges would require further research, a proof-of-concept for the technology has already been tested in shallow waters, and MIT’s team said that the UBL system achieved centimeter-level accuracy.