Radio waves can be transmitted through water, but conduction and absorption of the short wavelengths makes it incredibly inefficient. That’s particularly true in saltwater, which will quickly stop the penetration of radio waves. In this past, the solution has been to pump huge amounts of power into radio transmitters. Unlike radio waves, sound waves move quite well through water, because they have a lower frequency and therefore a longer wavelength. The problem, when it comes to tracking objects underwater, is that sound waves echo and create noisy data. At the same time, producing sound waves still requires power. That’s why researchers from MIT have developed new battery-free backscatter localization technology that could fuel undersea exploration.
Let’s say you wanted to build an unmanned robot for deep sea exploration. How would you communicate with it? How would you even keep tabs on its current location? GPS relies on radio waves being transmitted from satellites, and those don’t have enough power behind them to penetrate more than a few feet into sea water. A common solution is to use a tether — a long cable that connects the robot to a vessel on the surface. That has many limitations: it requires a ship to remain above the robot, it creates a hazard, and that cable increases the cost of the robot. When it comes to tracking sea life, such as migrating whales, a tether is completely unsuitable. This new technology has the potential to solve all of these problems using acoustic backscatter signals.
The term “backscatter” is most often used in relation to radio signals. A wireless router can, for example, transmit radio waves that are reflected by specific objects. Those reflected signals can be used to convey data and the location of the objects. This new technology, called Underwater Backscatter Localization (UBL), works in a very similar way, just using acoustic signals. The object to be tracked can be equipped with a piezoelectric sensor that picks up the sound waves and uses them to generate a small amount of electricity. It can then selectively reflect some of those sound waves, modulating them to encode binary data.
Echoes can create noise that makes that data difficult to decipher, but the team is overcoming that problem by transmitting at varying frequencies so that the sound waves return to the observation unit in different phases. Combined with a relatively low bitrate, this makes it possible to clean the echoes out of the data. Unfortunately, the echoes become more severe at shallow depths, forcing them to use a lower bitrate to transfer data. The team is working on improving that in order to make the technology more viable. If they’re able to achieve that, UBL will be absolutely indispensable for undersea research and exploration.