Hand signals are the sole way for the millions of individuals who partake in activities like snorkelling and scuba diving each year to convey safety and navigational information underwater.

Professional divers can use more than 200 signals in their vocabulary, although recreational dives may only use around 20. These signals can be used to indicate everything from the oxygen level to the presence of nearby aquatic creatures to the accomplishment of group duties.

These hand signals’ visual character makes them less effective at a distance and in low light. A possible substitute is a two-way text messaging, but this requires expensive, specialised technology that is not commonly accessible. University of Washington researchers demonstrated how to enable underwater texting on the billions of smartphones and smartwatches currently adopted using only software.

The group created AquaApp, the first smartphone application for acoustic-based networking and communication underwater that can be utilised with current devices like smartphones and smartwatches.

The scholars presented their paper at SIGCOMM2022 on August the 25th, providing details about the potential of AquaApp. Co-lead author Tuochao Chen, a doctoral student in the Paul G. Allen School of Computer Science & Engineering at the University of Washington noted: “Smartphones rely on radio signals like WiFi and Bluetooth for wireless communication. Those don’t propagate well underwater, but acoustic signals do”.

“With AquaApp, we demonstrate underwater messaging using the speaker and microphone widely available on smartphones and watches. Other than downloading an app to their phone, the only thing people will need is a waterproof phone case rated for the depth of their dive.”

The AquaApp interface allows users to choose from a total of 240 pre-set messages that match hand signals used by professional divers, with the 20 most popular signals displayed in a more accessible way for quick selection. Messages may also be filtered by eight other categories, such as directional indications, environmental conditions, and equipment status. The team had to overcome a number of technological obstacles that they had not before experienced on dry land when developing the app.

“The underwater scenario surfaces new problems compared to applications over the air,” explained co-lead author Justin Chan, a doctoral student at the University of Washington, Allen School. “For example, fluctuations in signal strength are aggravated due to reflections from the surface, floor and coastline. The motion caused by nearby humans, waves and objects can interfere with data transmission. Further, microphones and speakers have different characteristics across smartphone models. We had to adapt in real-time to these and other factors to ensure AquaApp would work under real-world conditions.”

Other difficulties included dealing with the current tendency for devices to rapidly shift position and proximity, as well as the various cases of noise the app might come across due to the presence of animals, vessels and even aircraft flying at low heights.

The team developed an algorithm that enables AquaApp to optimise the bitrate and acoustic frequencies of each transmission in real-time based on parameters such as noise, distance and variations in frequency response across devices. The process can be defined as such: when one user wishes to send a message to another device, their app first sends a brief note to the other device, known as a preamble. AquaApp on the receiving device will then run an algorithm to find the ideal conditions for receiving the preamble. Then, the app will communicate with the sender’s device to request the use of the same conditions when sending the actual message.

In order to facilitate messages between various devices, the researchers created a networking protocol to share access to the underwater network, resembling how WiFi networks regulate internet traffic. AquaApp’s local network may support up to 60 different users concurrently.

The team examined the practical applicability of the AquaApp system in six different sites. Sites included: under a bridge in calm water; a famous waterfront park with strong currents; in the proximity of a fishing pier of a busy lake; in a bay with powerful waves. The performance of the app was assessed by the researchers at depths and distances of up to 12 metres and 113 metres, respectively.

The system was continually run on two Samsung Galaxy S9 smartphones by the researchers, while the displays were turned on and the system’s maximum volume was on. This allowed them to assess AquaApp’s effect on battery life. Over the course of four hours, the software only reduced the smartphones’ battery power by 32%, which is within the maximum suggested dive duration for recreational scuba diving.

“AquaApp brings underwater communication to the masses,” commented senior author Shyam Gollakota, a University of Washington professor in the Allen School of Computer Science & Engineering. “The state of underwater networking today is similar to ARPANET, the precursor of the internet, in the 1970s, where only a select few had access to the internet. AquaApp has the potential to change that status quo by democratizing underwater technology and making it as easy as downloading software on your smartphone.”

The researchers’ study data and the open-source Android code can be found on the AquaApp website.

Sources: Tuochao Chen et al, Underwater messaging using mobile devices, Proceedings of the ACM SIGCOMM 2022 Conference (2022). DOI: 10.1145/3544216.3544258

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