Aquatic robots are busier than ever. They have seabeds to mine, cable pathways to plough, and marine data to gather. But they and their aquatic brethren—including submarines and scuba divers—still struggle to communicate.
For decades, global standards defining Wi-Fi and cellular networks have allowed people to exchange data over the air. But those technologies are worthless below the waves, and no such standards have existed for underwater communications.
Aquatic systems have instead used a mishmash of acoustic and optical signals to send and receive messages. However, manufacturers sell acoustic modems that operate at many different frequencies, which means those systems often can’t speak to each other.
“We live in a time of wild west communications underwater,” says João Alves, a principal scientist for NATO.
Now, Alves and other NATO researchers have established the first international standard for underwater communications. Named JANUS, after the Roman god of gateways, it creates a common protocol for an acoustic signal with which underwater systems can connect.
Acoustics has long been a popular medium for underwater communications. Generally, optical signals can deliver high data rates underwater at distances up to 100 meters, while sound waves cover much greater distances at lower data rates.
The main role of JANUS is to bring today’s acoustic systems into sync with one another. It does this in part by defining a common frequency—11.5 kilohertz—over which all systems can announce their presence. Once two systems make contact through JANUS, they may decide to switch to a different frequency or protocol that could deliver higher data rates or travel further.
In this way, Alves compares JANUS to the English language—two visitors to a foreign country may speak English to one another before realizing they are both native Spanish speakers, and switch to their native tongue.
Chiara Petrioli, a specialist in underwater sensors and embedded systems at La Sapienza, the University of Rome, says JANUS could be the first step toward an “Internet of Underwater Things"—a submerged digital network of sensors and vessels.
In addition to designating a frequency, JANUS also provides a modulation encoding scheme to describe how data should be encoded onto a sound wave, and describes the particular waveform that should be used (known as FH-BFSK). It also spells out which redundancies should be added to the data stream to minimize transmission errors.
In order to use JANUS, a system would first emit three optional tones to indicate that it intends to broadcast a JANUS data packet hitched to a sound wave. Then, the system would pause for about 400 milliseconds to allow other devices in its vicinity to “wake up.” Next, the system would broadcast a fixed series of tones to ensure both systems were properly synchronized to the JANUS protocol. Finally, the system would send the JANUS packet, consisting of 56 bits followed by a redundancy check, which tests for transmission errors.
The JANUS standard was developed by Alves’ team at NATO’s Centre for Maritime Research and Experimentation in La Spezia, Italy and sponsored by NATO’s Allied Command Transformation. It is the first underwater communications standard to be defined by an international body.
Milica Stojanovic, an expert in oceanic engineering at Northeastern University, expects other standards will soon follow. She says the 11.5 kHz frequency used by JANUS is great for transmitting data between 1 and 10 kilometers, but a lower frequency, perhaps 1 kHz, would be better for sending data over longer distances of 10 to 100 km.
Even with JANUS and other standards, any future underwater Internet will probably be cursed by far lower data rates than modern Wi-Fi or cellular networks. Sound travels at much lower frequencies, and on much longer waves, than the signals used for consumer electronics. Though sound waves travel faster in water than on land, they still travel more slowly through water than radio waves through air.
To develop JANUS, Alves’ team relied on the Littoral Ocean Observatory Network, a collection of tripods that NATO researchers have placed on the seafloor in the harbour of La Spezia, Italy. Each tripod emits acoustic signals to other tripods, which send performance reports to researchers through undersea cables. Those reports helped the team understand how fluctuations in water temperature, and other environmental changes, will affect JANUS signals.
The tripods also allowed researchers to build a JANUS receiver, advanced versions of which could minimize decoding errors and account for the Doppler effect. The Doppler effect describes shifts in sound waves caused by motion, such as the whirl of an ambulance siren as it drives by.
In another series of tests, researchers aboard the research vessel Alliance, a NATO ship operated by the Italian Navy, measured the performance of JANUS signals along the surface of the ocean.
Buoys would act as transeivers between RF signals and JANUS’ sonic messages. Illustration: NATO
Once deployed, aquatic systems could use JANUS to send data directly to each other, or to “gateway buoys” bobbing on the water’s surface. The buoys could then use radio waves to relay that data to nearby control centers.
In one demonstration, Alves’ group helped the Portuguese Navy set up a buoy that converted data about the positions and speeds of nearby ships to JANUS. The buoy rebroadcast this information to Portuguese submarines lurking below.
Based on their work, Alves says submarines could also use JANUS to issue calls for help to ships and rescue crews. “Using an open scheme like JANUS to issue distress calls would increase incredibly the chances of those being picked up,” he says.
Now that JANUS is available, manufacturers of aquatic systems must decide whether or not to adopt it. Alves is confident they will, and Petrioli, who contributed feedback to the development of JANUS, agrees that adoption is essential to the industry’s future.
But Stojanovic is not so sure. “If there starts to develop a serious market, then everybody will have to play to the same tune,” she says. “If not, and everybody finds their own niche market with their own protocols, then they will do that.”
An abridged version of this post appeared in the September 2017 print issue as “A Language for the Internet of Underwater Things.”