Published On: Thu, May 10th, 2018

A better way to transmit messages underwater

RADIO waves cannot penetrate water, so cannot be used for submarine communication. That is why the sea is probed by sonar, not radar. But, as people and their machines venture ever farther into the deep, ways of building underwater communications networks would be welcome. And researchers at Newcastle University, in England, led by Jeff Neasham, think they have just the thing to build them with: an acoustic “nanomodem”.

Existing underwater modems, which transmit and receive data via sound, are power-hungry (consuming up to two watts when receiving messages, and as much as 35W when transmitting) and expensive (costing between £5,000 and £15,000, or $ 7,000-20,000). Dr Neasham’s nanomodems consume only ten milliwatts when listening, and 1W when broadcasting. They cost about £50 a pop. They are also, being about the size of a matchbox, a tenth as big and heavy as the conventional variety. But they suffer from no diminution in range. They are able, as an existing modem is, to broadcast over a distance of up to 2km. That range can, moreover, be extended by deploying a number of them as a network in which each talks to its neighbours, recording messages and passing them on. Existing modems can do this too, in principle. In practice their cost restricts the size of the network.

These paragons of underwater communication consist of a low-cost microprocessor (a baby version of the processor found in most smartphones) and two customised amplifiers—one to transmit signals and one to receive them. The transmission rate is a mere 40 bits per second, but that is a consequence of the spread-spectrum technique used to broadcast those bits, which trades speed for resistance to interference. The ocean is a noisy place, but broadcasting the same message on several frequencies increases the chance it will get through on at least one of them. Spread-spectrum broadcasting thus compensates for a nanomodem’s low power.

Around 200 of Dr Neasham’s nanomodems are already being tested, in several projects. One, which started in January, is a whale watch organised by the Natural Environment Research Council, a British government agency. The plan is to survey sites where offshore wind farms might be built, to assess the risk of any development there interfering with local cetaceans.

Such surveys are done by dropping sensors to the sea bed, to record the sounds made by whales and dolphins when they are navigating, hunting and talking to each other. This gives an indication of which species are present, and in what numbers. In the past, such surveys have been difficult and expensive. The sensors have had to log and store the animals’ noises for weeks or months after deployment, and have then had to been recovered in order to have their data read. Adding a nanomodem to a sensor means the data it collects can be retrieved remotely, whenever convenient (a process made even easier when the modems are part of a network, and can thus pass their data to a single retrieval point). There is therefore no need to recover the devices when a survey is over.

Another use of Dr Neasham’s nanomodems is on submarine drones, known as AUVs (autonomous underwater vehicles). One such, the ecoSUB, made by ecoSUB Robotics, a British firm, is less than a metre long, weighs about 4kg, and is intended to work in groups, called shoals, monitoring pipelines and other pieces of underwater infrastructure. Fitting a nanomodem to each drone in a shoal will let it talk to the others, permitting shoal members to co-ordinate their activities.

Navigating such a shoal to its target, though, is a problem. It will tend to drift with the current and, when underwater, an AUV cannot listen to the radio signals transmitted by the satellite-based Global Positioning System (GPS) which most navigation now relies on. But Terry Sloane, ecoSUB Robotics’ boss, has an acoustic answer to this, as well. He plans to add a surface drone to the shoal, to pick up GPS signals and then broadcast its position acoustically to the underwater drones. The AUVs will thus know where they are.

Nanomodems could also help chart the ocean floor. Some 95% of the sea bed is unexplored, so Shell, a large oil company, is sponsoring a prize (the Ocean Discovery XPRIZE) for better ways to map it. One of the finalists in the competition, Team Tao, includes members from the nanomodem group at Newcastle. The Team Tao scheme involves an unmanned surface vessel releasing dozens of torpedo-like Bathypelagic Excursion Modules (BEMs), each 1.3 metres long. The BEMs drop to the sea bed as a shoal, scan the area with sonar, and return to upload their data and recharge their batteries. When underwater, they remain in the proper formation by exchanging information through their nanomodems.

Team Tao’s members estimate that their approach will cost a hundredth as much as a conventional survey ship, deploying a conventional AUV, would require to do the same job. That could open vast reaches of the sea floor to science and commerce.

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A better way to transmit messages underwater