Water bug

A sensor network that uses ultrasonic pulses to relay information through water could be used to protect nuclear power stations from terrorist attack, according to its US developers.

The sensor system, being developed at Penn State University, is based on a network of nodes capable of communicating with each other using short bursts of ultrasonic energy. Each node can house sensors to detect movement or harmful chemicals in shallow waters.

The network could be used to prevent terrorists attempting to plant explosives around the cooling water intakes of nuclear power plants, said Dr Craig Grimes, associate professor of electrical engineering and materials science and engineering at the university. ‘The US Department of Defence has been asking whether a system could be developed to sit passively in the water and then wake up and sound an alert if someone comes into the area on a canoe,’ he said.

To monitor a wide expanse of water such as a river, reservoir, lake or ocean bay, a network of sensors is needed. Radio signals cannot be used under water, so the nodes send information encoded in acoustic pulses.

But designing each sensor to communicate directly with a central computer or single node a large distance away would require a great deal of power, and water decreases the strength of acoustic signals.

As a result a relay system was developed, where information is passed from one node to another, said Grimes. ‘We wanted to develop a technology where we could throw into the water any number of sensors and have them communicate with each other, relaying the information back.’

Once the network has been placed in the water, an uplink node capable of communicating with a remote central computer broadcasts an ultrasonic signal with a coded message containing its identity. Any node close enough to receive the message records the uplink node as its parent, and then broadcasts a signal of its own, which is in turn detected by other nodes, until a cascade of parent nodes covers the entire system.

Each node sends its sensor information to its parent node, which passes on this data and its own information to its parent. Once the uplink node floating on the surface of the water has received all the information, it converts the acoustic signal to radio frequency and sends the data to the central computer.

The network could also be used to continuously monitor drinking supplies, rivers and lakes for industrial pollution and harmful agents, including the potential terrorist agent ricin.

Rivers in industrialised countries have been found to contain contaminants such as antibiotics, hormones and insecticides. But water supplies and rivers are analysed by hand, which is expensive and time consuming, and can miss periodic releases of pollution from factories or sewage treatment plants, said Grimes.