If you're watching a movie where an underwater submarine is communicating with an aeroplane, some creative licence is being taken – the sonar waves used by subs can't reach the air, and radio waves used by planes don't travel at all well underwater.
That's always caused problems for communications between underwater and airborne vehicles, as well as for the recovery of wrecked subs, ships, and aeroplanes. Submarines often have to surface to send messages, which risks also giving away their location.
However, a team of scientists at Massachusetts Institute of Technology (MIT) has now developed a system enabling subs and planes to communicate with each other, though for the time being the rate of data transfer is very slow, and only in one direction.
"Trying to cross the air-water boundary with wireless signals has been an obstacle," says senior esearcher Fadel Adib. "Our idea is to transform the obstacle itself into a medium through which to communicate."
The system – officially called translational acoustic-RF communication (TARF) – uses a sonar signal from a standard acoustic speaker to create tiny ripples on the surface of the water, ripples which wouldn't normally be noticed.
These ripples are about 100,000 times smaller than the smallest ocean ripples.
But with the right equipment – specifically a new extremely-high-frequency radar developed by the researchers – the ripples created by the audio signals can be picked up, because of the way they interfere with rebounding radio wave signals. They become the 1s and 0s of digital communication.
"The radar reflection is going to vary a little bit whenever you have any form of displacement like on the surface of the water," says Adib. "By picking up these tiny angle changes, we can pick up these variations that correspond to the sonar signal."
The ripples caused by the underwater sonar may be very, very small, but they're vibrating at a much higher frequency (travelling much faster) than normal waves. This means the new radar can spot them, using a carefully crafted algorithm.
Over the course of 500 test runs in a water tank and two swimming pools, TARF proved able to transmit a number of messages from the water to the air, including one that read: "Hello from underwater."
At the moment the new system can't transmit data very quickly – several hundreds of bits per second, so worse than an old dial-up modem, but about on a par with other underwater communication methods.
Plus, TARF only works in water where the surface waves are less than 16 centimetres (6.3 inches) high. However, it's a significant breakthrough in water-to-air communications, and the researchers are already working to refine and improve their methods.
Potentially, the new technique could one day enable underwater monitoring vessels to transmit data to drones overheard, without heading back to the surface, as well as recover lost flight recorders and keep subs and planes in contact, as we've already mentioned.
"It can deal with calm days and deal with certain water disturbances," says Adib. "But [to make it practical] we need this to work on all days and all weathers."
The research has not yet been published but is being presented at the SIGCOMM conference in Budapest.