The Navy is betting heavy on unmanned undersea vehicles, or UUVs, for minesweeping and a range of other intelligence operations. But these technologies face an inherent limitation: It's hard to send a communications signal through the water.
Radio frequency signals don't propagate well in the sea. That's why scuba divers use hand signals. In the case of UUVs, "it can mean that these vehicles may have to surface constantly in order to talk to one another," said Troy Olsson, a program manager in the Microsystems Technology Office at the Defense Advanced Research Projects Agency.
Olsson is leading a team in search of a solution to the underwater telecommunications conundrum. They hosted a proposer's day in January and have since been taking proposals for potential fixes.
Early-stage research and development is slated to begin this summer, with two subsequent phases of development leading to a potential product rollout in about four years. If successful, A Mechanically Based Antenna, or AMEBA, could expand wireless communication and data transfer into undersea and underground settings where such connections previously have been impossible.
While it’s inherent in the physics of radio frequency propagation that radio waves won’t carry underwater, there are other means of transmitting a signal. Light may be used as a communications tool, but as Olsson points out, the sender needs to know the receiver’s exact location. Sound might work, but acoustic signals are easily detected.
There is another route: One can build a very big antenna — but it has to be very big. The Navy constructed such a facility on a remote peninsula in Cutler, Maine, in the heat of the Cold War. The massive transmitter complex occupies 2,000 acres, with 26 towers up to 1,000 feet high, and operates with megawatt levels of power. Still, it can relay only a trickle of data.
DARPA researchers are taking a different avenue. They are investigating the possibility of transmitting via ultra-low-frequency electromagnetic waves, in the range of hundreds of hertz to 3 kilohertz, which can penetrate some distance through water, soil, rock and other materials.
A nearby band of very-low-frequency signals at 3 KHz to 30 KHz would open additional options, using the atmospheric corridor between the Earth’s surface and the ionosphere as a sort of highway to propagate signal halfway around the planet.
AMEBA aims to produce a communications tool that would be small and light enough to fit not just on a UUV but in the backpack of a war fighter working underground or on radio-inaccessible terrain. These units would generate signal by mechanically moving materials in which strong electric or magnetic fields were stored.
Imagine suspending a magnet in a vacuum, then giving it a spin.
"Once I started it spinning, it could theoretically spin for a very, very long time without my having to add extra energy," Olsson said. That unhindered spinning would form the basis for communication.
Newly emerging micro-electronic mechanical systems could make this possible, if the right suspension mechanism could be built.
"We’ve also seen some improvements in electric-type materials that make me believe those might also be practical solutions, particularly in the development of very highly charged storage capacity electrics. There is a lot of work that needs to be done, but there are emerging approaches," he said.
Much of the physics remains to be worked out, especially in terms of size, weight and power.
"The question would be, how low can you go? And that is a matter of how much you can reduce the mechanical dissipation, reduce the losses in the system," Olsson said. The longer you can keep the components spinning, the more efficient and effective the tool will be.
When the researchers talk about high-level conceptual physics, their ambition to "create a dipole moment," it can be easy to lose sight of the bigger picture. The math may be all about spinning magnets, but the impact is very real-world.
"This will bring a capability that does not exist today," Olsson said. "To be able to bring useful communications systems, to be able to do thinks like text and voice and data underground or underwater or deep inside buildings, that would be a big step forward."