The Buzz

The U.S. Military Might Be on the Verge of the Ultimate Naval Weapon

The Pentagon’s research entity and BAE Systems are working together to develop a next-generation undersea drone communications technology to help identify mines, find enemy submarines and surveil many items relevant to combat missions.

Using underwater acoustic signals, a surface buoy, beacon or “node,” and GPS signals in a coordinated fashion, the Positioning System for Deep Ocean Navigation (POSYDON) is able to quickly relay location coordinates from undersea drones on patrol to command and control systems on board a ship or submarine.

The program, now in a Phase I developmental effort, is a collaborative enterprise between industry and the Defense Advanced Research Projects Agency (DARPA).

POSYDON provides “omnipresent, robust positioning across ocean basins. By ranging to a small number of long-range acoustic sources, an undersea platform would be able to obtain continuous, accurate positioning without surfacing for a GPS fix,” DARPA developers explained.

While experts say there are some very low-frequency radios that can transmit some kind of signal undersea, submarines need to surface in order to achieve a strong radio frequency (RF) or GPS signal for on-the-spot data and communications.

“You can receive GPS at very shallow depths, but that is not relevant to where we operate. POSYDON brings a ‘GPS-like’ capability to submerged users,” said Lin Haas, program manager for the DARPA Strategic Technology Office, in a newly released agency podcast.

Military scientists and technology developers refer to the effort to establish connectivity in a “GPS-denied” environment as acquiring “precision, navigation and timing.”

However, the scientific challenges of bringing seamless connectivity undersea, similar to the way GPS functions on the surface, are substantial, Haas explained.

GPS signals work with algorithms able to compute the distance of an object by knowing the constant or “fixed” speed of light and the time of travel. If the length of travel is identified, along with the speed of a signal, then algorithms can quickly determine a precise distance, therefore identifying an object.

For example, an electromagnetic signal used by a radar system -- or laser from a weapon’s laser rangefinder -- would use the known speed of light, and time of travel, to quickly identify the location, shape or speed of an object.

However, with acoustic signals undersea, determining distance is much more complex, Haas explained.

“For GPS the speed of light is constant. That is not the case for underwater speed of sound. Underwater signals are a function of many things, primarily temperature and salinity. We have developed models that account for all these acoustic signals underwater. Underwater signals don’t travel in a single line,” Haas said.

As a result, there is no linear transmission from transmitter to receiver with acoustic signals.

“Acoustic signals will take many paths; the signal is refracted through temperature and pressure profiles. Algorithms can improve current models and develop new modes,” Haas added.

Therefore, underwater drones can use acoustic waves to relay real-time info back to submarines.

DARPA officials say BAE Systems, Raytheon BBN and Draper Laboratory are all working on the POSYDON program.

“GPS signals bounce off ocean surfaces and cannot penetrate seawater. The importance of POSYDON is to make sure that these UUVs [unmanned underwater vehicles] can really focus on their missions without having to periodically come to the surface for GPS to figure out exactly where they are,” said Geoff Edelson, director of Maritime Systems and Technology at BAE Systems.

The technology relies upon a kind of “triangulation,” Edelson explained. A GPS signal emerging from a satellite is sent to a surface node -- which then uses acoustic waves to connect with and locate an undersea drone.

“Many signals do not propagate under the sea. Light cannot travel very far and RF signals do not really propagate under the sea. With POSYDON, a GPS signal is replaced by low-frequency acoustic signals,” Edelson said.

The POSYDON effort is progressing through Phase I of a three-phased effort; Phase I involves modeling signal propagation channels, Phase II is intended to develop a single waveform and Phase III is aimed at building a complete prototype positioning system, according to DARPA.

“Right now we are analyzing data to ensure the concept has merit. We are going ‘point to point’ from a source to one or two receivers,” Edelson said.

The emergence of this technology, which is still likely several years away from operational use, is entirely consistent with the Navy’s undersea drone strategy. Undersea drones are increasingly critical countering emerging high-tech surface and sub-surface threats such as quieter, more advanced submarine technology and weapons being developed by potential adversaries.

UUVs that are better able to transmit information back to host platforms, and quickly provide their location data, can naturally assist in locating enemy targets, undersea mines and other items of critical relevance to Navy missions.

POSYDON technology, once operational, could work with existing platforms, such as Wave Gliders, designed to collect target and threat information, as well as oceanographic and hydrographic information.  

For instance, a current underwater drone called the Seaglider uses buoyancy and wings to achieve forward motion as opposed to an electrically driven propeller. For long periods of time it is able to gather oceanographic data, such as water column temperature or salinity, collecting the data and then sending it back.