The U.S. Navy has developed a system for nuclear subs to communicate with the rest of the world that formerly required the boat to navigate close to the surface in order to raise a radio antenna. The new system is called Deep Siren.

The system consists of a disposable buoy dropped into the water by an aircraft or over the side of a ship in the general area of where the sub is believed to be positioned. A 90 kilometer radius to the boat is required. The buoy sends out an acoustic signal that U.S. subs are equipped to receive. This coded message either orders the sub to get a radio antenna above water and call home, or simply delivers a brief message. The buoy also has satellite telephone capability — additional messages can be sent from anywhere to the sub. The sub is unable to send messages to the buoy as powerful sensors are required to pick up the signals. In the past, the only way to "page" submerged subs was via a large, shore based, low frequency, transmission system. This system was less reliable than the new one, although it did possess a much longer range.

Recently, the Navy successfully tested the other end of the system. The sub releases a similar buoy through its garbage chute. The buoy hovers below surface for a while, then rises to the surface and sends messages. The buoy delays surfacing until the sub is far enough away to prevent the buoy signal from revealing the exact location of the boat. The buoy then receives short messages and uses a sonar type device to send the data acoustically to the sub. Outgoing messages, which are sent via satellite, can be longer, and even include outgoing email from the crew to family. However, the acoustically transmitted messages are much shorter and include orders from surface ships or anyone in the chain of command to the sub commander.

Deep Siren will also be useful for American carrier task forces, which are usually accompanied by at least one SSN (nuclear attack sub.) As thermal layers make underwater transmissions vary a great deal in range, the buoy sends the command messages several times to insure that at least one gets through to the intended receiver. The buoy from the sub can remain active for several days should the sub remain in the area. Eventually, the buoy sinks itself. The Navy has not, as of this writing, committed to Deep Siren. Additional and more frequent tests are planned to test the system's reliability.

(Source: Strategy Page)