The U.S. Navy is considering new technology that will allow land-based officers to communicate with submarines with minimal disruption to the sub's operations and reduced risk of detection. The military hopes that an emerging tactical paging technology dubbed Deep Siren will allow fleet commanders anywhere in the world to instantly communicate with subs despite the latters' depth or speed.

Currently, vessels can only be contacted if they are on or near the surface, which is not only inefficient but dangerous for subs furtively trolling hostile waters. Deep Siren is designed to deliver communications using acoustic, expendable buoys that, when contacted via a communications satellite in the National Security Agency's Global Information Grid, can send and receive messages to and from submerged subs as far as 175 miles (240 kilometers) away depending upon acoustic propagation conditions.

"This is about bringing real-time communications to the sub, without latency," says Bill Matzelevich, a former Navy captain who retired in 2000 and is now a senior manager in government contractor Raytheon Company's Network Centric Systems group. The Navy in July awarded Raytheon a $5.2 million development contract to deliver a Deep Siren tactical paging system. "If you need to get a message urgently to a sub, you might have to wait eight hours for it to come close enough to the surface. A strike group commander may need to change direction and can't get this info to the sub immediately."

Messages to submarines are typically broadcast from onshore naval communication centers for a fixed amount of time--eight hours or so. For a sub to receive these radio-frequency or satellite messages, it must stop what it is doing within that time period, extend an antenna and rise to "periscope depth"— approximately 60 feet (18 meters) below the surface, which is shallow enough to use a periscope. During this time the sub may become more vulnerable to detection and may be more restricted in its ability to perform its mission.

Once at periscope depth, submarines tow a floating long-distance antenna behind them, but the data rates are generally slow and the wire used to tether the antenna to the sub restricts the vessel's agility. "You can only go so fast and so deep with this wire attached," Matzelevich says. "This is Word War II–era technology."

To communicate with a submerged submarine safely, a gateway mechanism is required to deliver messages deeper than periscope depth. The Deep Siren Tactical Paging system is comprised of a disposable gateway buoy with an antenna that gathers radio-frequency signals and converts them to Deep Siren acoustic signals that penetrate the water and are received by the submarine's sonar system. These acoustic signals are then converted on board the submarine to text messages with the Deep Siren receiver. The Deep Siren system also includes a portable transmit station which can be located on shore or carried on board a ship or airplane. "You want to have this be a global capability, where the buoy can be called from anywhere in the world," Matzelevich says.

Working with RRK Technologies, Ltd., in Glasgow, Scotland, and Ultra Electronics Maritime Systems in Dartmouth, Nova Scotia, Raytheon is developing a Deep Siren system that includes expendable buoys that are five inches (12.7 centimeters) in diameter and about 3.5 feet (one meter) long with antennas that receive signals from a constellation of Iridium Satellite, LLC, communication satellites. The buoys—designed to stay afloat for up to three days—can be ejected out of the sub's trash disposal unit without major modifications to the vessel. In this way, subs can set up their own acoustic networks without the need to tow an antenna.

The other components of Deep Siren include computers onboard subs and in communications facilities—which may be located ashore, or onboard ships or aircraft—to access messages, along with special software to interpret them. The software—written by RRK—matches different acoustic tones emitted by the buoys with a set of vocabulary words shared between the sender and receiver, performing the translation from words to tones and back to words again. This methodology allows communications to a submarine in a format similar to text messages that occur on a cell phone or PDA.

Deep Siren acoustic technology uses digital message processing to ensure that the receiver can move at a rate of greater than 30 knots (about 35 miles per hour) without incurring any measurable interference. Deep Siren uses digital signaling capabilities at lower frequencies—less than two kilohertz— and permits signal encryption to achieve secure sonar communications at a substantial range to a submarine at depth. Secure and encrypted signals permit more liberal communication from ship to submarine; enemy units may be able to pick up the signals, but they cannot decode them.

The Navy plans to conduct an at-sea military assessment of Deep Siren in June as part of its Communications at Speed and Depth initiatives.