June 30, 2020:
For the second time in the last fifteen years, a working communications system between submerged submarines and surface forces has been developed. The latest effort is LRAM (Long-Range Acoustic Messaging) developed by GeoSpectrum Technologies, a Canadian subsidiary of Israel defense manufacturer Elbit. LRAM, like similar earlier efforts, uses acoustic signals to transmit data underwater. This can be used to establish a one-way or two-way link over underwater distances up to a thousand kilometers. While such a system can be very useful in an emergency to contact a disabled sub on the sea bottom, or commercial research gear (fixed or mobile) navies have been looking for systems that can provide some communication with submerged subs, even if one way, in combat situations. LRAM seems capable of doing that and now navies are deciding whether to invest in equipping one or more of their subs for a test. The Canadian government has expressed interest in testing LRAM and if that is successful others will probably follow.
Since the 1990s the U.S. Navy has been trying to develop a practical method for communicating with submerged submarines. Much work went into finding ways to generate acoustic pulses that could be picked up over long distances. In 2008 American firm Raytheon completed the development of such a system, which enabled nuclear subs to communicate with the rest of the world that, normally, could not be done until the boat came close to the surface and poked a radio antenna above the surface. The Deep Siren, or "tactical paging system", provided a practical solution to the problem of communicating with a submerged sub. On paper, both LRAM and Deep Siren appear to operate in a similar fashion. The Deep Siren system consists of a disposable buoy, that is dropped in the water, by an aircraft or over the side of a ship, in the general area (within about 90 kilometers) where the sub is believed to be. The buoy sends out an acoustic signal that U.S. subs are equipped to automatically pick up. 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 a satellite telephone capability, so that additional messages can be sent from anywhere, to the sub. The sub cannot send messages to the buoy (because 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. These systems were less reliable than the new ones like Deep Siren, although these older systems had a much longer range of nearly 200 kilometers.
The navy successfully tested the other end of the system. To do this, the sub releases a similar buoy through its garbage chute. The buoy hovers for a while (so the sub can move away), then rises to the surface and sends its messages. Because of this, the buoy signal will not give away the exact location of the boat. The buoy then receives messages (short ones) and uses a sonar type device to send the data acoustically, for up to 90 kilometers, to the sub. Outgoing messages, which are sent via satellite, can be longer, and even include outgoing email from the crew to family. But the acoustically transmitted messages are much shorter, and include orders from the surface ships, or anyone in the chain of command, to the sub commander.
Deep Siren could also be useful for American carrier task forces, which are usually accompanied by at least one SSN (nuclear attack sub.) Because thermal layers make underwater transmissions vary a great deal in range, the buoy sends the command messages several times to ensure at least one gets through. The buoy from the sub can stay active for several days, if the sub is remaining in the area. But eventually, the buoy sinks itself.
The U.S. Navy spent about $10 million on Deep Siren to install it in some subs and test it. These tests continue, to see how reliable it would be under realistic conditions. Developer Raytheon has demonstrated Deep Siren for other navies but no one ever issued a press release about installing Deep Siren for regular use. If there were any problems with Deep Siren, there were no published details, possibly for security reasons. In the past new tech was installed in nuclear submarines with great secrecy and no publicity at all. No wonder subs are called the silent service.