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Siren of the Deep Sings Digital Song

December 2000
By Christian B. Sheehy

Experimental sonar enhances long-range communications capability on submarines.

A team of Scottish researchers is pursuing the design and development of an advanced sonar system that will enable personnel on board tactical surface and air units to communicate with submarines cruising at operational depths without revealing their positions. The technology addresses a growing demand for systems that can deliver critical data to hard-to-reach units to improve interoperability and unify command network connectivity.

Sound conduction limitations in an ocean environment usually restrict communications between surface ships and submarines to ranges of 10 kilometers or less because variations in water temperature, salinity and motion distort the signals. A sonar system called Deep Siren developed by RRK Technologies Limited, Glasgow, Scotland, could change this by using very low frequency sonar that would overcome these conditions and increase the range of data transmissions to between 70 and 200 kilometers. Submarines could maintain continuous connectivity with a task force while remaining on station throughout maneuvers.

Normal task force operations require submarines to ascend to a depth of 19.5 meters every 6 hours to send and receive mission updates over satellite radio. After assuming station at a designated cruising depth, they are essentially out of contact with a task force’s surface and air units. Traditional underwater telephony allows surface ships a communication range of only 5 to 10 kilometers before the signal fades due to attenuation. As a result, submarines must climb to periscope depth to exchange necessary information by radio antenna and then return to station. This entire process can take at least an hour and a half to complete, making submarines vulnerable to detection by enemy sonar.

According to Robert Kerr, president and founder of RRK Technologies, the Deep Siren system will improve integration between ships and submarines by providing a constant data pathway to submerged units while they remain on station. “Giving a task force the ability to communicate with submarines at any time during an operation while maintaining a patrol at a safe depth, away from possible enemy detection, is the main idea,” he offers.

Designed initially for use on surface vessels, Deep Siren technology began taking form in the early 1990s. A fundamental deficiency in communications connectivity between surface ships and submarines cruising at distant geographic coordinates raised some concerns. In wartime situations, for example, friendly units could easily become disoriented in relation to other units within a given task force. To address this issue, researchers in Glasgow began experimenting with ways sonar might be used for secure ship-to-ship communications.

Primarily used for object detection, sound waves can carry data over long distances. With this in mind, sonar engineers designed Deep Siren based on a three-part configuration. A ship-mounted transmitter sends an initial electronic signal to the second element, a transducer. This device converts the electrical signal to sound and emits a low frequency ping that can be picked up by the system’s third component, a fin-mounted receiver/transmitter located on a submarine.

Using a software program installed in a personal computer at both ship and submarine end points, the transmitter amplifies an electronic data message for release down a winch-controlled hard wire towline by means of a digital sound processor card. The signal is recognizable by a transducer housed in a stingray-shaped flotation device—called a tow body or batwing—that is towed 606 meters behind and 150 meters below the vessel.

According to Ian Smith, technical manager for RRK Technologies, water less than 50 meters deep is a poor environment for sound propagation due to variations in temperature, pressure and salinity in the layer near the surface. Clear sound transmission decreases throughout this layer due to attenuation and downward refraction which, in turn, shortens the range of sonar signals. Sudden changes in water temperature and salinity can redirect the path of sound waves into a downward refractory profile (DRP). Bent toward the sea floor, the sound trails off into the depths, never reaching its intended destination.

Using a thermograph, ships can determine the approximate depth of the bottom edge of this layer. To avoid DRP, the transducer is positioned below it, minimizing interference. Existing hull-mounted sonar systems avoid these variances by increasing transmission frequency to maintain an optimal pathway. The problem with this approach for submarine communications is that as sonar frequencies increase, the distance the signal travels decreases due to a rapid attenuation of energy into the environment, Smith indicates. The resulting change in signal pattern leads to the corruption of the carried message. Deep Siren receiver/transmitters can tolerate up to 25 percent signal corruption and still produce a clean transmission.

Once the amplified signal reaches the batwing, the internal transducer converts the digital message into low frequency sound waves. To direct the signal better, the tow body was designed with a hydrodynamic shape that allows the device to maintain a relatively level path through the water while minimizing drag. The towing ship’s velocity controls the forward momentum of the batwing, which ascends as the ship accelerates and descends as the ship decelerates. The winch can also be used to keep the transducer at an optimal depth by bringing in or letting out the towline, he notes.

Deep Siren technology, which takes advantage of the fact that low sonar frequencies travel farther than high ones, uses a very low frequency channel of less than 2 kilohertz. Sea trials conducted with the U.S. Navy in 1999 determined that under optimal water temperature and salinity conditions, the system propagates signals up to a range of 150 kilometers. Although information can be sent over longer distances, limitations on bandwidth availability at lower frequencies present problems for carrying high-data-rate transmissions.

“Because the density of water layers can vary due to temperature and salinity, sound can be distorted at the interfaces between these layers,” Smith explains. “The choice of frequency is dependent upon the amount of power available and the attenuation that the frequency is subject to.” The corruptibility of data transmissions traveling through the ocean has been the major obstacle to advances in sonar technology. Digitized signaling has done much to ensure that transmissions are received in their original format, he says.

In digital message coding, numeric values are assigned to each piece of data and remain with the data no matter how scrambled it can become during a transmission. Unlike analog transmissions that depend on medium conditions for the degree of clarity in which they are received, digital signals cannot be misinterpreted unless equipment fails or the message does not reach the receiver. Deep Siren technology uses digital message processing to ensure that the receiver can tolerate a relative Doppler advance or retreat rate of 32 knots without incurring any measurable interference.

In addition to the importance of message clarity, maintaining a secure communications environment between participating units during military operations is paramount to a mission’s integrity. “Before digital signal processing became widespread, analog systems were subject to interception by enemy sonar,” Smith notes. “At present, submarines communicate using radio systems at periscope depth or on the surface. Sonar systems are still used primarily for contact detection. Deep Siren uses today’s digital signaling capabilities at lower frequencies to achieve encrypted sonar communication at a substantial range to a submarine at depth.” The encryption of signals permits more liberal communication from ship to submarine. Enemy units can pick up the signals, but they cannot decode them.

The system on board a submarine receives data by way of hull-mounted sonar transducers. Omni transducers allow a 360-degree scan around the ship, absorbing any sound waves that contact the vessel’s skin. Once a signal is accepted, it is digitally authenticated and reconstituted by a transmitter/transducer into a readable message. Recognized by its frequency domain, data is processed first for source validity and then for content identification. Submarine receivers comprise both the transmitter and transducer elements of the Deep Siren system. They translate signals from sound to digital electronic format and then read the code presented.

“In most cases, the data sent to a submarine will not require an immediate response to the transmitting surface vessel,” Smith indicates. “A response to a message using sonar is not a requirement since the submarine’s position could easily be compromised by such a transmission. Submarines will normally ascend at given intervals to deliver return radio messages by satellite.” For radio communication with surface ships, an antenna must be projected out of the water for signal reception. High-definition radar, aircraft and lookouts can often see an antenna wake, so this maneuver is normally performed at night, he notes.

RRK Technologies also is experimenting with incorporating the Deep Siren transmitter and transducer assembly into an aircraft-dropped, A-sized sonobuoy for use in operational communications between a task force’s air and submerged units. The sonobuoy would be expendable and would contain prerecorded messages triggered by a radio signal from the aircraft. “In theory, a submarine could send a sonar response signal back to the transducer for projection as an electronic or radio signal to a ship or aircraft. In reality, however, the submarine would be putting a traceable signal into the water that could potentially give its coordinates away,” Smith explains.

Although the system has only been tested for use in ship-to-submarine communications, the basic design proved compatible in aircraft-to-submarine and submarine-to-submarine transmissions, Kerr says. As submarine-to-submarine communication is generally restricted to surface radio-to-radio transmissions, engaging in traceable sonar activity is possible but not practical because it could reveal key coordinate information to enemies within monitoring range. Deep Siren was designed with the capability to accommodate intersubmarine sonar connectivity; however, for these stealth-related reasons, testing is not being pursued in this area. A future fin-mounted transmitter-transducer element could support two-way submarine contact.

“The system remains primarily under consideration for its potential application to the military in the form of surface ship-to-sub signaling,” Kerr reports. “Testing for civilian uses may come in arenas such as seabed seismic research and oil drilling.” Extending a command’s ability to control the tactical aspects of an operation continues to be the primary aim for the implementation of Deep Siren technology. Further sea trials to test maximum range capabilities are scheduled for the spring of 2001. Civilian companies could begin bidding as early as next summer, Kerr adds.