Making waves: solutions for communications at depth

Improving the ability of a submarine to communicate with other naval assets when submerged has always represented a significant challenge to the sub-sea systems community, chiefly because the electrical conductivity of salt water creates an invisible shield that most electromagnetic transmissions find extremely difficult - if not impossible - to penetrate.

Traditionally, maritime commanders relied on one-way transmissions using relatively slow shore-to-submarine radio broadcasts, normally at frequencies in the Very Low Frequency (3-30 kHz) band, to pass orders and other mission-critical information to boats on covert operations.

Electromagnetic waves at these frequencies can penetrate water to a depth of between 10 m and 40 m, which is sufficient for submarines operating near the surface. Although vessels operating at greater depths can receive and transmit messages in other bands via cable-linked antenna buoys floating on or near the surface, this practice exposes them to a serious risk of detection by enemy sonar or radar.

Mechanically-generated sound waves, on the other hand, can travel very great distances in water. Data is encoded in the acoustic wave's amplitude, phase or frequency. Rudimentary analogue underwater telephones have been used with mixed success for many years; however, their ease of detection, propensity to distortion and data loss, and frustratingly poor range has necessitated the development of more capable systems.

Underwater acoustic communications can be affected by a range of influences including ambient noise, path loss, high and variable propagation delay, and the Doppler Effect (the change in frequency and wavelength of a wave as an observer moves relative to the source of the wave). All can result in degraded performance and interference in the transmission.

These factors in turn affect the spatial and temporal variability of the acoustic channel, making the bandwidth available for the acoustic channel very limited and dependent on range and frequency. A long-range system operating over tens of kilometres will use only a small bandwidth of a few kilohertz, while a short-range system will use many hundreds of kilohertz of bandwidth. Either system will still use a very low baud (bits per second) rate, but a consequence is the severely reduced ability to transmit larger volumes of data.

During the Cold War, nuclear-powered submarines from the Soviet Union, UK and United States navies were employed on long solo patrols with a requirement to remain silent throughout. Detailed pre-planning of missions was essential to allow boats to operate without the need for regular instructions and operational updates from headquarters.

Since 1991 and the demise of the Soviet Union, the role of the submarine has changed. The ability to influence littoral areas means that, as well as continuing its bluewater sea-denial strategy, the submarine has now become more involved in surface fleet operations and land-attack campaigns. In order to become a multirole platform and engage in fleet-based activities, the submarine must link up to the network-centric battlespace and communicate with naval or other assets on a regular basis and be able to send and receive relatively large amounts of data.

Image: The US Navy's Ohio-class guided-missile submarine USS Florida [SSGN 728] makes its way through Cumberland Sound to Kings Bay naval base in April 2006. The Communications at Speed and Depth programme will give improved capabilities to the Ohio class and other submarines. (US Navy) 496 of 3,693 words