Staying connected underwater

Written by Huma Siddiqui | Updated: Aug 18 2014, 06:40am hrs
Underwater radio communication is an important component of global technology in the 21st century, but at the same time continues to be a very challenging part as it has many uses in various spheres including underwater diving, deep sea exploration, oil exploration, naval submarine operations etc.

Communication with underwater submarines is difficult because radio waves dont travel well through thick electrical conductors like salt water. The obvious solution is to surface and raise an antenna above the water, then use ordinary radio transmissions. Early submarines had to frequently surface anyway for the oxygen needed by their diesel engines. During the Cold War, however, nuclear-powered submarines were developed that could stay submerged for months.

Now, with India planning a larger fleet of nuclear-powered submarines, which can prowl underwater for several months at a time, the Indian Navy has acquired a new advanced facility to communicate with the silent predators. India is among a handful of nations in the world that has state-of-the-art very low frequency (VLF) transmitting station. This new facility would provide a boost to the Navys ability to communicate with deployed ships and submarines on an uninterrupted basis throughout the year. Admiral RK Dhowan, Chief of the Naval Staff, inaugurated the new transmitting station at INS Kattaboman, Tirunelvelli, Tamil Nadu recently.

The new facility incorporates modern technology and will provide the Navy enhanced reach, redundancy and operational features. Indian Navy has an elaborate communication infrastructure, including modern satellite communication facilities, to link and network its deployed units around the world with their home bases and command and control centres. The new VLF station will strengthen this infrastructure and provide the Navy additional operational advantages.

The new facility equipment has been constructed by Larsen & Turbo divisions in Chennai and Bengaluru. Interestingly, the facility boasts of the highest masts structures in India, as well as several other unique engineering feats. VLF transmitters require huge antennae suspended high above the ground. These antennaes, in India, are developed by DRDO.

VLF radio waves (330 kHz) can penetrate seawater to a depth of approximately 20 meters. Hence a submarine at shallow depth can use these frequencies. A vessel more deeply submerged might use a buoy on a long cable equipped with an antenna. The buoy rises to a few meters below the surface, and may be small enough to remain undetected by enemy sonar.

Due to the low frequency, a VLF broadcast aerial needs to be quite big. In fact, broadcasting sites are usually a few square kilometers (or miles). This of course prevents such aerials being installed on submarines. Submarines only carry a VLF reception aerial, and do not respond on such low frequencies. So a ground-to-submarine VLF broadcast is always a one way broadcast, originating on the ground and received aboard the ship. If two-way communication is needed, the ship must surface and communicate on other, higher, frequencies (such as UHF or VHF).

A surfaced submarine can use ordinary radio communications. Submarines may use naval HF, VHF and UHF voice and teleprinter circuits. Where available, dedicated military communications satellite systems are preferred for long distance communications, as HF may betray the location of the submarine. The US Navys system is called Submarine Satellite Information Exchange Sub-System (SSIXS), a component of the Navy Ultra High Frequency Satellite Communications System (UHF SATCOM).

A couple of years ago, the Indian Navy had decided to set up a very low frequency (VLF) communication station near Hyderabad, due to its location, and keep tabs on Arabian Sea, Bay of Bengal and Indian Ocean.

Because of their large wavelengths, VLF radio waves can diffract around large obstacles and so are not blocked by mountain ranges, and can propagate as a ground wave following the curvature of the Earth. Transmitting antennas for VLF frequencies are very large wire antennas. They consist of a series of steel radio masts, linked at the top with a network of cables, shaped like an umbrella. VLF is used to communicate with submarines near the surface, while ELF is used for submerged vessels. VLF is also used in electromagnetic geophysical surveys.