Underwater Radio Communications using VLF/ELF

This is in continuation to already two articles published on Marine/Air Traffic communication and I thought it would be best to complete it like a trilogy by writing in the most complex form of radio communication, UNDERWATER RADIO PROPAGATION.

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 don't 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. In order to allow for communication with submerged submarines, several techniques are used.

VLF radio waves (3–30 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 / radar.

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).

Electromagnetic waves in the ELF frequency range (3–3000 Hz) (see also SLF) can travel through the oceans and reach submarines anywhere. Building an ELF transmitter is a formidable challenge, as they have to work at incredibly long wavelengths: The US Navy's system operates at 76 hertz the Soviet/Russian system (called ZEVS) at 82 hertz. The latter corresponds to a wavelength of 3658.5 kilometers. That is more than a quarter of the Earth's diameter. Obviously, the usual half-wavelength dipole antenna cannot be constructed, as it would spread across a large country.

A VLF Antenna site in US. A typical VLF Transmitter site occupies a huge surface area and stretches for kms mostly . An example is a Grid laid out in the form of a Six pointed star (Star of David), with the Main Transmitter Station (MTS) at the center and transmission towers at the points. There are 13+ Transmission towers totally. The grid is composed of a central mast and two concentric circles of six masts each.

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 Navy's system is called Submarine Satellite Information Exchange Sub-System (SSIXS), a component of the Navy Ultra High Frequency Satellite Communications System (UHF SATCOM).

A Radio room inside a submarine.

No wonder the Indian Navy attached high priority to submarine communications even decades ago and subsequently anticipated the importance of Very Low Frequency (VLF) underwater transmissions instantaneously. As part of an ambitious naval modernisation program, during the mid-1980s the Indian Navy had constructed a VLF broadcasting station in Tamil Nadu.

Although not publicly declared, it was reported that the United States, the undisputed leader of submarine communications actively collaborated in the project, which was completed in September 1986. This facility needs to be viewed as an “initial step” in the quest of development of underwater Very Low Frequency/Extremely Low Frequency (VLF/ELF) and laser communications for effective coordination of the submarines with the Indian NCA. The operational VLF facility is used by the Indian Navy to communicate with its SSK fleet of Russian Kilo Class and German Type 1500, with trailing communication buoys at periscope depth of 10 to 20-metres. After the nuclear-powered ATV become operational, the VLF facility will permit Indian National Command Authority to issue launch orders to the submerged ATVs at periscope depth.

INS Kattabomman, located at Tirunelveli, Tamil Nadu is home to a VLF (Very Low Frequency) station and communications network. It allows the Navy to communicate with its submarines, underwater, at long ranges. It can monitor surface vessels and can distinguish between merchant vessels and naval vessels over long distances in the Indian Ocean. It is the first of its kind in Asia and its technology was developed locally. VLF facilities exist at Chennai and Calcutta as well.

G.Vipin Shankar (SWL)