Sep 6, 2009

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)

Sep 2, 2009

Chennai Airport VHF Radio Communication Infrastructure having problems

Subsequent to the earlier article , this interesting article appeared in TOI newspaper today 2nd Sept 2009.

Pilots, ATC fume as radio at city airport cracks up
Arun Ram TNN Chennai: The sole area control VHF radio at Chennai airport, which monitors aircraft movements up to 200 miles, is cracking up in the face of increasing traffic, putting pilots and controllers under severe stress. Though the air traffic control (ATC) is using a second area control frequency on a trial basis, it is unable to make it fully functional because of a shortage of personnel. The situation could prove risky during monsoon, when pilots ask for urgent deviations to avoid turbulence. Unlike in Delhi and Mumbai (which have 2 frequencies) and Kolkata (which has 3), Chennai has only one frequency (118.9 Mhz) for area control.

Chennai handles about 350 take-offs and landings daily, and a controller has to monitor and interact with up to 30 aircraft during peak hours. This results in overlapping of conversation between several cockpits and the ATC. Pilots call it ‘stepping down’. “When I am stepped down, all I get to hear is a buzz,” a pilot told TOI. “Normally, it may not be dangerous as I come under the approach radar once I am 50 nautical miles from the tower. But during monsoon, when I may have to request for urgent deviations, it could prove shaky.” ATC officers said such overlaps often happen. “In two years, traffic has doubled but upgradation of gadgets and manpower has not happened,” said an ATC official.

Air traffic management executive director V Somasundaram told TOI that the Airports Authority of India was planning upgradation of facilities across the country, but denied having problems with the Chennai area control VHF. Radar blacks out for 4 hours Hundreds of passengers at Chennai airport were put to hardship as flight movements were disrupted for about four hours on Tuesday because of radar failure.

The approach radar at the ATC went blank after an earthmover snapped an underground cable. P 4 AIR POCKET Trouble in air as gadgets fail INADEQUATE MANPOWER DESPITE TRAFFIC DOUBLING IN 3 YEARS Chennai: Too few men grappling with too old machines — that’s what Chennai air traffic control is. While the approach radar conks off often, as it happened on Tuesday morning, the single area control VHF used to communicate with pilots up to 200 nautical miles is proving too inadequate for the exploding traffic. “It was okay to have just one VHF till three years ago, when the airport was handling only about 150 movements a day. The traffic has doubled since then. Though we are trying out a second frequency, 124.45 MHz, to monitor aircraft between 80 nautical miles and 40 nautical miles, we don’t have enough people to operate both the frequencies simultaneously. We need at least 200 controllers, but we have hardly 140 people working in four shifts,” said an air traffic official.

Unlike Chennai air traffic control, which has only one area control frequency, Delhi and Mumbai have two, and Kolkata has three. All hell breaks lose at the control tower when the ageing approach radar that monitors flight movements within a radius of 50 nautical miles also fails, sending the controllers scurrying for pen and paper to make manual calculations of aircraft positions. Adding to the controllers’ nightmare are the confusing call signs (code names) of aircraft. There is no uniformity in terms of number of digits; some have two digits, others three or four. Three different airlines have call signs such as 442, 2442 and 3442. “During peak hours, with the VHF acting up, we may not hear one digit and may end up giving instructions to the wrong pilot,” said an air traffic controller.

When contacted, air traffic management executive director V Somasundaram said he had not got any complaint on the area control VHF malfunctioning. “Anyway, we are planning to upgrade the system across the country. Chennai will have a new system called performancebased navigation, which would make air traffic control more effective from October. As for manpower, we keep enhancing the strength as per the requirements,” he said. Chennai ATC has a wide range, extending to the Andaman and Nicobar Islands in the east, Thiruvananthapuram in the south, Hyderabad in the north and almost till Goa in the west. Besides handling landings and takeoffs, Chennai also has to monitor movements along busy overhead flight paths like the Singapore-Dubai route. The control tower at the airport monitors aircraft movements within a radius of 10 nautical miles using 118.1 MHz frequency. Then there is the monopulse secondary surveillance radar (MSSR), called the approach radar, which has a range of 50 nautical miles. Beyond this and up to 200 nautical miles, the communication is handled by the area control VHF on 118.9 MHz frequency.

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