OPERATING ON ELF FREQUENCIES

Edited by Jerry Proc, VE3FAB


Most of us never venture into the 'basement' of the radio spectrum, yet some amazing and unusual techniques are used down there to communicate with submerged submarines.

In mid-ocean, the only means of communication with a submerged submarine is to use radio in the VLF or ELF bands. The ability of radio signals to penetrate water depends on the following factors:

* the strength of the signal.
* the frequency being received.
* the efficiency of the receiving antenna.
* the salinity of the water surrounding the submarine.

In the middle of the Atlantic Ocean, with a salinity of 3.2 %, a VLF signal will penetrate down to a depth of 10 to 20 meters, barely periscope-depth for a modern day submarine. In areas with less salinity, like the Mediterranean Sea or in the brackish waters of the Baltic Sea, it will be possible to receive the same signal  to a depth in excess of 40 meters. Near the Atlantic coast where the salinity is less due to the run off from fresh water rivers, the receive depth could be even greater.

If cruising too close to the surface, a submarine might compromise its position to the searching eyes of a spy satellite or A/S aircraft. To achieve one-way, long distance communications with submarines down to about 330 feet or deeper, it is necessary to resort to an Extremely Low Frequency (ELF - 300 Hz to 3 KHz) system. A system such as this is very expensive mainly because of the great length of the antenna. The US 'Austere ELF' system for example, has an antenna some 200 km (125 miles) in length. ELF has two major disadvantages. It suffers from the inevitable natural noise in that part of the spectrum and also has a very low data rate. The Austere system is capable of passing just one three letter group in 15 minutes. With three letters it would be possible to signal some 17,576 codes (26 letters by 26 letters by 26 letters = 17,576). The most important use of the ELF system would be to act as a 'bell ringer' by instructing the submerged submarine to approach the surface and deploy a float or mast mounted antenna to receive a higher level, encrypted HF or satellite signal.

The original plan for the ELF exciter called for a chain of transmitters positioned in a line over 1,000 miles in length, with an antenna working against a well grounded and buried counterpoise. The intent was to couple RF energy directly into the Earth's core at its natural resonant frequency. Since the right-of-way crossed many farms, there were many objections from dairy farmers claiming that the RF might sour milk so the actual antenna system, as installed, is only a fraction of its original intended length. Even with a 'shortened' antenna, the idea did work since both the USN and the Soviet navy both used the system to reach submerged submarines.

There are two reasons for the very slow data rate. First, at ELF frequencies, the modulation rate must be extremely low. Secondly, the "code" which is sent is a repeated, error correcting code. As a result, the bit rate is few bits per minute repeated until enough data is accumulated to let the receiver decide if a letter has been received successfully. At the end of 15 minutes, it's expected that three correct characters have completed the journey. All messages sent in this manner instruct the submarine to do something which would not be catastrophic if there were an error. (Example - copy the 0800Z schedule NOT launch all nuclear weapons).

In the United States military, the primary means of communicating with submerged submarines is the TACAMO (Take Charge and Move Out) system which uses a fleet of aircraft. Two aircraft are always airborne - one over the Atlantic and one over the Pacific. Other aircraft are stationed on the ground and they are on a 15 minute alert. The aircraft fly 10.5 hour missions, starting at one airfield and ending at another. Random patterns are flown to mislead any unauthorized observers. The TACAMO aircraft can receive and relay signals from a number of different ground command posts. Each aircraft is equipped with a 6.2 mile long trailing wire antenna (wound on a reel) and a 100 kw transmitter operating in the VLF region. When the aircraft has to transmit a message, it banks and proceeds to fly a very tight circle. That causes the trailing wire antenna to hang vertically below. Once the message is transmitted over the VLF downlink the aircraft resumes normal flight.

The TACAMO fleet was initially comprised of the Lockheed Hercules EC130 aircraft, but these are being gradually phased out and replaced with the Boeing 747 AWACS type aircraft. These aircraft have the capability to transmit a 200 kw signal using a 2.5 mile trailing antenna. The systems described above were documented in a book printed in 1985. Due to unavailability of information, I do not know if the same system is still in use in the 1990's, but the techniques, nonetheless are quite intriguing.

Somehow the difficulty of fitting a 160 meter amateur radio antenna in a restricted space seems trivial when compared to the problems of seriously operating on VLF and ELF frequencies.



References:

1) Max Walmer, An Illustrated Guide to Strategic Weapons. Arco Military Books. 1985.
2) John Peters K1ER, CO (Ret'd), USN Submarine Service
 

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