Project E.L.F.

THE SUBMARINE LONG WAVE
The U.S. Navy’s Extremely Low Frequency
Communication System
  

The U.S. Navy classifies their offensive submarines into two large groups: the nuclear patrol/attack submarines (SSN) and the nuclear ballistic missile submarines (SSBN). The use of submarines as part of our nuclear attack force has imposed requirements for communication systems unlike any before. The SSBN’s carry fifty percent of the total strategic nuclear warheads (TRIDENT II) and have been declared the most survivable components of the nuclear triad of US strategic deterrent forces because they are so hard to detect. (Kaufman-Stillwell, 1993) Although the missiles have no pre-set targets when the submarine goes on patrol, the SSBN’s are capable of rapidly targeting and deploying their missiles should the need arise. To maintain positive command and control of this force, the Navy requires SSBN’s to receive continuous communications so they can respond to an attack. At the same time, SSBN’s must remain deeply submerged to maintain their covert nature. (Jane’s, 1997)

For over forty years, the Navy has been researching and developing various technologies to communicate with these submarines. A broad range of radio frequencies (VLF; very low frequency, LF; low frequency, HF; high frequency, and UHF; ultra high frequency) are presently used by the Navy for command and control of the naval fleet, but only the VLF signal can penetrate seawater. Unfortunately, even VLF penetration is limited to about forty feet. (White, F.C., 1980) This forces submarines to operate close to the surface ("come shallow"), or to tow a cable type antenna, which slows their speed and subjects them to detection by surface and air ships.

In 1958, as a result of the limited ability of VLF waves, the Navy began researching the use of ELF electromagnetic waves for communication. The acronym ELF, which stands for Extremely Low Frequencies, is one of a number of band designators defined by the Institute of Electrical and Electronics Engineers (IEEE) to name bands or ranges of the electromagnetic frequency spectrum. Some of the other designators, along with services or applications that use that frequency range are given in the following summary:

The use of ELF transmissions is based on the principle that energy penetration into a conducting medium such as seawater is inversely proportional to wavelength. ELF Carrier frequency waves (30-300 Hertz) have extremely long wavelengths (between 2000 and 3000 miles long). These waves can penetrate the sea down to about 400 feet. (Reder, F.H., 1981) This allows submarines to operate at normal depths ("run deep") and still receive communications when necessary.

One of the difficulties associated with the use of ELF for communication purposes, is the problem of generating a useful signal. The physical size of an antenna that can produce a useable signal with reasonable efficiency is inversely proportional to the frequency. For example, an antenna used for cellular telephone frequencies, need only be a few inches long to be completely effective. At ELF, on the other hand, a reasonably efficient antenna must be quite large. In fact, the current ELF antennas evolved from the US Navy experiment Project Sanguine. Sanguine transmitted at 40 to 80Hz from a 222km dipole antenna high up in the Laurentian Shield in Wisconsin. Project Sanguine was decommissioned in 1977 and upgraded to the 45km Project Seafarer antenna at Clam Lake, Wisconsin. In 1980, a second Seafarer Project upgrade featuring 90km wires was installed at Sawyer Air Force Base near Republic, Michigan. (Pike, J.)

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