Submarine Communication Overview
The U.S. Navy operates two extremely low frequency (ELF) radio transmitters to communicate with its deep diving submarines. The sites at Clam Lake, Wisconsin and Republic, Michigan are operated by the Naval Computer and Telecommunications Area Master Station – Atlantic. The Clam Lake site, located in the Chequamegon National Forest in Northern Wisconsin, is the site where testing began for ELF communications more than 30 years ago. The submarine communication site has more than 28 miles of over-head signal transmission line that form part of the “electrical” antenna to radiate the ELF signal from the two-acre transmitting facility.
The Clam Lake ELF radio station broadcasts messages to the fleet as required by the Navy Submarine Broadcast Control Authority in Norfolk, Virginia or Pearl Harbor, Hawaii. For the U.S. submarine fleet to perform its mission, it must remain silent and be undetectable.
The Navy’s ELF submarine communication system is the only operational communications system that can penetrate seawater to great depths and is virtually jam proof from both natural and man-made interference. It is a critical part of America’s national security in that it allows the submarine fleet to remain at depth and speed and maintain its stealth while remaining in communication with the national command authority.
With other submarine communication systems, continuous communication is possible only when submarines deploy a receiving antenna while operating at or near the surface. This requirement imposes an enormous restriction upon the submarine's operating depth and its speed, as well as increasing its exposure to detection.
ELF Submarine Communication
The ELF submarine communication system permits submarines to receive communications without reducing speed or operating at the surface. Thus, the ELF system represents a critical safeguard against a scientific breakthrough in submarine detection by another nation using aircraft or satellite systems that exploit non-acoustic phenomena such as kelvin wakes and internal waves near the surface.
ELF submarine communication systems make use of a principle in physics where the attenuation of radio signals (electromagnetic waves) from seawater increases with the frequency of the signal. This means that the lower the frequency a radio transmission, the deeper into the ocean a useable signal will travel.
Radio waves in the Very Low Frequency (VLF) band at frequencies of about 20,000 Hertz (Hz) penetrate seawater to depths of only tens of feet. The Navy’s ELF system operates at about 76 Hz, approximately two orders of magnitude lower than VLF. The result is that ELF waves penetrate seawater to depths of hundreds of feet, permitting submarine communication while maintaining stealth.
Each ELF antenna works as an independent horizontal electric dipole. The two ELF transmitting sites synchronize their transmissions to provide greater coverage to most of the earth’s oceans in which United States submarines operate. They are located geographically to take advantage of the bedrock layer (Precambrian metamorphic) and overlying rocks (Paleozoic) of the Superior Upland shield. This geological formation channels ELF currents deep into the ground and effectively increases the size of the antenna for more efficient signal transmission. The conductivity of the bedrock layer helps to improve the efficiency of the antenna system (that is, the lower the conductivity, the more improvement in effective transmitted power).
The areas chosen for the ELF system have low conductivity rock (rock that does not conduct electricity well) that produce the best results for creating an ELF antenna. In these areas, electrical current flows deep into the ground (hundreds of meters) before returning to the opposite antenna terminal ground.
The eight-watt ELF signal radiates from the dual-site system and travels around the world through the atmospheric layer between the earth's surface and a zone of charged air particles known as the ionosphere. As these electromagnetic waves pass over the ocean’s surface, some of their energy passes into the ocean. This energy, or signal, reaches submarines almost worldwide at depths of hundreds of feet and traveling at operational speeds.
All Navy submarines are equipped with ELF receivers that can decode ELF transmissions. ELF broadcast signals provide a one-way message system to submarines that is slow, but reliable. The submarines can receive ELF messages but they cannot transmit ELF signals because of the large power requirements, the large transmitter size, and the large antenna required to transmit ELF. Submarine communications can occur on or near the ocean’s surface with higher data rate systems such as satellite communications systems.
Submarine Communication in Wisconsin and Michigan
The Northern Wisconsin area was selected as a location for the Navy’s ELF facility because of its geology. The low conductivity bedrock is important because the ELF wave uses the bedrock to help complete the signal path for the antennas. The Navy selected the Clam Lake site for submarine communications because of these geological conditions and the opportunity to conduct the early research work on federal land and minimize or eliminate the need to disturb landowners, homeowners, and communities.
The ELF antenna’s layout in the Chequamegon National Forest uses techniques such as “screening” (using trees, changes in geography, and changes in antenna direction) to improve the visual appearance of the system in the forest. The creation of the antenna right-of-way also played an important role in the in the State of Wisconsin's successful reintroduction of elk into Northern Wisconsin.
The antenna right-of-way is about 75 feet wide, allowing elk and deer to move freely and quickly through the Chequamegon National Forest in the area near the 28 miles of antenna lines. Additionally, the Navy’s maintenance cycle of clearing brush in the right-of-way continually renews young plants that are important to elk, deer and other wildlife. The rights-of-way for the antenna and the grounding array are open to the public.
During the late 1950s, researchers and scientists theorized that submarine communication ELF radio waves could deeply penetrate the oceans and would permit communications with deep-diving nuclear powered submarines. This theory suggested the potential for a unique capability not available with other radio frequencies, and the U.S. Navy began testing in the ELF radio spectrum.
The Navy needed to determine the feasibility of building such a submarine communication system for sending messages to submerged submarines such as the Polaris missile boats. If ELF transmissions worked as hoped, Navy submarines would not have to rise to or near the surface to receive messages from the national command authority. This would allow the submarines to remain hidden at depth, be more difficult to detect, and improve operational safety while maintaining a link to the national command authority.
The ELF system of today — a two-transmitter site submarine communication system with the transmitter facilities and antenna system above ground — was evaluated over other submarine communication systems and ultimately chosen for implementation. Initially, the ELF transmitter and antenna system was envisioned to be very large and capable of transmitting control orders to the submarine fleet.
Submarine Communication 1970s and 1980s
During the mid-to-late 1970s, the Wisconsin Test Facility was used to send messages during a number of tests conducted on submarines in both Atlantic and Pacific Oceans and under the Arctic ice cap to assess the utility of the system. The Environmental Impact Statement was prepared and supplemented as required by changes in the system’s concept. After analyzing the results of the research and the various systems that might be employed, the current ELF communications system was selected because it was the smallest and most cost-effective system meeting the Navy’s mission requirements.
In the mid-1980s, the Wisconsin Test Facility was upgraded and redesignated as the Wisconsin Transmitter Facility, and construction of a second transmitter facility 148 miles away in Republic, Michigan was proposed. In 1985, the Clam Lake site attained an initial operating capability. In the Fall of 1989, when the Michigan site became fully operational, the Wisconsin site was renamed the Naval Radio Transmitter Facility Clam Lake.
The entire ELF submarine communication system became fully operational Oct.1, 1989 when the two transmitter sites began synchronized transmitting of an ELF broadcast to the submarine fleet 24-hours a day, 7-days a week.
An Antenna Well Grounds System was introduced in the mid-1980s, as part of a transmitter facility upgrade at the end of several of the ELF antenna lines, with a resulting improvement in safety. Well Grounding Arrays require less maintenance, reduce ground surface electrical potentials and minimize potential impacts to local habitat.
Based on the success of this previous ground terminal upgrade work, the Navy anticipates replacing the other existing grounds for the antenna at the Clam Lake site in the next several years. In conjunction with these upgrades, the Navy will work with the U.S. Forest Service – the manager of the Chequamegon National Forest – to ensure the required ecological, environmental, cultural and historic requirements are met.