The Bat, the Whale, and the Secretive ELF
The world’s smallest flying mammal, the bat, has its own tiny sonar system. It emits sounds at high frequencies (around 20 kHz to 100 kHz) to find its way in the dark, locate tasty insects, and avoid predators. Bats also make sounds to communicate with one another. We can think of this as bat speech! Click on the below link to hear bats echolocating and communicating:
Sea creatures like whales also generate and listen for sounds. They use underwater echolocation techniques to navigate, hunt, and protect themselves, and sing “whale songs” to communicate with other whales over thousands of miles. You can listen to whales at this website:
https://www.environment.gov.au/marine/marine-species/cetaceans/whale-dolphins-sound
Like a bat and a whale, a submarine uses echolocation to sense things in its environment and avoid dangers, such as enemy submarines and torpedoes. A submarine’s active sonar emits pings in frequency ranges of 1 kHz to 10 kHz (wavelengths of about 1.5 to .15 meters) to listen for echoes off of other undersea objects. Its passive sonars quietly listen for other sources of sound in the ocean which may indicate the presence of a hazard.
Can an underwater submarine communicate over thousands of miles like a whale? Yes and no. Yes, a submarine can receive messages from a powerful land-based transmitter thousands of miles away. No, these communications don’t use sound waves like whales do. Instead, such messages are sent as electromagnetic (EM) waves that the submarine can detect.
How does this work? Remember that the wavelength of any wave is equal to its speed divided by its frequency. If it’s a sound wave, its speed is going to be something like 340 meters/second in air or 1500 meters/second in water, depending on conditions like temperature, pressure, salinity, etc. If it’s an EM wave, the wave will travel at the speed of light or 3 x 108 meters/second.
Submerged submarines can hear EM messages sent from land in the extremely low frequency (ELF) band (30 Hz – 300 Hz) and in the very low frequency (VLF) band (3 kHz -30 kHz). ELF and VLF waves have wavelengths of:
lELF = 107 – 106 meters = 10,000 – 1000 km = 6214 – 621.4 miles
lVLF = 105 – 104 meters = 100 – 10 km = 62.1 – 6.2 miles
Wow, based on what we already know about antennas, don’t ELF and VLF signals require huge antennas? Yes they do. Let’s look at a specific example.
Project ELF was an Extremely Low Frequency communications project undertaken by the U.S. Navy which became operational in 1989. Its goal was to communicate with deeply submerged submarines. The system consisted of two high power transmitters located 145 miles apart in Michigan and Wisconsin. The effective size of the antenna hosted across the two sites was big enough to allow an ELF wave of 76 Hz (and a wavelength of 2,452 miles!) to be heard by a submarine deep underwater. The submarine needed an antenna of several hundred yards in length to hear the signal. You can read more about ELF at:
https://pages.hep.wisc.edu/~prepost/ELF.pdf
The narrowness of the ELF band (300-30 = 270 Hz) constrains the data transmission rate severely. Just like we talked about before, this is another example where we see that channel capacity depends on bandwidth (narrow), signal power (millions of watts), and noise (variable). Estimates put the ELF project data rate in the range of 10-4 to 1 bit per second, and that’s a transmit-only communication to the submarine.
Why use ELF if so little data can be transmitted? The idea is that submarines can receive their regular communications via higher frequency radio waves when they are surfaced or near the surface. ELF provides a backup system to communicate with submarines that are more deeply submerged. An ELF message alerts a submarine that it needs to “phone home”. When it is safe to do so, the submarine can maneuver to a shallower depth or the surface so it can communicate using higher capacity RF communications channels.
How much information can be really be transmitted in a few symbols? Ah-ha, that’s where data compression comes in! As the article tells us, “the Navy feels that low data rates present no problems, since signals of just a few letters can convey quite a bit of pre-encoded information.”
There is one final note to be made here. Navies are very secretive about their submarines. They are also very secretive about their back-up communication bands like ELF and VLF. The cited article is a reliable source in the public domain about one U.S. Navy ELF system. I’m sure there are many others, but you’re probably not going to read about them on the internet!