Long-range communications at extremely low frequencies

Extremely low frequencies (ELF) are ideal for communicating with deeply submerged submarines because, below 100 Hz, electromagnetic waves penetrate deeply into seawater. Propagation at these frequencies takes place in the "waveguide" formed between the earth and ionosphere; low propagation losses allow nearly worldwide communication from a single transmitter. Atmospheric noise at these frequencies is caused entirely by lightning; the non-Gaussian nature of this noise has important implications for receiver design and performance. Because the ratio of ELF transmitter input power to radiated power is very low, it is important that the modulation/coding technique employed make very efficient use of received signal energy. Such an efficient receiver has been implemented on a small digital computer and has been used to receive messages aboard a submarine crossing the Atlantic Ocean.